Wallerian degeneration: It is a group of degenerative changes occur at the distal segment of the nerve fiber. It includes, swelling of the nerve terminals, disappearance of the secretory vesicles, breakdown of the neurofibrills, and lysis of myelin

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Wallerian degeneration It is a group of
degenerative changes occur at the distal segment
of the nerve fiber. It includes, swelling of the
nerve terminals, disappearance of the secretory
vesicles, breakdown of the neurofibrills, and
lysis of myelin sheath into fat droplets. The
remnants of degeneration are taken by the tissue
macrophage cells, and the neurolemmal membrane
becomes empty tube.
Retrograde degeneration It is a group of
degenerative changes occur in the cell body
includes, swelling of the cell and disappearance
of the dendrites, disappearance of Nissel
granules and fragmentation of Goli apparatus, the
nucleus becomes eccentric and may
disappear(chromatolysis) in case of cell death.
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Trans-degeneration It is the degeneration of
one neuron if another neuron degenerates. The
degenerated neuron should be the only stimulatory
neuron of this other neuron. This condition is
rare in the CNS, it occurs in the cells of the
lateral geniculate body if the fibers of the
optic nerve get damaged.
Regeneration It starts after 20 days and
completed within 80 days if the gap between the
two ends not more than 4mm.The proximal end of
the axon grows inside the neurolemmal tube, while
the cell body regains its normal shape and
structure.
Cross-regeneration It is the growth of the
proximal end into distal end of another neuron.
This could occur in sensory as well as motor
nerves. Regeneration occurs only in the fibers
with similar transmitter.
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Functional organization of the nervous system

• Sensory division
• receptors-----afferent----tract----brain
  centers
• Centers of integrative function
• spinal cord
• lower brain level
• higher brain level
• Motor division
• Integrating centers-----efferent------effector
  organ

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The receptors

• Definition
• 1-Terminals of sensory nerve as naked free nerve
  endings or of specialized shape.
• 2-Specialized neuro-epithelium as taste bunds at
  which the sensory nerve terminals are present.
• Function
• 1-Detect the stimulus (detector).
• 2-Transform the stimulus (physical) into
  electrical receptor potential (transformer).
• 3-Conduct the stimulus as R. potential into the
  afferent nerve as action potential (conductor)

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• Properties of the receptors
• 1-Specificity Mullers law
• Each receptor has its specific stimulus to which
  it is highly sensitive
• (adequate stimulus). The receptor responds to its
  adequate stimulus at low intensity.
• 2-Excitability
• Receptors are excitable structures, they respond
  to their stimuli (physical) by producing receptor
  potential (electrical), called receptor generator
  potential, when reaches certain magnitude
  (threshold valuefiring level) leads to action
  potential in the nerve (AP).

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• R. generator potentialDefinition It is a
  state of localized depolarization of the
  receptor, once reaches threshold value, it
  becomes able to produce an action potential in
  the nerve.
• It is studied in pacinian corpuscle, it is
  large receptor contains unmyelinated nerve
  ending, 1st. Node of ranvier and covered by
  concentric layers of connective tissue. At rest,
  the receptor potential is about -70mv. The
  adequate stimulus (pressure) produces change in
  the shape of the receptor that opens non-specific
  Na channels leading to Na influx. This produces
  local depolarization of the receptor, when it
  reaches 10mv, then the receptor potential becomes
  -60mv which is the threshold value to be
  conducted passively to the 1st. Node of ranvier
  to fire an action potential in the afferent
  nerve.
• When the stimulus produces more depolarization
  to the receptor (less than -60mv), the frequency
  of AP increases.The action potential of the
  nerve is due to activation of voltage-gated Na
  channels

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• Properties of generator potential
• 1- It is a local depolarization of the receptor.
• 2- Its duration is about 5msec. (AP duration is
  2msec).
• 3- Can be summated to reach threshold value
  leading to AP in the nerve.
• 4- It is not followed by absolute refractory
  period (ARP).
• 5- It does not follow all or non role.
• 6- It is conducted passively (electronic
  conduction) to the 1st. Node of ranvier.
• 7- It is due to activation of non-specific
  Na-channels.

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• Cont.Properties of receptors
• 3- Discharge of impulse
• Receptors change the frequency of discharge of
  impulse (rate of AP) in
• the nerve according to the intensity of the
  stimulus a process called
• frequency-modulation response.
• Weber-Fechners law
• It states that the frequency of discharge of
  impulses in the afferent
• nerve is proportional to the log intensity of the
  stimulus, that is when
• the intensity of the stimulus increases 1000
  times the frequency
• increases only 3 times (log 1000). So, the
  receptor compacts the
• intensity, which is called compression function
  of the receptor.
• Power principle
• R KSA
• R -----sensation S---- intensity K A
  are constant
• Significance It makes the nervous system able to
  discriminate an extremely wide range of stimuli
  intensities.

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• Cont. properties of receptors
• 4-Adaptation
• It is the decrease in activity of the receptor in
  response to maintained stimulus
• of fixed intensity.
• Mechanism
• 1- Gradual closure of the non-specific
  Na-channels.
• 2- Decreased excitability of the 1st. Node f
  Ranvier.
• 3- loss of stimulus energy to surrounding
  tissues.
• 4- Accomodation of the afferent nerve to receptor
  stimulation.
• NB adaptation differs from fatigue that occurs
  in muscle, since fatigue is due to repeated
  activity , due to accumulation of lactic acid,
  increases by hypoxia and of slow onset and
  recovery.

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Adaptation of receptor

• 1-Rapidly adapting receptors phasic R.
• The rate of decreased activity occurs rapidly as
  for touch receptors (Meissners and Pacinian
  corpuscles). It is to prevent unnecessary
  annoying continuous stimulation.
• 2-Intermediatly adapting receptors
• The rate of adaptation is less than the rapidly
  adapting R. as taste and smell receptors as well
  as thermo receptors(20-40C for warm receptor).
• 3-Slowly adapting receptors tonic R.
• These receptors show continuous activity to keep
  their important function as pain , proprioceptors
  in muscle to keep muscle tone and baroreceptors
  to regulate the BP

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Coding of sensory information

• It is the ability of the central nervous system
  to discriminate the modality, locality and
  intensity of the stimulus.
• Discrimination of modality
• It depends on the nervous pathway that links
  between the receptor and the higher center in the
  cortex. Each pathway is labeled for specific
  modality (labeled-line principle).
• Discrimination of locality
• It also depends on the pathway form the receptor
  to the center, at center the stimulus is
  projected to the area where the stimulated site
  is represented (law of projection)..
• NB After limb amputation, stimulation of
  receptors at the stimulated stump gives rise to
  sensation in the amputated limb (phantom-limb)
  because the cortex projects the stimulus to the
  area of the amputated limb that is still present
  in the cortex.
• Discrimination of intensity
• The strength of the stimulus depends on
• 1- number of stimulated receptors.
• 2-frequency of impulses in the afferent nerve.
• 3-condition of the center, is it facilitated or
  inhibited

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Classification of receptors

• 1-mechanoreceptors
• a-Touch R. in skin. bPressure R. in skin and
  deep structures. c- proprioceptors in muscles and
  joints. dBaro-R. in CVS. eStretch R. in lung
  and hollow organs. fsound R. in ear.
• 2-Thermoreceptors
• a-R. for warm. bR. for cold
• 3-Nociceptors
• -pain R.
• 4-Chemoreceptors
• a-Smell R. b- taste R. c- glucoreceptors in
  hypothalamus d-chemo-R to control respiration.
• 5-Electro-magnetic receptors
• a-rod-R b- cone-R. they present in retina for
  vision.

