Motor pathways Flashcards
1
Q
Upper and lower motor neurons
A
- Upper motor neurons (UMN): cell body in motor cortex and subcortical nuclei that synapse on anterior horn cells in the SC, or somatomotor neurons of cranial nerve nuclei
- For UMNs, the cell body and axon is within the CNS
- Lower motor neurons (LMN): anterior horn cells of the SC or somatomotor neurons of cranial nerve nuclei that send their axons to terminate on skeletal muscles @ NMJs
- For LMNs, the cell body is in CNS but the axons travel to the periphery
2
Q
Types of LMNs
A
- Somatomotor (alpha) neurons: found in SC and cranial nerve nuclei. Innervate skeletal muscle for voluntary contraction and provide trophic support (via tonic activation) for muscle survival
- Gamma motor neurons: found in the SC and brain stem, innervates muscle spindle fibers to maintain sensitivity to stretch reflexes
- Both types of LMNs are activated by UMNs when voluntary activity is initiated
3
Q
Location of LMNs
A
- LMNs of the neck, trunk and limbs (not head) are found in the SC
- LMNs that control proximal muscles are located medially in the ventral horn, while neurons controlling distal muscles are located laterally
- Cranial nerve nuclei in the brain stem contain that contain LMN to innervate skeletal muscles of the face are: CNs III, IV, V, VI, VII, IX, X, XI, XII (all but I, II, VIII- the ones that are only special senses)
4
Q
Location of UMNs
A
- Can be from 2 places: either the motor cortex or subcortical nuclei (red nucleus, reticular formation, vestibular nuclei)
- Motor cortex neurons can either be in the primary motor cortex or the FEF (frontal eye field), they send out axons that descend through the medullar pyramids (pyramidal tracts) to reach the SC or the cranial nerve
- Both the corticospinal tracts (for body) and corticobulbar tracts (for head) are pyramidal tracts
5
Q
Corticospinal tracts
A
- UMN axons pass through medullar pyramids and enter the SC
- The axons descend the SC until they reach the LMNs they innervate
- The LMNs innervate muscles of the neck, trunk, and limbs
6
Q
Corticobulbar tracts
A
- UMN axons end in the brainstem (some do travel down the medullar pyramids)
- They synapse on the somatomotor neurons of cranial nerves that innervate muscles of the head, larynx and pharynx
7
Q
Motor cortex 1
A
- Motor association cortices: planning and programming of motor activity, precedes the activation of primary motor cortex (PMC). Occurs in the supplementary motor area (SMA) and premotor area (PMA)
- Prefrontal cortex (PFC) is responsible for what the action will be, sends info to the SMA and PMA. PFC is anterior to motor association cortices and is not part of them
- In addition to the motor association cortices and PMC, motor contributions can come from the general sensory cortex and sensory association areas of the parietal lobe
8
Q
Motor cortex 2
A
- Primary motor cortex (PMC) gives origin to the corticospinal (CST) and corticobulbar (CBT) tracts
- Neurons of origin of CST and CBT are mostly found in the precentral gyrus (PCG) and paracentral lobule
- In the PMC there is somatotopic representation of the body parts (homunculus) with the feet and legs in the paracentral lobule (near longitudinal fissure) and hands and face at the lateral part of the precentral gyrus near the lateral sulcus
9
Q
Vascular supply to PMC
A
- The more medial part of the PMC (near the longitudinal fissure, controls the lower limbs) is supplied by the anterior cerebral arteries (ACA)
- The more lateral parts of the PMC (controlling the upper limbs and face) is supplied by the middle cerebral arteries (MCA)
- Areas of the PMC that represent different body parts are proportionate to the complexity of motor functions of that body part (i.e. the thumb/hand and muscles for speech take an exceptionally large amount of PMC)
10
Q
Corticospinal tract (CST) 1
A
- Axons of cortical neurons come together to form the corona radiata
- Below the corona the tract condenses into the posterior limb of the internal capsule
- Somatotopic arrangement in the posterior limb of the internal capsule for CST are fibers destined for the face/upper limbs are more medial anterior, while fibers destined for the leg/lower body are more lateral posterior
