motor systems Flashcards
skeletal muscle is innervated by what kind of neuron. Where do their cell bodies reside
alpha motor neurons (lower motor neurons) with cell bodies in ventral horn of spinal cord and brainstem cranial motor nuclei
functions of vestibular nucleus, resticular formation, superior colliculus and motor cortex
anticipatory motor control, rhythmic motor output such as locomotion, eye movements and voluntary motor control respectively
modulatory motor centers and functions
cerebellum acts as a feedback error correction circuit and computes short and long term corrections to these errors. Basal ganglia have roles in movement initiation and action selection as well as roles in motor learning, reinforcement and motivated behavior
somatotopic arrangement of alpha motor neurons
lower motor neurons are arranged somatotopically, with lateral musculature innervated by laterally situated motor neurons and medial muscles innervates by medially situated motor neurons.
What do the cervical and lumbar enlargements represent
enlarged motor neurons populatioins innervating the upper and lower limbs
What disease causes degeneration of alpha motor neurons
ALS
motor neuron pool
The population of α motor neurons that innervates the muscle fibers within a single muscle is called the ‘motor neuron pool’ for that muscle. The neurons of this neuron pool are within the spinal cord in rod like clusters running along the axis of the cord Each muscle fiber is innervated by only onemotor nueron, but each neuron can innervate multiple muscle fibers.
compare the motor units for large vs smal alpha motor neurons
small: innervates small number of muscle fibers, generating motor unit that forms small forces. Large: innervates a large number of muscle fibers forming a motor unit that generates large forces. Gradients btw small and large also exist
Size principle
Systematic recruitment of smaller motor units before larger motor units. This occurs b/c small neurons have high input resistance due to fewer membrane channels. A given synaptic current will induce a relatively larger voltage change in a small motor neuron compared to a large motor neuron, thus the small motor neuron will be brought to threshold faster, with less synaptic input
tonic muscle fibers
non spiking muscle fiber that shortens extremely slow and efficiently generates isometric tension with low fatigability. Found in spindles and extraocular muscles
slow twitch fibers
Have a high conc of myoglbin and mitochondria which aid in slow fatiguability
fast twitch oxidative fibers
Activate quickly, Have many mitochondria and fatigue moderately slowly
Fast twitch glycolytic fibers
activate quickly and fatigue rapidly due to few mitochondria and dependence on anaerobic glycolysis ATP generation
describe slow motor units
small motor neuron, small number of slow twitch muscle fibers, generates small forces, fatigue slowly, recruited first due to high input resistance, recruited during sustained activities such as maintaining posture
describe fast fatigue-resistant motor units
Intermediate sized motor neuron, intermediate number of fast twitch oxidative muscle fibers, generates large forces, fatigue slowly, recruited second
describe fast fatigable motor units
Large motor neurons, large number of fast twitch glycolytic muscle fibers, generate large forces, fatigue quickly, recruited last, recruited during running and jumping
What determines a muscles phenotypes (ie. Fast twitch vs slow) and can it be changed
The motor neuron activity governs muscle phenotype. Can be altered with chronic stimulation or exercise
What are muscle spindles
A special type of sensory receptor, called a proprioceptor, that is embedded within a muscle and detects muscle stretch. They are composed of specialized muscle fibes called intrafusal muscle fibers, which run in parallel with the main extrafusal muscle fibers.
How do muscle spindles signal stretch
through group 1a and group II sensory afferents (large, fast axons) which fire AP’s in response to mechanical stretch of the intrafusal muscle fibers.
- What is the function of gamma motor neurons?
Innervate intrafusal muscle fibers of the spindle. During voluntary contraction, gamma motor neurons fire with alpha motor neurons, shortening the extrafusal (normal skeletal muscle) and intrafusal (spindle) fibers together. This ensures that when the muscle is contracted it can still detect stretch
What are golgi tendon organs
Another type of proprioceptor- collagen structures that reside at the junction of a muscle and a tendon and signal via type Ib sensory afferents that wind around and within the collagen strands
- Compare and contrast muscle spindles and Golgi tendon organs. Include parallel versus series arrangement, the type of information they transmit (force vs. length), and the type of innervation they make on motoneurons.
