Neurotransmission, UMN, LMN

Question 1

graded potentials vs action potentials

Answer 1

graded potentials: vary in size, excitatory or inhibitory, result of individual ion channels opening, decrease over time and space… summation of excitatory graded potentials can trigger action potential if threshold is reached

action potentials: all-or-nothing, result of voltage-gated ion channels opening, only excitatory, propagate at full amplitude along length of axon

Question 2

temporal summation vs spatial summation

Answer 2

temporal summation = 1 excitatory presynaptic neuron firing rapidly

spatial summation = 2+ excitatory presynaptic neurons firing

recall graded potentials can trigger action potential if threshold is reached

Question 3

what are the dietary substrates of each of the following NT?
a. acetylcholine
b. glutamate, GABA
c. NE, dopamine
d. serotonin

Answer 3

a. choline —> acetylcholine
b. glutamate —> glutamate, GABA
c. tyrosine —> NE, dopamine
d. tryptophan —> serotonin

Question 4

which type of neurotransmitters are NOT stored in vesicles

Answer 4

gaseous NT (NO, CO, H2S)

Question 5

what are the steps of NT exocytosis? (4)

Answer 5

1. vesicle docks
2. SNARE complexes form to pull membranes together
3. entering Ca2+ binds to synaptotagmin
4. Ca2+-bound synaptotagmin catalyzes membrane fusion by binding to SNAREs and plasma membrane

Question 6

tripartate synapses

Answer 6

concept in synaptic physiology based on the demonstration of the existence of bidirectional communication between astrocytes and neurons

astrocytes provide physical/nutritional support, release gliotransmitters (D-serine, ATP), sequester Na+/K+ ions to control concentrations, metabolize NT

Question 7

what type of ionotropic post-synaptic receptor responds to glutamate?

Answer 7

NMDA (excitatory): voltage and ligand gated Ca2+ ion channel, blocked by Mg2+ at resting potential

inhibited by hallucinogenic drugs PCP (“angle dust”) and ketamine —> hallucinations, resembles schizophrenia

Question 8

what type of ion channel are the post-synaptic GABA (y-aminobutyric acid) and glycine receptors?

Answer 8

ligand-gated Cl- channels

(major inhibitory NT in CNS)

Question 9

what is the role of dopamine in the following pathways?
a. mesolimbic pathway
b. nigrostriatal pathway
c. mesocortical pathway

Answer 9

a. mesolimbic pathway (cingulate cortex + nucleus accumbens + amygdala + hippocampus): reward (addiction), schizophrenia, depression

b. nigrostriatal pathway (putamen + caudate): control of movement, Parkinson’s

c. mesocortical pathway (prefrontal cortex): working memory, schizophrenia, ADHD

Question 10

____ are branches from the anterior and posterior spinal arteries that encircle the spinal cord

Answer 10

vasocorona

Question 11

what is the difference between the medullary and radicular arteries which reinforce the ASA and PSA in the spinal cord?

Answer 11

anterior and posterior radicular arteries arise from segmental arteries, branches of aorta or internal iliac artery at each spinal level to serve respective roots and ganglia

medullary arteries are extensions that penetrate dura to augment the spinal cord blood supply

Question 12

the _________ is the major artery at left T9-T11 that supplies the majority of lumbar and sacral spinal cord

Answer 12

great radicular artery of Adamkiewicz

note fracture dislocations of vertebra can interfere with blood supply, and occlusion of aorta during surgery can produce ischemic damage (—> paralysis)

Question 13

CMAP (in context of neuro)

Answer 13

compound action potential - measured in motor nerve conduction test, recorded over muscle

surface electrodes stimulate skin over nerve

Question 14

motor unit =

Answer 14

motor neuron + all the muscle fibers it innervates

Question 15

what are the steps of the stretch reflex circuit (3)?

