Sensory and Motor Circuits Flashcards
There are four somatosensory circuits, what are they?
Lemniscal, Anterolateral (spinothalamic), spino-cerebellar, Trigeminal
Large motor neurons are especially vulnerable to insult from ischemia, toxicity, bacteria, etc, so that early symptoms of peripheral nerve disease often first show up how?
Epicritic rather than protopathic losses.
What is the gracile fasciculus?
Dorsal root axons from the lower trunk and limbs that enter at lower thoracic and lumbosacral levels.
What is the cuneate fasciculus?
Upper trunk and limb axons that enter the spinal cord at upper thoracic and cervical levels ascend as the cuneate fasciculus.
The gracile fasciculus and cuneate fasciculus if cut would cause what?
Epicritic losses ipsilaterally.
Where do the gracile fasciculus and cuneate fasciculus synapse? Where do they go from there?
In the medulla. The information is picked up by second-order neurons whose cell bodies lie in the gracile and cuneate nuclei. Axons of these cells exit the nuclei cross to the other side in the medulla at which time they are renamed the medial lemniscus.
What would damage to the right medial lemniscus produce?
Epicritic symptoms on the patient’s left side of the body.
Where does the medial lemniscus synapse? What happens to the information from there?
In the ventral posterolateral nucleus of the thalamus (VPL). Information is then picked up by the third-order neurons, which carry it to the post central gyrus (S1, or Brodmann’s area 3,2,1) via the internal capsule.
What do epicritic symptoms include the loss of?
Stereognosis: Cannot recognize tactile shapes placed in the hand (astereognosis)
Two-point tactility: cannot separate the location of 2 simultaneous touches near each other on the skin Position sense:cannot state the position of the limb without visual cues
Vibration: Insensitivity to high frequency stimulation
Simple touch: Largely intact, just less sensitivity.
The size of the dorsal root axon diameter in the Anterolateral system is _________, while in the Lemniscus system they are __________?
Small-diameter.
Large-diamter.
What happens if you damage the anterolateral tract on one side? Why?
It will cause protopathic symptoms on the opposite side of the body. This is because most of the AL tract neurons first cross the midline before ascending to the brain.
Anterolateral tract axons synapse diffusely and at the level in the brainstem, but where do most terminate? What is the fate of the rest of the axons?
In the reticular formation. A few make it to the thalamus where they terminate in the ventral posterolateral nucleus of the thalamus as well as sever other nuclei including the dorsomedial and intralaminar nuclei. Signals then pass via the internal capsule to primary somatosensory cortex (post central gyrus, SI, 3,1,2 of Brodmann)
What does reduced “protopathic” mean?
Pain: reduced or no sense of pain from a pinprick or from chronic internal pain.
Temperature: reduced or lost sense of warming and cooling of the skin.
Simple touch: Largely intact, but reduced sensitivity
In the spino-cerebellar system where do signals from muscle spindles and joint receptors enter the CNS?
Via the dorsal roots and after one or more synapses, spino-cerebellar tracts carry this information to the same side of the cerebellum.
In the spino-cerebella system axons from the leg and lower trunk ascend as part of the gracile fasciculus, where do the axons terminate?
They terminate in the dorsal nucleus of Clarke or Clarke’s nucleus in the thoracic cord. Second order neurons from Clarke’s n. ascend to the cerebellum along the lateral rim of the white matter as the dorsal spinocerebellar tract.
In the spino-cerebella system a second component from the arm and upper torso ascends with the cuneate fasciculus to the medulla, where do they synapse?
They synapse in the accessory (or lateral) cuneate nucleus. Second-order neurons carry the signals to the cerebellum as the cuneo-cerebellar tract.
Why are the clinical effects of pathology of the spino-cerebellar system rarely seen?
They are rarely damaged in isolation.
In the Trigeminal system where do axons of the trigeminal nerve conveying epicritic sensation synapse? Where do second-order neurons travel?
In the principal or chief sensory nucleus of CN 5 and in the pontine part of the spinal nucleus of CN5. Second-order neurons leave these nuclei, cross the midline, and join the medial lemniscus en route to the thalamus (ventral posteromedial nucleus). Subsequent neurons forward the signals to the post central gyrus.
What is the pathology of the pontine portion of the trigeminal system?
There will be epicritic losses of the head and neck. The symptoms are on the same side if the pathology lies before the crossing (in the trigeminal nerve or nuclei). Pathology after the crossing (medial lemniscus, thalamus, cortex) will cause losses on the opposite side of the patient’s face.
Where do axons from the second portion of the trigeminal nerve travel? What information do they contain?
They convey protopathic information and descend toward the spinal cord as the spinal tract of CN5, synapsing along the way in the adjacent caudal portion of the spinal nucleus of CN5. Second-order neurons leave the caudal part of the spinal nucleus, cross the midline, and project largely to the reticular formation.
Describe the pathology of the caudal portion of the spinal trigeminal tract (in medulla) and nucleus of CN5:
Cause protopathic losses on the same side of the head. Pathology after the crossing (ie. from the reticular formation), particularly at a pontine or midbrain level, will affect the opposite side of the head.
A third sensory component of the trigeminal nerve carries proprioreceptive input from the face. What path do the axons take?
Axons in the trigeminal nerve are continuous with the neurons of the mesencephalic nucleus of CN5, these axons, called the mesencephalic tract of CN5 continue through the mesencephalic nucleus of CN5 to synapse with neurons of the motor nucleus of CN5 for jaw reflexes, and via the reticular formation before projecting to the cerebellum.
