Somatosensory System

Question 1

Organization of the Somatosensory System

Answer 1

Somatosensory System
Somatosensation: All over body
- Tells us what the body is up to and what’s going on in the environment by providing bodily sensations such as:
Touch, temperature, pain, position in space, and movement of the joints
- Allows us to distinguish between what the world does to us and what we do to it

Question 2

Somatosensory Receptors and Perception

Answer 2

Sensitivity to different somatosensory stimuli is a function of the kinds of receptors
Humans have two kinds of skin (the largest sensory organ!)
- Hairy skin
Relatively low sensitivity
- Glabrous skin
Skin that does not have hair follicles but contains larger numbers of sensory receptors than do other skin areas (e.g. palm of hands/feet, lips)
- Sensitive to wide range of stimuli

Question 3

Three Main Types of Somatosensory Perception

Answer 3

Nocioception
Perception of pain and temperature
Hapsis
Perception of fine touch and pressure, (e.g. grasp, manipulate)
Proprioception
Perception of the location and movement of the body

Question 4

Nocioceptors

Answer 4

Nocioceptors: free nerve endings

• sharp/dull pain and heat/cold
• Damage to dendrite or surrounding cells release chemicals that stimulate dendrite and produces an action potential

Question 5

Haptic receptors

Answer 5

Haptic receptors: dendrite attached to hair, connective tissue, or dendrite encased in capsule of tissue

• Distinguish touch, pull, vibration, flutter, indentation
• Mechanical stimulation, produces action potential
• Composition of capsule determines the type of mechanical energy conducted

Question 6

Proprioceptors

Answer 6

Movements stretch the receptors to mechanically stimulate dendrites and producesan action potential

Question 7

Sensory receptors and adaptation

Answer 7

Slow:
•signals the presence of a long sustained stimulus
(e.g. pain, long pinch)
Rapid:
•Motions of object on skin
•gives good responses to repeated stimuli
(e.g. vibration, tickle)

Question 8

Dorsal-Root Ganglion Neurons

Answer 8

• The dendrite and axon are continuous and carry sensory information from the skin to the CNS via the spinal cord
• The tip of the dendrite is responsive to sensory stimulation
• Each spinal cord segment has one dorsal-root ganglion on each side that contains many dorsal-root ganglion neurons
• In the spinal cord, the axons of these neurons may synapse onto other neurons or continue up to the brain

Question 9

Deafferentiation

Answer 9

Loss of incoming sensory input usually due to damage to sensory fibers; also loss of any afferent input to a structure

Question 10

Somatosensory Pathways to the Brain - Dorsal Spinothalamic Tract

Answer 10

1) Dorsal Spinothalamic Tract
Carries haptic and proprioceptive information
2) Axons from the dorsal-root ganglion neurons enter the spinal cord and ascend ipsilaterally until they synapse in the dorsal column nuclei (base of brain)
3) Axons from the dorsal column nuclei cross over to the opposite side of the brain and project up through the brainstem as part of a pathway called the medial lemniscus
4)Axons synapse with neurons located in the ventrolateral nucleus of the thalamus, which projects to the somatosensory cortex and motor cortex

Question 11

Somatosensory Pathways to the Brain - Ventral Spinothalamic Tract

Answer 11

1) Carries nocioceptive information
2) Axons from the dorsal-root ganglion neurons enter the spinal cord and cross over right away and synapse onto neurons on the contralateral side
3) Axons from contralateral spinal cord then ascend to the brain where they join with other axons forming the medial lemniscus, eventually synapsing with neurons located in the ventrolateral nucleus of the thalamus
4) Neurons from the thalamus then project to the somatosensory cortex

Question 12

Spinal Reflexes

Answer 12

Monosynaptic Reflex
Reflex requiring one synapse between sensory input and movement
Example: Knee-jerk reflex

Question 13

The Vestibular System and Balance

Answer 13

Within each ear, there is a vestibular organ that contains:
Three semicircular canals
Otolith organs (utricle and saccule)

Vestibular organs have two functions
Tell us the position of the body in relation to gravity
Signal changes in the direction and speed of head movements

Question 14

How it works

Answer 14

1) When the head moves, fluid (endolymph) located within the semicircular canals pushes against hair cells, which causes bending of the cilia located on top of the hair cells
2) Responds to ‘angular acceleration’
3) Bending of cilia leads to receptor potentials in the hair cells and action potentials in the cells forming the vestibular nerve
4) The direction in which the cilia are bent determines whether the hair cell becomes depolarized or hyperpolarized

Question 15

Two main somatosensory areas in the cortex

Answer 15

Primary Somatosensory Cortex

• Receives projections from the thalamus
• Brodmann’s areas 3-1-2
• Begins the process of constructing perceptions from somatosensory information

Secondary Somatosensory Cortex

• Located behind the primary somatosensory cortex
• Brodmann’s areas 5 and 7
• Continues the construction of perceptions, projects to the frontal cortex

Question 16

Wilder Penfield (1930s)

Answer 16

Stimulated the cortical surface with large diameter electrodes and recorded patient responses. Wanted to locate source of seizures.