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Different types of receptors
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Somatic sensation

• Mechanoreceptive sensation
• 1-Touch
• A-Crude touch
• It is felt from skin, poorly localized and tested
  by moving a piece of cotton over the skin.
• Receptors free nerve ending and hair end organ.
• Afferent A-delta fiber.
• Tract ventral-spinothalamic.
• Center for perception Thalamus.
• B-Fine touch
• 1-Tactile localization. 2-Tactile
  discrimination.
• 3-Sterognosis . 4-Texture
  discrimination

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Types of sensations

• 1-Somatic originates from skin and deeper
  structures.
• It is divided into a) Mechnoreceptive. B)
  Thermoreceptive. C) Pain.
• 2-Visceraloriginate from viscera.
• 3-Special senses originate from eye, ear and
  nose.
• 4-Organic(vegetative) for hunger, thirst and sex
  desire.

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B-Fine touch

• Receptors Meissners corpuscle and Merkels
  disks.
• Afferent A-beta fiber
• Tract Gracile and cuneate.
• Center Sensory cortex.
• 1-Tactile localization
• It is the ability to precisely localize the
  stimulated site. It is done by using a marker
  pen.
• 2-Tactile discrimination (2-point
  discrimination)
• It is the ability to discriminate 2 points of
  stimulation at the same time providing the
  distance between the two stimuli is not less than
  threshold value. It is highly developed in finger
  tips, lips and tip of nose 2-3mm, while it is
  less developed at shoulders and back 60mm. It is
  tested by using a compass

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• Cont. 2-point discrimination
• NB
• - If the two stimulated points distance is less
  than the threshold value, the stimuli converge to
  a single neuron and reach the cortex as a single
  stimulus.
• - It is highly developed at finger tips and
  lips due to rich nerve supply with few
  convergence, while it is less developed in back
  and shoulders due to few innervations and more
  convergence.
• -2-points discrimination could be applied for
  other sensations as for vision to test visual
  acuity.

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3-Stereognosis

• It is the ability to identify the nature of a
  familiar object (key) without seeing it. It is
  high quality sensation in which different
  receptors are stimulated as touch, deep pressure
  and thermal receptors .It is an educated
  sensation that needs the sensory cortex and
  previous experience.
• Tract of conduction gracile and cuneate.
• Center sensory cortex.
• 4-Texture of material
• It is a type of stereognosis to identify texture
  of the material (cotton, wool or silk), its
  characters are similar to that of stereognosis.

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2-Vibration sense

• It is the ability to feel the sensation of
  vibration (thrill). It is tested by putting the
  handle of a tuning fork over a bonny prominence
  to magnify the vibration waves.
• Receptors Pacinian corpuscle, stimulated by
  vibration up to 700Hz.
• Meissners corpuscle, stimulated by vibration up
  to 80Hz.
• Pacinian and Meissners corpuscles are rapidly
  adapting receptors. Vibration is not a maintained
  stimulus, it is a rapidly repeated stimulation,
  therefore, the receptors remain stimulated during
  the whole time of vibration.
• Tract gracile and cuneate.
• Center sensory cortex.
• NB Vibration sense is used to test the function
  of gracile and cuneate tract, absence of
  vibration sense is most likely an indication of
  damage of the tract.

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3-Sense of Pressure

• It is the ability to discriminate between weights
  without lifting them. It is tested by placing
  different weights on a supported hand. The
  weights stimulate superficial and deep receptor
  to give rise the sensation of pressure.
• Receptors Pacinian corpuscles Ruffinis end
  organ.
• Afferent A-beta fiber
• Tract gracile and cuneate
• Center sensory cortex

4- Sense of muscle tension
It is the ability to discriminate different
muscle tensions due to its contraction, it is
tested discriminating different weights lifted by
the muscle Receptor Golgi tendon organ Afferent
A-beta fiber Tract gracile and cuneate Center
sensory cortex
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5-Proprioceptive sensation

• It is the sensation that originates from the
  muscles and joints to give rise the feeling of
  body position in space and the feeling of
  movements of different body parts. It is divided
  into
• A-Static proprioceptive sense of position
• It is the ability to feel the position of the
  body (without movement) and the position of
  different parts of the body in relation to each
  others
• It is tested by putting the arm of the subject in
  certain position and ask him to put the other arm
  in the same position, while closing his eyes.
• Receptors muscle spindle, Golgi tendon organ
  and Ruffinis ending,
• NB All of the previous receptors are slowly
  adapting.

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6-Itch and tickle sensation

• B-Dynamic proprioception kinesthetic sensation
• It is the ability to feel the direction and
  extent of the movement of body parts. It is
  tested by moving the subject finger or toe and
  ask him to tell the beginning and the end of the
  movement, while his eyes are closed.
• Receptors Pacinian corpuscle, Golgi tendon
  organ- like receptors
• NB the previous receptors are rapidly adapting.
• Afferent for proprioceptive sensation A-beta
• Tract gracile and cuneate
• Center sensory cortex

Tickle sensation could cause to laugh, while itch
is annoying sensation form irritated
skin. Receptor free nerve ending. Afferent
C-unmyelinated fibers. Tract Ventral
spinothalamic. Center Thalamus. NB itching
initiate scratch reflex that suppresses itching
through lateral inhibition mechanism (see later)
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Thermoreceptive sensation

• Receptors
• Interoceptors present in hypothalamus and
  internal organs. It records the core (internal)
  body temperature.
• Extroceptors present in the skin and records the
  skin temperature.
• Cold sensation
• Receptors Krauses end bulb and free nerve
  endings.
• They are stimulated by temperature range of
  10C-35C and maximum at 25C.
• Paradoxical cold sensation
• It occurs at temperature of 45C -50C.
• Afferent A-delta for Krause end bulb.
• C-thin unmyelinated fiber for free
  nerve ending.
• Tract lateral spinothalamic.
• Center Thalamus for crude sensation.
• Sensory cortex for fine sensation.

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Warmth sensation

• Receptors free nerve endings
• They are stimulated by temperature range of 25C
  -45C and maximum at 37C.
• Afferent C-thin unmyelinated fibers.
• Tract lateral spinothalamic.
• Center Thalamus for crude sensation.
• Sensory cortex for fine sensation.

