Somatosensory Lab 1 and 2

Where do somatosensory axons from the body end in the thalamus? Where to axons from the face end in the thalamus?

VPL: Body VPM: Face (See image in the lab) Note that the taste nucleus is the very pale area ventral to the VPM. The somatotopy of the medial lemniscus is maintained, with face medial, then hand/arm, then foot dorsolaterally (that's why it makes sense that VPM, which is more Medial, gets the face while the other gets the body)

What are the proprioceptive afferents?

We have just reviewed cutaneous mechanoreceptive afferents (SA1 afferents, SAII, RAI, RAII, as well as hair related afferents). Proprioceptive afferents help us determine where we are in space. 1. Golgi tendon organ (FORCE) - sensitive to muscle force (A-alpha, Ib) 2. Muscle spindle - respond to muscle LENGTH AND VELOCITY and are controlled by motor axons. (Either A-alpha if primary spindle afferent, or A-beta if secondary spindle afferent) 3. Join angle afferents - fire in respond to changes in joint angle. A-beta (Group II)

Pacinian corpuscle. The soma of this receptor is in the capsule and this projects to the cuneate nucleus in medulla (if receptor is located in the hand).

What structure is shown in this image? The soma of this receptor is where? If receptor is located in hand, to what brainstem nucleus does this project to?

Answers: a) Fasciculus gracilis b) DRG cell bodies c) Loss of sensation would be experienced on ipsilateral side d) Level of spinal cord is at lumbar e) injury was below the level, because DRGs give rise to fasciculus gracilis f)

When a patient survives spinal cord transection for an extended period, the transected axons degenerate and the region of degenerating axons can be detected through loss of myelin. Figure 6-15 shows a myelin-stained section of human spinal cord, many years after a spinal cord injury. In the region marked by arrows, what axon tract has degenerated? Where are the cell bodies that give rise to these axons? A loss of sensation would be expected for what part of the body? What level of the spinal cord is shown? Was the injury above or below the level shown? Explain the rationale for your answer.

What other areas does S1 project to?

1) One stream of axons go to Brodman's area 5/7 in parietal lobe. ("Where and how" pathway, activated when animal reaching for object 2) Second stream projects to SII and temporal cortex (What pathway, similar to 3b of S1). If you ablate SII, it takes much longer to learn cutaneous input and joint position... can't perform a wide range of tactile function.

What are the main sites that trigeminal afferents travel to in the brainstem (i.e. nuclei)? (i.e. names of the nuclei)? What is another name for the spinal trigeminal tract termination point?

1. SENSORY (MECHANO/PROPRIOCEPTION): Large diameter cutaneous afferents terminate in the principal sensory nucleus in the PONS. These CROSS at the level of the pons to join the medial lemniscus (NOT dorsal columns - we call medial lemniscus because these are the axons coming from second order neurons after having crossed midline). 2. MOTOR Muscle spindle afferents from mastication muscles terminate in the mesencephalic nucleus of V (essentially motor nucleus), located more medially. These are displaced ganglion cells (PNS) and the only example of ganglion in the CNS. * JAW JERK REFLEX* 3. PAIN/TEMP: Spinal trigeminal tract descend from DRGs carrying axons that enter in with other tracts from that level, but descend to the closed medulla/ cervical spinal cord and synapse here around C3. (Also called pars caudalis). They cross and join the anterolateral pathway/spinothalamic pathway formed by the pain/thermosensation fibers from the body.

What would you see if you ablated area 3b? Area 1? Area 2? Area 3a?

3a - this is for PROPRIOCEPTION; you might be able to move fingers but wouldn't know where they're in space. 3b - all tactile function lost, spatial tuning (can you feel embossed letters (aka spatial)? can you detect differences in cutaneous input over time (slipped glass)?) 1 - TEXTURE 2 - 3d shape perception is impaired; this is because this area gets proprioceptive (3a)/joint angle and cutaneous (3b) input, and thus are involved in detecting 3-D shapes (i.e if you indented the digit, which involves a cutaneous sensation as well as muscle spindles and joint angle receptor activation

What is area 3a? 3b and 1? Area 2?

