Chapter 19 - Control of Movement

The ____ takes in _____, processes, and produces output

nervous system; sensory information

All externally observable behavior is the result of what?

effector activation

What are some effectors?

skeletal muscles

smooth muscles

glands

chromatophores

What does behavior =?

muscle contractions

What is a spinal reflex?

a simple, graded, automatic response to a specific sensory stimulus

What mediates spinal reflexes and where?

mediated by neural circuits w/in the spinal cord

Is conscious involvement/brain input needed for spinal reflexes?

NO

What is afferent?

carrying signals TOWARDS CNS (sensory!)

What is efferent?

carrying signals AWAY from CNS (motor!)

What are interneurons?

a neuron that stays entirely w/in the CNS, connecting sensory & motor neurons

What is divergence?

one presynaptic neuron contacts many postsynaptic neurons

What is convergence?

one postsynaptic neuron receives input from many presynaptic neurons

What is reciprocity?

muscles are arranged in antagonist pairs (flexor vs extensor) & any motor signal that activates one group will inhibit the opposing group at the same time

Which neurons enforce reciprocity?

inhibitory interneurons

The primary input to spinal motor neurons is what?

descending from the brain, making sensory reflex input secondary

Are reflexes independent loops?

NO, they’re embedded in larger system

What is the main function of reflexes?

to provide feedback that corrects/modulates movements commanded by brain

What is the stretch reflex?

a muscle is stretched by an external force, load, or tap on tendon.

What are muscle spindles?

specialized receptor organs that monitor muscle length

What are contained in muscle spindles?

intrafusal muscle fibers, which are NOT the working fibers

What are the working fibers called?

extrafusal fibers

What is the central region of the intrafusal fiber wrapped with?

1a afferent sensory neurons

What is the path of the stretch reflex?

muscle stretched —→ spindle stretched —-→ 1a afferent fires

Where does the 1a axon fire onto?

alpha motor neurons of the same muscle

The pathway of alpha motor neurons firing:

alpha motor neuron fires —→ extrafusal fibers contract —→ muscle shortens —→ stretch relieved

Where does the 1a axon enter the spinal cord?

via dorsal root

What is happening at the same time as the stretch?

1a axon excites inhibitory interneurons to stop the alpha motor neurons of the antagonist muscle

What does the flexion reflex do?

protects you from painful/noxious stimuli

What are flexion-reflex afferents?

sensory neurons with endings in skin, muscle, & joints

what is REQUIRED for flexion-reflex afferents?

at least one layer of interneurons needed to connect to motor neurons

Flexion reflex pathway:

interneurons excite flexor motor neurons of stimmed limb —→ flexors contract —-→ limb pulled away

What happens at the same time as flexion reflex occurs?

inhibitory interneurons suppress extensor motor neurons of same limb —→ extensors relax —→ limb lifts & withdraws

What is the crossed extension reflex?

partner to flexion reflex, it allows the opposite limb to hold position/take body weight

Where do flexion-reflex afferents synapse for the crossed extension reflex?

onto interneurons that cross the MIDLINE of the spinal cord

What is the gamma motor system?

The ability for the brain to actively adjust its sensitivity by sending motor commands to the spindle itself

What problem does the gamma motor system solve?

when a muscle contract & shortens, the spindle would normally go slack and not sense any stretch, and the system prevents that.

What two types of motor neurons are used in the spinal cord?

alpha motor neurons: innervate extrafusal fibers; produce force/movement

gamma motor neurons: innervate contractile ends of intrafusal fibers; do NOT produce force/movement

Pathway of gamma activation:

gamma motor neuron fires —→ contractile ends of intrafusal fiber contract —→ central sensory region is stretched —→ 1a afferent fires

What is gain control?

when the spindle stays sensitive throughout the movement

What does alpha activation do?

decreases spindle firing (unloads spindle)

What does gamma activation do?

increases spindle firing (loads spindle)

What is alpha-gamma coactivation?

its every voluntary command from the brain simultaneously activating both alpha and gamma motor neurons; either correction is needed or not needed (picking up a box that WAS light vs THOUGHT to be light, actually heavy)

What is chained-reflex hypothesis?

when each movement triggers a sensory signal that reflexively triggers the next movement; aka peripheral control hypothesis

What is an example of chained-reflex hypothesis?

locust about to take flight

What is a central pattern generator (CPG)?