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Cont.Thermoreceptive sensation

• NB
• -The thermoreceptors are moderately adapting
  receptors, however, the warmth receptors are more
  rapidly adapting than cold receptors.
• -Cold receptors are more numerous.
• -Crude thermoreceptive sensation can not
  discriminate the difference between fine grades
  of temperature specially at 20 C -40 C. it is
  sensation of either warm or cold, it is perceived
  at the level of thalamus.
• -Fine thermoreceptive sensation can discriminate
  between fine grades of temperature specially
  between20C -40C, it is perceived at the level
  of sensory cortex

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Types of the afferent nerve fibers

• Types of nerve fibers
• l-A-alph for proprioception, diameter 12-20um,
  conduction velocity 80-120m/sec.
• II-A-beta for fine touch, stereognosis,pressure
  and vibration, diameter 6-12um, conduction
  velocity 35-75m/sec
• III-A-delta for fast pain, temperature and crude
  touch,diameter1-6um, conduction velocity
  5-30m/sec.
• IV-C-unmyelinated for slow pain, temperature,
  itch and tickle, diameter less than 1um,
  conduction velocity 0.5-2m/sec.

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The ascending pathways

• Lateral spinothalamic
• It transmits, pain, temperature and sexual
  sensations. It is divided into two tracts 1)
  paleospinothalamic tract 2)neospinothalamic
  tract.
• Paleospinothalamic tract
• It transmits slow pain and crude temperature.
• 1st. Order neuron
• Starts from the receptors, then the afferent
  mainly C-fiber enters the spinal cord, ascend for
  few segments forming Lissuars tract then relays
  on collection of cells in the posterior horn
  called substantia gelatenosa of Rolandi (SGR).

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Cont. paleospinothalamic tract

• 2nd.Order neuron
• Starts from SGR cross to opposite side in front
  to the central canal of the spinal cord then
  ascend in the lateral column of the spinal cord
  forming the tract to end at
• 1-Priaqueductal gray area in midbrain.
  2-Reticular formation of brain stem.
  3-non-specific thalamic nuclei mainly the
  intralaminar.
• 3rd.order neuron
• Starts from thalamic nuclei to terminate into
  wide cortical areas
• The fibers that terminate in thalamus are
  responsible for sensation of slow pain and crude
  temperature.
• The fibers terminate in reticular formation and
  cortex produce arousal effect to different
  cortical areas.

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Neospinothalamic tract

• It transmits fast pain and fine temperature
  sensations
• 1st.order neuron
• Starts from receptors, the afferents are A-delta
  fibers, enters the spinal cord ascends for few
  segments forming Lissuars tract to terminate on
  posterior horn cells.
• 2nd.order neuron
• Starts from the posterior horn cells , cross to
  opposite side in front to central canal of the
  spinal cord forming the tract to end at the
  ventro-postro-lateral nucleus (VPLN) of the
  thalamus.
• 3rd.order neuron
• Starts from the thalamus to terminate at the
  sensory cortex.
• NBThe fibers of the lateral spinothalamic tract
  crosses in the spinal cord in front close to the
  central canal, dilatation of the canal leads to
  early manifestation of the tract damage.

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Ventral spinothalamic tract

• It transmits crude touch, itch and tickle
  sensations
• 1st.order neuron
• Starts from receptors, the afferents are A-delta
  and C-fibers, enter spinal cord, ascend for few
  segments forming Lissuars tract, then terminates
  at posterior horn cells at the main sensory
  nucleus of the dorsal horn.
• 2nd.order neuron
• Starts from the main sensory nucleus, cross to
  opposite side in front to central canal of the
  spinal cord forming the tract to end at VPLN of
  thalamus.
• 3rd.order neuron
• Starts from thalamus (VPLN) and ends at sensory
  cortex (area1-2-3)
• NB The ascending fibers of both the ventral and
  lateral spinothalamic tracts in brain stem form
  the spinal leminiscus

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Ascending and descending pathways
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Gracile and cuneate tract

• The tract transmits fine touch, proprioceptive,
  pressure, muscle tension, stereognosis, texture
  of material and vibration sensations.
• 1st.order neuron
• Starts from the receptors, enters the spinal
  cord, ascend without crossing in the posterior
  column to form the tract to relay in gracile and
  cuneate nuclei in the medulla oblongata.
• 2nd.order neuron
• Starts from gracile and cuneate nuclei in
  medulla, cross to opposite side, ascend in brain
  stem as medial leminiscus to terminate into VPLN
  of thalamus.
• 3rd.order neuron
• Starts from thalamus to end in sensory cortex

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Cont. gracile and cuneate tract

• NB
• -The tract is formed from the 1st. Order neuron,
  it ascends in the spinal cord at the same side
  (epsilateral) without crossing.
• -Gracile tract transmits the sensation from the
  lower part of the body while, cuneate transmits
  the sensation from the upper part.
• -Some fibers pass from both gracile and cuneate
  nuclei in medulla forming the external arcuate
  fibers to enter the epislateral cerebellum
  through the inferior cerebellar peduncle to
  inform the cerebellum about the body position and
  movements

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Spinocerebellar tracts

• They are two tracts, Ventral and dorsal
  spinocerebellar tracts both transmit
  subconsciously proprioceptive signals (position
  and movement) to the cerebellum.
• Ventral spinocerebellar tract
• 1st.order neuron
• Starts from receptors, enters spinal cord to
  relay on Clarks cells in the posterior horn
• 2nd.order neuron
• Starts from Clarks cells then partially cross
  and ascend in the lateral column of the spinal
  cord to enter the cerebellum on both sides
  through the superior cerebellar peduncle to relay
  in the cerebellar cortex.
• 3rd.order neuron
• Starts from the cerebellar cortex and end in deep
  cerebellar nuclei. On both sides

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Cont.spinocerebellar tract

• Dorsal spinocerebellar tract
• 1st.order neuron
• Starts from receptors, enter spinal cord to relay
  on Clarks cells.
• 2nd.order neuron
• Starts from Clarks cells ascend on the same side
  as a tract, enters the cerebellum through the
  inferior cerebellar peduncle to end in the
  cerebellar cortex of the same side.
• 3dr.order neuron
• Starts from the cerebellar cortex to end in deep
  cerebellar nuclei cortex of the same side.

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Pain Sensation

• Pain sensation indicates the presence of some
  sort of pathology, it is a warning sign
  stimulating the individual to look for the cause
  and treat it. It is a protective sensation that
  prevents the progression of the pathology.
• Pain receptors Free naked nerve endings,
  chemical, mechanical and thermal endings that
  stimulated by chemical, mechanical and thermal
  stimuli respectively.
• Mechanism of stimulation The affected tissue
  releases chemical substance (s) as K, histamine,
  bradykinin, prostaglandin, proteolytic enzymes,
  able to stimulate pain receptors and induce pain.

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Cont.pain

• Character of pain sensation
• It is widely distributed all over the body.
• Needs strong stimulus (noxious).
• Initiated by nonspecific stimulus.
• Initiates predominating (prepotent) reflex that
  inhibits other reflexes present simultaneously.
• It can be perceived at the level of thalamus and
  at the level of sensory cortex.
• Initiates autonomic response (sympathetic or
  parasympathetic).
• Leads to emotional and behavior changes either
  excitement or collapse.