3a: Muscle spindles (proprioception).N ote that this is closest to motor cortex, so can remember that is involved with regulation our muscles in space. 3b and 1: This is where cutaneous spatial and temporal information is first processed in the cortex. SA1 (Merkel's) go here (Braille, spatial) and RA systems (Meissner's, Pacinians) go here (grasped objects slip... temporal) 2: Both cutaneous and proprioceptive, from the above parts of the thalamus. And joint angle. You can remember that 3a and 2 are both kind of by sulci, so they get "deep" input aka muscle spindles, joint angle. While 3b/1 are more near the surface of the postcentral gyrus and thus get cutaneous input.

What defines a modality? What are the components of a labeled line that bring the modality to the somatosensory cortex?

A somatosensory modality is a stimulus transduced by a certain receptor type (vision by RGCs, for instance). Each modality is sent to higher stations in the brain (aka somatosensory cortex) via a set of primary, second order, and third order neurons. Primary: DRG (body afferents) or trigeminal ganglion (face afferents) Second order neuron: SC and brainstem. These receive inputs from ganglion and send their own axons to thalamus. CROSSING of fibers occur at second order level. Third order neurons: Neurons in THALAMUS send info to SI in postcentral gyrus of cerebral cortex

What are the four classes of fibers and their conduction speeds? What kind of afferents do these fibers carry?

A-alpha (1a/b); primary muscle spindle, Golgi tendon A-beta (II); cutaneous mechanoreceptors (Merkel's, Meissner's, Ruffinian's, Pacinians) A-delta (III); cold/prickling pain C fibers (IV); warm, itching, burning All the A fibers are mylenated, in descending amounts; C fibers are unmylenated. Thus, conduction velocity is fastest for alpha fibers and slowest for C fibers. * Can remember the function/speed of C fibers because this is kind of like an allergic rxn (skin gets warm and itches). Also can remember that alpha fibers carry Golgi tendon/primary muscle spindle because you need to know where you are in space almost instantaneously; you also need to be able to sense pressure pretty readily!)

Which axons would one expect to have larger cell bodies, A1 and A2 vs C fibers?

A1/A2 have larger cell bodies than C fibers. We know that A fibers are thicker than C fibers and the size of the cell body corresponds with this. This is why when large fiber afferents enter next to smaller ones at the DRG entry zone, we can see the difference. Larger fibers (carrying mechanoreceptive and proprioceptive input) appear lighter while the Lissauer's tract fibers (smaller diameter, with free nerve endings, carrying pain and temperature) come together close to each other.

What are the four main types of cutaneous mechanoreceptive afferents (in terms of their adapting ability)? a) What types of afferent fibers do they use? b) How big is their receptive field, and what do these help us perceive?

Adaptation refers to the ability of cells to return to baseline levels of stimulation after receiving some kind of impulse. If they go back to baseline quickly, they are rapidly adapting, whereas if they return slowly, they are slow adaptors. *ALL ARE A-beta fibers!!!* In general, the size of the receptive field reflects their sensitivity to different stimuli. 1. Slow adapting type 1 afferents (Merkel's; sweat glands). These have SMALLEST receptive fields and are dense so they can held detect detailed patterns; they respond to CURVATURE and help perceive form/texture. Example: Braille (surface features) 2. Rapidly adapting type 1 afferents (Meissner's); located just below epidermis. These respond to movement and dynamic skin deformation (grip, motion). Medium receptive field, large density. Example: Holding glass - Meissner receptors detect small slips and send message that slip has occurred to spinal cord. 3. Slow adapting afferents II (Ruffini's) - located in dermis/deeper tissue, these afferents respond to SKIN STRETCH (makes sense that receptive field is big because stretch spans quite a distance!) Example: Holding thumb and middle finger together while closing eyes 4. Rapidly adapting type II afferents (Pacinians) - the receptive field for this is the WHOLE hand because if you stimulate these anywhere with HIGH frequency vibration, they will respond. Example: Holding a tool, sensing vibration

What determines if fibers will be part of the fasiculus gracilis or cuneatus?

