Motor control and reflexes Flashcards
Where is the sensory input to alpha motor neurons from?
From muscle spindles – feedback from ipsi- and contralateral muscles
What initiates and controls voluntary movement in alpha motor neurons?
Upper motor neurons from the motor cortex and brain stem
What is the functions of interneurons in the spinal cord in alpha motor neurons?
Circuitry that generates spinal motor programs
What do muscles contain within them (sensory feedback)
Muscle spindles/stretch receptors (intrafusal fibres)
What are muscles innervated by at distal ends?
Gamma motor neurons
What wraps around centre and synapse on alpha motor neurons that innervate the muscle in the spinal cord?
1a sensory axons
What is the function of group I and II afferent axons?
Sensory - carry info to CNS
What occurs when muscle starts to stretch and muscle spindles are stretched?
Depolarisation of 1a sensory axons
What does depolarisation of 1a sensory axons occur via?
Via mechanosensitive ion channels
What occurs when 1a sensory axons deporarise?
Increases APs generated and synapse on alpha motor neurons thus increasing their AP frequency and causing muscle to contract to resist this change
What are the steps of the stretch reflex?
- Increased afferent to spinal cord
- Increased efferent output through alpha motor neurons
- Muscle contracts
- Firing rate of afferent sensory neuron decreases (negative feedback)
What do gamma motor neurons innervate?
Both ends of the muscle spindle
What do gamma motors neurons operate to do as muscle contracts?
To stimulate contractile portions of spindle to contract
What are the steps in muscle spindle innervation?
- Afferent input from sensory endings of muscle spindle fibre
- Alpha motor neuron output to regular skeletal muscle fibre
- Stretch reflex pathway
- Gamma motor neuron output to contractile end portions of spindle fibre
- Descending pathways coactivating alpha and gamma motor neurons
Unstretched muscle (action potentials)
Action potentials are generated at a constant rate in the associated sensory fibre.
Stretched muscle (action potentials)
Stretching activates the muscle spindle, increasing the rate of APs.
What does contraction of muscle cause? (muscle spindle)
Causes muscle spindle to slacken
What does activation of gamma-motor neurons
cause?
Causes contractile distal ends of spindle to contract
What do gamma-motor neurons ensure?
Ensures central ‘sensory’ portion remains taut
What would happen if only alpha motor neurons were activated?
Only the extrafusal muscle fibres contract. The muscle spindle becomes slack and no APs are fired. It is unable to signal further length changes.
What occurs when alpha-gamma coactivation occurs?
Both extrafusal and intrafusal muscle fibres contract. Tension is maintained in the muscle spindle and it can still signal changes in length.
What is the increase in AP frequency is proportional to?
Rate of stretch - detect velocity of muscle stretch
What is the resting discharge rate of firing of AP?
~10 / sec
What happens to AP when muscle spindle is stretched?
An increase in AP discharge
During very rapid stretch, how high can AP frequency rise?
As high as 500/sec
What can phasic receptors do?
Can rapidly adapt in response to stretch of muscle and muscle spindle
What are tonic receptors? (adaptation)
Only slow adaption in response to stretch of muscle and muscle spindle
What do secondary endings detect?
Detect absolute of muscle length
Why is proprioception from the Golgi tendon organs important?
For information on whole muscle tension – conscious level
How are muscle spindles physically related to muscle fibres?
Muscles spindles are parallel to muscle fibres
How are golgi tendon organs organised?
In series with muscle fibres
Where is the synapse with 1b sensory axons ?
On interneurons of ventral horn
What is the GTO primarily a sensor system for?
Tension
What type of fibres are golgi tendon organs? (and where)
Afferent (type 1b) fibres lying within the tendons
What do Golgi tendon organs respond to?
To alterations in muscle tension
What does muscle tension cause in tendon?
Causes tendon tightening and uplift to joint
What does increased tension in the tendon cause?
It stretches Golgi tendon organs
What does stretch intensity increase?