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• Types of pain
• There are two types of pain sensation
• 1-Fast pain
• Characters
• -Arises from skin.
• -initiated mainly due to mechanical and thermal
  stimuli.
• -Sharp pricking in quality.
• -Highly localized.
• -Conducted by A-delta afferent fiber, glutamate
  is the transmiter.
• -Transmitted by neospinothalamic tract.
• -perceived at the level of sensory cortex.
• -Rapidly perceived within 0.1sec form stimulation
• -Remains for short duration less than 1.0 sec.
• -depressed by pressure and hypoxia.
• -Initiates somatic reflex (withdrawal reflex).
• -Can be dissociated from fast pain.
• -not subjected to summation.

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• 2-Slow pain
• Character
• -Arise from skin and from deep structures.
• -It is burning or dull aching in quality.
• -Initiated mainly by chemical stimuli.
• -Poorly localized.
• -Conducted by C-afferent fibers, substance-P is
  the transmiter.
• -Transmitted by paleospinothalamic tract.
• -Perceived at subcortical level at thalamus
  (non-specific nuclei).
• -Perceived after 1.0 sec or more from
  stimulation.
• -Remains for few minutes.
• -Can be summated to give severe pain.
• -Initiates autonomic reflex.
• -Depressed by local anaesthesia.
• -Can be dissociated from fast pain

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Types of pain according to its site

• 1- Cutaneous pain. 2- Deep pain.
  3- Visceral pain.
• Cutaneous pain
• It could be fast or slow type, conducted by
  A-delta for fast and C-fiber for slow,
  transmitted both divisions of the lateral
  spinothalamic tract.
• Response to cutaneous pain
• Somatic response
• -Fast cutaneous pain initiates somatic protective
  flexion withdrawal reflex. This reflex is
  predominating (prepotent) that inhibits other
  simultaneously present reflex.
• Autonomic response
• -initiates sympathetic response, however severe
  pain produces parasympathetic response.
• Emotional response
• -Leads to excitement and restlessness, however
  severe pain could lead to collapse (shock).

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Cont.response to cutaneous pain

• Cutaneous hyperalgesia
• It is an area of skin with altered pain
  perception (increased sensitivity) due to
  pathological condition in skin as inflammation
  exposure to excessive sun, injury mechanical and
  chemical. It is of two types primary and
  secondary.
• Primary cutaneous hyperalgesia
• -It occurs at skin area affected by injury.
• -Pain threshold is lowered minimal stimulus
  ,even touch, leads to severe pain.
• -It is due to release of chemical substance (s)
  from the damaged cell substance-P that lowers
  pain receptor threshold.

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Cont. cutaneous hyperalgesia

• Secondary cutaneous hyperalgesia
• -It occurs in healthy skin area surrounding the
  damaged area.
• -Pain threshold is normal or even increased.
• -The central perception of pain is exaggerated
  painful stimulus produces sensation of
  exaggerated severe pain.
• -Its mechanism is through convergence
  facilitation . The impulses coming from area of
  secondary hyperalgesia converge to the same
  facilitated neurons in the spinal cord receiving
  impulses from area of primary hyperalgsia,
  facilitating (increasing) its response.
• NB secondary hyperalgesia can present alone
  without primary hyperalgesia as in case of
  visceral pain (referred).

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Deep pain

• This type of pain originates from muscles, joints
  and periostium.
• Characters
• 1-It is dull aching
• 2-It is poorly localized (referred).
• 3-Initiates parasympathetic response.
• 4-It leads to contraction of the underlying
  muscle.
• Cause
• -Trauma to muscle or bone affecting the
  periostium.
• -Inflammation.
• -Severe muscle spam
• -Ischemia to muscles
• NB periostium is richly supplied with pain
  receptors, on the other hand, bone contains no
  pain receptors it is insensitive to pain.

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Ischemic pain

• Cause
• Insufficient blood supply to the muscle during
  activity. It is due to disease affecting the
  vessel supplying the muscle (atheroscelerosis).
• Mechanism
• During muscle activity, muscle acidic
  metabolites accumulate (lactic acid) and
  stimulate the pain receptors, besides, the
  ischemic muscle releases pain inducing
  proteolytic enzymes. Lewis-P factor, K and
  bradykinin are known to produce pain when
  accumulate in the muscle.

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Cont. ischemic pain

• Intermittent claudication
• It is ischemic pain occur in skeletal muscle. The
  pain is felt after little exercise ( walking for
  short distance) forcing the subject to stop and
  raise up his leg to relieve the pain, then start
  to walk and stops agian due to reappearance of
  pain, and so on repeating the cycle.
• Angina pectoris
• It is pain due to ischemia of the cardiac muscle.
  It is due to coronary artery disease. Stimulation
  of the heart due to any cause, emotional,
  exercise leads to tachycardia. The cardiac muscle
  under such condition needs more blood supply to
  supply O2 and nutrient and to wash out the
  metabolites. Deficient blood supply causes
  accumulation of metabolites that gives rise to
  the feeling of pain. The pain is relieved when
  the heart returns to resting condition. It is the
  classical type of angina of effort.
• NB In ischemia, pain is felt during activity
  because , at rest, the blood supply can prevent
  the accumulation of metabolites and supply the
  needed O2.

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Visceral pain

• All parenchymal organs are poor in pain receptors
  as liver, kidney, spleen as well as lung alveoli
  and the visceral layers peritoneum, pleura and
  pericardium. It gives a sensation of burning
  pain, conducted through C-afferent fibers. On
  the other hand, organ capsules, parietal layers
  of pleura, pericardium and peritoneum and smooth
  muscle, all are rich in pain receptors. It causes
  colicky pain from smooth muscle or sharp acute
  pain from other structures. It is conducted
  through A-delta afferent fiber.
• Cause
• -Ischemia of smooth muscle.
• -Severe spasm or severe distension.
• -Inflammation, bacterial or chemical (perforated
  peptic ulcer).
• -Compression or infiltration of a viscus by tumor.

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Cont. visceral pain.

• Character
• -It is either dull aching, from organs or
  intermittent cramps (colic pain) from smooth
  muscle.
• -It is poorly localized and referred to skin
  (referred pain).
• Response
• Somatic response
• -contraction to the skeletal muscle over the
  affected area (guarding rigidity) anterior
  abdominal wall in appendicitis.
• Autonomic response
• -Parasympathetic stimulation, bradycardia,
  hypotension, salivation, nausea and vomiting
  (sickening-pain).

48
Afferent innervation to viscera

• Sympathetic
• It transmits pain from viscera present between
  the thoracic line and pelvic line (from lower
  esophagus to distal colon, kidney and ureter,
  body of the urinary bladder, lungs, uterus ,
  ovaries and fallopian tubes.
• Parasympathetic
• It transmits pain from organs above and below the
  area of sympathetic.
• Somatic
• - Phrenic nerve from center of diaphragm.
• - Intercostal nerves transmit pain from periphery
  of diaphragm and parietal pleura.

