Exam 2

Question 1

What is the major somatosensory relay nucleus of the thalmus?

Answer 1

The ventral posterior nucleus (VP nucleus) is the major somatosensory relay nucleus of the thalamus. It is the only site of termination of the medial lemniscus, and a major site of termination of spinothalamic fibers.

Question 2

Where do axons of VP neurons project to?

Answer 2

Axons of VP neurons project to the first somatosensory cortex (or SI) located in the caudal bank of the central sulcus and the postcentral gyrus, and to a part of the parietal operculum, to an area called the second somatosensory area (or SII).

Question 3

How is the VP nucleus divided?

Answer 3

The ventral posterior nucleus is divided into two subnuclei which are related to the somatotopic organization of the system. The ventral posterior lateral (VPL) nucleus receives medial lemniscal fibers originating from the contralateral dorsal column nuclei; the ventral posterior medial (VPM) nucleus receives lemniscal input originating from the contralateral trigeminal nucleus.

Question 4

Which fibers does the VPL receive?

Answer 4

The ventral posterior lateral (VPL) nucleus receives medial lemniscal fibers originating from the contralateral dorsal column nuclei.

Question 5

Which fibers does the VPM receive?

Answer 5

The ventral posterior medial (VPM) nucleus receives lemniscal input originating from the contralateral trigeminal nucleus.

Question 6

What is the smooth and uninterrupted somatotopic representation of the contralateral body on the VP?

Answer 6

In VPL, the leg is most lateral, followed medially by trunk, arm and neck; head and face in VPM, with inside of mouth most medial. There is a disproportionately large volume devoted to the representation of the foot, hand and lips since these areas have the highest density of innervation and therefore the greatest sensory acuity.

Question 7

What are the two types of neurons in the VP nucleus?

Answer 7

There are two types of neurons in any thalamic nucleus: relay neurons (which relay to the cortex, mostly) and intrinsic inhibitory interneurons, which use the neurotransmitter GABA. Relay neurons exhibit properties which mirror those of their input: lemniscal or spinothalamic (not both).

Question 8

What are some Lemniscal fiber properties?

Answer 8

• Great synaptic security (i.e. high fidelity of transmission)—reproduce faithfully temporal pattern of action potentials.
• Modality and place specificity (input from specific type of receptor)
• Neurons with same place and modality characteristics cluster together
• Surround inhibition (activation of one group of neurons accompanied simultaneously by inhibition of neighboring clusters.

Question 9

What are some Typical spinothalamic properties?

Answer 9

• Many fewer cells of this type and hard to drive (ie require intense stimuli)
• Large receptive fields
• Many respond specifically to noxious stimuli, others to non-noxious thermal stimuli (cooling largely), but can be both.

Question 10

What are the 4 areas in S1 and how are they organized?

Answer 10

There are four, separate architectonic areas which comprise SI: areas 3a, 3b, 1 and 2. There is a complete contralateral body-map in each of these areas, with leg represented most medially, face and head most laterally and trunk and upper limb in between. Like the thalamus, in each of these areas of SI, there is a comparable disproportionality in the volume devoted to feet, hands and lips.

Question 11

Which thalamocortical afferents terminate in Areas 3a and 2?

Answer 11

Areas 3a and 2: receive the axons from VP cells carrying proprioceptive information (i.e. from muscle spindle primary and secondary afferents and those from joints).

All four areas receive thalamic innervation representing spinothalamic input.

Question 12

Which thalamocortical afferents terminate in Areas 3b and 1?

Answer 12

Areas 3b and 1: receive the axons from VP cells carrying cutaneous tactile information (i.e. from A-beta fibers).

All four areas receive thalamic innervation representing spinothalamic input.

Question 13

How is SII organized?

