Chapter 1 A Perspective on Human Genetics

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Chapter 6A The Peripheral Nervous System
Afferent
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• Describe the components (afferent and efferent)
  of the peripheral nervous system. This will be
  measured by lecture and laboratory exams.

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Outline

• Pathways, perceptions, sensations
• Receptor Physiology
• Receptors have differential sensitivities to
  various stimuli.
• A stimulus alters the receptors permeability,
  leading to a graded receptor potential.
• Receptor potentials may initiate action
  potentials in the afferent neuron.
• Receptors may adapt slowly or rapidly to
  sustained stimulation.
• Each somatosensory pathway is labeled according
  to modality and location.
• Acuity is influenced by receptive field size and
  lateral inhibition.
• PAIN
• Stimulation of nociceptors elicits the perception
  of pain plus motivational and emotional
  responses.
• The brain has a built-in analgesic system.

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Peripheral Nervous System

• Consists of nerve fibers that carry information
  between the CNS and other parts of the body
• Afferent division
• Sends information from internal and external
  environment to CNS
• Visceral afferent
• Incoming pathway for information from internal
  viscera (organs in body cavities)
• Sensory afferent
• Somatic (body sense) sensation
• Sensation arising from body surface and
  proprioception
• Special senses
• Vision, hearing, taste, smell

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Perception

• Conscious interpretation of external world
  derived from sensory input
• Why sensory input does not give true reality
  perception
• Some information is not transduced
• Some information is filtered out
• Cerebral cortex further manipulates the data
• Sensation vs. perception

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What Do You Perceive?
Proof !
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Receptors

• Structures at peripheral endings of afferent
  neurons
• Detect stimuli (change detectable by the body)
• Convert forms of energy into electrical signals
  (action potentials)
• Process is called transduction

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Types of Receptors

• Photoreceptors
• Responsive to visible wavelengths of light
• Mechanoreceptors
• Sensitive to mechanical energy
• Thermoreceptors
• Sensitive to heat and cold
• Osmoreceptors
• Detect changes in concentration of solutes in
  body fluids and resultant changes in osmotic
  activity
• Chemoreceptors
• Sensitive to specific chemicals
• Include receptors for smell and taste and
  receptors that detect O2 and CO2 concentrations
  in blood and chemical content of digestive tract
• Nociceptors
• Pain receptors that are sensitive to tissue
  damage or distortion of tissue

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Shaft of hair inside follicle
Skin surface
Epidermis
Dermis
Myelinated neuron
Subcutaneous tissue
Merkels disc light, sustained touch
Ruffini endings deep pressure
Pacinian corpuscle vibrations and deep pressure
Hair receptor hair movement and very gentle touch
Meissners corpuscle light, fluttering touch
Figure 6-5 p190
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Muscle Receptors

• Two types of muscle receptors.
• Both are activated by muscle stretch, but monitor
  different types of information.
• Muscle spindles monitors muscle length.
• Golgi tendon organs detect changes in tension.

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Muscle spindle (proprioceptor)regulates rate of
change of length, And length
Golgi tendon organ
Type II sensory neuron
Spinal cord
Intrafusal muscle fibers
Nuclear bag fiber
Type lA sensory neuron
Nuclear chain fiber
Nuclei of muscle fibers
Motor end plate
Alpha motor neuron
Extrafusal muscle fibers
Gamma motor neuron
Like pg. 289
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Capsule
Alpha motor neuron axon
Intrafusal (spindle) muscle fibers
Gamma motor neuron axon
Contractile end portions of intrafusal fiber
Afferent neuron axons
Noncontractile central portion of intrafusal fiber
Extrafusal (ordinary) muscle fibers
Fig. 8-25a, p. 289
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Uses For Perceived Information

• Afferent input is essential for control of
  efferent output
• Processing of sensory input by reticular
  activating system in brain stem is critical for
  cortical arousal and consciousness
• Central processing of sensory information gives
  rise to our perceptions of the world around us
• Selected information delivered to CNS may be
  stored for further reference
• Sensory stimuli can have profound impact on our
  emotions

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Receptors

• May be
• Specialized ending of an afferent neuron
• Separate cell closely associated with peripheral
  ending of a neuron
• Stimulus alters receptors permeability which
  leads to graded receptor potential
• Usually causes nonselective opening of all small
  ion channels
• This change in membrane permeability can lead to
  the influx of sodium ions. This produces
  receptor (generator) potentials.
• The magnitude of the receptor potential
  represents the intensity of the stimulus.
• A receptor potential of sufficient magnitude can
  produce an action potential. This action
  potential is propagated along an afferent fiber
  to the CNS.

