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Unit VIII SPINAL NERVES, RECEPTORS AND REFLEXES

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Biology 220 Anatomy & Physiology I Unit VIII SPINAL NERVES, RECEPTORS AND REFLEXES Chapter 13 pp. 475-483, 491-507 E. Gorski/ E. Lathrop-Davis/ S. Kabrhel – PowerPoint PPT presentation

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Title: Unit VIII SPINAL NERVES, RECEPTORS AND REFLEXES


1
Unit VIIISPINAL NERVES,RECEPTORS AND REFLEXES
Biology 220 Anatomy Physiology I
Chapter 13 pp. 475-483, 491-507
E. Gorski/ E. Lathrop-Davis/ S. Kabrhel
2
Spinal Nerves
  • pass through intervertebral foramina (between
    vertebrae)
  • all are mixed (sensory/motor)
  • areas served
  • entire body
  • in head/neck
  • spinal nerves only provide innervation for
    sympathetic division of ANS
  • other head/neck innervation comes from cranial
    nerves)

3
Number and Naming
  • 31 pairs of nerves
  • cervical -- 8 pair
  • thoracic -- 12 pair
  • lumbar -- 5 pair
  • sacral -- 5 pair
  • coccygeal -- 1 pair
  • cervical (in red) are named after the vertebra
    (in blue) below them (except C8) others (eg., T1
    in orange) named after the vertebra above them

C1
C1
T1
C8
T1
Fig. 12.24, p. 463 see also Fig. 13.5, p. 491
4
Spinal Nerve Composition
  • Ventral root -- carries motor (efferent
    outgoing) fibers (axons) of lower motor neurons
    and autonomic motor neurons
  • Dorsal root -- carries afferent (incoming) fibers
    from sensory neurons
  • Dorsal root ganglion -- consists of cell bodies
    of first-order sensory neurons
  • sensory neurons are unipolar with a peripheral
    process extending from receptor toward cell body
    and a central process extending away from cell
    body and entering spinal cord

5
Spinal Nerve Branches
  • Spinal nerve formed from union of dorsal and
    ventral root continues for about 2 cm then
    branches into rami (mixed fibers)
  • a. dorsal ramus -- serves skin and muscles of
    back
  • b. ventral ramus -- primarily serves skin,
    muscles of anterior trunk and limbs
  • c. rami communicantes (singular ramus
    communicans) -- carry autonomic motor fibers
    between ventral ramus and autonomic ganglia

6
Spinal Rami
Dorsal Ramus
Ventral Ramus
Ramus communicans
See Fig. 13.6, p. 492 Fig. 14.6, p. 521
7
Regeneration of Neurons PNS
  • cell bodies -- if cell body is damaged, cell
    dies neurons downstream from damaged neuron
    may die as well.
  • peripheral axons (PNS)
  • portion of axon distal to site of damage is
    degraded within 1 week
  • neurolemma usually remains intact (depends on
    severity of damage myelin sheath degenerates)
  • axon regrows at about 1-5 mm per day
  • Schwann cells that form cord will eventually
    remyelinate regenerated axon

8
Regeneration of Neurons PNS
  • Steps
  • 1. macrophages enter area and phagocytize debris
    after Wallerian degeneration of distal axon and
    myelin sheaths (neurilemma remains intact)
  • 2. Schwann cells proliferate in response to
    mitosis-stimulating chemicals released by
    macrophages
  • 3. Schwann cells release nerve growth factor
    (NGF) and other chemicals that stimulate axonal
    growth
  • 4. Schwann cells also form cellular cords which
    axon follows during regeneration

9
Regeneration of Neurons PNS
Fig. 13.3, p. 482
10
Regeneration of Neurons CNS
  • CNS macrophages phagocytize debris more slowly
    than peripheral macrophages do
  • normally, neuron dies because oligodendrites
    (supplying myelin sheath) associated with damaged
    neuron die, so do not aid regeneration of axon
    (fiber)
  • nearby astrocytes may proliferate and produce
    scarring
  • myelin sheaths (oligodendrites) of other, nearby
    axons secrete inhibitory proteins or die
    (secondary demyelination)
  • note in experiments, macrophages transplanted
    from outside CNS can secrete proteins that
    inhibit inhibitory proteins

11
Nerve Plexuses
  • Plexuses formed from branching and rejoining of
    ventral rami in cervical, lumbar and sacral
    regions (not in thoracic region)
  • each nerve leaving plexus contains fibers from
    several spinal nerves
  • muscles of limbs receive innervation from more
    than one spinal nerve
  • Thoracic spinal nerves T2-T12 do not form
    plexuses they innervate external and internal
    intercostal muscles

