LECTURE 29
SENSORY SYSTEM;
SOMATIC SENSATIONS
By: Dr. Khaled Ibrahim
• At the end of the lecture, the students should be
able to
a) Classify receptors.
b) Explain the mechanism of generation of receptor potential.
c) Discuss the classification of somatic sensations.
d) Describe the properties of receptors.
Guyton and Hall- Textbook of Medical Physiology 12th edition,
Page 559-581.
Ganong’s Review of Medical Physiology; 23rd edition, page 149155, 173-179.
Definition:
 Functionally specific microscopic structures located at the
peripheral termination of an afferent (sensory) nerve.
Functions:
 Inform CNS about the changes in the surrounding
environment (= acts as detectors).
 Initiate reflex actions which aim to maintain homeostasis.
 Give rise to sensations.
Physiological (Functional) Classification
- According to the nature of the stimulus (=
specificity of the receptor):
• Receptors can be classified into:
1- Mechanoreceptors.
2- Chemoreceptros.
3- Thermoreceptors.
4- Photoreceptors.
5- Nociceptors.
1- Mechanoreceptors
* responds to mechanical stimuli which cause change in the shape of the
receptor (=deformation).
vessel wall
* they include:
urinary bladder.
1) Stretch receptors:
GIT
responds to stretch & are present in
alveolar wall
skeletal muscle
2) Tension receptors:
- responds to ↑ tension in the tendon of the skeletal muscle by strong
contrction or passive stretch.
- they are called "Golgi tendon organ".
3) Touch & pressure receptors:
-responds to light & moderate mechanical stimuli.
- present in the skin & subcutaneous tissues.
- e.g., pacinian corpuscle & Meissner's corpuscle.
4) Auditory & vestibular receptors:
- responds to sound waves & changes in posture.
- present in the labyrinth of the internal ear.
5) Joint receptors:
- detects joint movements & position.
- present in the joint ligaments and capsule.
2- Chemoreceptors
* They respond to chemical stimuli.
* They include:
External chemoreceptors
Internal chemoreceptors
- responds to external chemical - responds to endogenous
stimuli.
chemical stimuli.
- includes:
- includes:
a) taste receptors.
a) carotid & aortic bodies.
b) smell receptors.
b) respiratory & vasomotor
centers (O2, CO2, H+).
c) hypothalamic glucoreceptors.
3- Thermoreceptors
* They respond to changes in the temperature.
* They include:
Warm receptors
Cold receptors
Detects the drop of Detects the rise of
temperature in a range temperature in a range
of 35 to 15 c
of 30 to 45 c
4- Photoreceptors (electromagnetic or visual receptors)
present in the retina & stimulated by electromagnetic waves of light.
5- Nociceptors (pain) receptors
responds to any stimulus potent enough to cause tissue damage.
Properties of the Receptors
1) Specificity (= adequate stimulus).
2) Excitability (= receptor potential).
3) Adaptation.
1) Specificity (= adequate stimulus):
"Each receptor is most sensitive to only one
specific stimulus called adequate stimulus &
when stimulated, it gives rise to only one
modality of sensation whatever the way of
stimulation"
- This is referred to as "Muller's law of specific
nerve energy“.
Examples:
• Light -------> Retinal receptors --------> Visual sensation
• Sound waves ---> cochlea ----------------> hearing sense.
• Light mechanical st. --> touch receptor --> touch
sensation.
- This law is not absolute.
as some receptors may respond to stimuli other
than their adequate stimulus provided that
stimulus is strong enough to excite the receptor
BUT the sensation produced will be always the
same.
Example: a blow to the eye (mechanical stimulus
of high intensity) stimulates photoreceptors in the
retina which responds by seeing flashes of light.
2) Excitability (=receptor potential):
Excitability: is the ability of the receptor to respond to
stimuli & converting them to electrochemical energy (receptor
potential) used to generate nerve impulses. So, receptors act
as transducers.
Receptor (generator) potential: is a change in the potential (usually
depolarization) of the terminal unmyelinated region of the sensory nerve
fiber (receptor or transducer zone) as a result of its stimulation.
Ionic basis of receptor potential:
 Receptor potential is studied on Pacinian corpuscle as it is easily
dissected & large in size.
 Pacinian corpuscle is an encapsulated receptor present in the skin &
subcutaneous tissues and sensitive to touch & pressure.
Structure:
 the receptor is the ending of myelinated nerve fiber & is
surrounded by concentric layers of lamellae (like an onion) forming
a connective tissue capsule. The 1st node of Ranvier is included
within the capsule.
Mechanism:
 During rest, application of 2 microelectrodes (one on the
surface and the other is introduced inside the nerve) records a
RMP of -70mv.
 Application of adequate stimulus (pressure) -----> deformation
of capsule --> deformation of the nerve terminal -----> stretching
the terminal membrane to open ionic channels for Na+ ----> Na+
influx -----> a state of depolarization which is called receptor
potential.
 This creates a local circuit of electrotonic current which spreads
for a short distance to the 1st node of Ranvier (the point of
impulse initiation) -----> depolarization of the node. The strength
of this electrotonic current is proportionate to the amplitude of
the receptor potential.
 If the receptor potential amplitude is strong enough, It will
depolarize the 1st node of Ranvier to the threshold level ----->
generation of an action potential , which is then transmitted
along the sensory nerve fiber to the central nervous system. For
this, the receptor potential is alternatively referred to as the
“generator potential”.
