Chapter 12 somatosensation exam

Question 1

Somatic sensation is divided into what categories?

Answer 1

Touch, pain. temperature, and body position

Question 2

What are sensory receptor responsible for?

Answer 2

encode intensity, duration, position, and sometimes direction

Question 3

What are the two major types of skin?

Answer 3

Hairy and glabrous (hairless) such as the backs and palms of the hand, lips, and soul of the feet.

Question 4

What is the function of skin?

Answer 4

The outer layer of the skin is called the epidermis and the inner layer of the skin is called the dermis. Skin provides an essential protective layer, prevents body fluids from evaporating in hot environments, and provides our most direct contact with the world.

Question 5

What are mechanoreceptors?

Answer 5

Mechanoreceptors are sensitive to body distortion such as stretching or bending. They are spread throughout the body. At the heart of all the mechanoreceptors are unmyelinated axon branches that are sensitive to stretching, bending, pressure, and vibration. The receptors get depolarize therefore sodium channels open up.

Question 6

What is pacinian corpuscle?

Answer 6

Pacinian corpuscle is the largest mechanoreceptor which lies deep in the dermis and can be as long as 2mm and 1mm in diameter. They have higher density in the fingers.

Question 7

What are Ruffini’s endings?

Answer 7

Ruffini’s endings are types of mechanoreceptors found in both hairy and glabrous skin but they are smaller than pacinian corpuscles.

Question 8

What are meissner corpuscles?

Answer 8

Meissner corpuscles are types of mechanoreceptors found in the ridges of glabrous skin (the raised parts of the fingertips). They are one-tenth the size of pacinian corpuscles.

Question 9

What are Merkel’s disks?

Answer 9

Merkel’s disks are located within the epidermis. Each consists of a nerve terminal and a flattened non-neural epithelial cell (Mark cell)

Question 10

What are krause end bulbs?

Answer 10

Krause end bulbs lie in the border of dry skin and mucous membrane such as around the lips and genitals. The nerve terminals look like knotted balls of string.

Question 11

What are the receptor field sizes of the mechanoreceptors?

Answer 11

Meissner corpuscle and merkel’s disk have smaller receptor fields compared to Pacinian corpuscle and Ruffini’s endings

Question 12

What is the adaptation rate of the mechanoreceptors?

Answer 12

Meissner’s and Pacinian corpuscle tend to respond quickly at first but then stop firing even though the stimulus continues, they are said to be rapidly adapting. However, Merkel’s disk and Ruffini’s ending are slowly adapting and generate more sustained responses during a strong stimuli.

Question 13

What are receptor fields and how can they be mapped?

Answer 13

The receptor field is an area of the receptor array that upon stimulation generates a neural signal. Introducing a microelectrode into the median nerve of the arm, it is possible to record the action potentials from a single sensory axon and map its receptive field on the hand with a fine stimulus probe.

Question 14

Why are fingertips much better for Braille reading than the elbow?

Answer 14

1. There is a higher density of mechanoreceptors in the skin of the fingertip.
2. Fingertips are enriched with receptors with smaller receptor fields such as Merkel’s disk and Meissner’s corpuscle.
3. There is more brain tissue
4. High resolution discriminations (know exactly where something is happening)

Question 15

What are primary afferent axons?

Answer 15

Axons bring information from the somatic sensory receptors to the spinal cord or brain stem. They enter the spinal cord through dorsal roots and their cell bodies lie in the dorsal root ganglia.

Question 16

What is two-point discrimination?

Answer 16

The two-point discrimination is used to measure someone’s perception of how far apart are two separate points of contact on their skin

Question 17

What are hair follicles?

Answer 17

Hair grows from follicles embedded in the skin, each follicle is innervated by free the termination of a single axon wrap around the follicle or runs parallel to it. The bending of the hair causes deformation of the follicle and surrounding skin tissues which in turn stretches, bends, and flattens the nearby nerve endings which the increases or decrease their action potential firing frequency.

Question 18

Spinal segments are divided into which groups?

Answer 18

cervical, thoracic, lumbar, and sacral

Question 19

What is dermatome?

Answer 19

The area of skin innervated by the right and left dorsal roots of a single spinal segment is called a dermatome. To lose all sensation in one dermatome all three adjacent dorsal roots must be cut.

Question 20

What are shingles?

Answer 20

The skin innervated by the axons of one dorsal root is revealed by shingles in which all neurons of a single dorsal root ganglion become infected with a disease.

Question 21

What is the dorsal column-medial lemniscal pathway?

Answer 21

The pathway serving touch is called dorsal column-medial lemniscal pathway (information ascend to the cerebral cortex) It goes from dorsal column to the dorsal column nuclei to medial lemniscus to thalamus and the primary somatosensory cortex.

Question 22

What is medial lemniscus?

Answer 22

The axons of the dorsal column nuclei ascend within a conspicuous white matter tract called the medial lemniscus. The medial lemniscus rises through the medulla, pons, and midbrain, and its axons synapse upon neurons of the ventral posterior (VP) nucleus.

Question 23

What is the trigeminal touch pathway?

