BIO 476 Exam 2

motor unit

single alpha motor neuron and the muscle fibers it innervates

function of motor units

Coordinate contraction of different muscle groups

location of motor neuron bodies

spinal cord ventral horn

motor neuron pool definition

all the motor neurons innervating a single muscle

alpha motor neurons

Large neurons that innervate extrafusal fibers to generate muscle contraction

gamma motor neurons

Small neurons that innervate intrafusal fibers in muscle spindles

lower motor neuron innervation of distal muscles

lateral ventral horn; regulate control of limbs

Lateral ventral horn much larger in the cervical and lumbar enlargement

lower motor neuron innervation of proximal muscles

medial ventral horn; involved in postural control

the size principle of motor unit recruitment

as input strength on alpha motor neurons increases, smaller motor neurons are recruited and fire action potentials before larger motor neurons

Slow motor units (S)

Small α-MNs innervate small, weak muscle fibers very resistant to fatigue are recruited by low amount of synaptic input

Important for postural control and standing

Fast fatigue-resistant motor units (FR)

Intermediate α-MNs innervate motor units with twice the force of slow fibers

somewhat resistant to fatigue

Recruited by moderate synaptic input

Fast fatigable motor units (FF)

Largest α-MNs innervate motor units with high force, but fatigue rapidly

Recruited by high levels of input

Important for running, jumping

inputs that lower MNs integrate

Supraspinal long-range neuronal projections, local intraspinal projections, and sensory projections

locations of supraspinal long-range neuronal projections

Motor cortex and brainstem

locations of local intraspinal projections

Interneurons within spinal cord

contralateral/commisural, intersegmental/propriospinal, and intrasegmental

locations of sensory projections

DRG sensory neurons; deep tendon and other reflexes

function of intrafusal fiber contraction

innervated by gamma MNs; keep muscle spindle tensed over wide range of muscle lengths

muscle spindle gain control

Allows spindle to accurately convey stretch information at any muscle length

alpha gamma coactivation

Most motor commands activate both alpha and gamma motor neurons

Central Pattern Generators

Spinal/brainstem networks of interconnected excitatory and inhibitory neurons that produce oscillating, rhythmic output

play an important part in breathing and repetitive locomotive responses

upper motor neurons

Glutamatergic pyramidal neuron bodies in layer 5 of M1; One of largest somal sizes in the brain

Send topographically organized axonal projections to subcortical sites

Upper MNs controlling proximal and axial muscles

Descend bilaterally in medial tracts, innervate multiple segments, and aid in posture and balance

Upper MNs controlling distal muscles

Descend contralaterally in lateral tracts, innervate few segments, and aid in voluntary, skilled movements

location of upper MN projections

descend through internal capsule, with collateral branches in striatum, midbrain, thalamus

corticobulbar tract

innervates targets in pons and medulla to control face movements

lateral corticospinal tract

most of corticospinal axons cross to the contralateral side at the medullary/pyramidal decussation

ventral corticospinal tract

some axons go down ipsilateral side

muscle field

2-4 muscles activated by single upper MN

Concurrent activation of “ensembles” of upper motor neurons encode behaviorally relevant coordinated movements

Upper MN lesions

Weakness; mild/no atrophy; hyperactive deep reflexes after initial period of spinal shock; spasticity; Babinski’s sign and clonus; widespread distribution of impairment in body regions; impairment of fine voluntary movements

lower MN lesions

Weakness or paralysis; severe atrophy; Hypoactive superficial and deep reflexes; initial signs and symptoms persist; fasciculations and fibrillations; geographic distribution of impairment; impairments of reflexive and gross and/or fine voluntary movements

Basal Ganglia Motor Functions

motor initiation; activated by decision to move; involved in starting selected motor programs; inhibits motor programs that aren’t selected

basal ganglia anatomy

Dorsal striatum, ventral striatum, globus pallidus, subthalamic nucleus (STN), and substantia nigra

components of dorsal striatum

Caudate and putamen

components of ventral striatum

Nucleus accumbens

components of globus pallidus

external segment (GPe) and internal segment (GPi)

components of substantia nigra

pars reticulata (SNr) and pars compacta (SNc)

BG input

Cortico-striatal projection and SNc

Cortico-striatal Projection of BG

from motor planning areas in frontal cortex; glutamatergic

SNc Projection of BG

dopaminergic

Medium Spiny Neurons

receive glutamatergic input on dendritic spines from cortical neurons and dopaminergic input from the SNc; send GABAergic projections to GPe, GPi, and SNr

BG Output

GP and SNr

GP projections

External segment to STN; internal segment to VA/VL thalamus

SNr projections

superior colliculus

BG body movement loop

Motor/premotor/somatosensory cortex → putamen → lateral GPi → Ventral lateral and ventral anterior nuclei

BG oculomotor loop

Posterior parietal/prefrontal cortex → caudate → GPi/SNr → Mediodorsal and ventral anterior nuclei

BG prefrontal loop

Dorsolateral PFC → Anterior caudate → GPi/SNr → Mediodorsal and ventral anterior nuclei

BG limbic loop

Amygdal/hippocampus/orbitofrontal/anterior cingulate/temporal cortex → ventral striatum → ventral pallidum → Mediodorsal nucleus

disinhibitory circuit of BG direct pathway

GP is tonically active

Activation of MSNs releases tonic inhibition of thalamus → upper MN activation and movement initiation

