Sensory Systems 2: Somatic Senses and Mechanoreceptors

Somatic Senses Overview

BI20B2
Somatic Senses
Dr Derryck Shewan
d.shewan@abdn.ac.uk
Silverthorn, chapter 10
UNIVERSITY OF ABERDEEN

The somatosensory system is concerned with sensory information from the
skin, joints, muscles and viscera
Skin receptors (cutaneous exteroceptors) detect touch-pressure, vibration,
temperature, and pain
Muscle and joint receptors (proprioceptors) provide kinaesthetic
information (position and movement)
Somatic visceral receptors (interoceptors) whose input is detected at
conscious level are modally similar to cutaneous receptors but are lower in
density and the sensations are poorly localised
Somatosensory receptors are classified by modality, morphology, degree of
adaptation and subclassified by fibre type based on axon diameter and
conduction velocity

Sense Receptors Classification

Exteroreceptors

• Exteroreceptors
• Receptors for external
  stimuli
  - photoreceptors
  - hair cells in the inner ear
  - olfactory receptors
  - taste receptors
  - skin receptors
  - mechanoreceptors
  - thermoreceptors
  - nociceptors

Interoreceptors

• Interoreceptors
• Receptors for internal
  stimuli
  - GIT
  - respiratory tracts
  - cardiovascular system
  - pressure
  - pH
  - volume

Proprioceptors

• Proprioceptors
• Muscle reflexes and
  body position
  - most reflexes at spinal
  level
  - conscious awareness
  secondary
  - joint receptors

Cutaneous Receptors

Human sense of smell is pretty poor
What about sense of touch?
https://www.youtube.com/watch?
v=fHP-Td7LZw80

O
Merkel's disks
Touch
O
0
O
0
Epidermis
Ruffini's
corpuscles
Touch,
pressure
Nerves
Dermis
Pacinian
corpuscle
Pressure
Sweat
gland
Free nerve endings
Pain, itch,
temperature
LIFE: THE SCIENCE OF BIOLOGY, Seventh Edition, Figure 45.6 The Skin Feels Many Sensations
C 2004 Sinauer Associates, Inc. and W. H. Freeman & Co.
Meissner's
corpuscle
Sensitive
touch

PasSkimnecopassles
Epidermis
Dermis
Meissner's
corpuscle
Free
nerve
endings
Merkel
disk
Ruffini's
corpuscle
Subcutis
Pacinian
corpuscle
Smooth skin
Hairy skin

Pacinian Corpuscles

subcutaneous tissue in palms of
hands and soles of feet, joints,
genitals and GIT
- rapidly-adapting mechanoreceptors
- respond to vibration or tickle
single nerve ending
central clear
blood
supply
space
concentric lamellae of
fine connective tissue
fibres
large myelinated axons allow fast
propagation of axon potentials

Meissner's Corpuscles

1Meissner's corpuscles
Epidermis
Dermis
Meissner's
corpuscle
Free
nerve
endings
Merkel
disk
Subcutis
Ruffini's
corpuscle
50
C
Pacinian
corpuscle
Smooth skin
Hairy skin
subepidermal location in hands,
feet, forearm, lips and tip of
tongue
- rapidly-adapting mechanoreceptors
- respond to tapping
nerve ending
coils of nerve fibres
tactile corpuscle
connective tissue capsule

Other Mechanoreceptors

Other mechanoreceptors
Merkel cells
Epidermis
Dermis
Meissner's
corpuscle
Free
nerve
endings
Merkel
disk
Subcutis
Ruffini's
corpuscle
Pacinian
corpuscle
Smooth skin
Hairy skin
- basal layer of the skin
- free nerve endings that terminate in discs
- slow-adapting mechanoreceptors
- respond to skin indentation
- associated with whiskers in most
mammals, but not humans
Ruffini's corpuscle
- nerve endings branch within a single sheath of thick connective tissue
- slowly-adapting, with tonic resting firing rate - detect stronger pressures
Hair root nerve endings
- can be rapid or slow adapting AS fibres, depending on hair type
- some fire on hair displacement, not on hair release