Answer 23

The sensory connections of the trigeminal nerve are analogous to those of the dorsal roots. The large-diameter sensory axons of the trigeminal nerve carry tactile information from skin mechanoreceptors. They synapse onto second-order neurons in the ipsilateral trigeminal nucleus, which is analogous to a dorsal column nuclei. The axons from the trigeminal nucleus decussate and project into the medial part of the VP nucleus of the thalamus. From here, information is relayed to the somatosensory cortex.

Question 24

Where is the somatic sensory system located?

Answer 24

Most of the cortex concerned with the somatic sensory system is located in the parietal lobe. It is easy to find in humans because it lies on the postcentral gyrus (right behind the central sulcus).

Question 25

What happens in the primary somatic sensory cortex?

Answer 25

(1) it receives dense inputs from the VP nucleus of the thalamus (2) its neurons are very responsive to somatosensory stimuli (but not to other sensory stimuli) (3) lesions here impair somatic sensation (4) when electrically stimulated, it evokes somatic sensory experiences. (5) Also receives a dense input from the thalamus, however, this region is concerned with the sense of body position rather than touch.

Question 26

What is somatotopy?

Answer 26

nother way to map the somatosensory cortex is to record the activity of a single neuron and determine the site of its somatosensory receptive field on the body. The receptive fields of many S1 neurons produce an orderly map of the body on the cortex. The mapping of the body’s surface sensations onto a structure in the brain is somatotopy. The relative size of cortex devoted to each body part is correlated with the density of sensory input received from that part. same parts of the body, literally in parallel along adjacent strips of the cortex. The two somatotopic maps are not identical, but mirror images.

Question 27

What is agnosia?

Answer 27

Damage to posterior parietal areas can yield some bizarre neurological disorders such as agnosia, the inability to recognize objects even though simple sensory skills seem to be normal. People with aste-reognosia cannot recognize common objects by feeling them

Question 28

What is a neglect syndrome?

Answer 28

Parietal cortical lesions may also cause a neglect syndrome, in which a part of the body or a part of the world (the entire visual field left of the center of gaze, for example) is ignored or suppressed, and its very existence is denied. ood on one half of his plate, or attempt to dress only one side of his body. Neglect syndromes are most common following damage to the right hemisphere, and, fortunately, they usually improve or disappear with time.

Question 29

Why is post parietal cortex important?

Answer 29

the posterior parietal cortex seems to be essential for the perception and interpretation of spatial relationships, accurate body image, and the learning of tasks involving coordination of the body in space.

Question 30

What is noiception?

Answer 30

the sensory process that provides signals that trigger pain.

Question 31

How are noiceptors activated?

Answer 31

Nociceptors are activated by stimuli that have the potential to cause tissue damage. Tissue damage can result from strong mechanical stimulation, extremes in temperature, oxygen deprivation, and exposure to certain chemicals, among other causes.

Question 32

What are polymodal nociceptors?

Answer 32

The majority of nociceptors respond to mechanical, thermal, and chemical stimuli, and are therefore called polymodal nociceptors. Thus, there are also mechanical nociceptors, showing selective responses to strong pressure; thermal nociceptors, showing selective responses to burning heat or extreme cold and chemical nociceptors, showing selective responses to histamine and other chemicals.

Question 33

What is hyperalgesia?

Answer 33

Our body's ability to control its pain.

Question 34

What is analgesia?

Answer 34

capsaicin has a seemingly paradoxical clinical application. Applied in large quantities, it can cause analgesia, the absence of pain

Question 35

What are primary pain afferents?

Answer 35

The first pain sensation registered by noxious stimulation is mediated by fast A axons. The second, is longer lasting pain. The neurotransmitter of the pain afferents is glutamate; however, as mentioned previously, these neurons also contain the peptide substance P. P is required to experience moderate to intense pain.

Question 36

What are ascending pain pathways?

Answer 36

First, they differ with respect to their nerve endings in the skin. The touch pathway is characterized by specialized structures in the skin; the pain pathway has only free nerve endings. Second, they differ with respect to the diameter of their axons. The touch pathway is swift, using fat, myelinated A fibers; the pain pathway is slow, using thin, lightly myelinated A fibers and unmyelinated C fibers. Third, they differ with respect to their connections in the spinal cord. Branches of the A axons terminate in the deep dorsal horn; the A and C fibers branch, run within the zone of Lissauer, and terminate within the substantia gelatinosa.

Question 37

What is descending regulation?

Answer 37

One is a zone of neurons in the midbrain called the periventricular and periaqueductal gray matter (PAG). Electrical stimulation of the PAG can cause a profound analgesia that has some- times been exploited clinically.

Question 38

What is Melzack’s and Wall’s gate theory of pain?

Answer 38

he relay of nociceptive signals by the projection neuron is gated by the activity of an inhibitory interneuron. Activity in the non-nociceptive mechanoreceptor can suppress, or close the “gate” on, nociceptive signals before they can proceed to the spinothalamic tract. The signs indicate excitatory synapses and the  signs indicate inhibitory synapses.

Question 39

What drugs help with analgesia?

Answer 39

Small injections of morphine or endorphins into the PAG, the raphe nuclei, or the dorsal horn can produce analgesia. The opioid receptor antagonist naloxone can block the anal- gesic effect of the placebo, just as it antagonizes the effects of morphine, a true analgesic.