when a motor pattern is generated by a CNS circuit, not by a chain of reflexes

How is the brain involved in movement?

it initiates voluntary movements, plans/programs complex sequences, modulates/corrects ongoing movements, initiates locomotion via command signals

How is the spinal cord involved in movement?

contains CPGs for rhythmic locomotion, integrates descending commands w/ sensory feedback, & is final output to motor neurons

How is sensory input involved in movement?

it provides feedback about body position, movement errors, & external conditions, and can modulate both brain commands and spinal circuits

What is the primary motor cortex?

the main output station for voluntary movement - a map of the body

Where is the primary cortex located?

just anterior to the central sulcus (the groove that separates it from the somatosensory cortex)

What does the frontal cortex anterior to it form?

the premotor region

What is the output of the motor cortex neurons?

large pyramidal cells

What does the motor cortex also activate?

the brainstem motor nuclei via the corticobulbar tract

What does the cerebellum NOT do?

it does NOT initiate movement

What does the cerebellum do?

compares what the brain intended to do with the body, vs what the body is actually doing and corrects any differences

What happens if there’s damage to the cerebellum?

movements = clumsy, disordered, imprecise (ataxia)

movements accompanied by tremors

What are the 3 parts of the cerebellum?

vestibulocerebellum (posterior)

spinocerebellum (medial)

cerebrocerebellum (lateral)

What does the vestibulocerebellum handle?

interacts with the vestibular system & controls balance and eye movements

what does the spinocerebellum handle?

coordination of ongoing limb movement, receiving sensory feedback & motor commands, correcting movements in real time

what does the cerebrocerebellum handle?

involved in motor planning & sequencing, participates in cognitive functions

What are mossy fibers?

they bring information from the spinal cord, vestibular nuclei, & cerebral cortex; each mossy fiber diverges widely to excite granule cells

What are climbing fibers?

from the inferior olive, each makes a 1:1 excitatory synapse with a Purkinje cell, carries error signals

What do granule cells do?

their axons become parallel fibers —→ excite Purkinje cells

What do Purkinje cells do?

the only out of the cerebellar cortex —→ inhibitory —→ inhibit the deep cerebellar nuclei

What are the deep cerebellar nuclei?

the final output of the cerebellum —→ project to motor cortex, brainstem motor nuclei

How is a movement corrected?

the error occurs —→ climbing fiber fires —→ weakens parallel fiber connection that was active —→ Purkinje cell output changes —→ correction made

What are basal ganglia?

they select which movement to make and suppress all competing movements; they DO NOT execute movement

How do basal ganglia work?

primarily through inhibition, and most of their action involves disinhibiting a chosen movement rather than directly commanding it

What is the striatum?

it receives excitatory input from broad areas of the cerebral cortex

Basal ganglia direct pathway:

cortex —→ excites striatum —→ striatum inhibits GPi —→ GPi can’t inhibit thalamus —→ thalamus excites motor cortex —→ movement occurs

Basal ganglia indirect pathway:

cortex —→ excites striatum —→ striatum inhibits GPe —→ GPe can’t inhibit GPi —→ GPi stays active —→ thalamus remains suppressed —→ competing movements = prevented

Parkinson’s disease

Direct pathway = weakened, leading to less GPi inhibition, causing motor cortex to be less able to generate movements

Indirect pathway = strengthened, GPe suppressed more, GPi even more active = even less movement

The result of Parkinson’s:

akinesia - inability to initiate movements

Huntington’s disease:

caused by loss of striatal neurons, specifically the neurons of the indirect pathway (weakened, GPe not inhibited, GPi more inhibited = excess movement)

Results of Huntington’s:

hyperkinesia - excess, uncontrolled movements

chorea - involuntary jerky, coordinated movements

athetosis - slow writhing movements

Parkinsons’s vs Huntington’s

Parkinson’s = too much basal ganglia inhibition

Huntington’s = too little basal ganglia inhibition

Motor cortex involvement:

initiated voluntary movement; specifies direction, force, timing

premotor/SMA involvement:

plan & sequence complex movements; encode intention

Basal ganglia involvement:

select appropriate motor programs; suppress competing ones

cerebellum involvement:

coordinate timing, precision, error correction

brainstem centers involvement:

posture, tone, orienting movements, locomotor drive

spinal cord involvement:

final integrator of descending commands and reflexes