49
Referred pain

• It is a pain that is felt in skin area away from
  the affected viscus.
• The pain radiate to skin area which is developed
  from the same emberyonic segment that develops
  the affected organ (dematomal rule).
• Examples
• -Cardiac pain (angina pectoris) retrosternal,
  root of neck, epigastrium, left shoulder and
  inner part of left arm.
• -Gallbladder (cholecystitis) mid epigastrium,
  right shoulder and the tip of right scapula.
• -Kidney and ureter inguinal region and
  testicles.
• -Appendix (appendicitis) at umbilicus, later on,
  when the inflammation extends to parietal
  peritoneum, it becomes localized to right iliac
  fossa.
• -Gastric pain at epigastrium.
• NB pain originates from pain-sensitive viscera
  as the parietal pleura, parietal pericardium and
  the capsule of organs are usually localized to
  the affected viscus.

50
Mechanism of referred pain

• Convergence projection theory
• The afferent that supplies the skin area as well
  as the affected viscus developed from the same
  emberyonic segment. Afferents from both the skin
  segment and the affected viscus converge in
  spinal cord to the same neurons (pathway) and
  conducted to the brain. Brain is used to get pain
  from skin (dominant for pain), so, it projects
  the signals of pain from the viscus to the skin
  segment which is transmitted by the same
  pathway.
• NB Visceral pain is usually accompanied with
  secondary hyperalgesia
• Mechanism convergence facilitation theory
• The afferent from the affected viscus transmits
  pain signals to neurons in spinal cord (SGR)
  leading to their facilitation. Afferent for skin
  segment converge to the same facilitated neurons
  exaggerating their central effect. The central
  facilitation of the signals coming from skin
  leads to the feeling of hyperalgesia.

51
Referred pain
52
Pain analgesia system

• It is a collection of neurons present at
  different sites inside the CNS. It aims at
  blocking the pain conduction in the afferent
  pathway leading to analgesia.
• The system is formed of
• 1-Cortical limbic association areas and
  hypothalamic neurons in the periventricular
  nuclei release ß-endorphin as a transmitter.
• 2-Periaquidutal gray area in upper pons and
  midbrain releases enkephalin as a transmitter.
• 3-Raphe magnus nucleus in the lower pons releases
  serotonin as a transmitter.
• 4-Pain inhibitory complex in the spinal cord
  rleases enkephaline as a transmitter.

53
Endogenous opioid

• Opioids are peptide transmitters acting on
  morphin receptors present in the analgesia
  system. They are released from the neurons of the
  analgesia system.There are so many of them, but
  however, the most important are enkephlin,
  endorphin and dynorphin.
• Mechanism of stimulation of analgesia system
• Cortical stimulation stress analgesia
• In severe emotional condition as when a soldier
  get injured in a battle, the limbic system
  (endorphin neurons) stimulates, the hypothalamic
  periventricular nuclei (endorphin neuron), which
  in turn stimulates the periaquiductal area in
  upper pons (enkephalin neurons), to stimulate the
  raphe nucleus in lower pons (serotonin neurons),
  which in turn stimulates the pain inhibitory
  complex (PIC) in the spinal cord (enkephalin
  neurons). PIC once stimulated well produce
  presynaptic inhibition to SGR. PIC inhibits the
  release of P-substance from the nerve terminals
  conducting the pain, so, preventing the
  activation of SGR blocking the transmission of
  pain signals.

54
Cont.analgesia system

• Stress? Limbic cortex(endorphin) ? Hypothalamus
  periventricula n.(endorphin) ? midbrain
  periaquiductal area(enkephalin) ? pons raphe
  magnus n.(serotonin) ?
• Spinal cord PIC (enkephalin) ?presynaptic
  inhibition to SGR, block the conduction of pain.
• Spinal stimulation to analgesia system
• Rubbing the skin, using counter-irritant and
  acupuncture, all stimulate A-beta afferent and
  A-delta that transmit impulses to intermediate
  neuron (enkephalin or GABA), which in turn
  produces presynaptic inhibition to SGR.
• Rubbing
• Counter-irritant ? A-beta afferent(mechano-R)
  andA-delta?IMN ?pesynaptic inhibition to SGR
  acupuncture

55
Pain analgesia system
56
Spinal inhibition of pain
57
The synapse

• Definition
• It is the junction between nerve terminals of one
  neuron (presynaptic) and the cell body
  (postsynaptic) of another neuron.
• Types
• 1-Axo-denderetic. 2-Axo-somatic. 3-Axo-axonic
  (common and effective).
• The presynaptic neuron has about 1000 terminals
  each ends by a knob. The post synaptic neuron
  receives terminals from many presynaptic neurons,
  so that, each postsynaptic neuron receives about
  10000-100000 synaptic knobs
• The space between the per and postsynaptic
  neurons is called synaptic cleft it is about
  30-50nm.

58
Synaptic convergence
59
Mechanism of synaptic transmission

• The conduction of the response from the
  presynaptic neuron to the postsynaptic neuron
  occurs as a result of release of a chemical
  transmitter from presynaptic terminals over the
  postsynaptic neuron.
• Release of the chemical transmitter
• The transmitter is present in vesicles at the
  knobs. The vesicles are bound to the knob
  cytoskeleton by a protein called synopsin.
• As a result of nerve stimulation Ca2 influx at
  nerve terminals occurs leading to separation of
  synopsin from the vesicles. The vesicles move
  towards releasing site at the knob membrane and
  released by exocytosis, it is Ca2-dependent
  excocytosis process.
• The released transmitter acts on its specific
  ligand-gated channel (receptor) to induce
  postsynaptic potential, which is either
  stimulatory or inhibitory depending on the
  released transmitter as well as the activated
  receptor.

60
Post and pre-synaptic potential

• 1-Excitatory postsynaptic potential EPSP
• It leads to depolarization of the postsynaptic
  membrane, when reaches threshold value initiates
  an action potential. The released excitatory
  transmitter (Ach) opens Na-channels at the
  postsynaptic membrane, Na influx produces the
  depolarization. Also, excitatory transmitter acts
  by activating Ca2 -channels
• 2-Inhibitory postsynaptic potentialIPSP
• It leads to hyperpolarization of the postsynaptic
  membrane. The released inhibitory transmitter
  opens either Cl- or K channel or inactivate
  Ca2-channels.
• Glycine opens Cl- -channels, GABAA opens Cl-
  -channels and GABAB opens K-
• channels.

