Motor systems Flashcards
what ion leak channel stabilises the membrane potential in the musculature
chloride
what neural structures are involved in the control of movement
cerebellum basal ganglia motor cortex brainstem centres local circuit neurons lower motor neurons
without stimulus what are the characteristics of the muscle
muscle is relaxed, high membrane conductane and negative membrane potential (-70 to -90mV)
muscles are sitmulated to contract by what
cholinergic (ACh) neuronal inputs at neuro-muscular junction (nmj)
muscles are stimulated to contract by what
cholinergic (ACh) neuronal inputs at neuro-muscular junction (nmj)
there is no direct inhibition of muscle so how are they inhibited
inhibition of motor neurons in the spinal cord (glycinergic)
skeletal muscle force is controlled by
rate of stimulation of motor units
number of motor units stimulated
properties of motor units stimulated
what are lower motor neurons
neurons located in the ventral horn of the spinal cord - direct innervation of muscle - occur in pools driving different muscles - topographically organised (eg proximal to distal)
how can motor pools be visualised and defined
by retrograde labeling of motor neurons after tracer injections into individual muscles
or injection of antibodies against transcription factors expressed in motor neurons
what makes up one motor unit
ONE α–motor neurone PLUS all its muscle fibres
each muscle fibre is only innervated by,,,,
….one alpha-motor neuron
ONE α–motor neurone can innervate…
…many muscle fibres
at low frequencies of stimulation each action potential in the motor neurons results in…
a single twitch of the related muscle fibres
at high frequency stimulation what happens in the electrical profile of the muscle
the twitches sum to produce a force greater than that produced by single twitches
what is unfused tetanus
high frequency stimulation of the muscles causes a greater force (than individual twitches) to be produced but a partial relaxation of the muscle fibers still occurs between the twitches.
what is fused tetanus
highest rates of motor neuron activation where the individual twitches are no longer apparent and the force produced is great
forces of muscle contraction increases with _________
stimulation rate
what is temporal summation of muscle contraction
Rapidly repeated stimuli lead to increased force as muscle does not fully relax between stimuli leading to smooth sustained contraction
how quick are the reflex responses in myelination axons
80-120m/sec
types of motor units differ in what characteristics
in response, forces and fatiguability
myosin heavy chain subtypes
what are the three main types of motor unit and what are the differences
type1 – slow twitch/oxidative /high myoglobin / darker red/ fatigue resistant
type2A – intermediate/ fast fatigue resistant
type2X - fast twitch /glycolytic/low myoglobin / pale/ fast fatigable
which motor units are recruited under which basic behavioural conditions
Slow (S) motor units provide the tension required for standing. Fast fatigue-resistant (FR) units provide the additional force needed for walking and running. Fast fatigable (FF) units are recruited for the most strenuous activities, running and jumping
how is more voluntary forces produced in muscles
spatial summation - recruitment of parallel motor units is progressively increased
what neural structures are involved in spinal reflexes
local circuit neurons and lower motor neurons
what can induce stereotypical spinal reflex activity
sensory inputs to the same level of the spinal cord as the motor neurons
what are the main two mechanosensory pathways and what info do they carry
the dorsal column-medial lemniscus pathway - mechanosensory info from the posterior third of the head and rest of the body
the trigeminal pathway - mechanosensory info from the face
what is the purpose of the knee-jerk reflex
to help maintain an upright posture in the face of unexpected changes.
describe a simple reflex circuit (eg knee jerk)
Stimulation of peripheral sensors (a muscle stretch receptor in the knee jerk) initiates receptor potentials that trigger action potentials that travel centrally along the afferent axons of the sensory neurons.
This information stimulates spinal motor neurons by means of synaptic contacts.