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Afferent terminals
Rate of neurotransmitter release at afferent
terminals
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Afferent fiber potential (mV)
Afferent fiber
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Frequency of action potentials in afferent fiber
Sensory receptor
Receptor potential (mV)
Rest
Magnitude of receptor potential
Stimulus strength
Stimulus
On
On
Off
Off
Time (sec)
Stimulus strength
Figure 6-3 p189
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Conversion of Receptor Potentials into Action
Potentials
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Receptors

• May adapt slowly or rapidly to sustained
  stimulation
• Types of receptors according to their speed of
  adaptation
• Tonic receptors
• Do not adapt at all or adapt slowly
• Muscle stretch receptors, joint proprioceptors
• Phasic receptors
• Rapidly adapting receptors
• Tactile receptors in skin

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Phasic- Membrane potential drops More rapidly
(intensity i.e pressure)
Tonic -Takes longer for the membrane Voltage to
drop (maintaining the signal i.e position)
Fig. 6-5, p. 185
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Somatosensory Pathways

• Pathways conveying conscious somatic sensation
• Consists of chains of neurons, or labeled lines,
  synaptically interconnected in particular
  sequence to accomplish processing of sensory
  information
• First-order sensory neuron
• Afferent neuron with its peripheral receptor that
  first detects stimulus
• Second-order sensory neuron
• Either in spinal cord or medulla
• Synapses with third-order neuron
• Third-order sensory neuron
• Located in thalamus

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Table 6-1 p192
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Fig. 5-11, p. 145
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Acuity

• Refers to discriminative ability
• Influenced by receptive field size and lateral
  inhibition

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Lateral inhibition
Fig. 6-7, p. 187
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Pain

• Primarily a protective mechanism meant to bring a
  conscious awareness that tissue damage is
  occurring or is about to occur
• Storage of painful experiences in memory helps us
  avoid potentially harmful events in future
• Sensation of pain is accompanied by motivated
  behavioral responses and emotional reactions
• Subjective perception can be influenced by other
  past or present experiences

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• Cortex
• Higher processing
• Basal nuclei
• Control of movement, inhibitory, negative
• Thalamus
• Relay and processing of sensory information
• Awareness, a positive screening center for
  information
• Hypothalamus
• Hormone secretion, regulation of the internal
  environment
• Cerebellum
• Important in balance and in planning and
  executing voluntary movement
• Brain Stem
• Relay station (posture and equilibrium), cranial
  nerves, control centers, reticular integration,
  sleep control

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Pain

• Presence of prostaglandins (lower nociceptors
  threshold for activation) greatly enhances
  receptor response to noxious stimuli
• Role of asprin
• Nociceptors do not adapt to sustained or
  repetitive stimulation
• Three categories of nociceptors
• Mechanical nociceptors
• Respond to mechanical damage such as cutting,
  crushing, or pinching
• Thermal nociceptors
• Respond to temperature extremes
• Polymodal nociceptors
• Respond equally to all kinds of damaging stimuli

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Table 6-2 p194
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Pain

• Two best known pain neurotransmitters
• Substance P
• Activates ascending pathways that transmit
  nociceptive signals to higher levels for further
  processing
• Glutamate
• Major excitatory neurotransmitter
• Brain has built in analgesic system
• Suppresses transmission in pain pathways as they
  enter spinal cord
• Depends on presence of opiate receptors
• Endogenous opiates endorphins, enkephalins,
  dynorphin

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Higher processing of pain

• Substance P
• Different destinations
• Cortex localizes the pain
• Thalamus- perception of pain
• Reticular formation- increases alertness
• Hypothalamus/limbic system- emotional and
  behavioral responses

• Glutamate
• AMPA receptors
• Aps in the dorsal horn
• NMDA receptors
• Ca entry makes dorsal horn neuron more sensitive

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(Localization of pain)
Somatosensory cortex
(Perception of pain)
Higher brain
Thalamus
(Behavioral and emotional responses to pain)
Hypothalamus limbic system
Brain stem
Reticular formation
( Alertness)
Noxious stimulus
Spinal cord
Afferent pain fiber
Dorsal horn excitatory interneurons
Substance P
Nociceptor
(a) Substance P pain pathway
Figure 6-9a p195
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Periaqueductal gray matter
Reticular formation
Medulla
No perception of pain To thalamus
Inhibitory interneuron in dorsal horn
Endogenous opiate
Noxious stimulus
Opiate receptor
Transmission of pain impulses to brain blocked
Afferent pain fiber
Dorsal horn excitatory interneurons
Substance P
Nociceptor
(b) Analgesic pathway
Figure 6-9b p195