At lumbar and T6 or below, the afferent fibers go up the dorsal columns in the fasiculus gracilis (essentially becoming the medial part of the dorsal columns). As you ascend (i.e. T5 and above) afferents go up the dorsal columns in the fasiculus cuneatus, getting afferents from shoulders, arms, and hands (and ribs), which join the dorsal columns more laterally. Thus, gracile nuclei get sensory information from trunk, legs, and feet; cuneate nuclei get sensory information from shoulder, arms, and hands.

At the level of the medulla, what is the arrangement of the gracile, cuneate, and spinal trigeminal nucleus (i.e. medial/lateral)? What is the arrangement of the median lemniscus at the level of open medulla?

Closed medulla: Gracile most medial to spinal trigeminal nucleus most lateral -- this makes sense when you think about where the afferents join the spinal cord (early for lower limb, then upper limb, then finally nociception from face). Note that all input at this point is ipsilateral: the nuclei on the right side of the body/face are receiving sensory stimulation from that side. Open medulla: Tracts have feet at the ventral end, hands at dorsal end (based on crossing of internal arcuates) [Note: Body has mechano/proprioceptors, while face only has nociception here; the sensory input for the face only comes in at the level of the pons]. This was the level where we had our fists up (to represent vertical medial lemniscus), and the Fist represented head while elbow represented foot/lower limb

What is responsible for phantom limb pain?

Cortical plasticity. If you were to amputate someone's 3rd digit, the sensory afferents coming from that digit would not longer send info to the cortex via the thalamus. Adjacent digits (2/4) have afferents that do still project to the cortex, and over time will start to take over the area that the 3rd digit used to be represented by. Because the territory that should be represented by missing limb is now represented by adjacent territories -- due to the great plasticity of cortex -- an amputee will still feel sensation of that third limb (can thus still perform tasks).

What is the DRG, what is its structure, and what does it do?

DRG neurons are COLLECTIONS of neurons in the periphery (hence, ganglion). NOT A SINGLE NEURON! They are derived from neural crest and are located along spinal cord. Thus, a ganglion at a specific level will carry a ton of different sensory stimuli. These neurons are the PRIMARY sensory neurons of different stimuli, ranging from mechano/touch to pain/temperature. They have an axon that splits by the soma (pseudounipolar), and one branch goes to the periphery to sense the stimuli (essentially acts like a dendrite). The other branch goes toward the brain/spinal cord. Peripheral process: Sensitive to specific stimuli that generate AP's (sensory part of DRG)... a stimuli --> hits receptor which is close to DRG --> activates DRG. Central process: carry information to the spinal cord and brainstem

How is the spinal cord arranged in terms of locations of gray/white matter?

Gray matter is located inside and white matter is outside. Gray matter consists of 3 areas: dorsal horn (from alar plate - sensory neurons, ascending pathway to brain); intermediate gray (interneurons); ventral horn (motor neurons from basal plate, which exit via ventral root to innervate muscle, and interneurons). White matter consists of axons and glia -- axons bring afferents that go from periphery or SC to higher levels of CNS, or axons that go from higher levels and synapse on inter/motor neurons in SC

How does the medial lemniscus change at the level of the pons? What other somatosensory input is added at this level? Where do you see feet vs hands?

In the pons the medial lemniscus rotates from being oriented vertically to more horizontally -- think of the image of your feet slipping out from under you, making you go from standing to fallen position. (Thus, the head/shoulders goes medially, feet most laterally. The principal sensory nucleus of V sends of its axons in pons, and these cross midline to join medial lemniscus at the MEDIAN edge. Thus, each 1/2 of the medial lemniscus has a full representation of the contralateral face --> feet sensory stimuli (crossing for body occurred in closed medulla, and now crossing for face occurs in pons). Note that axons from spinothalamic and pars caudalis of spinal trigeminal nucleus move close to medial lemniscus Feet are lateral, hands medial (contra)

What makes the pacinian corpuscle rapidly adapting?

Its connective tissues surrounding the central axon (as opposed to multiple nerves innervating Merkel's cells), which is wrapped like an onion. When there is a vibration, this leads to shift in fluid in the capsule -- they move to the side, and now there is less pressure on the nerve fiber, which deforms the fiber, and Na+ enters (it can fire and rapidly adapt). If you peeled away these layers, it would be slowly adapting. Thus, the ability of this receptor to be rapidly adapting depends on the layers. High frequency/vibration.