12
Cervical Plexus
  • Formed from the ventral rami of C1-C5
  • found in neck region under sternocleidomastoid
    muscle
  • sensory innervation from skin on back of neck,
    ear, over parotid gland, shoulder
  • motor innervation to lower muscles of neck, deep
    muscles of neck, trapezius, sternocleidomastoid
  • Phrenic nerve provides motor (and sensory)
    innervation to diaphragm

Fig. 13.7, p. 494
13
Brachial Plexus
  • Formed from ventral rami of C5-T1 (may get fibers
    from C4 and/or T2)
  • located in the inferior lateral neck and axillary
    region
  • serves arm and lateral shoulder, chest
    (pectoralis major and minor muscles)
  • Major nerves
  • Median
  • motor to flexor muscles of forearm (flexor carpi
    radialis, flexor digitorum superficialis)
  • sensory from skin of lateral 2/3 of hand

Fig. 13.8, p. 496
14
Brachial Plexus Major Nerves
  • Radial
  • motor to posterior arm extensors (triceps
    brachii), wrist extensors, supination
    (supinator), and thumb abduction
  • sensory from posterior arm
  • Ulnar
  • motor to flexor muscles of anterior forearm
    (flexor carpi ulnaris) intrinsic muscles of hand
    (other than palm)
  • sensory from medial 1/3 of hand (anterior and
    posterior)

Fig. 13.8, p. 496
15
Brachial Plexus Major Nerves
  • Axillary
  • motor to deltoid and teres minor
  • sensory from skin of shoulder region
  • Musculocutaneous
  • motor to flexor muscles of arm (biceps brachii,
    brachialis)
  • sensory from anterolateral forearm (varies)

Fig. 13.8, p. 496
16
Lumbar Plexus
  • Formed from ventral rami of L1-L4
  • Femoral nerve
  • motor to anterior muscles of thigh (thigh
    flexion and knee extension quadriceps femoris)
  • sensory from medial leg and foot, anterior thigh
  • Obturator nerve
  • motor to thigh adductors (adductor magnus,
    brevis, longus)
  • sensory from skin of medial thigh

Fig. 13.9, p. 498
17
Sacral Plexus
  • Formed from L4-S4
  • Sciatic nerve
  • motor to posterior thigh (hamstrings), posterior
    and anterior lower leg (gastrocnemius, soleus,
    tibialis anterior) and adductor magnus (posterior
    portion)
  • sensory from posterior skin

Fig. 13.10, p. 500
18
Sacral Plexus
  • Superior gluteal nerve
  • motor to gluteus medius
  • Inferior gluteal nerve
  • motor to gluteus maximus

Fig. 13.10, p. 500
19
Dermatomes
  • Area of skin innervated by cutaneous branches of
    single spinal nerve (all spinal nerves except C1)
  • dermatomes
  • trunk -- C2-T12
  • arm -- ventral rami of C5-T1 (sometimes T2)
  • anterior leg -- ventral rami L1-S1

Fig. 13.11, p. 501
20
Dermatomes
  • posterior leg and perineum -- ventral rami of
    S2-S5

Fig. 13.11, p. 501
21
General Sensory Receptors
  • Classification based on
  • 1. Location
  • 2. Type of stimulus detected
  • 3. Structural complexity

22
1. Location
  • exteroceptors -- at body surface (skin) detect
    pain, temperature, pressure, touch
  • interoceptors -- internal organs, blood vessels
  • proprioceptors -- measure degree of stretch in
    muscles, tendons, joints, ligaments

23
2. Type of Stimulus Detected
  • mechanoreceptors -- respond to touch, pressure,
    vibration, stretch (when they or surrounding
    tissues are deformed)
  • thermoreceptors -- respond to heat or cold
  • photoreceptors -- respond to light energy
    (light-gated channels)
  • chemoreceptors -- respond to chemicals in
    solution (taste, smell, blood chemistry
    chemically-gated channels)
  • nociceptors -- pain (chemicals released during
    tissue damage)
  • itch receptors - respond to histamine
  • NOTE over stimulation of any receptor causes pain

24
3. Structural Complexity
  • Complex receptors
  • associated with special senses (sight, hearing,
    smell, taste)
  • involve more complex structures and tissues
  • Simple receptors -- most common
  • modified dendritic endings of sensory neurons
    associated with general senses
  • found in epithelia, muscles, and connective
    tissues
  • free dendritic endings or encapsulated receptors
    (mechanoreceptors)

25
Free Dendritic Endings
  • naked dendritic endings
  • most are unmyelinated fibers (type C)
  • examples
  • free nerve endings -- pain, temperature, some
    mechanoreceptors (Merkel discs)
  • root hair plexuses -- mechanoreceptors of hair
    movement