Golgi tendon organs: situated in series with muscle and tendon, preferentially signal muscle tension during muscle contraction, type Ib sensory afferents, maintains muscle force. Spindles: situated in parallel with muscle, preferentially signal passive stretch which lengthens the muscle before the tendon, type Ia sensory afferents, maintains muscle length
- Describe the key elements of the stretch reflex. Include the sensory ending, the sensory neuron, and the motor neuron.
Muscle is stretched > 1a sensory neuros sense the stretch in the spindle > sensory info relayed to populations of alph motor neuron in spinal cord > alpha motor neuron contracts the stretched muscle (i.e. the “homonymous” or “synergist” muscle).
In addition to the motor neurons innervating the muscle in which the spindle resides (homonymous motor neurons), name two other sets of neurons innervated by the afferent endings in a stretch reflex
- inhibitory interneurons 2. excitatory interneurons
Reciprocal innervation
Type Ia afferents in stretch reflex directly contact inhibitory interneurons within the spinal cord that inhibit motor neurons controlling the antagonist muscle. This causes relaxation of the antagonist muscle at exactly the same time as the synergist muscle contracts.
flexor extensor coupling
Ib afferents from the GTOs directly contact inhibitory and excitatory interneurons in spinal cord. This protects muscles from over exertion by relaxing the synergist (homonymous) muscle and contracting th antagonist.
Normal function of the stretch reflex
Maintains muscle tone- resistance of muscle to stretch.
Causes of hypotonia and hypertonia
hypotonia: damage to the Ia sensory afferents innvervating the spindles or the alpha motor neurons innervating the muscle. Hypertonia: damage to descending motor pathways that influence the spinal cord premotor circuits
Basal firing rate of Ia afferents
Even when the spindle is flaccid, there is a basal rate of firing. So, the Ia can signal both stretch and shortening by increasing or decreasing firing rate, respectively. This allows for maintenance of tone
How do our muscles correct for errors
Via co-activation of alpha and gamma motor neurons. Gamma fibers are activated with voluntary movements, so mismatches btw expected and actual muscle stretch can be detected rapidly and used to correct errors in motor output (ie. Anticipating that an item will be heavy, but lifting it and finding out it is actually not heave). If the spindles are overcontracted relative to the muscle fiber, there is a drop in Ia afferent firing which rapidly reduces alpha motor neuron drive and reduces muscle contraction.
Are gamma motor neurons activated during reflexive contractions
No. 1a sensory afferents directly contact alpha, not gamma, motor neurons
crossed extension reflex
Cutaneous sensory receptors (such as nociceptors) detect pain > innervate spinal interneurons > coordinate extensor relaxation/ flexor contraction on the side of the stimulus, and extensor contraction/ flexor relaxation on contralateral side. (ie. If you step on a tack, you shift weight to alternate leg while lifting the pricked foot)
- What are central pattern generators and where are they located?
Circuits that generate complex coordinated movements like locomotion (coordinated extension and flexion) and swimming. Sensory and central input are not necessary for CPGs. Inhibitory interneurons are involved, such that when the extensor is activated, the flexor is not activated and vice versa
Describe the “clock” of the central pattern generators
The clock (or rhythm generator) innervates and drives interneuronal networks that amplify the clock signal and distribute it appropriately to coordinate muscle contraction and relaxation. Descending pathways can affect clock rate and motor patterns
How do central pattern generators for both limbs interact
They are linked by commissural fibers which coordinate between limb use
descending Pathways of the vestibular nucleus
- via the medial vestibulospinal tract to the medial spinal cord where they regulate head orientation and neck muscle activation; and 2. via the lateral vestibulospinal tract to the lateral motor pools controlling proximal limb musculature.
