Sensory and motor systems

Question 1

What are the primary afferent axons?

Answer 1

The skin is richly innervated by axons that course through the vast network of peripheral nerves on their way to the CNS. Axons bringing information from the somatic sensory receptors to the spinal cord or brain stem are the primary afferent axons of the somatic sensory system.

Question 2

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

Answer 2

The primary afferent axons enter the spinal cord through the dorsal roots; their cell bodies lie in the dorsal root ganglia.

Question 3

How are different sizes of axons designated?

Answer 3

By two sets of names: using Arabic and Greek letters and Roman numerals. In order of decreasing size, axons from skin sensory receptors are usually designated Aa, AB, Ad, and C; axons of similar size, but innervating the muscles and tendons, are called groups I, II, III, and IV. Group C (or IV) axons are, by definition, unmyelinated axons, while all the rest are myelinated.

Question 4

Which types of axons transport touch, pain, temperature sensation and itch?

Answer 4

Recall that the diameter of an axon, together with its myelin, determines its speed of action potential conduction. The smallest axons, the so-called C fibers, have no myelin and are less than about 1 um in diameter. C fibers mediate pain, temperature sensation, and itch, and they are the slowest of axons, conducting at about 0.5–1 m/sec.

On the other hand, touch sensations, mediated by the cutaneous mechanoreceptors, are conveyed by the relatively large AB axons, which can conduct at up to 75 m/sec.

Question 5

What are the spinal segments? Name their groups

Answer 5

Each spinal nerve, consisting of dorsal root and ventral root axons, passes through a notch between the vertebrae (the “back bones”) of the spinal column. There are as many spinal nerves as there are notches between vertebrae. The 30 spinal segments are divided into four groups, and each segment is named after the vertebra adjacent to where the nerves originate:

cervical (C) 1–8,
thoracic (T) 1–12,
lumbar (L) 1–5, and
sacral (S) 1–5.

Question 6

How is the segmental organisation of spinal nerves and the sensory innervation of the skin are related?

Answer 6

The area of skin innervated by the right and left dorsal roots of a single spinal segment is called a dermatome; thus, there is a one-to-one correspondence between dermatomes and spinal segments. When mapped, the dermatomes delineate a set of bands on the body surface, as shown in docs.

Question 7

What happens when a dorsal root is cut?

Answer 7

When a dorsal root is cut, the corresponding dermatome on that side of the body does not lose all sensation. The residual somatic sensation is explained by the fact that the adjacent dorsal roots innervate overlapping areas. To lose all sensation in one dermatome, therefore, three adjacent dorsal roots must be cut.

Question 8

What is meant by the “horses tail”?

Answer 8

The spinal cord in the adult ends at about the level of the third lumbar vertebra. The bundles of spinal nerves streaming down within the lumbar and sacral vertebral column are called the cauda equina (Latin for “horse’s tail”). The cauda equina courses down the spinal column within a sack of dura filled with cerebrospinal fluid (CSF).

Question 9

In what operation is this sack of dura in the cauda equina utilised?

Answer 9

In a method called lumbar puncture (also called a spinal tap), used to collect CSF for medical diagnostic tests, a needle is inserted into this CSF-filled cistern at the midline.

If the needle is inserted a little off center, however, a nerve can be touched. Not surprisingly, this causes a sensation of sharp pain in the dermatome supplied by that nerve.

Question 10

WHat is the spinal cord composed of?

Answer 10

The spinal cord is composed of an inner core of gray matter, surrounded by a thick covering of white matter tracts that are often called columns. Each half of the spinal gray matter is divided into a dorsal horn, an intermediate zone, and a ventral horn

Question 11

What are meant by second order sensory neurons?

Answer 11

The neurons that receive sensory input from primary afferents are called second-order sensory neurons. Most of the second-order sensory neurons of the spinal cord lie within the dorsal horns.

Question 12

What happens after the large, myelinated AB axons conveying information about a touch to the skin enter the dorsal horn? (2)

Answer 12

They branch, One branch synapses in the deep part of the dorsal horn on second-order sensory neurons. These connections can initiate or modify a variety of rapid and unconscious reflexes.

The other branch of the AB primary afferent axon ascends straight to the brain. This ascending input is responsible for perception, enabling us to form complex judgments about the stimuli touching the skin.

Question 13

Does information about touch or vibration of the skin, pain and temperature take the same path to the brain?

Answer 13

Information about touch or vibration of the skin takes a path to the brain that is entirely distinct from that taken by information about pain and temperature.