- Important: motor axons in internal capsule run with sensory (superior internal capsule) and run w/o sensory (inferior internal capsule)
- Therefore lacunar infarcts of internal capsule may affect just motor (inferior) or both sensory and motor (superior)
11
Q
Corticospinal tract (CST) 2
A
- The CST then descends through the middle 3/5ths of the crus cerebri in the midbrain. In the crus cerebri the upper limb fibers are medial to the lower limb fibers
- The CST descends to the pons where it traverses the ventral part of the pons. There is no clear somatotopic localization
- In the upper medulla the CST axons gather at the pyramid. 75-90% of CST axons decussate in the medullary pyramid, at the medullary-SC junction
12
Q
Corticospinal tract (CST) 3
A
- After decussating, the axons form and descend in the lateral corticospinal tract in the lateral funiculus (region of white matter just lateral to the dorsal horns)
- This is the split btwn the lateral (crossed) vs anterior (uncrossed) CST
- The anterior CST are the remaining fibers that descend the SC in the medial part of the ventral funiculus: just anterior to the white commissure and medial/anterior to the ventral horns (most medial and anterior part of the SC)
13
Q
Lateral CST
A
- LCST is located in lateral funiculus of SC, axons leave the tract at all levels and synapse on LMNs in the anterior horn on the same side (contralateral to side of origin)
- The LMNs innervated by axons in LCST usually then innervate muscles in the distal parts of the limbs
- At any level below the decussation (i.e. the SC), damage to the LCST will affect the LMNs and skeletal muscles on the same side of the lesion
- Above the pyramidal decussation, damage to this pathway will affect LMNs and skeletal muscles on the contralateral side of the lesion
14
Q
Anterior CST
A
- Found only in the cervical and upper thoracic levels
- Axons may end contralateral to the side of origin, by crossing thru the white commissure to synapse on the medial group of LMNs in the anterior horn of the other side of the SC
- Many axons do not cross thru the white commissure, and synapse on medial group of LMNs in the ipsilateral anterior horn
- Therefore the tract ends bilaterally
15
Q
Corticobulbar tract (CBT) 1
A
- Axons originate from neurons in the PMC (regions that represent the face and head), close to the lateral sulcus
- They descend in the corona radiate and intermingle w/ CST
- In the internal capsule the CBT and CST are still intermingled. The CBT lies at the genu (flexure) of the posterior limb of the internal capsule, posterior and medial to the CST fibers
16
Q
Corticobulbar tract (CBT) 2
A
- In the midbrain the CBT fibers are medial to the CST fibers, as they flow through the crus cerebri
- In the basal part of the pons and the pyramid, the CST and CBT are intermingled and indistinguishable
- Axons in the CBT leave the tract at the level of the cranial motor nuclei (in the brainstem) they will innervate
- These cranial motor nuclei are at many different levels, and the CBT does not extend much below the medulla except to innervate the nucleus of XI in the upper cervical regions (C2-5)
17
Q
Locations of cranial nerve motor nuclei
A
- Midbrain: nucleus of III (move eye) in upper midbrain, nucleus of IV (move eye) in lower midbrain
- Pons: nucleus of V (mastication) in midpons, nucleus of VI (move eye) in lower pons, nucleus of VII (facial expression) in lower pons
- Medulla: nucleus of IX and X (nucleus ambiguous, muscles of larynx and pharynx) in upper medulla, nucleus of XII (muscles of the tongue) in the upper medulla
- Cervical SC: nucleus of XI (SCM and trapizious) in C2-5
18
Q
Terminations of CBTs for bilateral innervation of muscles
A
- CBTs for CN V, IX, X and XI end bilaterally on cranial motor nerve nuclei (to innervate the jaw, larynx, pharyngeal, and palatine muscles)
- This means that the left CBT for V will synapse on both the left and right CN V motor nuclei (and the right CBT for V will do the same)
- Axons destined for each pair of nuclei leave the tract immediately above the nuclei
- Because of the bilateral innervaiton pattern, unilateral UMN lesions of the CBT may not show clinical deficits in CN V, IX, X, and XI