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.

Ahmed A. E. Eljack 2

Sensory Receptors:

■ They are specialized neural structures.

■ Types of sensory receptors:

– Mechanoreceptors.

– Thermoreceptors.

– Nociceptors.

– Electromagnetic receptors.

– Chemoreceptors.

Ahmed A. E. Eljack 3

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).

Ahmed A. E. Eljack 4

Characteristics of Stimuli:

■ Modality: refers to the type of stimuli.

■ Intensity.

■ Duration.

■ Location.

■ Transduction (important!).

■ Receptive field.

Ahmed A. E. Eljack 5

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.

Ahmed A. E. Eljack 6

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).

Ahmed A. E. Eljack 7

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.

Ahmed A. E. Eljack 8

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).

Ahmed A. E. Eljack 9

Ahmed A. E. Eljack 10

Ahmed A. E. Eljack 11

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.

Ahmed A. E. Eljack 12

MODALITIES OF SENSATIONS

Ahmed A. E. Eljack 13

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.

Ahmed A. E. Eljack 14

Ahmed A. E. Eljack 15

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.

Ahmed A. E. Eljack 16

Proprioception:

■ Static proprioception: The perception of

orientation of different body parts.

■ Dynamic proprioception: Rate of movement.

Ahmed A. E. Eljack 17

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)

Ahmed A. E. Eljack 18

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.

Ahmed A. E. Eljack 19

Ahmed A. E. Eljack 20

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.

Ahmed A. E. Eljack 21

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.

Ahmed A. E. Eljack 22

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.

Ahmed A. E. Eljack 23