How is proprioceptive information carried to the cerebellum from the lower and upper limbs?

LOWER LIMBS: Muscle spindle afferents (proprioceptive) coming from the lumbar and sacral regions are carried up by fasciculus gracilis to this area (kind of in between dorsal and ventral horns) by the thoracic cord, which we can tell by the presence of its lateral horns. Axons from this nucleus DO NOT cross but instead travel in dorsal spinocerebellar tracts to the ipsilateral cerebellum (exception to the rule that 2nd ary neurons cross). UPPER LIMBS: Axons from fasciculus cuneatus synapse in external cuneate (or lateral or accessory cuneate) located in medulla. Secondary neurons send of axons that go in the cuneocerebellar tract (not spinocerebellar) to the ipsilateral cerebellum. In this way, cerebellum gets proprioceptive information from spindle afferents that are detecting changes in stretch in both upper and lower limbs, which is important in helping us balance and maintain our coordination.

What are the similarities between the nociceptive pathway for the face versus the body? What about for the somatosensory pathways?

Lissauer's tract = spinal tract of V. Note that these appear lighter because they're C fibers and unmylenated. Lamina 1/2/5/SG and spinothalamic system = pars caudalis of spinal trigeminal nucleus (For somatosensory body, mechano/proprioceptors --> DCML pathway; for face, these same receptors carry stimulus to the principal sensory nucleus)

What are the two main categories of stimuli in the somatosensory system? How do these sensory afferents differ?

Mechanical stimuli: Touch (cutaneous receptors) and proprioception (receptors in muscles, tendons, and joints) Pain/temperature stimuli: Nociceptors and thermoreceptors These sensory afferents differ in their pathway (from periphery to cortex) and degree of mylenation.

Give a brief summary of how the medial lemniscus forms and rotates up until the midbrain.

Medial lemniscus forms in an orderly pattern, with feet first and upper limb last. This initially maintains somatotopy (body map). In the pons, the medial lemniscus rotates into a horizontal orientation with the feet lateral, and axons from the main sensory nucleus of the trigeminal cross the midline and add the face representation to the medial edge of the medial lemniscus.

How does the medial lemniscus rotate in the midbrain? Where do you see feet vs hands?

Most lateral parts of the lemniscus (feet sensory) rotate more dorsally, while the medial parts (head sensory) rotates more ventrally. This leads to a 90 degree bend in the lemniscus, more close to the dorsolateral region. This is the orientation that axons enter the thalamus: most medial/ventrally from main sensory nucleus of V, intermediate position from cuneate nucleus, and axons from gracile nucleus forming most lateral/dorsal part of the medial lemniscus. Feet dorsolateral, hands ventral

Are Meissner's only innervated by A fibers? What exactly is a Meissner corpuscle and Merkel neurite complex? Why does it make sense that merkel receptive fields are smaller?

No - also by C fibers. Note that receptor is way out at tip of epidermis, unmylenated. The "corpuscle" refers to the fact that each afferent branches to several different corpuscles; there are also many different afferents going to different merkel's cells. Thus, the point is that these receptors are getting innervation from multiple afferents. Merkel fields might be smaller because they are trying to discern patterns, smaller edges. If their field was HUGe they would have more sensitivity but would be able to less effectively define the edges of small patterns. From this difference in receptive field, we can see how Merkel's has better resolution than Meissner's which is better than Pacinians.

Are all neurons in DRG the same size?

No. They have different size cell bodies which reflects their overall function; also reflects the fact that you need to synethesize more protein in order to build longer, thicker axons (hence, bigger soma) Large soma - heavily mylenated, conduct rapid afferents (alpha) Medium - large mylenated axons (beta) Small sonata - small and lightly mylenated (delta) or unmylenated, slow conducting fibers (C fibers) This is because we want afferent sensory information to reach us but at different times -- we need to immediately know where we are in space, but might want some delay with pain stimulus.

How does Lissauer's tract appear in myelin stains, and why?

Pale, due to the lightly myelenated A-delta fibers or unmylenated C fibers.