Answer 13

Somatotopic organization, inputs and organization of SII cortex

There is only a single representation in SII, but it is bilateral (formed by callosal axons which integrate the two body-halves found in each hemisphere). The face is represented anteriorally, the leg most posteriorally. The same type of information as in SI is also found in SII, and the organization of inputs and intrinsic connections which forms a “cortical column” in SI is also found in SII.

Question 14

What comprises the somatosensory Thalamus?

Answer 14

Somatotopic organization, inputs and organization of SII cortex

There is only a single representation in SII, but it is bilateral (formed by callosal axons which integrate the two body-halves found in each hemisphere). The face is represented anteriorally, the leg most posteriorally. The same type of information as in SI is also found in SII, and the organization of inputs and intrinsic connections which forms a “cortical column” in SI is also found in SII.

Question 15

What do axons from the spinothalamic tract and from the spinal nucleus of V carry and where do they terminate?

Answer 15

Axons from the spinothalamic tract and from the spinal nucleus of V, representing pain and temperature information, terminate broadly in somatosensory and other thalamic nuclei.

Question 16

What is the path of sensory receptors for the face?

Answer 16

Trigeminal ganglia-> brainstem-> thalamus-> SI cortex.

Question 17

What is the path of sensory receptors for the body?

Answer 17

Dorsal root ganglia-> spinal cord-> thalamus-> SI cortex

Question 18

What does the DC-ML system do?

Answer 18

Mechanosensation. DORSAL COLUMN-MEDIAL LEMNISCAL SYSTEM:

Medial division: touch, pressure & vibration (A fibers) position & movement (Group I, II fibers)

Question 19

What does the Spinothalamic (anterolateral) system do?

Answer 19

Pain and temperature. ANTEROLATERAL SYSTEM:

Lateral division: pain & temperature, crude touch (A & C fibers; Group III, IV fibers)

Question 20

What do exteroreceptors do?

Answer 20

Exteroceptors: receptive to stimuli on or beyond the body surface.

Question 21

What do Interoceptors do?

Answer 21

Interoceptors: receptive to stimuli arising within the body itself.

Question 22

What are Proprioceptors?

Answer 22

Proprioceptors: special group of interoceptors which are receptive to the position of the body, head or limbs in space.

Question 23

What are free nerve endings?

Answer 23

Free nerve endings are the majority of sensory receptors in the skin. These terminal branches lose all coverings (including Schwann cell wrapping) and penetrate into the epidermis almost as superficially as stratum corneum. They display no obvious structural specialization, but evidence suggests that different fibers respond preferentially to painful stimuli, or warmth or cold, or to mechanical displacement of the skin. Free nerve endings are the endings of unmyelinated parent axons (called “C” fibers—see section III below) or thinly myelinated parent axons (A-delta fibers).

Question 24

What are Unencapsulated endings with accessory structures ?

Answer 24

Unencapsulated endings with accessory structures are terminal axon branches which end in intimate association with other cell types found in the skin. Hair receptors are one example: every hair follicle is innervated by several unmyelinated branches which encircle the follicle. The terminal axon membrane is embedded in the glassy membrane of the follicle, and is activated by hair deflection. Other examples include the Merkel’s touch corpuscle, in which a terminal axon branch expands to form a flattened disc that is closely apposed to a modified epidermal cell (a Merkel cell). Merkel cells are connected to neighboring cells via desmosomes, and cytoplasmic protrusions of the Merkel cell enclose the terminal disk of the axon. The axons which end in a Merkel’s corpuscle discharge when pressure is applied directly to the corpuscle.

Question 25

What are encapsulated endings?

Answer 25

Encapsulated endings are ones in which the terminal axon ends inside a distinct connective tissue capsule; these endings are often specialized for determining the direction or type of displacing force that acts on the contained sensory nerve terminals. In general, the capsule is formed by the overlapping processes of several cells, often fibroblast-like cells, which form concentric lamella separated by fluid, resembling an onion. Examples in the skin and joints include Pacinian corpuscles and Ruffini endings—these are innervated by the largest diameter, myelinated axons (A-beta fibers), and encode vibratory or pressure information, respectively. In skeletal muscle, encapsulated mechanoreceptors include the muscle spindle and, in the myotendinous junction, the Golgi Tendon Organ (GTO)—these latter two are discussed fully in the motor lectures.