AP frequency
How are afferent signals delivered and interpreted?
centrally and interpreted consciously
What muscle spindles fire when muscle is stretched?
(1a) and GTO (1b) fire (GTO to a lesser extent)
What happens to muscle spindle activity during active contraction?
It decreases
Why does GTO become more active?
Due to tension created at either end of contracting muscle
What tendon involved knee jerk reflex?
Patellar Tendon
What does passive stretching of extensor, which stretches muscle spindles induce?
Induces contraction of extensor and inhibition of flexor
Extensor muscles -> (full)
Axon of sensory neuron (peripheral process) - ce;; body of sensory neuron - interneuron - cells bodies of motor neurons - axons of motor neurons - flexor muscles
What do alpha motor neurons receive input from?
- Primary sensory axons,
- Descending axons from brain
- Collaterals of lower motor
neuron axons.
What does the arrangement of alpha motor neurons allow?
Allow coordinated motor programs to be generated in response to many inputs
Where do collaterals of 1a sensory axons from muscles spindles synapse?
Via inhibitory interneurons on alpha motor neurons of antagonist muscle
Where do nociceptive axons terminate?
On excitatory or inhibitory interneurons in spinal cord
What is the function of ipsilateral side?
- Inhibits activity of extensor muscle
2. Stimulates contraction of flexor muscle
Why do nociceptor fibres stimulate extensors and cause relaxation of flexors?
- To withdraw limb on ipsilateral side
- To stabilise limb on contralateral side and ensure weight-bearing
- To ensure stabilisation of organism
What is the function of nociceptor fibres?
To stimulate extensors and cause relaxation of flexors
What does planning of movement require?
Input about current body position, visual and somatosensory information
Where is the motor cortex located?
On the frontal lobe, anterior to the central sulcus
What are the areas that are believed to be important in planning of movement?
Pre and supplementary motor areas
What are the areas of central control of movement?
- Cortex and basal ganglia (forebrain)
- Motor cortex and cerebellum
- Brain stem and spinal cord
What are the principal descending pathways?
- Corticospinal (pyramidal)
- Brainstem
return 29
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What are the three motor control systems?
- Spinal reflexes
- Corticospinal pyramidal motor system
- Extrapyramidal system
What is the role of spinal reflexes?
Posture adjustment, withdrawal reflexes
What is the role of the corticospinal pyramidal motor system
Fine, discrete voluntary movement
What is the role of the extrapyramidal system
Overall posture, involuntary maintenance
What is the effect of motor neuron damage?
Flaccid paralysis, no reflexes
What is the effect of cerebellum/basal nuclei damage?
Uncoordinated voluntary movement
What is the effect of damage to higher cortical command centres?
Inertia, inability to translate voluntary planning into motor activity
What does sensory information do?
It informs central input
What does central input do?
It varies
What does the motor unit output pattern affect?
Skeletal Muscle Activity
What does skeletal muscle activity determine?
Observable motor responses
What would alpha motor neuron activation alone cause?
A decrease in 1a sensory activity as muscle spindle would be slack; contraction would cease as muscle spindle would be flaccid
What would gamma motor neuron activation alone cause?
An increase in 1a sensory activity as muscle spindle (centre) is stretched; induces contraction of muscle
What is required to maintain muscle tone.
A balance between alpha and gamma motor neuron activation.
What does layer V of the brain surface receive input from?
From other cortical areas and thalamus
What do layer V of the brain surface send and where?
Send axon collaterals to subcortical sites involved in sensorimotor processing (brain stem)
What do layer V pyramidal cells project to?
To spinal cord
What is subcortical input from?
Basal ganglia
Where does basal ganglia receive input from?
Cerebral, prefrontal and parietal cortex
What does input from basal gangle initiate?
Activity of motor cortex in initiating movement
What is the basal ganglia composed of?
Caudate nucleus, putamen, globus pallidus, subthalamic nucleus and substantia nigra
When does initiation of movement begin?
When activation of SMA is boosted beyond some threshold