Where is information sent once it reaches SI from the thalamus?

Parietal lobe: integration of visual/somatosensory info occurs to construct image of body in space (and also to initiate and direct movement) SII cortex: input from here goes to the insula (by temporal lobe) and temporal lobe for object recognition/working memory This is parallel processing, in which thalamocortical neurons send simultaneous projections to primary areas. These areas send info to temporal lobe (perception, facial recognition, working memory) and parietal lobe (sensory control of movement) in order to generate a picture of where you are in order to move.

Where is the somatosensory cortex that VPM/VPL project to? What are the four regions of the SI cortex and in what way are they similar?

Postcentral gyrus. SI cortex is divided into 4 areas, each with its own map of the body: 3a, 3b, 1, 2 (Brodmann's areas). Each carries a somatotopic representation of the body, where the face is most lateral and body/feet are most central (because that's the way the neurons from VPL and VPM project to the cortex -- note that while the representation of the VPL/VPM is similar to the medial lemniscus, the representation on the cortex totally flip!

What is the second somatosensory cortex and what is its purpose/location?

Receive output projections from 4 areas of SI cortex, and is on the upper bank of the lateral sulcus. Primarily cutaneous and proprioceptive input. Know that neurons in these secondary areas have receptive fields that are more complex than in the SI cortex. This reflects the fact that as you go to higher levels, sensory information and receptive fields are more complex.... this is likely because it's no longer the original stimulus that you're transducing, but a) one that's reduced in stimulus, b) one that has been transduced by secondary and third neurons that are now carrying it to higher levels, and these neurons can have their own sets of connectivity and impulse strength.

How would vibratory information from your hands travel to the brain (only discuss path outside brain)?

Sensory info --> DRG located at T5 or above --> afferent travels in dorsal columns (fasiculus cuneatus) to one of two places. First: cuneate nuclei in closed medulla. Secondary neurons here send off axons that decussate (internal arcuate fibers) and ascend on the opposite side, in the median lemniscus, to the thalamus. The second site of termination (besides cuneate nucleus) is the accessory/lateral/external cuneate nucleus, which is where muscle spindle afferents travel and terminate; from here they follow the cuneocerebellar tract, which conveys proprioceptive information from the upper body to ipsilateral cerebellum.

How does somatosensory information enter the thalamus? How is the thalamus organized?

Somatosensory information enters the thalamus along separate paths and remains segregated in the thalamus. It's organized as if it has a bunch of discrete sections with are responsible for getting signals and transmitting them to the cortex -- i.e. instead of one structure, like multiple structures working together with the same overall function. Specifically, the thalamus is broken up into discrete nuclei with their input/outputs. The inputs define the properties of the nucleus while the outputs determine which areas of the cortex are sensory (i.e. what is primary visual is because it's innervated by retinal recipient nucleus of thalamus).

What is the monosynaptic reflex arc and how does it work?

The DRG innervates stretch receptors (muscle spindles) in striated muscle. When a muscle is stretched, it causes a signal --> DRG --> carries this impulse to the ventral horn of the spinal cord, where it synapses on an alpha motor neuron. This motor neuron then responds to the stretch by contracting the muscle. (Ex: quad reflex) The two main neurons involved are thus the DRG and alpha motor neurons. DRG cell bodies are outside spinal cord (but are still located somewhat ventrally) and thus considered PNS, while alpha motor neuron cell bodies are located more inside the cord and are considered CNS. The trigger zone of each neuron responsible for initiating and conduction an action potential is located right after cell body by axon hillock, where the maximum # of VG-sodium channels are located and thus, depolarization can occur.

Which fiber type has the highest threshold for electrical stimulation? Which fibers do pressure blocks affect most, and which ones do anesthetics affect most?

The last group (C/IV) has the highest threshold for electrical stimulation, and pressure blocks affect larger fibers first and then successively smaller ones Anesthetics affect unmylenated and smaller mylenated fibers.

In sum, what are the inputs of VPM and VPL and where do they project to?

