Chapter 9 - Somatic Sensory System: Touch and Proprioception

Question 1

What is the cause of the variable performance on two-point discrimination tasks observed when testing different regions of the body?

Answer 1

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.

Question 2

On the —–, stimuli are perceived as distinct if they are separated by roughly 2mm, but the same stimuli applied to the —- are not perceived as distinct until they are at least 40 mm apart!

Answer 2

Fingertips, forearm.

Question 3

Sensory afferents are differentiated by the temporal dynamics of their response. What are the two different groups, and what is the primary difference between them?

Answer 3

1. Rapidly adapting afferents
2. Slowly adapting afferents
   They are different in how fast they adapt to stimulation. Rapidly adapting afferents are thought to be particularly effective in conveying information about changes in ongoing stimulation such as those produced by stimulus movement. Slowly adapting afferents are better suited to provide information about the spation attributes of the stimulus, such as size and shape.

Question 4

Which cutaneous sensory afferents are most closely studied?

Answer 4

Those that are found in the glabrous (hairless) portions of the hand.

Question 5

Name four different classses of mechanoreceptive afferents that innervate the glabrous skin of the hand.

Answer 5

1. Merkel cell afferents
2. Meissner afferents
3. Pacinian afferents
4. Ruffini afferents

Question 6

Rapidly adapting or slowly adapting?

1. Merkel cell afferents
2. Meissner afferents
3. Pacinian afferents
4. Ruffini afferents

Answer 6

1. Merkel cell afferents - SLOW
2. Meissner afferents - RAPID
3. Pacinian afferents - RAPID
4. Ruffini afferents - SLOW

Question 7

What is the role of proprioceptors?

Answer 7

The role of proprioceptors is primarily to give detailed and continuous information about the poisition of the limbs and other body parts in space.

Question 8

How do proprioceptors in muscle spindles work?

Answer 8

Sensory afferents are coiled around the central part of the intrafusal spindle and, when the muscle is stretched, the tension on the intrafusal fibers activates mechanically gated ion channels in the nerve endings, triggering action potentials.

Question 9

Mention three different classes of proprioceptors found in muscles and tendons.

Answer 9

1. Muscle spindles.
2. Goldi tendon organs
3. Joint receptors

Question 10

What are Golgi tendon organs?

Answer 10

Goldi tendon organs are encapsulated afferent nerve endings located at the end of a muscle and a tendon.

Question 11

What is a tendon?

Answer 11

A tendon (or sinew) is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. (NO: sene)

Question 12

What is the name of the central pathway that conveys tactile information from the body?

Answer 12

The doral column-medial lemniscal system

Question 13

What is the name of the central pathway that conveys tactile information from the face?

Answer 13

The trigeminothalamic system.

Question 14

The axons of the cutaneous mechanosensory afferents enter the …

Answer 14

The axons of the cutaneous mechanosensory afferents enter the spinal cord through the dorsal roots.

Question 15

The axons of the cutaneous mechanosensory afferents enter the spinal cord through the dorsal roots; the majority ascend (contra/ipsilaterally) through …

Answer 15

The axons of the cutaneous mechanosensory afferents enter the spinal cord through the dorsal roots; the majority ascend ipsilaterally through the dorsal columns.

Question 16

The axons of the cutaneous mechanosensory afferents enter the spinal cord through the dorsal roots; the majority ascend ipsilaterally through the dorsal columns to a nuclei in …

Answer 16

The axons of the cutaneous mechanosensory afferents enter the spinal cord through the dorsal roots; the majority ascend ipsilaterally through the dorsal columns to the lower medulla, where they synapse on neurons in the dorsal column nuclei.

Question 17

The dorsal columns of the spinal cord. That’s columns, plural. How many are there, and why?

Answer 17

The dorsal columns on the spinal cord are topographically organized such that the fibers conveying information from loewr limbs lie most medial and travel in a circumscribed bundle known as fasciculus gracilis, while those that convey information from the upper limbs, trunk, and neck lie in a more lateral bundle known as the fasciculus cuneatus.

Question 18

The dorsal columns on the spinal cord are topographically organized such that the fibers conveying information from loewr limbs lie most medial and travel in a circumscribed bundle known as fasciculus gracilis, while those that convey information from the upper limbs, trunk, and neck lie in a more lateral bundle known as the fasciculus cuneatus. Where do these axons first synapse?

Answer 18

In the lower medulla, in two different nuclei - depending on the column (gracilis or cuneatus).

Question 19

The dorsal columns carrying information from mechanosensory afferents synapse in the gracile and cuneate nucleus in the lower medulla. Where do the second-order neurons synapse?

Answer 19

They synapse contralaterally in the ventral posterior lateral nucleus (VPL) in the thalamus.

Question 20

As the second-order neurons, carrying mechanosensory information from the body, leave the gracile and cuneate nuclei and cross the midline they form a tract known as the medial lemniscus. Give some funfacts about that name.

Answer 20

Lemniscus means “ribbon”, because the crossing looks like the crossing of two ribbons. In anatomy this crossing is called a decussation, from the Roman numeral X, or decem (10).

Question 21

Third-order neurons, carrying mechanosensory information from the body leave the ventral posterior lateral nucleus (VPL) of the thalamus and synapse on …

Answer 21

Third-order neurons, carrying mechanosensory information from the body leave the ventral posterior lateral nucleus (VPL) of the thalamus and synapse on the primary and secondary somatosensory cortex.

Question 22

The cutaneous mechanoreceptor information from the face is conveyed centrally along which nerve?

Answer 22

The trigeminal ganglion (cranial nerve V).

Question 23

The cutaneous mechanoreceptor information from the face is conveyed centrally along the trigeminal ganglion and terminate in …

Answer 23

the pons.

Question 24

The cutaneous mechanoreceptor information from the face is conveyed centrally along the trigeminal ganglion and terminate in the pons. Second-order neurons carrying this information ascend and terminate in …

Answer 24

the ventral posterior medial (VPM) nucleus of the thalamus.

Question 25

Third-order neurons, carrying mechanosensory information from the face leave the ventral posterior medial (VPM) nucleus of the thalamus and synapse on … …

Answer 25

neurons in the primary and secondary somatosensory cortices.

Question 26

What is the gyrus and lobe of the primary somatosensory cortex?

Answer 26

The primary somatosensory cortex in humans (also called SI) is located in the postcentral gyrus of the parietal lobe.

Question 27

The thalamus is not just a relay, and the somatosensory cortices are not just sensory processing. What is often forgotten about the cortex?

Answer 27

The descending neurons from the cortex to the thalamus, vastly outnumbers the ascending neurons to the cortex.