61
Cont.presynaptic potential

• 3- presynaptic Facilitation
• A facilitatory (stimulatory) neuron releases
  stimulatory transmitter (serotonin) over the
  presynaptic neuron, leading to closure
  (inactivation) of its K-channels. Closure of
  K-channels prolongs the duration of action
  potential in presynaptic neuron activating Ca2
  -voltage-gated channels increasing Ca2 influx.
  Ca2 influx stimulates the release of the
  transmitter from the presynaptic knobs.
• 4-Presynaptic inhibition
• An inhibitory neuron releases inhibitory
  transmitter (enkephalin) over the presynaptic
  neuron leads to inactivation of its Ca 2
  channels. Inhibition of Ca 2 influx inhibits the
  release of the transmitter from the presynaptic
  neuron.
• NB enkephalin is released from the presynaptic
  PIC in analgesia system, it inhibit the release
  of p-factor from pain-conducting afferent and
  therefore prevents the activation of SGR blocking
  pain conduction.
• Serotonin produces presynaptic facilitation
  essential for the process of memory.

62
Properties of synapse

• 1-one way conduction from pre to post synaptic
  neuron.
• 2-Delay of conduction of about 0.5msec. Time
  needed to produce postsynaptic potential.
• 3-Summation of the effect of presynaptic neurons,
  it is important to produce effective
  transmission, it is of two types
• i-Temporal it is the summation of the effect of
  repeated stimuli in a single neuron.
• ii-Spatial it is the summation of the effects of
  neurons stimulated at the time.
• 4-Fatigue, it is the decreased activity after
  repeated stimulation due to depletion of the
  transmitter.
• 5-After-discharge, it is the prolonged activity
  of the postsynaptic neuron after stoppage of the
  stimulus due to prolonged release of the
  stimulus.
• 6-Sensitivity to
• - pH alkalosis increases synaptic transmission.
• - O2hypoxia decreases synaptic transmission.
• - Drugs caffeine, strychnine increase, while
  tranquilizers and anaesthetics decrease synaptic
  transmission.

63
Chemical transmitters

• The transmitters are of two types
• 1-small molecules and rapidly acting as Ach,
  amines (EN, NE, Dopamine) and amino acids
  (glycine and GABA).
• 2-Large molecules and slowly acting as opioids,
  GIT hormones, ATP, substance-P and
  neuropeptide-Y.
• Acetylcholine
• It is excitatory transmitter present in spinal
  cord, basal ganglia and cortex. It acts on
  M1receptor and essential for the intellectual
  functions as memory, arousal state and motor
  function. Deficiency in cortex leads to Alzheimer
  syndrome.
• Epinephrine and norepinephrine
• They are excitatory present in brain stem and
  hypothalamus. They are important for improving
  the mood and preventing fatigue. Deficiency leads
  to behavior changes as in schizophrenia.

64
Cont.ch.transmitters

• Dopamine
• It is inhibitory transmitter present in basal
  ganglia, hypothalamus and limbic system. Its
  deficiency in basal ganglia leads to
  parkinsonism. In hypothalamus, it acts as
  prolactin inhibitory factor and its absence leads
  to amenorrhea galactorea syndrome.
• Serotonin
• It is present in hypothalamus and brainstem
  mainly raphe nucleus. It stimulates prolactin
  release, reduce food intake, blocks pain
  conduction, and induce sleep.
• Gama-amino-butyric acid
• It is inhibitor acts as presynaptic inhibitor,
  needs pyridoxine (Vit.B6) for its synthesis.
  Deficiency of the vitamin decreases the formation
  of GABA and leads to convulsions.
• Glycine
• It is inhibitor acts as postsynaptic inhibitor.
  It activates Cl-channels and increases Cl-
  influx.
• Glutamat and asparate
• Glutamate constitute about 75 of the excitatory
  transmitters in brain. It is essential for memory
  and learning. Excess glutamate induces neuronal
  damage.

65
Processing the signal inside the CNS

• The input signal once enter the CNS is subjected
  to certain changes (processing) according to the
  nature of the input signal in order to give a
  purposeful output signal or significant input
  effect. These changes occurs due to synapse
  between the neuron transmit the input signal with
  group of neurons inside CNS called neuron pool.
  The neuron that transmit the input signal may
  relay over all neurons of the neuron pool or
  over some of it. The changes (processing) that
  occur at the neuron pool include
• 1- Divergence the number of output neuron is
  greater than the input neuron. It serves to
  spread the response (pyramidal tract supply so
  many motor neurons in pinsal cord (AHC) to supply
  the muscle, one pyramidal cell supply 1000 muscle
  fiber.
• 2- Convergence The number of input signals are
  greater than the output signals. It helps the
  process of summation and localize the site of
  response (at certain muscle).

66

• 3- After discharge It prolongs the duration of
  output signal.
• Mechanism The presence of intermediate neurons
  that act as interneuronal barrage. The
  intermediate neurons arranged in two forms i-
  Parallel multiple chain circuit.
  Ii-Reverberating circuit.
• 4- Shortening of signal it is to shorten the
  time of the active neuron. It can be produced
  through
• i- Negative feedback inhibition It is through
  inhibitory intermediate neuron (Renshaw cell).
  AHC transmit excitatory impulses stimulating
  intermediat inhibitory Renshaw neuron, which in
  turn produces feedback inhibition to AHC shorten
  its time of activity.

ii- Negative feedforward inhibition The
stimulated inhibitory intermediate neuron
inhibits (forward) the output signals from
another excited neuron shorten its time of
activity. It occurs mainly in cerebellum. 5-
Discharge zone and subliminal fringe zone The
neuron that transmit the input signal synapse
with collection of neurons inside CNS. The
neurons present at the center of the collection
receives many terminals and discharge an output
signal, it forms the discharge zone, while the
neurons at the periphery of the collection
receives few terminals and become only
facilitated and unable to discharge an output
signal, it forms the subliminal (facilitated)
zone.
67
6- Sharpening of signal

• It is done to inhibit the unwanted signals
  preventing the unnecessary crowding of signals
  and, at the same time acts to sharpen the wanted
  signals. The mechanism is through the process of
  lateral inhibition the neuron that transmit the
  input signal gives collateral that synapse with
  intermediate neurons , which in turn produces
  inhibition to neurons lateral to the neuron that
  transmit the input signal as for Renshaw cell
  which produces lateral inhibition to neurons
  lateral to the stimulated AHC. This occurs to
  sharpen the somatic sensory signals, visual and
  auditory signal.

68
Spinal cord reflexes

• They are divided into 1- Superficial. 2- Deep.
  3- Visceral.
• The superficial reflexes
• 1- Planter reflex scratch the lateral border of
  the foot from down upward and medially. The
  response is ventro-flexion of all toes. In upper
  motor neuron lesion , the response shows Babinski
  sign which is dorsiflexion of big toe (pyramidal
  lesion) and fanning of the other toes
  (extrapyramidal lesion) pyramidal tract lesion.
  Lesion to area-4 produces only dorsiflexion of
  the big toe, while lesion to area-6 produces only
  fanning of the other toes. Babinski sign occurs
  normally in infants below one year old due to
  undeveloped pyramidal tract and also in adult
  during deep sleep.
• Center L5-S1.2
• 2- Abdominal reflex scratching the abdominal
  wall in the upper and lwer quadrant leads to
  contraction of the underlying muscle and
  direction of the umbilicus towards the
  contraction. It is absent in upper motor neuron
  lesion.
• Center upper quadrants 7-10Th, while lower
  quadrant 10-12Th

69
Cont. spinal reflexes

• 3- Cremasteric reflex scratch the skin of upper
  and medial side of the thigh leads to upward
  movement of the corresponding testis.
• Center L 1-2
• 4- Anal reflex scratching the skin around the
  anus leads to contraction of the external anal
  sphincter.
• Center S 3-4
• 5- Flexion withdrawal reflex Painful (noxious)
  stimulus produces flexion movement of the
  stimulated limb. It is a protective reflex to
  keep the limb away from the stimulus.
• 6- Crossed extensor reflex It is reflex
  extension of one limb when the corresponding limb
  is reflexely flexed. It is a supportive reflex to
  keep the posture.
• 7- Positive supporting reaction Deep pressure to
  the sole (body weight) produces reflex
  contraction of the limb muscles both flexors and
  extensors. It is a supporting reflex to keep the
  posture.
• Center L 1-5, S 1.