Somatosensory Receptor Classification by Modality

Somatosensory system receptor classification

Modality	Receptor type	Fiber type & conduction velocity
Touch	Rapidly adapting mechanoreceptor, e.g. hair follicle receptors, bare nerve endings, Pacinian corpuscles	AB, 6-12 um diameter 33-75 m s-1
Touch & pressure	Slowly adapting mechanoreceptors, e.g. Merkel's disks, Ruffini corpuscles Bare nerve endings	AB. 6-12 um diameter 33-75 m s-1 A0, 1-5 um diameter 5-30 m s-1
Vibration	Meissner's corpuscles Pacinian corpuscles	Aß, 6-12 um diameter 33-75 m s-1
Temperature	Cold receptors Warm receptors	A0, 1-5 um diameter 5-30 m s-1 C-fibers, 0.2-1.5 um diameter 0.5-2.0 m s-1
Pain	Bare nerve endings-fast 'pricking' pain Bare nerve endings-slow burning pain, itch	Aº, 1-5 um diameter 5-30 m s-1 C fibers, 0.2-1.5 um diameter 0.5-2.0 m s-1

Cutaneous Mechanoreceptors Function

Cutaneous mechanoreceptors
Rapidly adapting cutaneous
mechanoreceptors signal the
onset and offset (phasic) of a
stimulus and give rise to
sensations such as vibration,
touch, and movement
Slowly adapting
mechanoreceptors
continuously signal (tonic) the
intensity of the stimulus and
give rise to the sensation of
pressure

(a) Tonic receptors are slowly
adapting receptors that respond
for the duration of a stimulus.
(b) Phasic receptors rapidly adapt to a
constant stimulus and turn off. They fire
once more when the stimulus turns off.
Stimulus
Stimulus
Receptor
Receptor
potential
Action
potentials
in sensory
neuron
Axon of
sensory
neuron
Time
>
Time
Copyright @ 2007 Pearson Education, Inc., publishing as Benjamin Cummings.

Receptive Fields and Spatial Discrimination

Cutaneous mechanoreceptors
Cutaneous receptors with small
receptive fields are involved in
fine spatial discrimination,
whereas receptors with larger
receptive fields are less spatially
precise
Overlap of receptive fields
allows lateral inhibition to occur
in the ascending pathways and
increase sensory acuity

(a)
(b)
Copyright 2007 Pearson Education, Inc., publishing as Benjamin Cummings.
Stimulus
Stimulus
Pir
Skin
A
B
C
Tonic level
A
B
c
Primary neuron
response is
proportional to
stimulus strength.
Secondary
neurons
Pathway closest
to the stimulus
inhibits
neighbors.
A
B
C
Tonic level
Inhibition of lateral
neurons enhances
perception of
stimulus.
Frequency of action potentials
Tertiary
neurons
A
B
C
Copyright @ 2007 Pearson Education, Inc., publishing as Benjamin Cummings.
Frequency of action potentials
Primary
sensory
neurons

Phasic and Tonic Responses

Phasic & Tonic Responses
Skin indentation
Mechanical
stimulus
20 Hz
100 Hz 300 Hz
I
250 um
Pacinian
corpuscle
Rapidly adapting
mecha no rece ptor
Slowly adapting
mecha no receptor
0
100 200
ms
The response of a Pacinian corpuscle, a rapidly adapting mechanoreceptor,
and a slowly adapting mechanoreceptor to pressure stimuli applied to the
skin. Note that the Pacinian corpuscle responds to maintained skin
indentation with a single action potential while the slowly adapting
mechanoreceptor continues to generate action potentials. Only the
Pacinian corpuscle responds to the 300 Hz vibration of the skin.

Thermoreceptors

Thermoreceptors
Epidermal.
Dermis
Meissner's
corpuscle
Merkel
disk
Free
nerve
endings
Sub utis
Ruffini's
corpuscle
Pacinian
corpuscle
Smooth skin
Hairy skin
Thermoreceptors
- throughout epidermis
- spot-like receptive fields
- cold receptors
- excited by fall in temperature
- myelinated fibres
stological section
Epidermis
- warm receptors
- excited by rise in temperature
- unmyelinated fibres
- fire constantly and indefinitely
- firing rate dependent on
temperature

