Somatosensory system - motor system

Question 1

How is the somatosensory system organised?

Answer 1

All over the body. Tell us what the body is up to & what’s going on in the environment by providing sensations e.g. touch, temperature, pain, position in space, & movement of joints. It has a closer relationships with movement than the other senses do.

Question 2

What are somatosensory receptors & perception?

Answer 2

Receptors are all over: skin, muscles, tendons, joints. The more receptors you have, the more sensitive you are to stimuli. Sensitivity to different somatosensory stimuli is a function of the kinds of receptors.

Question 3

What are the two types of skin humans have?

Answer 3

Hairy skin (relatively low sensitivity) and Glabrous skin (skin that does not have hair follicles but contains larger numbers of sensory receptors than other skin areas. Sensitive to a wide range of stimuli).

Question 4

What part of the body is most sensitive?

Answer 4

The fingertips are the most sensitive - have a higher density of mechanoreceptors, receptors with small receptive fields, and the brain devotes a lot of resources to this area. Glabrous skin has more sensitivity and is on parts of the body used for exploration.

Question 5

What are the three main types of somatosensory perception?

Answer 5

1. Nocioception - perception of pain & temp.
2. Hapsis - perception of fine touch & pressure (e.g. graph, manipulate)
3. Proprioception - perception of the location & movement of the body.

Question 6

What are nocioceptors?

Answer 6

Have free nerve endings. Causes sharp/dull pain and heat/cold. Damage to dendrite or surrounding cells release chemicals that stimulate dendrite and produces an AP. Either damage to free nerve ending or a chemical signal that triggers it, and sends a signal to the spinal cord.

Question 7

What are haptic receptors?

Answer 7

Dendrite attached to hair, connective tissue, or dendrite encased in capsule of tissue. Distinguish touch, pull, vibration, flutter, indentation. Mechanical stimulation, produces AP. Composition of capsule determines the type of mechanical energy conducted.

Question 8

What are proprioceptors?

Answer 8

Movements stretch the receptors to mechanically stimulate dendrites and produces an AP. Have muscle spindles (muscle stretch), golgi tendon organs (tendon stretch), and joint receptors (joint movement).

Question 9

What do somatosensory receptors tell us about a sensory events?

Answer 9

When it occurs, and whether it is still happening.

Question 10

How does it do this?

Answer 10

1. Rapidly Adapting Receptor - body sensory receptor that responds briefly to the beginning and end of a stimulus on the body.
2. Slowly Adapting Receptor - body sensory receptor that responds as long as a sensory stimulus is on the body.

Question 11

What are slow sensory receptors and adaption?

Answer 11

Signals the presence of a long sustained stimulus (e.g. pain, long pinch). Includes free nerve endings for pain and temperature.

Question 12

What are rapid sensory receptors and adaption?

Answer 12

Motions of object on skin gives good responses to repeated stimuli (e.g. vibration, tickle).

Question 13

What are Dorsal-Root Ganglion Neurons?

Answer 13

The dendrite and axon are continuous & carry sensory info from the skin - the CNS via the spinal cord. Tip of 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 14

What are three types of dorsal-root ganglion neurons?

Answer 14

1. Proprioceptive & Haptic Neurons - carry information about location & movement (proprioception) & touch & pressure. Large, well-myelinated axons (fast).
2. Nocioceptive neurons - pain & temp information. Small axons with little or no myelination (slow).
3. Deafferentiation - loss of incoming sensory input usually due to damage to sensory fibres: also loss of any afferent input to a structure.

Question 15

What are consequences of deafferentiation of dorsal-root ganglion neurons?

Answer 15

Patient G.O. lost sensation due to peripheral neuropathy after influenza. Did not lose motor control, but simple actions for prolonged periods (right) would require visual feedback. Could not perform many daily tasks. Afferent feedback is required for fine movements.