Ascending pathways of the vestibular nucleus
Projections go to cranial nuclei III (oculomotor), IV (trochlear) and VI (abducens) to regulate eye movements
Vestibular ocular reflex
produces eye movements that counter head movements to keep gaze fixed. Vestibular nuclei project bilaterally to abducens nuclei > abducens nuclei cross again > On the side contralateral to the activated vestibular neurons, its axons excite motor neurons that contract the lateral rectus of the contralateral eye and the medial rectus of the ipsilateral eye. On the side ipsilateral to the activated vestibular nucleus, its axons inhibit motor neurons that control the medial rectus muscle of the contralateral eye and relax the lateral rectus muscle of the ipsilateral eye
Functions of reticular formation and location
Reticular formation is in the midbrain tegmentum. Functions: Modulatory (cardiovascular control, respiratory control, sensorimotor reflexes, eye movement coordination, sleep wake regulation) and premotor functions
How does reticular formation participate in movements
- Mesencephalic locomotor region sends fibers to reticular formation to control locomotor speed. 2. Anticipatory responses to voluntary movement that help maintain postural control/balance (ie. Flexing leg muscles in anticipation of a change in center of mass while lifting a weight with biceps)
Function of superior colliculus
aka tectum. Computes a map merging auditory and visual space. Descending projections of the colliculuspinal tract target motor neurons in the axial muscles of neck to generate coordinated orienting responses. Ie. If siren goes off, you orient your gaze towards the stimulus
motor cortex function and pathway
voluntary movement. Primary motor cortex (precentral gyrus, BA 4) > internal capsule > cerebral peduncle (ventral surface of midbrain) > become pyramids in medulla > cross midline at caudal medulla then form lateral corticospinal tract which innervates alpha motor neurons controlling hand/finger movement and interneurons that control mootor neurons in ventral horns OR uncrossed fibers form ventral corticospinal tract which innervates motor neurons that control axial and proximal limb muscles
motor maps
map of motor fields of human motor cortex showing organized movements (not muscles) as M1 microzones A disproportionate M1 surface is devoted to fine motor tasks.
Premotor cortex
Just anterior to primary motor cortex, makes up 25% of corticospinal tract. Involved when movement is initiated by an external cue (such as being asked to move your hand to a light stimulus). Also has mirror neuron activity where neural responses are triggered in response to watching an action (ie. Jims premotor cortical neurons may be responsive to watching Amy reach for an apple. These same neurons are active if Jim reaches for an apple)
Supplementary motor cortex
involved in self cued movements. Activated during mental rehearsal of movement.
motor cortex plasticity
If stroke damages part of motor cortex, adjacent areas to the damaged region will sprout new connections to subserve motor control over the affected body part. Partial to complete receovery is possible. Practic can also expand a region of the cortex, for example in musicians or blind people reading braille.
Brain machine interfaces
robotic devices that use electrical activity in motor cortex to control robotic limbs. 1. neural activity recorded from motor cortex by implanted electrodes. 2. Neural recording extracts the motor plan of the neuron. 3. This info is used to control physical entity such as computer cursor or robotic limb
constraint induced movement therapy
enhances recovery after stroke- use of the limb most damaged by a stroke exclusively can lead to enlargement of that areas representation in primary motor cortex
upper motor neuron symptoms
contralateral flaccidity, weakness, spasticity (increased muscle tone, hyperactive deep reflexes, clonus or involunatry movements), babinski sign, loss of voluntary movements
lower motor neuron symptoms
paralysis, weakness, loss of deep reflexes, decreased muscle tone, muscle atrophy, fasciculations
function of reticulospinal tract
anticipatory motor control, locomotor initiation
function of vestibulospinal tract
unconscious responses to vestibular stimulation
function of colliculospinal tract (aka tectospinal)
orientation of gaze and stance to a stimulus in space
