Chapter 12a - Touch

Question 1

What does somatic sensation enable us to do?

Answer 1

To feel, to ache, to sense hot and chill, and to know what the body parts are doing.

Question 2

What are some of the specialities of the somatic sensory system?

Answer 2

1) The receptors are located all over the body, instead of being localized in a few areas.
2) It responds to many kind of stimuli, so it is actually 4 senses (temperature, touch, pain, body position)

Question 3

What is the sense of body position called?

Answer 3

Proprioception.

Question 4

What are the two major types of skin?

Answer 4

1) Hairy

2) Glabrous (hairless)

Question 5

What are the two layers that the skin has?

Answer 5

1) The epidermis, or the outer layer

2) The dermis, or the inner layer

Question 6

What are mechanoreceptors?

Where are they located?

Answer 6

Sensory receptors that are sensitive to physical distortion such as bending or stretching.

They are located throughout the body, on the skin and inside the body.

Question 7

What is the pacinian corpuscle?

Answer 7

The largest and best-studied mechanoreceptor.

Each human hand has about 2500 Pacinian corpuscles, with the highest densities in the fingers.

Question 8

Where is the pacinian corpuscle, and how big is it?

Answer 8

Deep in the dermis. It can be up to 2mm long and 1mm in diameter.

Question 9

What are rapidly adapting, and what are slowly adapting mechanoreceptors?

Answer 9

Rapidly adapting: Meissner’s corpuscle, Pacinian corpuscle.

Slowly adapating: Merkel’s disks, Ruffini’s endings

Question 10

What are follicles?

Answer 10

Cavities of the skin where hairs grow from.

Question 11

How do the mechanical sensitivities of the different mechanoreceptors differ from each other?

Answer 11

They are sensitive to vibrations of different frequencies.

For example, Meissner’s corpuscles respond best around 50 Hz (but can also trigger at 1-10 Hz), and Pacinian corpuscles respond best around 200 - 300 Hz.

Question 12

How does the capsule of the Pacinian corpuscle affect its receptivity?

Answer 12

The capsule is not necessary for mechanoreception – the receptor works even without it.

However, without the capsule, the receptor adapts more slowly; its response is prolonged and not as “sharp” as normally.

Thus, the capsule apparently makes the corpuscle insensitive to low-frequency stimuli.

Question 13

How can the mechanosensitive ion channels’ gating be affected by various forces?

Answer 13

1) By the membrane itself, when it is stretched or bent
2) Through connections between the channels and the extracellular proteins or intracellular cytoskeleton components (e.g., actin, microtubules)
3) Mechanical stimuli may also trigger the release of second messengers (e.g., DAG, IP_3) that secondarily regulate ion channels.

Question 14

Describe two-point discrimination briefly.

Answer 14

Two-point discrimination is the ability to discern whether a small touch on a skin (e.g., from the tip of a pen) comes from one or two nearby sources.

Different parts of the body have different two-point discrimination sensitivities. The fingers, lip, and big toe have high sensitivities, whereas the back, forearm and calf do not, for example.

Question 15

What are 4 reasons for the fingertips being better at reading Braille?

Answer 15

1) There is a much higher density of mechanoreceptors on the fingertips than other places of the body.
2) The fingertips have a lot of mechanoreceptors that have small receptive fields (e.g., Merkel’s disks).
3) There is more brain tissue devoted to the sensory information of the fingertips per square mm compared to other body parts.
4) There may be special neural mechanisms devoted to high-resolution discriminations.

Question 16

What are primary afferent axons of the somatic sensory system?

Answer 16

Axons that bring information from the somatic sensory receptors to the spinal cord or brain stem.

Question 17

Where do the primary afferent axons enter the spinal cord, and where are their cell bodies?

Answer 17

• They enter the spinal cord through the dorsal roots

- They have their cell bodies in the dorsal root ganglia

Question 18

How many pairs of dorsal and ventral roots is in the spinal cord? What are these called?

Answer 18

30 pairs. These are the 30 spinal segments.

Question 19

What are the names and ranges of the spinal segments, from the TOP?

Answer 19

(C) Cervical, 1-8
(T) Thoracic 1-12
(L) Lumbar, 1-5
(S) Sacral, 1-5

Cervical, Thoracic, Lumbar, Sacral, 8, 12, 5, 5

Question 20

What is a dermatome?

Answer 20

The area or skin innervated by the right and left dorsal roots of a single spinal segment.

There is a 1-1 correlation between dermatomes and spinal segments.

Question 21

What happens if a dorsal root is cut? Why is this unexpected considering the 1-1 correlation between dermatomes and spinal segments, and when would the expected result happen?

Answer 21

If a dorsal root is cut, not all sensation is lost from the dermatome connected to it.

This is caused because adjacent dorsal roots innervate overlapping areas. In order to lose all sensation from one dermatome, three adjacent dorsal roots need to be cut.

