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BIO2305
Peripheral Nervous System (PNS) Afferent Division Part 1
PNS – all neural structures outside the brain and spinal cord. Provides links to and from the external
environment.
Includes:
 sensory receptors
 peripheral nerves
 associated ganglia
 motor endings.
Afferent pathway - transmit impulses from PNS to the CNS
Properties of Sensory Systems
• Stimulus
– Internal--interoceptors
– External --exteroceptors
– Energy source—heat, light, sound, pressure
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Receptors - Afferent pathway ( from PNS to CNS)
– Sense organs
– Transducer
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CNS integration
Perception – conscious interpretation of external world created by the brain
Survival depends upon sensation and perception
Sensation is the awareness of changes in the internal and external environment
Perception is the conscious interpretation of those stimuli
Sensory Receptors - Structures specialized to respond to stimuli. Activation of sensory receptors results in
depolarizations that trigger impulses to the CNS.
*The realization of these stimuli, sensation and perception, occur in the brain
Classification of receptors can be classified by stimulus
• Mechanoreceptors – respond to touch, pressure, vibration, stretch, and itch
• Thermoreceptors – sensitive to changes in temperature
• Photoreceptors – respond to light energy (e.g., retina)
• Chemoreceptors – respond to chemicals (e.g., smell, taste, changes in blood chemistry)
• Nociceptors – sensitive to pain-causing stimuli
• Osmoreceptors – detect changes in concentration of solutes, osmotic activity
Receptor Classification by Stimulus Type
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Generator potentials
– Occur in specialized nerve endings
– Stimulus opens ion channels in receptor causing local current flow
– Local current flow opens ion channels in afferent neuron AP generating region
– If threshold reached, AP is generated
Receptor potentials
– Occur in separate receptor cells
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Stimulus opens ion channels in receptor causing graded membrane potential
Receptor cell releases chemical messenger
Chemical messenger opens ion channels in afferent neuron AP generating region
If threshold reached, AP is generated .
Receptors
• The receptor must have specificity for the stimulus energy
• The receptor’s receptive field must be stimulated
• Stimulus energy must be converted into a graded potential
• A generator potential in the associated sensory neuron must reach threshold
Stimuli exist in a variety of energy forms or modalities – heat, light, sound, pressure, chemical etc.
Transduction – the process of converting energy forms into electrical signals via a generator potential
which triggers an action potential if it it large enough to reach threshold. A generator potential is a type of
graded potential similar to a EPSP
Special Senses--External Stimuli
• Vision
• Hearing
• Taste
• Smell
• Equilibrium
Somatic Senses—Internal Stimuli
• Touch
• Temperature
• Pain
• Proprioception
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Somatic Pathways
First-order neurons – soma reside in dorsal root or cranial ganglia, and conduct impulses from
the skin to the spinal cord or brain stem
Second-order neurons – soma reside in the dorsal horn of the spinal cord or medullary nuclei and
transmit impulses to the thalamus or cerebellum
Third-order neurons – located in the thalamus and conduct impulses to the somatosensory cortex
of the cerebrum
Sensory Modalities
Location
– Lateral inhibition
– Receptive field
Intensity
Duration
Tonic receptors
Phasic receptors
Adaptation
*Receptive field = area within which a receptor can detect a stimulus
Lateral inhibition; to facilitate localization and sharpen contrast, the most strongly activated pathway at
the center inhibits the less excited pathways from the fringe areas
Adaptation occurs when sensory receptors are subjected to an unchanging stimulus
Receptor membranes become less responsive, receptor potentials decline in frequency or stop. Adaptation
occurs in the receptor, not the CNS
- Tonic receptors do not adapt at all or very slowly, important when maintaining information
about a stimulus is valuable – stretch, pain receptors
- Phasic receptors – rapidly adapt, useful in situations where it is important to signal a change in
stimulus – tactile (touch) receptors
Adaptation mechanisms
Mechanical – specialized receptor ending consists of concentric layers of connective tissue.
Sustained pressure causes layers to slip, dissipating stimulus intensity
Chemical – Na+ channels that initial opened are slowly inactivated
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Tactile Receptors (pressure)
• Mechanoreceptors
• Free nerve endings
– Lamellated (Pacinian) corpuscles - rapidly adapting skin receptor that detects pressure
and vibration.
– Corpuscle of touch (Meissner‘s) - receptor for discriminative touch
– Type I cutaneous (Merkel) receptors for discriminative touch
– Type II cutaneous(Ruffini) receptor for continuous touch sensation
– Baroreceptors – receptors to detect pressure changes in blood vessels
Proprioceptors - located in muscles, tendons, joints and internal ear and provide information about body
position and movement.
- Muscle spindle - 3 - 10 specialized muscle fibers called intrafusal fibers, oriented parallel to regular
muscle fibers, ends of spindle are anchored to the endomysium and perimysium. Muscle spindles
monitor changes in length of muscle by responding to the rate and degree of change.
- Tendon Organs (Golgi tendon organs) - consists of sensory fiber penetrating a thin capsule of
connective tissue and entwining around a few collagen fibers, found at the junctions of a tendon with a
muscle, help protect tendons and associated muscles from damage due to excessive tension or stretching
Chemical Detection
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Stretch Reflex--Primary purpose is to resist tendency for passive stretch of extensor muscles
by gravitational forces when person is standing upright
Classic example is patellar tendon, or knee-jerk reflex
Nociceptors- protective mechanism
Pain –
Three types of receptors (nociceptors) - mechanical, thermal, polymodal. All are naked nerve endings
and do not adapt. All can be sensitized by prostaglandins (increase pain). Prostaglandins derived from
lipid bilayer of membrane released from damaged tissues
Mechanical (crushing, cutting, pinching) and thermal (extreme temperatures) are transmitted over small
myelinated A-delta fibers – 30m/sec fast pain pathway
Polymodal repond to all kinds of damaging stimuli and is carried by small unmyelinated C-fibers
12m/sec slow pain pathway
Processing of pain - Afferent pathway
1st order pain neurons use substance P and glutamate as neurotransmitters
Stimulus > nociceptor – substance P > spinal cord neuron >brainstem > thalamus > somatosensory cortex
RAS
Hypothalamus
Glutamate released fro primary afferent pain terminals – major excitatory NT (similar to LTP mechanism)
exaggerated sensitivity of an injured area to subsequent stimulus.
Built in analgesic system- periaqueductal gram matter >> RAS .>> Endogenous opiate >> Inhibitory opiate
receptor>> suppresses release of substance P