Question 14

What is the pathway serving touch called? Describe the route

Answer 14

The pathway serving touch is called the dorsal column– medial lemniscal pathway.

Question 15

Describe the route of the dorsal column– medial lemniscal pathway

Answer 15

The ascending branch of the large sensory axons (AB) enters the ipsilateral dorsal column of the spinal cord, the white matter tract medial to the dorsal horn. The axons of the dorsal column terminate in the dorsal column nuclei, which lie at the junction of the spinal cord and medulla. This is a fast, direct path that brings information from the skin to the brain without an intervening synapse.

At this point in the pathway, information is still represented ipsilaterally. However, axons from cells of the dorsal column nuclei arch toward the ventral and medial medulla, and decussate. 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 of the thalamus. Thalamic neurons of the VP nucleus then project to specific regions of primary somatosensory cortex, or S1.

Question 16

What are the dorsal columns composed of?

Answer 16

primary sensory axons, as well as second-order axons from neurons in the spinal gray matter.

Question 17

What is the issue with the term ‘relay nuclei’?

Answer 17

It is tempting to assume that sensory information is simply transferred, unchanged, through nuclei in the brain stem and thalamus on its way to the cortex, with the actual processing taking place only in the cortex. In fact, this assumption is demonstrated by the term relay nuclei, which is often used to describe specific sensory nuclei of the thalamus such as the VP nucleus.

However, In both dorsal column and thalamic nuclei, considerable transformation of information takes place. As a general rule, information is altered every time it passes through a set of synapses in the brain. In particular, inhibitory interactions between adjacent sets of inputs in the dorsal column–medial lemniscal pathway enhance the responses to tactile stimuli. Some synapses in these nuclei can also change their strength, depending on their recent activity.

Question 18

What is the purpose of the trigeminal touch pathways?

Answer 18

Somatic sensation of the face

Question 19

What supplies input regarding somatic sensation of the face?

Answer 19

Mostly by the large trigeminal nerves (cranial nerve V), which enter the brain at the pons. There are twin trigeminal nerves, one on each side, and each breaks up into three peripheral nerves that innervate the face, mouth areas, the outer two-thirds of the tongue, and the dura mater covering the brain.

Additional sensation from the skin around the ears, nasal areas, and pharynx is provided by other cranial nerves: the facial (VII), glossopharyngeal (IX), and vagus (X).

Question 20

Describe the sensory connections of the trigeminal nerve

Answer 20

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 nucleus. 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 21

Where is the somatosensory cortex located?

Answer 21

Most of the cortex concerned with the somatic sensory system is located in the parietal lobe. Brodmann’s area 3b, now regarded as the primary somatosensory cortex (S1), is easy to find in humans because it lies on the postcentral gyrus

Other cortical areas that also process somatic sensory information flank S1. These include areas 3a, 1, and 2 on the postcentral gyrus, and areas 5 and 7 on the adjacent posterior parietal cortex

Question 22

Why is area 3b regarded as the primary somatosensory cortex? (4)

Answer 22

Area 3b is the primary somatic sensory cortex because

(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; and
(4) when electrically stimulated, it evokes somatic sensory experiences.

Question 23

Area 3a also receives a dense input from the thalamus. Why is this region different to 3b?

Answer 23

This region is concerned with the sense of body position rather than touch.

Question 24

Where do areas 1 and 2 receive input from and how do the contents of these projections differ?

Answer 24

Areas 1 and 2 receive dense inputs from area 3b. The projection from 3b to area 1 sends mainly texture information, while the projection to area 2 emphasizes size and shape.

Question 25

Describe describe two ways in which the layers of the somatosensory cortex are similar to those of the visual cortex

Answer 25

As is the case for visual and auditory cortex, the thalamic inputs to S1 terminate mainly in layer IV. The neurons of layer IV project, in turn, to cells in the other layers.

Another important similarity with other regions of cortex is that S1 neurons with similar inputs and responses are stacked vertically into columns that extend across the cortical layers

Question 26

Describe two ways in which the somatosensory system was mapped

Answer 26

Electrical stimulation of the S1 surface can cause somatic sensations localized to a specific part of the body. Systematically, moving the stimulator around S1 will cause the sensation to move across the body. American-Canadian neurosurgeon Wilder Penfield, working at McGill University from the 1930s through the 1950s, actually used this method to map the cortex of neurosurgical patients.

Another 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.

