Neurotransmission, UMN, LMN Flashcards
graded potentials vs action potentials
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
temporal summation vs spatial summation
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
what are the dietary substrates of each of the following NT?
a. acetylcholine
b. glutamate, GABA
c. NE, dopamine
d. serotonin
a. choline —> acetylcholine
b. glutamate —> glutamate, GABA
c. tyrosine —> NE, dopamine
d. tryptophan —> serotonin
which type of neurotransmitters are NOT stored in vesicles
gaseous NT (NO, CO, H2S)
what are the steps of NT exocytosis? (4)
- vesicle docks
- SNARE complexes form to pull membranes together
- entering Ca2+ binds to synaptotagmin
- Ca2+-bound synaptotagmin catalyzes membrane fusion by binding to SNAREs and plasma membrane
tripartate synapses
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
what type of ionotropic post-synaptic receptor responds to glutamate?
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
what type of ion channel are the post-synaptic GABA (y-aminobutyric acid) and glycine receptors?
ligand-gated Cl- channels
(major inhibitory NT in CNS)
what is the role of dopamine in the following pathways?
a. mesolimbic pathway
b. nigrostriatal pathway
c. mesocortical pathway
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
____ are branches from the anterior and posterior spinal arteries that encircle the spinal cord
vasocorona
what is the difference between the medullary and radicular arteries which reinforce the ASA and PSA in the spinal cord?
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
the _________ is the major artery at left T9-T11 that supplies the majority of lumbar and sacral spinal cord
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)
CMAP (in context of neuro)
compound action potential - measured in motor nerve conduction test, recorded over muscle
surface electrodes stimulate skin over nerve
motor unit =
motor neuron + all the muscle fibers it innervates
what are the steps of the stretch reflex circuit (3)?
- muscle spindle is excited by stimulus of muscle stretch
- Ia afferent sensory neuron makes direct (monosynaptic) excitatory synapse onto motor neurons of same muscle
- 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
which nerve roots are being tested in the following reflexes?
a. biceps
b. brachioradialis
c. triceps
d. patellar
e. achilles
a. biceps: C5-6
b. brachioradialis: C5-6
c. triceps: C7-8
d. patellar: L3-4
e. achilles: S1
what type of sensory afferent nerves respond to pain stimulus in the flexor withdrawal reflex?
A-delta afferents: cause ipsilateral limb to flex, contralateral limb to extend
(removes a limb from a painful stimulus)
how is muscle weakness scored on physical exam?
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
what are muscle fibrillations?
Short-duration, spontaneous biphasic or triphasic potentials produced by single muscle fibers. These are indicative of a denervated muscle.
detected via EMG (electromyography)
what are 2 signs of denervation when a muscle is a rest, as recorded by EMG (electromyography)?
- fibrillation: Short-duration, spontaneous biphasic or triphasic potentials produced by single muscle fibers
- 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
explain how muscle histology changes with chronic denervation
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
what occurs in spinal muscle atrophy (SMA I-IV)? what is the cause?
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
Werdnig-Hoffmann disease
“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)
apraxia is caused by lesions in which areas of the cortex?
premotor or posterior parietal cortex
apraxia = difficulty in using body part to perform complex voluntary actions
how are lower motor neurons somatotopically organized in the spinal cord?
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
what is the functional difference between the lateral and anterior corticospinal tracts?
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
through which portion of the internal capsule does the corticospinal tract run? how is it somatotopically organized?
posterior limb, which is connected to anterior limb via genu
moving posteriorly from genu to posterior limb is face —> arm —> trunk —> leg innervation
where does the corticospinal tract terminate?
terminates on interneurons and alpha motor neurons in ventral horn
what is the functional difference between the lateral and medial vestibulospinal tracts?
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)
what is the function of the reticulospinal tract?
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
how do the reticulospinal tract and lateral vestibulospinal tract work together to help maintain posture during movement?
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
what is the function of tectospinal tract?
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
where does the rubrospinal tract originate? And what is its function?
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)
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
c. rubrospinal tract: originates in red nucleus of midbrain, facilitates flexor muscles > extensors, contributes to distal control of limbs (receives input from cerebellum)
describe 2 ways/characteristics in which hypertonia in UMN lesions presents?
- velocity dependent: less resistance to slow movement compared to fast
- clasp-knife response: initial resistance followed by inhibition of the muscle (possibly due to golgi-tendon response)
what type of nerve injury does spreading of movements indicate?
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
what is Hoffman’s Sign and what does it indicate?
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
how will the following parameters differ between UMN and LMN lesions?
a. weakness
b. atrophy
c. fasciculations
d. reflexes
e. tone
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
how do the following drugs treat spasticity due to UMN lesion?
a. baclofen
b. diazepam
c. gabapentin
d. botulinum toxin
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
what type of diseases are leukodystrophies?
inherited (mostly autosomal recessive) dysmyelinating disease due to abnormal myelin synthesis or turnover
may involve lysosomal or peroxisomal enzymes
what are the 3 ways in which cerebral palsy presents, and which is most common?
- spastic (pyramidal): most common
- dyskinetic (extrapyramidal)
- ataxia (extrapyramidal)