Answer 15

1. muscle spindle is excited by stimulus of muscle stretch
2. Ia afferent sensory neuron makes direct (monosynaptic) excitatory synapse onto motor neurons of same muscle
3. motor neurons contract

note Ia afferent also synapses on Ia interneuron, which inhibits (relaxes) antagonist muscle

also note gamma-motor neurons function to increase excitability of muscle spindle (but do NOT cause fibers to contract)

finally, note the Ib inhibitory reflex is stimulated by muscle tension and causes muscle relaxation

Question 16

which nerve roots are being tested in the following reflexes?
a. biceps
b. brachioradialis
c. triceps
d. patellar
e. achilles

Answer 16

a. biceps: C5-6
b. brachioradialis: C5-6
c. triceps: C7-8
d. patellar: L3-4
e. achilles: S1

Question 17

what type of sensory afferent nerves respond to pain stimulus in the flexor withdrawal reflex?

Answer 17

A-delta afferents: cause ipsilateral limb to flex, contralateral limb to extend

(removes a limb from a painful stimulus)

Question 18

how is muscle weakness scored on physical exam?

Answer 18

0: No contraction
1: Muscle flicker, no movement
2: Movement, but not against gravity
3: Movement against gravity, not against resistance
4: Movement against some resistance
5: Normal

Question 19

what are muscle fibrillations?

Answer 19

Short-duration, spontaneous biphasic or triphasic potentials produced by single muscle fibers. These are indicative of a denervated muscle.

detected via EMG (electromyography)

Question 20

what are 2 signs of denervation when a muscle is a rest, as recorded by EMG (electromyography)?

Answer 20

1. fibrillation: Short-duration, spontaneous biphasic or triphasic potentials produced by single muscle fibers
2. positive sharp waves: sharp positive (downward) deflection followed by a slower upward deflection

These are thought to represent an unstable muscle fiber cell membrane. They occur in peripheral nerve injuries, axonal neuropathies, motor neuron disorders and some myopathies

Question 21

explain how muscle histology changes with chronic denervation

Answer 21

Normal: mosaic/checkerboard pattern of distribution of fiber types because single motor neurons spread through muscle.

Damage to motor neurons —> muscle atrophy, each affected fiber is the same type.

Atrophied, denervated fibers: reinnervated by axons from other motor neurons that supply adjacent fibers. This changes fiber types, resulting in more fiber type grouping

Question 22

what occurs in spinal muscle atrophy (SMA I-IV)? what is the cause?

Answer 22

group of diseases caused by degeneration of the anterior horns, due to mutation of survival motor neuron protein (SMN1) on chromosome 5

begins in infancy, progressive

Question 23

Werdnig-Hoffmann disease

Answer 23

“acute infantile spinal muscle atrophy”

most common type of spinal muscle atrophy (SMA, type I) with autosomal recessive pattern

degeneration of anterior horns —> weakness, muscle wasting (limbs, respiration, bulbar muscles)

Question 24

apraxia is caused by lesions in which areas of the cortex?

Answer 24

premotor or posterior parietal cortex

apraxia = difficulty in using body part to perform complex voluntary actions

Question 25

how are lower motor neurons somatotopically organized in the spinal cord?

Answer 25

LMN innervating extensors are more anterior, LMN innervating flexors are more posterior

medial pathway to medial ventral horn projecting to axial and proximal/postural motor neurons, bilateral

lateral pathways to lateral ventral horn projecting to limbs and distal motor neurons, unilateral

Question 26

what is the functional difference between the lateral and anterior corticospinal tracts?

Answer 26

lateral corticospinal tracts crossover at pyramidal decussation and innervate contralateral side distal muscles

anterior corticospinal tract (only 15% of fibers) remain ipsilateral and innervate proximal limbs and trunk muscles

Question 27

through which portion of the internal capsule does the corticospinal tract run? how is it somatotopically organized?

Answer 27

posterior limb, which is connected to anterior limb via genu

moving posteriorly from genu to posterior limb is face —> arm —> trunk —> leg innervation

Question 28

where does the corticospinal tract terminate?

Answer 28

terminates on interneurons and alpha motor neurons in ventral horn

Question 29

what is the functional difference between the lateral and medial vestibulospinal tracts?