The action potentials triggered by the synaptic potential in motor neurons travel peripherally in efferent axons, giving rise to muscle contraction and a behavioral response.
how many muscle spindles do the gastronemius and vastus lateralis have respectively
gastrocnemius has ~150 spindes
vastus lateralis ~450
what are muscle spindles
proprioceptors that consist of intrafusal muscle fibers enclosed in a sheath (spindle)
stretch receptors that signal the length and changes in length of muscles. They lie within an independent capsule, parallel to the main muscle.
what are intrafusal muscle fibres, what are the two types
skeletal muscle fibers that serve as specialized sensory organs (proprioceptors). They detect the amount and rate of change in length of a muscle. They constitute the muscle spindle, and are innervated by both sensory (afferent) and motor (efferent) fibers
- nuclear chain fibres
- nucelar bag fibres
what two types of sensory nerve endings are associated with muscle spindles
- flower spray endings (at ends)
- Annulospiral endings (in center)
what are extrafusal muscle fibres
skeletal muscle fibers that are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement - comprise the bulk of muscle and form the major force-generating structure
which motor neurons innervate which muscle fibres
alpha (α) motor neurone axon from cell bodies in Ventral Horn to EXTRAFUSAL muscle fibres
gamma (γ) motor neurone to INTRAFUSAL muscle fibres controls region of sensitivity
how are each of the 1a primary and 2 secondary afferent involved in sensory feedback from the muscle spindle
Ia afferents from bag fibres: AP output sensitive to rate of change of length (phasic)
II afferents come from chain fibres: sensitive to length. (tonic)
how do gamma efferent control sensitivity of muscles
via stimulation of intrafusal fibres
what are the relative frequencies of action potentials in the sensory, motor (extensor and flexor) and inter neurons after a stretch reflex pathway is activated
sensory neurons receive an immediate increase that fades
motor neurons of extensors have no change then up their action potential just before the leg extends similarly to interneurons - due to activation of excitatory synapses
motor neurons of flexors increase APs momentarily then dont have any until leg extends due to activation of inhibitory synapses
local circuits produce what type of reflexes
rapid but steroetypes reflexes
what are the inputs and outputs of spinal reflexes
Sensory information INPUT via Dorsal Root Ganglia
OUTPUT from motor neurones in Ventral Horn
Sensory information INPUT and Motor OUTPUT via α-motor neurones at same spinal cord level
what does the hard wired synaptic excitation of inhibitory interneurons do (myotactic reflexes)
reduces ability of the oppposing motor neuron to be activated (in knee jerk the flexor)
what is the clasp knife reflex
If tension in a muscle becomes so great that damage is likely to occur the motoneurones are suddenly inhibited, causing the muscle to relax to protect the muscle from further damage
how is the clasp knife reflex produced
by activation of the Golgi tendon organs which are located in tendons at the ends of the muscle -in series with the contractile elements. These activate 1b inhibitory’ interneurones in the cord via sensory 1b fibres to suppress flring of the motoneurones activating the muscle.
what is the crossed extensor reflex and what is the associated circuitry
a reflex in which the contralateral limb compensates for loss of support when the ipsilateral limb withdraws from painful stimulus in a withdrawal reflex.
sensory input: A delta sensory fibre via DRG
excitatory and inhbitroy interneurons
motor output: motor neuron to withdraw one leg and extend the other
what are central pattern generators
- spinal cord circuitry that underlies patterns of locomotion
neuronal circuits that when activated can produce rhythmic motor patterns such as walking, breathing, flying, and swimming in the absence of sensory or descending inputs that carry specific timing information.