70
Flexor withdrawal and crossed extensor reflexes
71
Cont. spinal reflexes

• Scratch reflex stimulation of the skin by moving
  object (insect) or itch leads to scratch reflex
  to inhibit he irritating stimulus.
• Corneal reflex touch of the cornea produces
  blinking response.
• Center afferent 5th.CN (tigeminal n.), efferent
  7th.CN (facial).
• NB Corneal reflex is a superficial reflex , its
  center is in brainstem.
• Visceral reflexes
• 1-Micturition R. 2- Defecation R. 3-
  erection R.
• The center of visceral reflexes is S 2,3,4.
• Deep reflexes
• Stretch reflex myotatic reflex
• Stretch of the muscle produces reflex contraction
  of the muscle. The muscle resist changes in its
  length. The receptor is muscle spindle, once
  stimulated transmit signals to AHC to produce
  muscle contraction.

72
Muscle spindle

• It is of about 4-10mm. Long and formed of about
  10 intrafusal muscle fibers divided into two
  types
• 1-nuclear bag fiber it has central swollen
  nucleated part and peripheral contractile part,
  each spindle contains 2 fibers. It is of 7-8mm.
  Long and 25um in diameter
• Innervation
• Afferent primary ending, Ia fiber (A-alpha)
  rapidly conducting form annul-spiral ending
  around the central non-contractile swollen part.
• Efferent small(3-6um) gamma motor fiber. It
  forms about 30 of the total motor fibers in the
  ventral horn. It is Gamma-d (dynamic) that forms
  end-plate ending at the peripheral contractile
  part of the intrafusal muscle fiber.

73
Cont.muscle spindle

• 2- Nuclear chain fiber each spindle contains
  about 4-8 fibers, it is without central swelling,
  but with non-contractile central part, its ends
  attach to the nuclear bag.It is of 3-4mm. Long
  and 12um in diameter
• Innervation
• Afferent- Primary ending Ia fiber form
  annulospiral ending as for nuclear bag.
• - Secondary ending, type II (A-beta fiber) form
  flower spray ending at the periphery of the
  fiber.
• Efferent Gamma-s (static) fiber ends as trail
  ending at the peripheral contractile part.
• NB - The nuclear bag fiber is supplied with only
  one afferent (Ia-fiber) and with dynamic gamma
  efferent.
• - The nuclear chain fiber is supplied with two
  afferents (Ia and II fibers) and with static
  gamma efferent .

74
Mechanism of action of muscle spindle Stretch
reflex

• The intrafusal muscle fibers of the muscle
  spindle are stimulated by either stretch of the
  whole muscle or by contraction of the peripheral
  contractile part of the intrafusal fiber. Gamma
  efferent produces contraction to the peripheral
  parts of the intrafusal muscle fibers leading to
  their stimulation.
• Stimulated intrafusal muscle fibers transmit
  impulses through their afferent to stimulate AHC
  to produce muscle contraction (extrafusal
  fibers).
• Stretch of the muscle or stimulation of gamma
  efferent ?stretch of the central part of the
  intrafusal fiber ? transmit impulses through
  afferent ?stimulate AHC ?muscle contraction
  (extrafusal fibers).
• NB- factors stimulate gamma efferent lead to
  stimulation of stretch reflex.
• - Stretch R. is the only monosynaptic
  reflex in the body.

75
Types of stretch reflex

• Static stretch reflex Muscle tone
• The muscles are in state of stretch because
  during development the bone growth is more than
  the muscle growth so, the muscle is stretched
  from both its ends the origin and insertion. The
  stretch of the muscle produces stretch to the
  nuclear chain fibers which in turn stimulates
  mainly its flower spray, sending impulses in
  afferents type II-fiber to stimulate AHC leading
  to muscle contraction.
• The antigravity muscles they are the muscles
  that keep the posture against the pulling effect
  of gravity. They are under much stretch and
  therefore, developed much tone. During the erect
  posture, the antigravity muscles are flexors of
  the upper limb, extensors of the lower limb,
  anterior abdominal wall muscle and muscles of the
  back.
• NB During rest the muscles are in a state of
  continuous contraction (muscle tone) due to the
  static stretch reflex.
• Flower spray endings supply the nuclear chain
  fibers are slowly adapting and can remain
  stimulated during the maintained stretch of the
  muscle, while the annulo-spiral ending in the
  nuclear bag are rapidly adapting so they become
  inactive during the maintained muscle stretch.

76
Cont. stretch reflex

• Dynamic stretch reflex Tendon jerk
• when the muscle is suddenly stretched as by
  taping its tendon, the nuclear bag fibers with
  their annulospiral endings are stimulated and
  transmitting impulses through Ia fast fibers
  (70-120m/sec.) to stimulate the AHC leading to
  sudden muscle contraction called tendon jerk. The
  nuclear bag fiber is rapidly adapting, it is
  stimulated to sudden unmaintained stimulus.
• Conclusion Muscle tone is a static stretch
  reflex, it is a state of maintained muscle
  contraction during rest due to stimulation of the
  intrafusal nuclear chain fiber with its flower
  spray endings.
• Tendon jerk is the dynamic stretch reflex due to
  sudden muscle stretch stimulating the intrafusal
  nuclear bag fiber with its annulo-spiral endings

77
Gamma efferent

• They are small motor(3-6um) fibers arising from
  the ventral horn of the spinal cord, they
  constitute about 30 of the total motor fibers in
  the ventral horn. They supply the intrafusal
  fibers of the muscle spindle leading to
  contraction of the peripheral contractile part of
  the fibers. Increased activity of gamma efferent
  increases the activity of stretch reflex (dynamic
  and static).
• The activity of gamma efferent is affected by
  supra spinal centers.
• There are two types of gamma fibers
• 1- static supplying the nuclear chain fiber and
  end as trail ending.
• .
• 2- dynamic supplying the nuclear bag fiber and
  end as end plate Gamma-alpha-loop
• ? Gamma? ?Intrafusal fibers ? ?afferent(Ia,II) ?
• ? AHC