The trigemina/gustatory pathway go to the VPM, and bring face sensation and taste. These areas project to primary somatosensory cortex. (Remember: Makeup goes on face) The Spinothalamic tracts (carrying pain and temperature from body AND face) and DCML go to the VPL, carrying pain/temperature, touch, vibration, and proprioception. This also ends in the primary somatosensory cortex. [Note that pain/temperature fibers terminate in intralaminar complex, caudal to VPL]

What is the path of small diameter fibers coming from the lower limb to the brain?

Thermoreceptive/nociceptive afferents --> DRG to Lissaeur's tract (essentially just dorsal columns for temperature and pain) located a bit more medially to the larger diameter fibers. Axons from these DRGs will synapse in the spinal cord at the level of Lissauer's they enter in (i.e. C5) but also at spinal levels above/below (i.e. C3-7), which results in broad surface of skin feeling painful stimuli. Neurons synapse at Lamina 1 or 2, as well as substantia gelatinosa, as well as lamina 5 and IG of the dorsal horn. From here, secondary neurons travel from Lamina 1, V, and IG cross the midline and ascend in anterolateral tracts or spinothalamic tracts. From these tracts, axons can terminate in the thalamus (most), reticular formation of pons and midbrain (arousal to painful stimuli), or in periaqueductal gray of midbrain

What are thermoreceptive afferents?

These are either cold (A-delta) or warm (C-fiber) afferents; their name reflects the temperature at which they fire most. The main difference is that cold receptors are more sensitive to sudden cooling (think of jumping in a pool) as opposed to warm receptors, which are equally sensitive to dramatic or gradual cooling. *If you gradually reduce temperature, might see that the rate of firing is the same at 10 or 30 degrees, but the pattern of firing may differ.

What is the pathway of the large diameter fibers in somatic sensation (outside the brain) from the LEG?

These start from receptors (mechanoreceptors, proprioceptors) that sense touch, vibration, stretch, and position in space and transmit signals to the DRG (by way of one of its axons). The DRG (primary neuron) sends its axons in the dorsal columns where they ascend. These dorsal columns (either called fasciulus gracilis, which runs medially, or fasiculus cuneatus, which runs laterally -- depends on where they come from) can terminate in one of three places. In this case, the impulse travels in fasciculus gracilus since it begins in the leg. a) ventrally at alpha motor neurons --> activate stretch reflex (i.e. tendon reflex) b) Clarke's nucleus at the thoracic level --> dorsal spinal cerebellar tract to ipsilateral cerebellum for position of lower limbs b) continue to the closed or caudal most part of the medulla (i.e. where it meets spinal cord) where they terminate in gracile or cuneate nucleus. From here, axons synapse, and cell bodies within these nuclei (2nd order - THUS THEY CROSS) give rise to axons that CROSS the midline. These are called internal arcuate fibers (see arrows in picture) which go to the medial lemniscus (wedged-shape above pyramids and medial to olives) and then posterior thalamus

Why do we divide the SI cortex into these four areas?

They are functionally different, in that they respond to different afferents. (Thus it makes sense why they would have a full representation of the body in each, because they are responding to different modalities across the whole body). They also have differences in cellular composition. The most dramatic one is when layer IV in area 3b turns less robust in area 1 Finally, they have differences in connectivity, as we can see from the fact that area 2 gets input from 1, 3a, and 3b -- and thus serves as the source of somatosensory information to the motor cortex.

What is the purpose of the trigeminal ganglion? How is it similar to the somatosensory system in the body in terms of its path and the primary, secondary, and tertiary levels of neurons?

Trigeminal ganglion provide sensory/pain/temp info to the FACE, as opposed to DRGs which provide sensory/pain/temperature information from the BODY. The cell bodies of the trigeminal ganglia are analogous to the DRG and give rise to a central and peripheral afferent or trigeminal NERVE (to the face). The face afferent has not one but three divisions (V1, V2, V3 for opthalmic, maxillary, and mandibular distributions) which reflects sensory stimulation from face, mouth, even up to the ear. While the peripheral branch goes to the face, the central axon of the trigeminal ganglia goes to the brainstem, where it synapses on two nuclei (almost 3). Second order neurons decussate and carry impulses along with other ascending tracts to terminate on the thalamus, adjacent to medial lemniscus (sensation from body) and spinothalamic tract (pain/temperature)