Question 22

What is the condition called shingles?

Answer 22

A condition where all the neurons of a single dorsal root ganglion become infected with a virus. This demonstrates the dermatomes quite well.

Question 23

What are the cauda equina?

Answer 23

Latin for horse’s tail, they are the bundles of spinal nerves streaming down within the lumbar and sacral vertebral columns.

Question 24

What is a lumbar puncture, and what is involved in it?

Answer 24

Collection of cerebrospinal fluid with a needle. This process is also called a spinal tap.

In a lumbar puncture, the sack of cauda equina (called the lumbar cistern) near the midline is punctured, as it contains CSF.

Question 25

What is the basic structure of the spinal cord?

Answer 25

It has an inner core of grey matter, surrounded by a thick covering of white matter tracts that are often called columns.

Each half of the spinal grey matter is divided into a dorsal horn, an intermediate zone, and a ventral horn.

Question 26

What are second-order sensory neurons? Where are they located?

Answer 26

Neurons that receive sensory input from primary afferents.

Most of the second-order sensory neurons of the spinal cord lie in the dorsal horns.

Question 27

What is the pathway in the nervous system that serves touch?

Answer 27

The Dorsal Column-Medial Lemniscal Pathway

Question 28

Where are the 3 nerves of the the Dorsal Column-Medial Lemniscal Pathway?

Answer 28

1. Dorsal root ganglion
2. Cuneate OR Gracile nucleus
3. Ventral posterolateral nucleus

Question 29

What is the major route of the Dorsal Column-Medial Lemniscal Pathway?

Answer 29

1. Sensory receptors
2. Large dorsal root axons
3. Dorsal column
4. Dorsal colum nuclei (Cuneate / Gracile)
5. Medial lemniscus
6. Thalamus (Central posterolateral nucleus)
7. Primary somatosensory cortex S1

Question 30

At which point of the neural pathway does somatosensory information start to traverse contralaterally instead of ipsilaterally?

Answer 30

From the ventral and medial medulla.

Question 31

What is the medial lemniscus and where is it?

Where do its axons synapse?

Answer 31

A white matter tract that goes through the medulla, pons, and midbrain.

Its axons synapse upon neurons of the Ventral Posterior (VP) nucleus of the thalamus.

Question 32

Is the information altered when it goes through the neural pathways? If so, where especially?

Answer 32

Yes, the information is altered generally every time it goes through a synapse.

In particular, inhibitory interactions between adjacent sets of inputs in the dorsal column-medial lemniscal pathway enhance responses to tactile stimuli.

Question 33

What supplies the somatic sensations of the face?

Answer 33

Mostly the large trigeminal nerves (cranial nerve V) which enter the brain at the pons.

Question 34

What is the structure of the trigeminal nerves?

Answer 34

There are twin trigeminal nerves, one on each side, and they both branch out into three peripheral nerves.

These three peripheral nerves innervate the face, mouth areas, the outer two-thirds of the tongue, and the dura mater covering the brain.

Question 35

What nerves provide sensation around the ears, nasal areas, and pharynx?

Answer 35

The facial (VII) glossopharyngeal (IX) and vagus (X) cranial nerves.

Question 36

What are the areas of the cortex that deal with the somatic sensory system?

Answer 36

The main area is Brodmann’s area 3b, and located in the parietal lobe. It is also called the somatosensory cortex.

This area lies on the postcentral gyrus, right behind the central sulcus.

Additional areas: 3a, 1, and 2 on the postcentral gyrus, and areas 5 and 7 on the adjacent posterior parietal cortex.

Question 37

Which layer of the somatic sensory cortex S1 receives the most thalamic input?

Answer 37

Layer IV, where the input is projected onto other layers.

Question 38

What is cortical somatopy?

Answer 38

The mapping of the body’s surface sensations onto a structure in the brain.

Remember, electrical stimulation of the S1 surface can cause somatic sensations localized to a specific part of the body.

Question 39

What is cortical map plasticity?

Answer 39

The phenomenon that if a certain body part is removed that has a specific area on the somatosensory cortex, that area will likely respond to stimulation of adjacent areas.

Cortical maps are dynamic and adjust depending on the amount of sensory experience. This kind of plasticity is widespread in the brain.

Question 40

What is agnosia?

Answer 40

The inability to recognize objects even though simple sensory skills seem to be normal.

Question 41

What is neglect syndrome?

Answer 41

A syndrome often caused by parietal cortical lesions, in which a part of the body or a part of the world is ignored or suppressed, and its very existence is denied.

They are most common following damage to the right hemisphere, and often improve or disappear with time.

Question 42

What is the posterior parietal cortex essential for?

Answer 42

For the perception and interpretation of spatial relationships, accurate body image, and the learning of tasks involving coordination of the body in space.