Question 26

What are Pacinian corpuscles and Ruffini endings and what do they do?

Answer 26

They are encapsulated endings in the skin and joints. these are innervated by the largest diameter, myelinated axons (A-beta fibers), and encode vibratory or pressure information, respectively.

Question 27

How is intensity encoded?

Answer 27

By the frequency of action potentials and the numbers of fibers recruited

Question 28

How is location perceived?

Answer 28

Location is perceived by the position and size of the neuron’s receptive field;

Question 29

What determines duration and rate of change?

Answer 29

duration and rate of change is determined by the discharge characteristics of the axon: slowly adapting (will continue to discharge action potentials as long as stimulus is applied) vs. rapidly adapting (discharges during onset, offset or changes in intensity).

Question 30

Describe Group I afferents.

Answer 30

Muscle afferents. Group I (Ia) (Ib). 
receptor types: primary endings in muscle spindles (Ia)
GTOs (Ib)
There are no equiv cutaneous afferents. 
Diameter (uM): 13-20
Conduction velocity (m/sec): 80-120

Question 31

Describe Group II afferents.

Answer 31

Muscle afferents.
Receptor types: secondary endings
in muscle spindles

Equiv. cutaneous afferents: Aß (beta)
Receptor types: Most encapsulated and unencapsulated endings
Diameter (uM): 6-12
Conduction velocity (m/sec): 35-79

Question 32

Describe group III and IV afferents.

Answer 32

Muscle afferents.
Rec types: nociceptors (muscle)

Eq. Cutaneous afferents: for Group III: Aδ (delta) For Group IV: C
Rec types: Thermoreceptors,
nociceptors, small diameter mechano-receptors-many are free nerve endings

III/Adelta Diameter (uM): 1-5
III/ADelta Conduction velocity (m/sec): 5-34

IV/C Diameter (uM): 0.2-1.5
IV/C Conduction velocity (m/sec): 0.5-2

Question 33

What is the mylination status of different afferent fibers?

Answer 33

By definition, Group IV (muscle) and “C” fibers (cutaneous) are unmyelinated; the rest are myelinated heavily (Group I, II, Aß) or thinly (Group III, Aδ).

Question 34

What are the two great ascending systems of axons that represent the somatosensory system in the spinal cord?

Answer 34

The dorsal column-medial lemniscal (DC-ML) system and the spinothalamic (also known as the anterolateral) system. The ascending pathways are composed of the central processes of DRG cells (the DC-ML pathway) and/or the axons of second, third, or greater-order spinal neurons (found in both the DC-ML system and the spinothalamic system) which receive their information from DRG cell axons.

Question 35

What is the dorsal root entry for different afferent fibers?

Answer 35

Dorsal root entry—each dorsal root filament organizes its various afferent fibers according to size/function as they enter the spinal cord:

• Medial division: larger afferents (Groups I, II and A-beta)—proprioception and low-threshold cutaneous mechanoreceptors.
• Lateral division: smaller afferents (Groups III, IV, A-delta and C)—pain and temperature largely, also inputs from viscera and some high-threshold tactile input.

Question 36

Describe the medial division branches.