Thermoreceptor Classification

Somatosensory system receptor classification

Modality	Receptor type	Fiber type & conduction velocity
Touch	Rapidly adapting mechanoreceptor, e.g. hair follicle receptors, bare nerve endings, Pacinian corpuscles	AB, 6-12 um diameter 33-75 m s-1
Touch & pressure	Slowly adapting mechanoreceptors, e.g. Merkel's disks, Ruffini corpuscles Bare nerve endings	AB. 6-12 um diameter 33-75 m s-1 A0, 1-5 um diameter 5-30 m s-1
Vibration	Meissner's corpuscles Pacinian corpuscles	Aß, 6-12 um diameter 33-75 m s-1
Temperature	Cold receptors Warm receptors	A0, 1-5 um diameter 5-30 m s-1 C-fibers, 0.2-1.5 um diameter 0.5-2.0 m s-1
Pain	Bare nerve endings-fast 'pricking' pain Bare nerve endings-slow burning pain, itch	Aº, 1-5 um diameter 5-30 m s-1 C fibers, 0.2-1.5 um diameter 0.5-2.0 m s-1

L
(a)
38-43ºC
1 s
Cold receptor
Warm receptor
15
12
6
3
0
Ť
13
23
33
43
Temperature (*C)
(b)

Pain Receptors (Nociceptors)

Pain receptors
Message is received in the thalamus and cerebral cortex
Tissue-damaging stimulus
activates nociceptors
Descending pathway
Message carried
to spinal cord
From brain
To brain
Nociceptors
Dorsal horn
Muscle fiber
Nociceptors
- all skin layers
- small receptive fields
- C-terminals
- mechanoreceptors stimulated by firm
pressure or sharp penetrating objects
- myelinated (AS) or unmyelinated (C)
fibres
- thermal or mechanothermal receptors
- unmyelinated (C) fibres
- excited by high temperatures (+42℃)
- inflammation chemicals

Pain Receptor Classification

Somatosensory system receptor classification

Modality	Receptor type	Fiber type & conduction velocity
Touch	Rapidly adapting mechanoreceptor, e.g. hair follicle receptors, bare nerve endings, Pacinian corpuscles	AB, 6-12 um diameter 33-75 m s-1
Touch & pressure	Slowly adapting mechanoreceptors, e.g. Merkel's disks, Ruffini corpuscles Bare nerve endings	AB. 6-12 um diameter 33-75 m s-1 A0, 1-5 um diameter 5-30 m s-1
Vibration	Meissner's corpuscles Pacinian corpuscles	Aß, 6-12 um diameter 33-75 m s-1
Temperature	Cold receptors Warm receptors	A0, 1-5 um diameter 5-30 m s-1 C-fibers, 0.2-1.5 um diameter 0.5-2.0 m s-1
Pain	Bare nerve endings-fast 'pricking' pain Bare nerve endings-slow burning pain, itch	Aº, 1-5 um diameter 5-30 m s-1 C fibers, 0.2-1.5 um diameter 0.5-2.0 m s-1

Proprioceptors and Reflexes

Golgi Tendon Organs

Proprioceptors
Nerve fiber
Tendon
bundles
Organ of Golgi, showing
ramification of nerve-fibrils
Muscular fibers
Golgi tendon organs
- junctions of tendons and
muscle
- Ib and II afferents
- stretch reflex

Figure 45.7 Stretch Receptors Are Activated when Limbs Are Stretched
Golgi tendon organs
Muscle
Load on muscle
Golgi
tendon
organ
Firing of
Sensory neuron
Time
Sensory neuron
Tendon

Stretch Reflex

The stretch reflex
Sensory neuron
Alpha motor neuron
Extensor muscles activated
Muscle
spindle
Inhibitory neuron
Flexor muscles inhibited
Stimulus
Response
Efferent nerves
Afferent nerves

Neuromuscular Spindles

Proprioceptors
Nerve fiber
Tendon
bundles
Organ of Golgi, showing
ramification of nerve-fibrils
Muscular fibers
FIG. 939 .- Middle third of a terminal plaque in the muscle spindle of an adult cat. (After Ruffini.)
Golgi tendon organs
- junctions of tendons and
muscle
- Ib and II afferents
- stretch reflex
Neuromuscular spindles
- nerve endings encircle
intrafusal muscle fibres
- Ia afferents
- crossed extension reflex

Figure 45.7 Stretch Receptors Are Activated when Limbs Are Stretched
Muscle spindles
Muscle
Muscle
spindle
Sensory
neuron
Sensory neuron
Stretch
Firing of
Time