what targetted neurotechnology restores walking in humans with a spinal injury (2018 research)
Using an implanted pulse generator with real-time triggering capabilities, they delivered trains of spatially selective stimulation to the lumbosacral spinal cord with timing that coincided with the intended movement - able to reestablish adaptive control and partially regain voluntary control
where are upper motor neurons located
in the cerebral cortex and brainstem
what brain structures have descending control of patterns of locomotion
BRAIN STEM CENTRES: (midbrain, pons, medulla) Multiple Sub-Cortical systems produce the postural control and co-ordinated patterns of locomotor activity
BASAL GANGLIA and CEREBELLUM dynamically modulate Cortical and Brain Stem centre activity
CORTEX: limited direct affect on lower motor neurone activity for axial and proximal muscle activity but control ‘intentional’ activity & fine control of distal muscles (eg. fingers)
what structure controls intentional activity and fine control of distal muscles
the cortex
what are the tagrets of the basal ganglia relay the modulatory motor effects
VA/VL complex - The ventral anterior and ventral lateral thalamic nuclei (VA/VL complex) are the targets of the basal ganglia, relaying the modulatory effects of the basal ganglia to upper motor neurons in the cortex.
what does the basal ganglia do in motor control
controls the intiation of movement
what structures are involved in the basal ganglia pathway of movement initiation
substantia nigra, globulus pallidus, putamen, caudate nucelus and the subthalmic nucelus
what structure decides to initiate movement
the cortex
what is the order of structures involved in maintaining stationary position
cerebral cortex -> striatum -> globus pallidus -> thalamus -> back to cerebral cortex - releases tonic inhibition (indirect pathway)
what is the direct pathway to the intiation of movement
cortex signal activate striatum -> strial GABAergic medium spiny neurons transiently inhibit the globus pallidus -> this transient inhibition of GP temporarily disinhibits the thalamus -> active thalmus can now singal to the cortex
what does the substantia do in motor control
substantia nigra dopaminergic activity excites the direct pathway and inhibits the indirect pathway from the cortex
what is the direct pathway (basal ganglia)
Releases tonic inhibition - The direct pathway starts from the cortex and projects to the striatum (caudate nucleus and putamen) with excitatory glutamatergic (glu) neurons. The neurons from the striatum, which are inhibitory GABAergic, send their axons to the medial (internal) globus pallidus and substantia nigra, pars reticulata (SNr). These send signals to the thalamus which send excitatory pathways to the cortex (prefrontal, premotor and supplementary cortex) where they affect the planning of the movement by synapsing with the neurons of the corticospinal and corticobulbar tracts in the brainstem and spinal cord.
what does the indirect pathway do (basal ganglia)
increases inhibition of upper motor neurons - globus pallidus is chornically active and inhibiting the thalamus
what happens to structures sin the brain in Parkinsons disease
reduced excitatory drive from thalamus leads to reduced motor cortex activity and and hypokinetics
Globus pallidus chronically inhibits thalamus and reduces excitatory dirve to cortex
Loss of striatal GABAergic output prevents switching off (transient inhibition) of the chronically active globus pallidus (internal)
Loss of DA neurons of substantia nigra - leads to dysregulation of striatal GABAergic output from medium spiny neurons
It is as if all of the motor programs stored in cortex are constantly inhibited by the indirect pathway, with not enough excitation of the direct pathway for the desired motor program to become activated.
parkinsons disease is characterised by what
the loss of dopaminergic neurons in the substantia nigra pars compacta.
what 3 three ways can you treat parkinsons disease and how does this impact the brain chemistry
L-Dopa - supplements dopaminergic function (these signals turn the direct pathway on )
Pallidotomy - reduce the globus pallidus inhibitory output to the thalamus
Deep brain stimulation - stimulation to modulate pathways
what happens to the structures in the basal ganglia pathways with Huntingtons disease
Loss of GABAergic Medium Spiny N. of Striatum
Reduces inhibition of Globus Pallidus (external segment)
Prevents switching-off of Globus Pallidus (external seg) so chronic inhibition of subthalamic nuc.
