PNS – Afferent Division Sensory Physiology

Part I

Peripheral Nervous System

• PNS – all neural structures outside the brain and spinal cord

• Includes sensory receptors, peripheral nerves, associated ganglia, and motor endings

• Provides links to and from the external environment

Organization of the Nervous System

Figure 8-1: Organization of the nervous system

• Stimulus– Internal– External– Energy source

• Receptors - Afferent pathway– Sense organs– Transducer

• CNS integration

Properties of Sensory Systems

From Sensation to Perception

• 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: Transducers

• Transduction - stimulus energy converted into information processed by CNS

• Sensory receptors are structures specialized to respond to stimuli, activation results in– Ion channels or second messengers that initiate

membrane potential change is sensory receptors– Depolarizations trigger impulses to the CNS

• The realization of these stimuli, sensation and perception, occur in the brain

Sensory Receptor Types

Receptor Classification• 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

• 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

Conversion of Receptor and Generator Potentials into Action Potentials

• 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

Receptor Potential Generator Potential

Sensory Pathways• Stimulus as physical energy sensory receptor

– Receptor acts as a transducer

• Intracellular signal usually change in membrane potential

• Stimulus > threshold action potential to CNS

• Integration in CNS cerebral cortex or acted on subconsciously

Sensory Pathways – External Stimuli

• Vision

• Hearing

• Taste

• Smell

• Equilibrium

• Somatic Senses

• Touch• Temperature• Pain• Proprioception

Somatic Senses – Internal Stimuli

Figure 10-10: The somatosensory cortex

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

Figure 10-9: Sensory pathways cross the body’s midline

• Modality – type of stimulus• Location

– Coded by site of the stimulated receptor– Precision of location called acuity,

• Receptive field• Lateral inhibition

• Intensity– Increased stimulus results in a larger

receptor potential leading to a higher frequency of action potential

– Stronger stimuli also affect a larger area and recruit a larger number of receptors

• Duration - Adaptation– Tonic receptors– Phasic receptors

Sensory Coding

Receptive Fields of Sensory Neurons

Figure 10-2

Receptive Field: Two-point discrimination

Lateral Inhibition

Figure 10-6: Lateral inhibition

Sensory Coding: Stimulus Intensity & Duration

• Intensity - coded by number of receptors activated and frequency of action potentials

• Duration - coded by duration of action potentials

• Some receptors can adapt or cease to respond

Figure 10-7

Duration

Amplitude

Time (sec)5 10 0 5 10 0 5 100

Threshold

(a)

(b)

Stimulus 20

-20-40-60-80

Mem

bra

ne

po

ten

tial

(m

V)

Mem

bra

ne

po

ten

tial

(m

V) 20

-20-40-60-80

0

0

Longer andstronger stimulus

0 5 10 0 5 10 0 5 10

Threshold

Figure 10-7: Sensory coding for stimulus intensity and duration

Adaptation

• Adaptation occurs when sensory receptors are subjected to an unchanging stimulus– Receptor membranes become less responsive– Receptor potentials decline in frequency or stop

• Tonic receptors – do not adapt or adapt very slowly

• Phasic receptors – readily adapt

Sensory Adaptation

• Tonic receptors (Pain):– Produce constant rate of

firing as long as stimulus is applied

• Phasic receptors: – Burst of activity but

quickly reduce firing rate (adapt) if stimulus maintained.

– Sensory adaptation: cease to pay attention to constant stimuli.

Adaptation

• Receptors responding to pressure, touch, and smell adapt quickly

• Receptors responding slowly include Merkel’s discs, Ruffini’s corpuscles

• Pain receptors and proprioceptors do not exhibit adaptation

• 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

Touch (pressure)

• Muscle spindle – In muscles– Sense stretch

• Golgi tendon organ– Near tendon– Sense force

• Joint receptors– Sense position

& pressure

Proprioceptors

Muscle Spindle Structure

• Consist of collections of specialized muscle fibers known as intrafusal fibers– Lie within spindle-

shaped connective tissue capsules parallel to extrafusal fibers

– Each spindle has its own private efferent and afferent nerve supply

– Play key role in stretch reflex

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

Pain

• Nociceptors

• Reflexive path

• Fast pain

• Slow pain

Thalamus

SensoryReceptor

Sensoryaxon

Spinalcord

DRG

Nociceptive Transmission Pathway

• A-Delta – Small, thinly myelinated.– 10 % sensory pain fibers.– Conduct at 5-30 m/sec.– Mechanical and thermal

stimuli.– Sensations of sharp,

pricking pain.• C Fibers

– Small, unmyelinatd fibers.– 90% of afferent sensory

fibers.– Conduct at 0.5-2.0 m/sec.– Mechanical, thermal,

chemical.– Long lasting, burning pain.

Fibers• A-Delta

– Small, thinly myelinated.– 10 % sensory pain fibers.– Conduct at 5-30 m/sec.– Mechanical and thermal stimuli.– Sensations of sharp, pricking pain.

• C Fibers– Small, unmyelinatd fibers.– 90% of afferent sensory fibers.– Conduct at 0.5-2.0 m/sec.– Mechanical, thermal, chemical.– Long lasting, burning pain.

A and C Nociceptors Mediate Pain

Firstpain Second

pain

Time

Painintensity

C-fiber

A fiber

• Key nociceptor transmitter is substance P.– Activates ascending pathways that

transmit nociceptor impulses.

• Glutamate:– Binds to AMPA receptors, increases

permeability, increasing likelihood of AP.– Binds to NMDA receptors increases

excitability of dorsal horn neurons.

Neurotransmitters in Spinal Cord

Spinal Cord: Excitatory TransmittersSpinothalamic

Tract

1o Afferent fiber

DRG

Substance P

2nd Order Neuron

Glutamate