The Somatic Sensory System

Question 1

What are mechanoreceptors?

Answer 1

Sensory receptors that respond to mechanical pressure or distortion

Relay extracellular stimulus to intracellular signal transduction through mechanically gated ion channels

Question 2

What are the two types of mechanoreceptor?

Answer 2

Slowly adapting receptors - always fire when the stimulus is present (best for providing information about the spatial attributes of the stimulus such as size and shape
Rapidly adapting receptors - only fire at the very start of a stimulation (best for providing information about changes in ongoing stimulation such as those produced by stimulus movement)

Question 3

Name the rapidly adapting receptors for tactile stimulation

Answer 3

Meissner corpuscle
Pacinian corpuscle

Question 4

What are the Meissner corpuscles?

Answer 4

• Rapidly adapting
• Most common in smooth hairless skin
• 40% innervation of hand
• For textured objects moving across the skin
• Detection of slippage between hand and object - important for grip

Question 5

What is the Pacinian corpuscle?

Answer 5

• Rapidly adapting
• More sensitive than Meissner
• Fine textured surfaces
• 15% of hand innervation
• Produce sensation of vibrations or tickle
• Important for the skilled use of tools

Question 6

Name the rapidly adapting receptors for tactile stimulation

Answer 6

Meissner corpuscle
Pacinian corpuscle

Question 7

Name the slowly adapting receptors for tactile stimulation

Answer 7

Ruffini’s corpuscles
Merkel’s disks

Question 8

What are Ruffini’s corpuscles?

Answer 8

• Slowly adapting
• 20% receptors in hand
• Unsure of function
• Something to do with proprioception
• Conformation of the hand in space

Question 9

What are the Merkel’s disks?

Answer 9

• Slowly adapting
• 25% receptors in hand
• Dense in finger tips
• Stimulation produces sensation of light pressure
• Role in detection of shapes, edges and rough textures
• For Braille reading

Question 10

What detects pain and temperature in the skin?

Answer 10

Free nerve endings

Question 11

What determines mechanosensory discrimination across the body surface?

Answer 11

1. Receptor density (Regional variations in the average size of afferent receptive fields which reflect the density of afferent fibres suppling the area)
2. Receptive field size (the area of the skin surface over which stimulation results in a significant change in the rate of action potentials)

Question 12

What are proprioceptors?

Answer 12

Provide information about mechanical forces arising from the body itself

Give continuous information about position of the limbs and other body parts in space

Question 13

What are the main types of proprioceptors?

Answer 13

Muscle spindles
Golgi tendon organs

Question 14

What are muscle spindles?

Answer 14

Stretch detectors - they sense how much and how fast a muscle is lengthened or shortened

Question 15

What are Golgi tendon organs?

Answer 15

Proprioceptors which receive information from the tendon and senses tension
Does not respond to passive stretch of muscle only to active contraction

Question 16

What are the somatic sensory pathways?

Answer 16

The Dorsal Column-Medial Lemniscus System - upper and lower body - fine touch, 2-point discrimination, conscious proprioception, vibration
The Trigeminal System - face
Spinothalamic - pain and temperature

Question 17

Describe the dorsal column-medial lemniscus system

Answer 17

Mechanosensory receptors in upper body travel through the cuneate tract to the cuneate nucleus in the medulla, decussates and then ascends up to the thalamus and then the primary somatic sensory cortex

Mechanosensory receptors in lower body travel through the gracile tract to the gracile nucleus in the medulla, decussates and then ascends up to the thalamus and then the primary somatic sensory cortex

Question 18

What are dermatomes?

Answer 18

Area of skin that is mainly innervated by afferent nerve fibres from the dorsal root of any given spinal nerve

substantial overlap between segments especially for touch, vibration and pressure

Question 19

Describe the trigeminal system

Answer 19

Mechanosensory receptors from face travel through the trigeminal ganglion at the level of the pons
Synapse in the principal nucleus of trigeminal complex and then decussate across the pons
Ascend in the trigeminothalamic tract to the ventral posterior medial nucleus (VPM) of the thalamus
Synapse and then project to the primary somatic sensory cortex

Question 20

What are the areas of the somatic sensory cortex?

Answer 20

3a = proprioceptive
3b & 1 = cutaneous stimuli
2 = proprioception and tactile stimuli

Question 21

What are the receptive fields for 3b, 1 & 2?

Answer 21

3b = single finger
1 & 2 = multiple fingers
1 = directions of movement
2 = complex stimuli such as specific shape

Question 22

What effects do lesions in areas 3b, 1 and 2 have?

Answer 22

3b = severe deficit on both texture and shape
1 = texture discrimination
2 = finger coordination and in size and shape discrimination

Question 23

What is sensory transduction?

Answer 23

The process of converting energy of a stimulus into an electrical signal

Question 24

Why do measures of two-point discrimination vary dramatically across the skin surface?

Answer 24

Regional differences in receptive field size and innervation density are the major factors that limit the spatial accuracy with which tactile stimuli can be sensed

On the fingertips, stimuli are perceived as distinct if they are separated by roughly 2mm but the same stimuli applied to the upper arm are not perceived as distinct until they are at least 40mm apart

Question 25

What sensory afferents are known to detect painful sensations?

Answer 25

Nociceptors which terminate in the skin as free nerve endings

Question 26

Describe the spinothalamic pathway?

Answer 26

Lateral spinothalamic tract
Information from tactile receptors travel to first order neurons in the dorsal root ganglion
Decussate across the spinal cord, ascend to the ventral posteriolateral nucleus (VPL) of the thalamus, then ascend to the somatosensory cortex

Anterior spinothalamic tract
First order neurons synapse in the nucleus proprius and the second order neurons decussate via the anterior white commissure
Ascend synapsing in the VPL of the thalamus

Question 27

What is the function of the spinothalamic pathway?

Answer 27

Nociceptive, temperature, crude touch and pressure