REDUCES subthalamic excitation of GP internal segment - less inhibition of Thalamus
ALLOWS increased Thalamic excitation of Cortex
Problems with STOPPING movements (chorea/hyperkinetic)
what does the cerebellum do in terms of motor function
integration of sensory feedback into coordinated movement
what are the major structures of the cerebellar system
the cerebellar cortex, the deep cerebellar nuclei, and the ventroanterior and ventrolateral (VA/VL) complex
what are the effects of cerebellar dysfunction
balance problems and gait disorders along with difficulties in coordination resulting in ataxia, uncoordinated movements, imbalance, speech problems(dysarthria), visual problems (nystagmus) and vertigo
what is a pallidotomy
surgical procedure which involves destroying a tiny area in a part of the brain called the Globus Pallidus interna
- offered to patients with parkinsons disease to alleviate some symptoms
what infomration is being inputted to and outputted by the cerebellum
Inputs:
Intentional actions from Cortex
Actual actions from periphery
Output:
Correction signals into brain stem motor centres
what pathways signal to the cerebellum for comparison of intention to actual movement
Cortical projections to the cerebellum are made via relay neurons in the pons. These axons then cross the midline within the pons and run to the cerebellum via the middle cerebellar peduncle. Axons from the inferior olive, spinal cord, and vestibular nuclei enter via the inferior cerebellar peduncle.
what is the output pathway of the cerebellum to the cerebral cortex
The axons of the deep cerebellar nuclei cross in the midbrain in the decussation of the superior cerebellar peduncle before reaching the thalamus then passing to the primary motor and premotor nucelus
what neurons input ‘actual’ sensory information (actual actions from theperiphery) to the cerebellum
Mossy Fibre Input from pontine nuclei (cerebral cortex), spinal cord, vestibular system
Climbing Fibre input from inferior olive
what neurons input ‘intentional’ information (intentional actions from cortex) to the cerebellum
INPUT onto the Purkinje cell from local circuit neurons (basket and stellate cells) and other Purkinje cells
what neurons carry the output ‘correction’ information from the cerebral cortex
OUTPUT of Purkinje cells to deep cerebellar nuclei sends “correction” signal to Brain stem centres and motor cortex
where do primary motor cortex axons terminate
among pools of local circuit neurons in the ventral horn of the spinal cord
what do upper motor neurons in the Reticulospinal pathway in the brain stem control
axial and proximal muscles controlling posture and balance –
what is the corticospinal tract
the major neuronal pathway providing voluntary motor function. This tract connects the cortex to the spinal cord to enable movement of the distal extremities.
describe the pathway of each the lateral and ventral corticospinal tract
Neurons in the motor cortex give rise to axons that travel through the internal capsule and coalesce on the ventral surface of the midbrain, within the cerebral peduncle. These axons continue through the pons and come to lie on the ventral surface of the medulla, giving rise to the pyramids.
Most of these pyramidal fibers cross in the caudal part of the medulla to form the lateral corticospinal tract in the spinal cord.
Those axons that do not cross descend on the same side and form the ventral corticospinal tract
what are the medullary pyrimids
paired white matter structures of the brainstem’s medulla oblongata that contain motor fibers of the corticospinal and corticobulbar tracts
what is the corticobulbar tract
part of the pyramidal tract - a two-neuron path which unites the cerebral cortex with the cranial nerve nuclei in the brainstem involved in motor function - carries upper motor neuron input to motor nuclei of trigeminal, facial, glossopharyngeal, vagus, accessory, and hypoglossal nerves
what does the lateral cortico spinal tract do
– Fine control of DISTAL limb muscles
what does th eventral/anterior corticospinal tract do
innervates medial aspect of ventral horn to control AXIAL (core) muscles and PROXIMAL limb muscles
where do the upper motor neurons cross to the opposite side of the spinal cord
in the spinomedullary junction
what are upper motor neuron disease characterised by
change in muscle tone
increased deep tendon reflexes (hyperreflexia)
loss of voluntary control
what are lower moton neuron diseases characterised by
areflexia (loss of relfexes)
flaccidity
severe atrophy of the muscle (due to loss of trophic factors coming form the lower neurons)
brainstem activity is dynamically modulated by what
the brain stem and cerebellum