Answer 36

Medial division: medial-division fibers usually divide into three types of branches: 1) one branch courses ventrally, through the grey matter of the same spinal segment as the segment of entry, and terminates (if cutaneous afferents) in either laminae III/IV (nucleus proprius) or (if muscle afferents) in laminae VII (Clarke’s column) or IX (motoneuron pools); 2) one branch descends caudally a couple of segments (traveling in the fasciculus interfascicularis and fasciculus septomarginalis) terminating in the same layers as above; 3) a long branch ascends in one of the dorsal columns (fasciculus gracilis if below T6, fasciculus cuneatus if T6 or above) to the medulla. Fibers in the spinal dorsal columns are somatotopically organized: sacral regions are represented by fibers lying most medially, next to the dorsal median septum, then, on moving laterally, foot, thigh, lower trunk (in the gracile fasciculus), upper trunk, upper limbs and neck (in the cuneate fasciculus). Note that each of these branches remains on the same side of the cord as that at which the dorsal roots enter.

Question 37

Describe the lateral division branches.

Answer 37

Lateral division: fibers accumulate on the surface of, and immediately ventral-lateral to, the surface of the dorsal horn. There, they form Lissauer’s tract (or dorsolateral tract). In this tract, the C fibers and A-delta fibers branch over 2-5 segments and enter the ipsilateral dorsal horn, terminating on second-order neurons of laminae I and II mostly (substantia gelatinosa).

Question 38

Describe the Dorsal column-medial lemniscal pathway.

Answer 38

Dorsal column-medial lemniscal pathway: this pathway is the continuation of the fibers which form the medial division of the dorsal roots. It is the principal pathway through which information about position, movement, touch and vibration is conveyed to higher perceptive centers of the brain (e.g. the cerebral cortex)—that is, it is concerned with discriminative aspects of somatic sensation. Fibers of the gracile and cuneate fasciculi ascend on the side ipsilateral to the side at which they entered. All of these fibers then terminate in the medulla, in the respective gracile nucleus or cuneate nucleus (which collectively are called the dorsal column nuclei, or DCN). Somatotopy conveyed by the ascending fibers is preserved in the dorsal column nuclei. In addition, neurons in the DCN retain the modality specificity of the axons which innervate them, they also retain the place specificity (i.e. there is little convergence of fibers, thereby the receptive field characteristics of the fibers are faithfully transmitted to the DCN neurons. Finally, information is rapidly and faithfully transmitted—that is, there is a great deal of synaptic security. These characteristics: somatotopy, place and modality specificity and synaptic security are the primary ones of the whole system, from periphery to cortex.

In the next step of this pathway, DCN neurons send their axons across the midline, and continue to ascend on the opposite side of the brain. The crossing is called the great sensory decussation, and the contralateral ascending tract is called the medial lemniscus. The axons which form the medial lemniscus terminate in the thalamus.

Question 39

Describe the Spinothalamic system.

Answer 39

Spinothalamic system: this pathway is the continuation of the fibers which form the lateral division of the dorsal roots. It is the principal pathway through which information about pain and temperature is conveyed to higher brain regions. Spinal neurons in laminae I, which receive their information directly from A-delta and C fibers, but also spinal neurons in laminae V, which are not directly contacted by lateral-division fibers, are the major contributers to the spinothalamic system. In contrast to the DC-ML pathway, the spinothalamic system is crossed in the cord, and ascends on the side opposite to the one in which the parent cell bodies reside (and opposite to the side of entry of the dorsal root fibers). Axons cross the midline within a segment of their origin, by traversing a narrow space between the central canal and the ventral median fissure—a region called the ventral white commissure. They then ascend in the contralateral anterolateral funiculus. Traditionally, in humans, it has been customary to recognize a lateral spinothalamic tract containing fibers conveying pain and temperature and a more medial ventral spinothalamic tract containing fibers conveying innocuous tactile information.

The spinothalamic tract fibers can ascend directly to the thalamus without synaptic interruption. Functionally, and in contrast to the DC-ML system, receptive fields are large and overlapping, and thus localization is much cruder in this system than the high spatial resolution characteristic of the DC-ML system.

Question 40

What is the ventral white commissure?

Answer 40

A region where axons in the spinothalamic system cross the midline within a segment of their origin, by traversing a narrow space between the central canal and the ventral median fissure. Therefore the axons signal on the contralateral side of the input.