Peripheral Nervous System- Afferent Division (Somatic)

Content

Peripheral Nervous System
Sensory Systems
Perception
Receptors Classified by Stimulus
Sensory Modalities
Adaptation
Proprioceptors
Nociceptors (Pain Receptors)
Processing of Pain

Peripheral Nervous System

• Is made up of 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
• The afferent pathway of the Peripheral Nervous System transmits impulses from PNS to the CNS

Properties of Sensory Systems

• Stimulus
• Internal--interoceptors
• External --exteroceptors
• Energy source—heat, light, sound, pressure

• Receptors - Afferent pathway ( from PNS to CNS)
• Sense organs
• Transducer

• CNS integration

Perception

• Is the 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

• Are 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

Receptors 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

Receptors Classified by Stimulus Type

• 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
• 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

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

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

• 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 (Pain Receptors)

• Carry out a protective mechanism           
• Three types of 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

 

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