SOMATOSENSORY SYSTEM

SOMATOSENSORY SYSTEM

DEFINITION AND TYPES OF SENSATIONS

Somatosensory system is defined as the sensory system associated with different parts of the body.

Sensations are of two types:

1. Somatic sensations

2. Special sensations.

1. Somatic Sensations

Somatic sensations are the sensations arising from skin, muscles, tendons and joints. These sensations have specific receptors, which respond to a particular type of stimulus.

2. Special Sensations

Special sensations are the complex sensations for which the body has some specialized sense organs.

These sensations are usually called special senses. Sensations of vision, hearing, taste and smell are the

special sensations.

TYPES OF SOMATIC SENSATIONS

Generally, somatic sensations are classified into three types:

1. Epicritic sensations

2. Protopathic sensations

3. Deep sensations.

1. Epicritic Sensations

Epicritic sensations are the mild or light sensations. Such sensations are perceived more accurately.

Epicritic sensations are:

i. Fine touch or tactile sensation

ii. Tactile localization

ii. Tactile discrimination

iv. Temperature sensation with finer range between 25°C and 40°C.

2. Protopathic Sensations

Protopathic sensations are the crude sensations. These sensations are primitive type of sensations.

Protopathic sensations are:

i. Pressure sensation

ii. Pain sensation

iii. Temperature sensation with a wider range, i.e. above 40°C and below 25°C.

3. Deep Sensations

Deep sensations are sensations arising from deeper structures beneath the skin and visceral organs.

Deep sensations are:

i. Sensation of vibration or pallesthesia, which is the combination of touch and pressure sensation

ii. Kinesthetic sensation or kinesthesia: Sensation of position and movements of different parts of the

body. This sensation arises from the proprioceptors present in muscles, tendons, joints and ligaments.

Proprioceptors are the receptors, which give response during various movements of a joint.

Kinesthetic sensation is of two types:

a. Conscious kinesthetic sensation b. Subconscious kinesthetic sensation. Impulses of this sensation are called nonsensory impulses.

iii. Visceral sensations arising from viscera.

Synthetic Senses

Synthetic senses are the sensations synthesized at cortical level, by integration of impulses of basic

sensations. Two or more basic sensations are combined in some of the synthetic senses. Best examples of synthetic senses are vibratory sensation, stereognosis and two-point discrimination.

SENSORY PATHWAYS

Nervous pathways of sensations are called the sensory pathways. These pathways carry the impulses

from receptors in different parts of the body to centers in brain.

Sensory pathways are of two types:

1. Pathways of somatosensory system

2. Pathways of viscerosensory system.

Pathways of somatosensory system convey the informa tion from sensory receptors in skin, skeletal

muscles and joints. Pathways of this system are constituted by somatic nerve fibers called somatic

afferent nerve fibers. Pathways of viscerosensory system convey the information from receptors of the viscera. Pathways of this system are constituted by visceral or autonomic fibers.

 Somatosensory Pathways

Each sensory pathway is constituted by two or three groups of neurons:

i. First order neurons

ii. Second order neurons

iii. Third order neurons.

Pathways of some sensations like kinesthetic sensation

have only first and second order neurons.

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