Question 27

What is the mapping of the body’s surface sensations onto a structure in the brain called?

Answer 27

Somatopy

Question 28

How is the somatopic map of rodents different to that of humans?

Answer 28

The large facial vibrissae (whiskers) of rodents receive a huge share of the territory in S1, while the digits of the paws receive relatively little. Remarkably, the sensory signals from each vibrissa follicle go to one clearly defined cluster of S1 neurons; such clusters are called barrels. The somatotopic map of rodent vibrissae is easily seen in thin sections of S1; the five rows of cortical barrels precisely match the five rows of facial vibrissae

Question 29

What happens to the somatotopic map in cortex when an input, such as the finger, is removed?

Also describe the study which attempted to answer this question

Answer 29

First, the regions of S1 sensitive to stimulation of the hand in an adult owl monkey were carefully mapped with microelectrodes. Then, one finger (digit 3) on the hand was surgically removed. Several months later, the cortex was again mapped. The cortex originally devoted to the amputated digit now responded to stimulation of the adjacent digits There clearly had been a major rearrangement of the circuitry underlying cortical somatotopy.

Question 30

In the amputation experiment, the cause of this map rearrangement was the absence of input from the missing digit. What happens when the input activity from a digit is increased?

Answer 30

To answer this question, mon- keys were trained to use selected digits to perform a task for which they received a food reward. After several weeks of this training, microelectrode mapping experiments showed that the representation of the stimulated digits had expanded in comparison with the adjacent, unstimulated ones. These experiments reveal that cortical maps are dynamic and adjust depending on the amount of sensory experience.

Question 31

What is meant by phantom limb syndrome? What could cause it?

Answer 31

A common experience among amputees is the perception of sensations coming from the missing limb when other body parts are touched. These “phantom limb” sensations are usually evoked by the stimulation of skin regions whose somatotopic representations border those of the missing limb; for example, feeling can be evoked in a phantom arm by stimulating the face.

Functional brain imaging reveals that the cortical regions originally devoted to the missing limb are now activated by stimulating the face. While this plasticity may be adaptive in the sense that the cortex does not go unused, the mismatch between sensory stimulation and perception in amputees shows that it can lead to confusion on how signals from S1 should be interpreted.

Question 32

What changes are observed as information passes through the cortex and receptive fields enlarge?

Answer 32

the character of neuronal receptive fields tends to change; For example, neurons below the cortex and in cortical areas 3a and 3b are not sensitive to the direction of stimulus movement across the skin, but cells in areas 1 and 2 are. The stimuli that neurons prefer become increasingly complex.

Certain cortical areas seem to be sites where simple, segregated streams of sensory information converge to generate particularly complex neural representations.

Question 33

When we discussed the visual system, we saw this in the complex receptive fields of area IT.

Which area is similar to that in the somatosensory cortex?

Answer 33

The posterior parietal cortex is also such an area. Its neurons have large receptive fields with stimulus preferences that are a challenge to characterize because they are so elaborate.

Moreover, the area is concerned not only with somatic sensation but also with visual stimuli, movement planning, and even a person’s state of attentiveness.

Question 34

What does damage to the posterior parietal cortex lead to? (2)

Answer 34

Damage to posterior parietal areas can yield agnosia, the inability to recognise objects even though simple sensory skills seem to be normal. People with astereognosia cannot recognise common objects by feeling them (e.g., a key), although their sense of touch is otherwise normal and they may have no trouble recognising the object by sight or sound. Deficits are often limited to the side contralateral to the damage.

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.

Question 35

In addition to the mechanosensitive touch receptors we have described so far, what else does somatic sensation depend strongly on?

Answer 35

Nociceptors, the free, branch- ing, unmyelinated nerve endings that signal that body tissue is being damaged or is at risk of being damaged.

Question 36

How does activation of skin nociceptors produces two distinct perceptions of pain?

Answer 36

Ad and C fibers bring information to the CNS at different rates because of differences in their action potential conduction velocities. Accordingly, the activation of skin nociceptors produces two distinct perceptions of pain: a fast, sharp, first pain followed by a duller, longer lasting second pain. First pain is caused by the activation of Ad fibers; second pain is caused by the activation of C fibers.

Question 37

Where do these small fibers have their cell bodies and where do they synapse?