78
Supra spinal centers

• They are group of centers present in brainstem
  and in cortex. They are divided into facilitatory
  and inhibitory centers.
• The supra spinal facilitatory centers
• 1- Pontine facilitatory reticular formation
  (PRF) activates directly gamma efferent.
• 2- Neocerebellum transmits impulses to PRF
• 3- Motor area-4 transmits impulses to PRF and to
  alpha motor neurons.
• 4- Lateral vestibular nucleus transmits impulses
  to alpha motor neuron and to PRF.
• 5 Caudate nucleus transmits impulses to PRF,
  Vestibular nucleus and to inferior olivary
  nucleus
• 6- Inferior olivary nucleus transmits impulses
  to cervical gamma and alpha motor neurons

79
Cont.supraspinal centers

• Surpaspinal inhibitory centers
• 1- Medullary inhibitory reticular formation
  (MIRF) It is activated by impulses received from
  other centers, then transmit to gamma efferent.
• 2- Red nucleus in midbrain transmits directly to
  gamma and also to alpha motor fibers.
• 3- Lentiform nucleus transmits to MIRF and also
  inhibits the vestibular nucleus.
• 4- Cortical suppressor area- 4S and area- 6
  Inhibit both alpha and gamma motor neurons.
• 5- Paleocerebellum transmits to MIRF and
  inhibits the vestibular nucleus.
• NB Supraspinal facilitatory centers stimulate
  the gamma efferent leading to increase in both
  dynamic and static stretch reflex.
• Supraspinal inhibitory centers inhibit gamma
  efferent leading to inhibition of both the
  dynamic and the static stretch reflex.

80
Inverse stretch reflex Lengthening reaction ve
induction reflex
Gamma alpha co-activation
Activation of supraspinal facilitatory center
activates both alpha (AHC) and gamma motor
neurons. As a result of activation of AHC, the
muscle contracts, so muscle spindle becomes short
and inhibited, to prevent the inhibition of
muscle spindle during muscle contraction, gamma
efferent is stimulated simultaneously with the
stimulation to AHC. It is called gamma-alpha
co-activation.

• Excessive stretch of the muscle produces reflex
  muscle relaxation. The reflex contain two
  synapses, it is a protective reflex prevents
  muscle disruption due to excessive stretch.
• Excessive stretch increases muscle tension leads
  to stimulation of Golgi tendon organ, tension
  receptor present as knobby nerve ending at the
  tendon. It transmit impulses in A-alpha (Ib)
  fiber to the spinal cord to inhibit the AHC
  leading to muscle relaxation.

81
Function of stretch reflex

• 1- It maintains the posture it is due to
  increasing the muscle tone in the antigravity
  muscles.
• 2- It leads to smooth voluntary movement
  Damping effect
• In absence of the reflex (cut the afferent
  nerve of the muscle), the movement becomes jerky.
  The AHC transmits intermittent signals to the
  muscle , stretch reflex through alpha-gamma loop
  modifies AHC impulses to give smooth contraction
  (signal averaging function of muscle spindle), it
  damps the jerky movements
• 3- It increases the force of muscle contraction
  Servo-assist
• During muscle contraction, gamma efferents
  are also stimulated, and in turn stimulate
  stretch reflex which add more stimulation to AHC
  and more muscle contraction. It is useful during
  lifting heavy weigh, stretch reflex increases the
  force of contraction without the need of motor
  cortex signals (load reflex).

82
Function of muscle tone

• 1- Maintain the body posture against the pulling
  effect of gravity.
• 2- Maintain the venous return and lymph flow, it
  helps continuous movements of the venous blood
  and lymph.
• 3- Maintain resting heat production to keep
  normal body temperature. In cold weather, the
  muscle tone increases to increase the rate of
  heat production.
• 4- Maintain the viscera in their position against
  the pulling effect of gravity, it prevents
  visceroptosis.

83
Tendon jerk

• It is dynamic stretch reflex, its receptor is the
  rapidly adapting nuclear bag fiber with its
  annulo-spiral nerve ending.
• It is done by taping the muscle tendon in a
  muscle with relatively stimulated stretch reflex.
  The muscle responds by brisky contraction.
• Types
• 1-Knee Jerk tap the patellar tendon while the
  knee is semiflexed (Partial stretch to
  quadriceps) leads to sudden contraction of the
  quadriceps muscle. Center L 2, 3, 4.
• 2-Ankle jerk tap the tendo-achillis while the
  foot is dorsiflexed, it leads to sudden
  contraction of calf muscles. Center S 1, 2.
• 3-Biceps jerk tap the biceps tendon while the
  examiner thumb is over the tendon, elbow is
  semiflexed forearm is pronated and supported by
  the examiner hand, it leads to sudden contraction
  of biceps muscle. Center C 5, 6.
• 4-Triceps jerk tap the triceps tendon, while the
  elbow is flexed and pronated, it leads to sudden
  contraction of triceps muscle. Center C 6, 7.
• 5- Jaw jerk tap over the chin while the examiner
  index finger over the chin, the mouth is
  slightly opened, it leads to contraction of
  masseter muscle and elevation of the jaw. Center
  trigeminal nerve (CN-5).

84
Clinical significance of tendon jerk

• 1- Determines the level of lesion, in upper
  lumbar lesion of spinal cord, the knee and ankle
  reflexes are lost, while at sacral lesion the
  ankle reflex is lost with intact ankle reflex.
• 2- Diagnosis of some neurological diseases as
  follow
• A- The muscle tone and tendon jerks increases in,
  upper motor neuron lesion (pyramidal tract),
  lesion to area-6, hyperthyroidism, tetany and in
  excitement.
• B- The muscle tone and tendon jerks decreases in,
  lower motor neuron lesion, lesion to area-4,
  neocerebellar syndrome (pendular knee jerk),
  hypofunction of thyroid gland (myxedema), during
  deep sleep and during anaesthesia.
• C- Muscle tone and tendon jerk are absent in
  peripheral neuritis and in tabes dorsalis due to
  interruption of the reflex arch.
• NB usually the tone and jerk reflexes are
  affected both similarly Increase and decrease
  together except in pakinsonism which is
  accompanied with increased mucle tone, while the
  tendon jerk reflexes not increased (hypertonia
  without hyper-reflexia)

85
Descending tracts

• The pyramidal tract Cortico-spinal tract
• Function
• 1-Initiates fine skilled discrete voluntary
  movements as movements of the fingers.
• 2-Transmit facilitatory impulses to alpha and
  gamma motor neuron
• Origin
• 1- About 30 take origin from cortical motor
  area-4 (only 3 are thick fibers originate from
  big Betz cells.
• 2-About 30 take origin from premotor area-6 and
  from supplementary motor area.
• 3-About 40 take origin from somatic sensory
  areas (1,2,3).
• NB About 97 of the fibers are thin less than
  4um.
• Pathway
• The fibers are collected from the cortical origin
  forming corona radiata, then pass through the
  internal capsule occupying the genu and the
  anterior2/3 of the posterior limb. The fibers
  then descend in brain.

86
Cont. pyramidal tract

• At midbrain cortico-nuclear tract