13 - Reflexes, Control of LMN, Drugs of Spasticity and Fatigue

Question 1

What is the muscle stretch reflex (with pathway description)?

Answer 1

Maintaining constant muscle length
Sensory ending of neuromuscular spindle -> Ia sensory neuron (in DRG) -> anterior horn motor cell body (glutamate) -> alpha motor neuron -> voluntary muscle

Question 2

What is the Golgi tendon organ reflex (with pathway description)?

Answer 2

Maintains constant muscle tension (if msc contracts, relaxes)
Golgi tendon organ -> Ib afferent -> inhibitory interneuron (glycine) -> alpha motor neuron -> muscle
(Note: discharge of potential causes inhibition of alpha motor neuron, causing relaxation)

Question 3

What neurotransmitter drives the muscle stretch reflex’s motor cell body?

Answer 3

glutamate

Question 4

What neurotransmitter drives the inhibitory interneuron in the Golgi tendon organ reflex?

Answer 4

glycine

Question 5

What is reciprocal innervation/inhibition?

Answer 5

Combination of the muscle stretch reflex and Golgi tendon organ reflex pathways, where the antagonistic muscles of an afferent volley are inhibited
Uses the same inhibitory interneuron (glycine) as the Golgi tendon organ reflex

Question 6

What is the vestibulo-spinal reflex?

Answer 6

When tilting the head to one side, ipsilateral extension of the body occurs to maintain upright posture

Question 7

What are the descending extrapyramidal pathways (definition only)?

Answer 7

Motor tracts that do not course through pyramids of the medulla

Question 8

Another name for the descending pyramidal system

Answer 8

Upper motor neuron (UMN)

Question 9

Describe the descending pyramidal system pathway

Answer 9

Starting at the precentral gyrus: primary motor cortex -> corona radiata (centrum semi-ovale) -> posterior limb of internal capsule -> crus cerebri (midbrain) -> basilar portion of pons -> medullary pyramid -> corticobulbar and corticospinal tracts

Question 10

Describe the corticobulbar tract

Answer 10

Starting at the precentral gyrus: PMC -> corona radiata -> posterior limb of internal capsule -> crus cerebri -> basilar part of pons -> medullary pyramids -> CN V, VII, IX, X, XI, XII
CNVII and trapezius msc: contralat innervation
CNXI (SCM): ipsilat innerv
CNV, IX, X: bilat innerv
CNXII: bilat innerv with preference of contralat innerv

Question 11

Describe the lateral corticospinal tract

Answer 11

Starting at the precentral gyrus: PMC -> corona radiata -> posterior limb of internal capsule -> crus cerebri -> basilar part of pons -> medullary pyramids -> pyramidal decussation -> gray matter of anterior horn -> alpha motor neurons (ventral)

Question 12

Characteristics of UMN lesions

Answer 12

Weakness (upper extrem: extensor weakness>flexors, lower: flexors>extensors, hypertonia with spasticity (may have “clasp-knife” phenomenon), increase in muscle stretch reflexes with possible clonus, Babinski sign, possible “triple flexion” response
Note: acute UMN may present intially with hypotonia and hyporeflexia with (sometimes) lack of Babinski; signs become expected after a couple of weeks

Question 13

Decorticate posture vs decerebrate posture

Answer 13

Decorticate posture: flexor posturing of upper extremities and extensor posturing of lower extremities; due to bilateral lesions of pyramidal/extrapyramidal fibers at or above rostral midbrain
Decerebrate posturing: extensor posturing of all 4 limbs; due to bilateral lesions of pyramidal/extrapyramidal fibers below rostral midbrain (usually)