26
Simple Encapsulated Receptors
  • Meissners corpuscles -- fine touch in skin
  • Krauses end bulbs -- touch in mucous membrane of
    mouth and eyes (conjunctiva), lips
  • Pacinian corpsuscles -- pressure and vibration in
    dermis, tendons, joint capsules also found in
    viscera
  • Ruffinis endings -- deep pressure and stretch in
    dermis, hypodermis, joint capsules

27
Simple Encapsulated Receptors
  • muscle spindles -- stretch in skeletal muscle
  • Golgi tendon organs -- stretch in tendons
  • joint kinesthetic receptors (Pacinian and Ruffini
    corpuscles, free dendritic endings, Golgi
    tendon-like organs) -- stretch, joint position,
    motion
  • located in joint capsule of synovial joint
  • important of proprioception

28
Reflexes
  • Rapid, predictable, unlearned, involuntary motor
    response to specific stimuli
  • integrates sensory input with motor output
    without involving higher centers
  • used clinically to test condition of nervous
    system

Fig. 13.12, p. 502
29
Components of the Reflex Arc
  • 1. receptor -- specialized dendritic end
  • 2. sensory neuron -- unipolar neuron carrying
    incoming impulse from receptor
  • 3. integrating center within gray matter of
    spinal cord
  • single synapse between sensory and motor
    monosynaptic
  • with interneuron polysynaptic
  • 4. motor neuron -- carries outgoing impulse to
    effector
  • 5. effector -- muscle or gland that responds to
    motor impulse

30
Types of Spinal Reflexes
  • Stretch reflex
  • Patellar reflex
  • Achilles' reflex
  • Flexor reflex
  • Crossed extensor reflex
  • Deep tendon reflex
  • Superficial reflex
  • Plantar Reflex (Babinski sign)

Lab
31
Stretch Reflex
  • protects against excessive stretch of muscle
  • e.g., patellar reflex (knee jerk) and Achilles
    reflex
  • monosynaptic and ipsilateral (same side)
  • events
  • gt stretch in muscle (tapping patellar ligament)
  • gt activates muscle spindle
  • gt sends impulse via sensory neuron
  • gt enters via dorsal root to synapse with motor
    neuron
  • gt motor impulse carried out by motor neuron
    through ventral root
  • gt impulse reaches stretched muscle (quadriceps
    femoris) causing it to contract

32
Patellar Reflex
  • integration with antagonist (opposing) muscle is
    polysynaptic involving interneuron (inhibitory)
  • sensory neuron also synapses with interneuron to
    antagonistic muscle (hamstrings)
  • interneuron inhibits motor neuron of antagonist
    (decreases efferent impulses to antagonist to
    limit its contraction while agonist is
    contracting)

Fig. 13.15, p. 505
33
Flexor ReflexWithdrawal Reflex
  • polysynaptic, ipsilateral
  • initiated by painful stimulus, which results in
    contraction of flexor to withdraw body part from
    stimulus
  • may be intersegmental (may involve more than one
    spinal cord segment)
  • may involve more than one muscle group
  • e.g., moving hand away from flame

Fig. 13.17, p.507
34
Crossed Extensor Reflex
  • e.g., arm being grabbed (also stepping on Lego)
  • two components
  • 1) ispilateral flexor reflex
  • afferent fibers (via dorsal root) --gt ipsilateral
    interneuron (gray matter of spinal cord) --gt
    ipsilateral flexor reflex (contraction of flexor
    on same side)
  • 2) contralateral extensor reflex
  • same afferent fibers --gt interneurons on opposite
    side (cross through gray commissure
    contralateral) --gt inhibit motor neuron to
    flexor on opposite side and stimulate motor
    neuron to extensor

35
Crossed Extensor Reflex (cont)
Fig. 13.17, p.507
36
Deep Tendon Reflex
  • protects against over-stretching of tendon
  • causes inhibition (relaxation) of contracting
    muscle stimulation of ipsilateral antagonist
  • steps
  • gt stimulation of Golgi tendon receptors
  • gt afferent impulses to spinal cord enter through
    dorsal root
  • gt synapse with interneurons to motor neuron of
    contracting muscle causing inhibition and
    stimulates motor neuron to antagonist muscle
    causing contraction

37
Deep Tendon Reflex
Fig. 13.16, p. 506
38
Superficial Reflexes
  • gentle cutaneous stimulation of skin results in
    reflexive contraction of nearby muscles
  • e.g., plantar reflex
  • tests integrity of spinal cord from L4- S2
  • tested by moving blunt instrument from heel
    toward toes then medially
  • result should be slight flexing (curling) of toes
  • damage to primary motor cortex or corticospinal
    tract results in positive Babinski sign (great
    toe dorsiflexes, other toes spread)
  • normal in babies because their nerves are not
    fully myelinated (24 months)
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