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Physiology of Sensory

Receptors

Dr Reem Abraham

Learning

Objectives

1. Apply the functional anatomical organization of the

somatosensory division to neural perception and processing of

sensory information

2. Identify the location and apply the distinct physiological function

of alpha and gamma motor neurons to the myotatic reflex.

3. Identify the modalities and physiological characteristics of the

various sensory receptors

At the end of this lecture, students should be able to:

Afferent sensory input:

- conveyed to brain via ascending tracts

Motor output:

- conveyed via descending tracts

Spinal afferents and

efferents

Primary sensory neuron cell bodies are found in the dorsal root ganglion (DRG);

send projections into the dorsal horn to synapse with their secondary neuron

Secondary neurons send information

to higher brain structures or to ventral

horn for spinal reflexes

CLASSIFICATION OF RECEPTORS

• (^) Based on origin of stimulus

1. • Exteroceptors

2. • Interoceptors

3. • Teleceptors

4. • Proprioceptors

• (^) Based on adequate stimulus

1. Mechanoreceptors –cutaneous receptors for touch and pressure & others like

Baroreceptors, Muscle spindle

2. Thermoreceptors - Cold & warm receptors

3. Chemoreceptors : respiratory chemoreceptors

4. Photoreceptors/ electromagnetic receptors - rod & cones

5. Nociceptors– pain receptors

• (^) Noxious stimuli: extremely painful stimuli, for eg. extreme cold, extreme heat

CUTANEOUS MECHANORECEPTORS

1. Pacinian corpuscle : encapsulated receptors found deep in the

skin throughout the body; Deep pressure, fast vibrations (High

frequency repetitive stimulation)

2. Meissner corpuscle : movement of objects over the surface of

the skin, slow vibrations

3. Merkel disc : determining the texture (Touch), sustained

pressure

4. Ruffini corpuscle : sustained pressure, vibration, joint rotation

5. Hair follicle receptor : motion

6. Free nerve endings: pain and temperature

Pain receptors are called nociceptors

Adaptation of Receptors

Slowly Adapting Receptors:

• (^) Muscle spindle
• (^) Pain receptors
• (^) Golgi tendon organ
• (^) Ruffini endings
• (^) Merkel discs
• (^) Temperature receptors
• (^) Chemoreceptors

and baroreceptors

o (^) Pacinian

corpuscle

o (^) Meissner’s

corpuscle

o (^) semicircular

canals (ear)

Principle sensory

modalities

Copyright © 2021 by Saunders, an imprint of Elsevier Inc.

Sensory receptor potential:

Exemplified by mechanical

distortion

• (^) Receptor potentials/generator

potentials: electrotonic, local

potentials that when

summated can trigger an

action potential

• (^) Receptor potential due to influx of

Na+ through voltage-gated Na+

channels

• (^) Receptor potential in turn induces

a local circuit of current flow

Receptor

potentials

Sensory transduction: Sensory

receptors transduce sensory stimuli

into receptor potential

• (^) is mediated through opening or closing of

specific ion channels Mechanical distortion in a

Pacinian corpuscle → ↑Na

+

conductance.

The action potentials are propagated to

dorsal root ganglion (no synapse) and is

transmitted to spinal cord/brain to secondary

sensory neurons

Action Potentials

Primary sensory neuron

Secondary

neuron

Copyright © 2021 by Saunders, an imprint of Elsevier Inc.

ionship between receptor

ntials/generator potentials and action potentials

Receptor potentials: nonpropagated

depolarizing potential recorded in a sensory

organ after an ADEQUATE STIMULUS is

applied

Receptor potentials are not action

potentials

Action Potentials occur only when

receptor potential reaches the

threshold

• ↑stimulus intensity → ↑ receptor

potential MAGNITUDE → after reaching

threshold  action potential

generated in sensory neurons

As the receptor potential

• (^) Each receptor is specific for a

specific stimulus

Copyright © 2021 by Saunders, an imprint of Elsevier Inc.

Receptive field: Two-Point

Discrimination

Lateral inhibition: helps to

localize the site of stimulus

application

‒the capacity of an excited neuron

to reduce activity of neighboring

neurons

‒allows the detection of two

distinct stimuli that occur closely

together

• (^) blocks lateral spread of the

excitatory signals and,

therefore, increases the degree of

contrast in the sensory pattern

perceived in the cerebral cortex

• (^) occurs at every synaptic level

of the dorsal column system

Lateral inhibition present = 2 point

discrimination

Transmission of signals to the cortex from two adjacent pinpoint stimuli. The blue curve represents

the pattern of cortical stimulation without “surround” inhibition, and the two red curves represent the

pattern when “surround” inhibition does occur.

• (^) Two-point threshold test tests the integrity of the dorsal column

(medial lemniscus) system

Copyright © 2021 by Saunders, an imprint of Elsevier Inc.

Identifying the location

of stimuli

is a function of

Dermatomes

Figure 48-14 Modified from Grinker RR, Sahs

AL: Neurology, 6th ed. Springfield, Ill: Charles

C. Thomas, 1966

The dermatome is somatotopographically

represented in the primary somatosensory

cortex.

• (^) Dermatomal rule is the basis of referred

pain

• (^) Dermatome—area of skin supplied by sensory

neurons (Each spinal nerve innervates a

“segmental field” of the skin)

• (^) For eg. When pain is referred, it is usually

to a structure that developed from the

same embryonic segment or dermatome

Stretch Reflex (Myotatic

reflex)

Monosynaptic reflex

Stretch reflex: When a skeletal

muscle with an intact nerve

supply is stretched, it contracts

Components:

Stimulus: Stretch

• (^) Receptor: Muscle spindle

Afferent/Sensory fibers: Ia fibers

Centre: Spinal cord

Efferent/Motor fibers: γ-motor

neuron fibers to intrafusal

fibers, α-motor neuron fibers

to extrafusal fibers

Effector organ and response:

skeletal muscle contraction

Function: maintains muscle

Stretch Reflex

(Myotatic reflex)

 RECIPROCAL

INHIBITION

• When a stretch

reflex occurs,

agonists contract

(excited) and

antagonists relax

(inhibited)

• Neuronal circuit

that causes this

reciprocal

relation is called

reciprocal

innervation.