Answer 37

Like the AB mechanosensory fibers, the small-diameter fibers have their cell bodies in the segmental dorsal root ganglia, and they enter the dorsal horn of the spinal cord. The fibers branch immediately, travel a short distance up and down the spinal cord in a region called the zone of Lissauer, and then synapse on cells in the outer part of the dorsal horn in a region known as the substantia gelatinosa

Question 38

What is the neurotransmitter of the pain pathways and what other substance is contained in the neurons?

Answer 38

The neurotransmitter of the pain afferents is glutamate; however, as mentioned previously, these neurons also contain the peptide substance P. Substance P is contained within storage granules in the axon terminals and can be released by high-frequency trains of action potentials. Recent experiments have shown that synaptic transmission mediated by substance P is required to experience moderate to intense pain.

Question 39

What gives rise to the phenomena of referred pain?

Answer 39

Nociceptor axons from the viscera enter the spinal cord by the same route as the cutaneous nociceptors. Within the spinal cord, there is substantial mixing of information from these two sources of input. This cross-talk gives rise to the phenomenon of referred pain, where visceral nociceptor activation is perceived as a cutaneous sensation.

The classic example of referred pain is angina, occurring when the heart fails to receive sufficient oxygen. Patients often localise the pain of angina to the upper chest wall and the left arm.

Question 40

How do the touch and pain pathways differ in respect to their nerve endings in the skin?

Answer 40

The touch pathway is characterised by specialised structures in the skin; the pain pathway has only free nerve endings.

Question 41

How do the touch and pain pathways differ with respect to the diameter of their axons?

Answer 41

The touch pathway is swift, using fat, myelinated AB fibers; the pain pathway is slow, using thin, lightly myelinated Ad fibers and unmyelinated C fibers.

Question 42

How do the touch and pain pathways differ with respect to their connections in the spinal cord?

Answer 42

Branches of the AB axons terminate in the deep dorsal horn; the Ad and C fibers branch, run within the zone of Lissauer, and terminate within the substantia gelatinosa.

Question 43

In what pathway is information about pain (as well as temperature) in the body is conveyed from the spinal cord to the brain via?

Answer 43

Information about pain (as well as temperature) in the body is conveyed from the spinal cord to the brain via the spinothalamic pathway

Question 44

Describe the route of the spinothalamic pathway

Answer 44

Unlike the dorsal column–medial lemniscal pathway, the axons of the second-order neurons immediately decussate and ascend through the spinothalamic tract running along the ventral surface of the spinal cord.

As the name implies, the spinothalamic fibers project up the spinal cord and through the medulla, pons, and midbrain without synapsing, until they reach the thalamus (Figure 12.30). As the spinothalamic axons journey through the brain stem, they eventually come to lie alongside the medial lemniscus, but the two groups of axons remain distinct from each other.

Question 45

How can this organization can lead to a curious, but predictable, group of deficits when the nervous system is impaired?

Answer 45

For example, if half of the spinal cord is damaged, certain deficits of mechanosensitivity occur on the same side as the spinal cord damage: insensitivity to light touch, the vibrations of a tuning fork on the skin, the position of a limb. On the other hand, deficits in pain and temperature sensitivity will show up on the side of the body opposite the cord damage.

Question 46

What other signs give additional clues about the site of the spinal cord damage?

Answer 46

motor deficiency and the exact map of sensory deficits, For example, movements will be impaired on the ipsilat- eral side. The constellation of sensory and motor signs following damage to one side of the spinal cord is called Brown–Séquard syndrome.

Question 47

Is there a trigeminal pain pathway?

Answer 47

Yes, pain (and temperature) information from the face and head takes a path to the thalamus that is analogous to the spinal path. The small-diameter fibers in the trigeminal nerve synapse first on second-order sensory neurons in the spinal trigeminal nucleus of the brain stem. The axons of these cells cross and ascend to the thalamus in the trigeminal lemniscus.

Question 48

How do the spinothalamic tract and trigeminal lemniscal axons in the thalamus compare to those of the medial lemniscus? Is this reflected in the cortex?

Answer 48

The spinothalamic tract and trigeminal lemniscal axons synapse over a wider region of the thalamus than those of the medial lemniscus. Some of the axons terminate in the VP nucleus, just as the medial lemniscal axons do, but the touch and pain systems still remain segregated there by occupying separate regions of the nucleus. Other spinothalamic axons end in the small intralaminar nuclei of the thalamus.

From the thalamus, pain and temperature in- formation is projected to various areas of the cerebral cortex. As in the thalamus, this pathway covers a much wider territory than the cortical connections of the dorsal column–medial lemniscal pathway.