Question 16

What is the dorsal spinothalamic tract? (somatosensory pathway to the brain)

Answer 16

Carries haptic & proprioceptive info. Axons from the dorsal-root ganglion neurons enter the spinal cord & ascent ipsilaterally until they synapse in the dorsal column nuclei (base of brain). 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. Axons synapse with neurons located in the ventrolateral nucleus of the thalamus, which projects to the somatosensory cortex and motor cortex.

Question 17

What is the ventral spinothalamic tract (somatosensory pathway to the brain)

Answer 17

Carries nocioceptive information. Axons from the dorsal-root ganglion neurons enter the spinal cord and cross over right away and synapse onto neurons on the contralateral side. 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. Neurons from the thalamus then project to the somatosensory cortex

Question 18

What is the result of the arrangement of somatosensory pathways to the brain?

Answer 18

Two separate pathways convey somatosensory information - haptic-proprioceptive & nocioceptive. Because of this arrangement, unilateral spinal-cord damage results in distinctive sensory losses to both sides of the body below the site of injury. Unilateral damage causes loss of fine-touch & pressure sensation on the same side of the body below the cut, and loss of pain & temp sensation on the opposite side of the body below the cut.

Question 19

What is a monosynaptic reflex?

Answer 19

Reflex requiring one synapse between sensory input & movement, e.g. knee-jerk reflex. Other more complex spinal reflexes involve connections among sensory neurons, interneurons, and motor neurons (multisynaptic connections).

Question 20

What is the vestibular system?

Answer 20

Somatosensory system that comprises a set of receptors in each inner ear that respond to body position and to the movement of the head.

Question 21

What is involved with the vestibular system and balance?

Answer 21

Within each ear, there is a vestibular organ that contains three semicircular canals and otolith organs (utricle & saccule). Vestibular organs have two functions: tells us the position of the body in relation to gravity, and signal changes in the direction & speed of head movements.

Question 22

What is involved with the vestibular system & balance?

Answer 22

When the head moves, fluid located within the semicircular canals pushes against hair cells, causing bending of the cilia located on top of the hair cells. This leads to receptor potentials in the hair cells & Paps in the cells forming the vestibular nerve. Direction of the bend determines whether the hair cell becomes polarised or hyper polarised.

Question 23

What happens when the hair is tilted?

Answer 23

When the head is tilted, the gelatin & otoconia push against the hair cells, which alters the rate of action potentials in cells that form the vestibular nerve.

Question 24

What are the two main somatosensory areas in the cortex?

Answer 24

1. Primary somatosensory cortex - receives projections from the thalamus. Brodmann’s areas 3-1-2. Begins the process of constructing perceptions from somatosensory info.
2. 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 25

What is the somatosensory homunculus?

Answer 25

Wilder Penfield (1930s) stimulated the cortical surface with large diameter electrodes (working on epilepsy). Work suggests there was a single ‘somatosensory homunculus’ - representation of the human body in the cortex. Useful concept for understanding functional layout of the cortex.

Question 26

What was Jon Kass’ (1980) work on the somatosensory homunculus?

Answer 26

Used small electrodes & precise recording techniques: better than Penfield. In these studies they measured if neurons responded to light touch of the skin surface, manipulation of joints and muscles, etc.

Question 27

What is hierarchical organisation of the somatosensory cortex?

Answer 27

Sensory information might be segregated because we often need to distinguish among different kinds of sensory stimuli coming from different sources. We need to be able to distinguish between an external agent or stimulation coming from muscles produced by our own movements.

Question 28

What are effects of damage to the somatosensory cortex?

Answer 28

Damage to the primary somatosensory cortex results in impairment in pressure sensitivity, hapsis (ability to identify objects by touch), & simple movements (e.g. reaching & grasping).

Question 29

Is reorganisation following damage possible?

Answer 29

Pons & colleagues, 1991. Following damage to the arm, the cortex that was devoted to the arm becomes sensitive to the face - ‘plasticity’.