2. the somatosensory system

THE SOMATOSENSORY

SYSTEM

Ahmed A. E. Eljack 1

Objectives:

By the end of this session you should be able to:

■ List the different types of sensory receptors.

■ List the different types of nerve fibers and their properties.

■ Discuss the characteristics of different stimuli.

■ Discuss the role of somatosensory cortex in perception of

sensations.

■ Discuss the different modalities of sensation.


Sensory Receptors:

■ They are specialized neural structures.

■ Types of sensory receptors:

– Mechanoreceptors.

– Thermoreceptors.

– Nociceptors.

– Electromagnetic receptors.

– Chemoreceptors.


Types of Nerve Fibers:

■ Either myelinated or unmyelinated.

■ Classified numerically into: Ia, Ib, II, III, and

IV (the only unmyelinated nerve fiber).

■ Classified alphabetically into: Aα, Aβ, Aδ,

and C (the only unmyelinated nerve fiber).


Characteristics of Stimuli:

■ Modality: refers to the type of stimuli.

■ Intensity.

■ Duration.

■ Location.

■ Transduction (important!).

■ Receptive field.


Spatial and Temporal Summation:

■ In spatial summation, the increase in signal

strength is transmitted using a larger

number of fibers.

■ In temporal summation, the increase in

signal strength is transmitted by increasing

the frequency of nerve impulses in each

fiber.


Receptor Potential and Transduction:

■ The receptor potential is the change in the membraneelectrical potential of the receptor when a stimulus excites thereceptor.

■ Transduction is initiated by:

– Mechanical deformation (opens ion channels in thereceptor’s membrane).

– Chemicals (opens ion channels).

– Change in the temperature of the receptor (changes thepermeability of the membrane).

– Electromagnetic radiation (changes the membranecharacteristics).


Receptor Potential and Transduction:

■ Sensory receptors adapt, they can be

divided into”

– Slow adapting receptors (like

nociceptors).

– Rapidly adapting receptors (like

pacinian corpuscles).

■ Adaptation occurs in different ways.


The Somatosensory Cortex:

■ Lies in the anterior parietal lobe (postcentral gyrus).

■ Divided into somatosensory area I (more important

and usually discussed) and somatosensory area II.

■ It shows somatotopic organization with some parts

represented in larger areas.

■ Amorphosynthesis is losing the ability to recognize

complex objects (in case of sensory association

area damage).


Important Laws Regarding somatic Sensations:

■ Law of Projection: sensations are projected

to the receptors.

■ Labeled line principle: the type of sensation

felt when a nerve fiber is stimulated is

determined by the point in the nervous

system to which the fiber leads.


MODALITIES OF SENSATIONS


Tactile Sensations:

■ Include touch, pressure, and vibration. They are mechanical sensations.

■ Two-point discrimination.

■ Transmitted through the anterolateral and dorsal column systems

■ Tactile receptors are:

– Free nerve endings.

– Meissner’s corpuscle.

– Expanded tip tactile receptors.

– Hair end-organ (hair follicle).

– Ruffini’s endings.

– Pacinian corpuscle.


Proprioception:

■ Mechanical sensation of the position.

■ Conscious proprioception

– Mediated mainly by free nerve endings.

– Transmitted by the dorsal column-medial lemniscus

system

■ Nonconscious proprioception

– Mediated mainly by muscle spindles and Golgi tendon

organs.

– Transmitted by the anterior and posterior

spinocerebellar tracts.


Proprioception:

■ Static proprioception: The perception of

orientation of different body parts.

■ Dynamic proprioception: Rate of movement.


Pain:

■ Detects damage to the tissues (protective mechanism).

■ Stimuli include:

– Mechanical stimuli.

– Chemical stimuli..

– Thermal stimuli

■ Mediated by free nerve endings and transmitted through theanterolateral system. The main neurotransmitter is substanceP (glutamate is also involved).

■ Divided into:

– Sharp pain (through type Aδ nerve fibers).

– Chronic pain (through type C nerve fibers)


Pain:

■ The severity depends mainly on the rate of destruction.

■ Thalamus plays an important role.

■ The analgesia system suppress the pain input (through

serotonin and enkephalin). It is composed mainly of:

– The periaqueductal gray matter and periventricular

areas in the midbrain.

– The raphe magnus nucleus and the nucleus reticularis

paragigantocelluraris.

– Pain inhibitory complex in the dorsal horn of the spinal

cord.



Siegel, A., & Sapru, H. N. (2015). Essential

Neuroscience. Lippincott Williams &

Wilkins. p257

Pain:

■ Clinical association:

– Referred pain.

– Visceral pain

– Phantom limb.

– Hyperalgesia.

– Thalamic pain syndrome.

– Neuralgia.

– Headache.


Thermal Sensations

■ Divided into:

– Cold sensation (free nerve endings transmitted

in type C nerve fibers).

– Warmth sensation (Myelinated nerve endings

transmitted in type Aδ nerve fibers).

■ Cold receptors are more than warmth receptors (3

to 10 times).

■ Mediated through the same pathways of pain.


Further Reading:

■ Barret, K.E, Barman, S.M, Boitano, S, Brooks, H.L. Ganong's

Review of Medical Physiology. (24th ed.). : McGraw-Hill; 2012.

■ Costanzo, L.S. Physiology. (5th

ed.). Philadelphia: Saunders; 2014

■ Hall, J.E. Guyton and Hall Textbook of Medical

Physiology. (12th ed.). Philadelphia: Saunders; 2011.

■ Eljack, A. A. E. Eljack’s Lecture Notes in Neuroscience.

Khartoum; 2015

■ Siegel, A, Sapru, H.N. Essential Neuroscience. (3rd

ed.). Philadelphia: Lippincott Williams & Wilkins; 2015.


2. the somatosensory system

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