BG direct pathway movement facilitation

MSN → GPi → Thalamus → Cortex

facilitates movement initiation

indirect BG pathway

MSN → inhibits GPe → excites STN → excites GPi → inhibits thalamus → inhibits movement

function of indirect BG pathway

Antagonizes direct pathway; suppresses movement

focused selection hypothesis

Direct pathway promotes intended motor program initiation

Indirect pathway inhibits broad set of unintended motor programs

modulatory effect of D1

D1 receptors activate cAMP and enhance direct pathway activity

Modulatory effects of D2

D2 receptors decrease cAMP and inhibit indirect pathway activity

GPi function

inhibits thalamus

GPe function

inhibits GPi and STN

STN function

excites GPi

VA/VL complex of thalamus function

excites motor cortex, premotor cortex, and supplementary motor areas

direct pathway with dopamine input

glutamate from cerebral cortex and D1 excite MSNs → increased MSN activity → increased GPi inhibition → disinhibition of VA/VL → increased activity in VA/VL complex → increased frontal cortex activity

indirect pathway with dopamine input

D2 receptors inhibit MSNs → decreased GPe inhibition → decreased STN inhibition → decreased GPi activation → decreased thalamic inhibition → increased movement

hypokinetic direct and indirect pathways of BG

degenerated SN → decreased dopamine → increased GPe → increased STN inhibition → increased activation of GPi → increased thalamic inhibition → reduced excitation of frontal cortex

decreased inhibition of GPi by D1 → increased thalamic inhibition → reduced excitation of frontal cortex

hyperkinetic direct and indirect pathways of BG

MSNs of indirect pathway degenerate → decreased GPe inhibition → increased STN inhibition → increased GPi inhibition by GPe and decreased GPi activation by STN → decreased tonic inhibition of thalamus → increased excitation of frontal cortex

function of cerebellum

coordination on ongoing movement and modifies descending motor commands

cerebrocerebellum

guides fine motor control movements such as upper limb movements and fine motor control of hands

spinocerebellum

controls medial muscles, coordinating limb/trunk movements, adjusting posture, and error correction of movement

vestibulocerebellum

maintaining balance, spatial orientation, and visual tracking

contralateral cerebellar input

Pons, Inferior Olive, and Middle Cerebellar Peduncle

ipsilateral cerebellar input

Spinocerebellar Tract, accessory Cuneate Nucleus, and vestibular nuclei

cerebellar output to contralateral forebrain via cerebrocerebellum

Cerebrocerebellum → dentate nucleus → premotor cortex (motor planning)

cerebellar output to contralateral forebrain via spinocerebellum

Spino cerebellum → interposed and fastigial nuclei → motor cortex and brainstem (motor execution)

cerebellar output to contralateral forebrain via vestibulocerebellum

Vestibulocerebellum → Vestibular nuclei → Lower motor neurons in spinal cord and brainstem (balance and VOR)

cerebellar output to ipsilateral brainstem via superior colliculus

Cerebellar cortex → fastigial DCN → inferior cerebellar peduncle → Superior colliculus → Reticular formation → anterior medial white matter of spinal cord → lower motor neurons in medial ventral horn

cerebellar output to ipsilateral brainstem via reticular formation

Cerebellar cortex → fastigial DCN → inferior cerebellar peduncle → reticular formation → anterior medial white matter of spinal cord → lower motor neurons in medial ventral horn

cerebellar output to ipsilateral brainstem via vestibular nuclei

Cerebellar cortex → inferior cerebellar peduncle → vestibular nuclei → anterior medial white matter of spinal cord → lower motor neurons in medial ventral horn

cerebellar output to contralateral brainstem

contralateral cerebellar cortex → dentate DCN → superior cerebellar peduncle → crosses midline → Superior colliculus → Reticular formation → anterior medial white matter of spinal cord → lower motor neurons in medial ventral horn

purkinje cell

GABAergic; large soma and extensive spines; provides output from cerebellar cortex to deep nuclei

granule cell

Glutamatergic; small, many neuronal subtypes

mossy fiber

Afferent axons to cerebellum from all sources except for inferior olivary nuclei; enter via the inferior middle cerebellar peduncles

parallel fiber

bifurcated axons of cerebellar granule cells that extend along molecular layer of cerebellar cortex where they synapse on dendritic spines of Purkinje cells.

climbing fiber

Axons that originate in inferior olivary nuclei, ascend through inferior cerebellar peduncle, and make terminal arborizations that invest proximal dendritic trees of Purkinje cells

cerebellar mossy fiber input

Pontine nuclei send mossy fiber projections, Excitatory synapse on granule cells, Granule cell parallel fibers synapse on Purkinje cell (PC) dendritic spines, and trigger simple spikes

cerebellar climbing fiber input

Inferior olivary neurons send climbing fiber projections, excitatory synapse on PC dendrites, and trigger complex spikes that modify parallel fiber-PC efficacy. complex spikes first fire heavily then produce an AP at 1-2hz for short time

cerebellar loops as a comparator

If cortical inhibitory loop and deep excitatory loop activity balanced, then motor commands maintained

purkinje cell LTD

If error detected then climbing fibers trigger complex spikes, climbing fiber activity causes Ca2+ influx into Purkinje cell leading to LTD of parallel fiber synapses, and weakening of cortical inhibitory loop enhances DCN output, which is relayed to motor cortex to alter motor output

cerebellar ataxia

involves irregular movements that lack coordination; failure in correcting errors during a motor command

dermatome

DRG sensory neurons

where somatosensory reception begins

Meissner corpuscles

detects motion across skin; fast adapting

Merkel cells

detects points, edges, curves; slow adapting

Pacinian corpuscle

detects skin vibration; fast adapting; large receptive field

qualities of rapidly adapting receptors

qualities of slowly adapting receptors

qualities affecting sensory perception

mechanosensory transduction

Muscle Spindle

Golgi Tendon Organs

Ruffini Corpuscle

axon of gamma motor neuron

group Ia afferent axons