 Then, the 1st node of Ranvier is repolarized. If the receptor
potential is still maintained above the threshold level, it reexcites
the node to generate another impulse ----> Repetitive generation
of action potentials along the sensory nerve terminal..
Characters of receptor potential:
1) Does not obey all or none. So, it is a graded potential.
i.e., the amplitude of receptor potential is proportional to the
intensity of the stimulus. Stronger stimulus opens more Na channels
----> more depolarization & vice versa.
2) Has no refractory periods, So, it can be summated by:
 Temporal summation: with repeated stimulation at the same site
 Spatial summation: with multiple stimuli applied at the same
time.
3) Passively conducted to the 1st node of Ranvier & its amplitude
is decreased by time.
4) not blocked by local anesthetics which can block action
potential.
5) its duration is 5ms which is longer than duration of action
potential which is 2 ms only.
The rate (frequency) of discharge of impulses
from a receptor along its afferent nerve is
directly proportional to the logarithm of intensity
of stimulus.
* This is known as "Weber-Fechner" law.
Mechanism:
↑intensity of the stimulus -----> ↑amplitude of the receptor potential ----->
↑velocity of electrotonic current conduction -----> faster depolarization.
Significance:
This compress wide range of stimuli into a narrow range of impulse
discharge allowing CNS to discriminate wide range of stimuli. This is known
as "compression function of the receptor“.
3) Adaptation
Definition:
 It is decline in the rate of discharge from a receptor in spite of constant
stimulation.
The rate of this decline is called the "rate of adaptation".
 Almost all sensory receptors adapt when they are exposed to continuous
stimulation. i.e., their rate of discharge rises to a peak level then
gradually decreases with time.
 According to rate of adaptation, receptors are classified into:
1- Rapidly-adapting receptors.
2- Slowly-adapting receptors
1) Slowly-adapting (Tonic) receptors:
 These receptors continue to discharge impulses (relatively at a constant
rate) as long as the stimulus is applied.
i.e., the receptor discharge reaches the peak at the onset of stimulation then
continues to discharge at a lower rate as long as the stimulus is applied.
Examples:
a) Mechanoreceptors: muscle spindle, Golgi tendon organ, vascular
baroreceptors.
b) Chemoreceptors: carotid & aortic bodies.
c) Pain receptors.
Physiological significance:
a) maintain sensory information: they continuously detect the intensity &
duration of their stimuli. So, CNS is continuously informed about the state
of the body & the condition of the surrounding environment.
b) subserve vital functions: as
- Maintenance of body posture & equilibrium by discharge of muscle
spindle & Golgi tendon organ.
- Maintenance of blood pressure & heart rate by discharge of baro- &
chemoreceptors.
- protection of the body against harmful stimuli by discharge of pain
receptors.
2) Rapidly-adapting (phasic) receptors:
 These receptors discharge to a peak level at the onset of stimulation then
after a very short period, their rate of discharge drops to about zero
inspite of constant stimulation.
 They re-discharge again on removal or changing the velocity of the
stimulus.
Examples:
Pacinian corpuscle, Meissner's corpuscle, hair receptors, and some joint
receptors.
Mechanism of adaptation: 1) Adaptation of mechanoreceptors:
Adaptation due to vesico-elastic properties of lamellae of the
capsule:
* Pressure applied to the outside of the corpuscle ----->
compression of its lamellae -----> deformation of the nerve ending
-----> opening of the Na+ channels ----> receptor potential and
action potential occurs.
* But, within a small fraction of a second the viscous
fluid
within the corpuscle redistributes -----> the pressure becomes
essentially equal on all sides of the nerve terminal ----> the
nerve terminal returns back to its resting shape ----> closure of
Na+ channels & the receptor potential is stopped.
* When the applied pressure is released, the lamellae of
the corpuscle spring back (move rapidly) to their original
form ---> lamellae are compressed again in a different
direction -----> deformation of the nerve terminal, and
generation of receptor potential and action potential
once more.
* Again within a fraction of a second
redistribution
occurs, thus the effect of the stimulus is lost.
In this way both the application and release of pressure
result in discharge of action potentials.
Physiological significance:
 They detect:
- onset of stimulation.
– Termination of stimulation.
- Changing the velocity of the stimulus.
So, they inform CNS about the dynamic properties of the stimulus. i.e.,
the rate or the velocity of the stimulus.
Sensation means: Conscious awareness of a particular
feeling produced by the stimulation of a certain type of
receptors by its adequate stimulus.
Classification of sensations
General
arises
from
widely distributed
receptors all over
the body.
Special
Emotional
* vision
e.g.: fear, anxiety,
* taste-smell
sadness ………..
* hearing & sense
of equilibrium.
General Sensations
Somatic sensations
* arises from the
somatic
structures
(skin,
skeletal
muscles, joints &
periosteum)
* carried by somatic
nerves
Visceral sensations
* arises from the
viscera.
*
carried
by
autonomic nerves
*
e.g.
pain
,
temperature,
distention….etc.
Organic sensations
* thirst
* hunger
* sexual desire
Somatic sensations
1- Mechanoceptive 2- Thermoceptive
a) Tactile (touch – * warm
pressure – vibration) * cold
b)
proprioceptive
(position
–
and
movement)
3- Pain sensation