Answer 29

lateral vestibulospinal tract - projects ipsilaterally to medial LMN to proximal muscles, projects to entire spinal cord, powerful facilitation to extensor/antigravity muscles of limbs and trunk

Medial vestibulospinal tract - only to cervical spinal cord, projects bilaterally to restore head position in response to acceleration (vestibulocervical reflex)

Question 30

what is the function of the reticulospinal tract?

Answer 30

2 bilateral pathways that begin in pontine and medullary reticular formation and extend entire spinal cord, part of medial motor systems

receive input from premotor cortex and innervate motor neurons to maintain posture and gait with changing locomotion

example of “feedforward” postural preparation for movements

Question 31

how do the reticulospinal tract and lateral vestibulospinal tract work together to help maintain posture during movement?

Answer 31

reticulospinal tract influences voluntary movement, reflex activity, muscle tone, producing an excitatory effect on flexor motor neurons

lateral vestibulospinal tract produces excitatory effect on extensor/anti-gravity motor neurons

both pathways are tonically active, influencing excitability of motor neurons

Question 32

what is the function of tectospinal tract?

Answer 32

originates in superior colliculus and crosses in the midbrain, extends to cervical spinal cord - part of medial motor system

Generates orienting movements of the head turn to visual or auditory stimuli, contributes to coordination of the eyes and head

Gives projections to CN3, 4, and 6 for eye movements in response to head movement or visual/auditory stimuli

Question 33

where does the rubrospinal tract originate? And what is its function?

Answer 33

originates in red nucleus of the midbrain, crosses in midbrain

travels alongside lateral corticospinal tract, but only extends to cervical spinal cord

facilitates flexor muscles > extensors, contributes to distal control of limbs (receives input from cerebellum)

Question 34

which of the following spinal cord tracts is NOT medial, but rather, runs laterally alongside the lateral corticospinal tract?
a. reticulospinal tract
b. vestibulospinal tract
c. rubrospinal tract
d. tectospinal tract
e. anterior corticospinal tract

Answer 34

c. rubrospinal tract: originates in red nucleus of midbrain, facilitates flexor muscles > extensors, contributes to distal control of limbs (receives input from cerebellum)

Question 35

describe 2 ways/characteristics in which hypertonia in UMN lesions presents?

Answer 35

1. velocity dependent: less resistance to slow movement compared to fast
2. clasp-knife response: initial resistance followed by inhibition of the muscle (possibly due to golgi-tendon response)

Question 36

what type of nerve injury does spreading of movements indicate?

Answer 36

spreading of movements = movement of one part of the body produces movements in other parts of the body (they may mirror)

sign of upper motor neuron syndrome (lesion)

may also see decreased ability to isolate a movement

Question 37

what is Hoffman’s Sign and what does it indicate?

Answer 37

hold the patient’s finger loosely and flick the fingernail downward, which will cause the finger to rebound into extension… positive test if the thumb flexes and adducts

sign of upper motor neuron syndrome, in which there is decreased ability to isolate movements

Question 38

how will the following parameters differ between UMN and LMN lesions?
a. weakness
b. atrophy
c. fasciculations
d. reflexes
e. tone

Answer 38

a. weakness - both
b. atrophy - LMN
c. fasciculations - LMN
d. reflexes - increased in UMN lesion, decreased in LMN lesion
e. tone - increased in UMN lesion, decreased in LMN lesion

Question 39

how do the following drugs treat spasticity due to UMN lesion?
a. baclofen
b. diazepam
c. gabapentin
d. botulinum toxin

Answer 39

a. Baclofen: GABA(B) agonist
b. diazepam: GABA(A) agonist

c. gabapentin: anti-epileptic, voltage-gated Ca2+ channel blocker

d. botulinum toxin: prevents ACh release from NMJ

Question 40

what type of diseases are leukodystrophies?

Answer 40

inherited (mostly autosomal recessive) dysmyelinating disease due to abnormal myelin synthesis or turnover

may involve lysosomal or peroxisomal enzymes

Question 41

what are the 3 ways in which cerebral palsy presents, and which is most common?

Answer 41

1. spastic (pyramidal): most common
2. dyskinetic (extrapyramidal)
3. ataxia (extrapyramidal)