Neuromotor System Flashcards
What are some functions of the skeletal muscle (‘effector organ of the neuromotor system’)?
- movement (including vocalisation and respiration)
- maintenance of posture
- stabilises joints
- generates heat
- venous return
- lymphatic drainage
- reproduction
- digestion
- excretion
What are some features of the neuromuscular synapse/junction?
- biggest chemical synapse in the body
- excitable synapse
- can undergo reinnervation
What is the process of generating action potentials?
- AP arrives at motor nerve terminal
- AP triggers opening of VGCCs, entry of Ca2+
- Ca2+ dependent release by exocytosis of ACh from synaptic vesicles (at active zones)
- Ach traverses the synaptic cleft to bind to its receptor a ligand gated ion channel
- This binding opens the ion channels to cause a large movement of Na+ in and a small movement of K+ out of the muscle cell
- The result is depolarisation. This sets up a local current flow between the depolarised post-synaptic membrane and adjacent membrane of muscle
- This local current flow opens voltage gated Na+ channels in the adjacent membrane
- Resulting entry of Na+ causes the resting membrane potential to rise - thus triggering generation of muscle AP
- ACh is subsequently destroyed by acetylcholinesterase (an enzyme located in the synaptic cleft)
How does rate modulation affect APs?
increase the frequency of muscle action potentials, which lead to increased muscle fibre tension
What is a motor unit?
1 motor neuron to the number of muscle fibres it innervates
What is a general rule for the size of motor units?
Larger motor units: more power, less control
Smaller motor units: less power, more control
What does motor unit recruitment mean?
the activation of additional motor units to accomplish an increase in the contractile strength (tension) in a whole muscle
What is Henneman’s size principle?
For a given synaptic input, motor neurons with the smallest cell bodies are activated first and large motor neurons are activated last
What are the three muscle fibre types, and what size neurons are they determined by?
Slow twitch (Myosin 1) - small motor neurons
Fast twitch Fatigue Resistant (myosin 2A) - medium sized motor neurons
Fast twitch Fatigable (Myosin 2B) - large sized motor neurons
What is the biggest determinator of muscle grouping?
neurological activity
What are the three levels of input control to motor neurons?
- Input of afferent (sensory) - reflexes
- Primary motor cortex (direct - hierarchical) - pyramidal
- Multi-neuronal motor system (indirect - parallel) - extra pyramidal
What information do sensory neurons bring from sensory receptors?
Muscle length - (muscle spindles)
Velocity of muscle contraction - muscle spindles
Muscle tension - Golgi tendon organs
Joint position - free nerve ending in joints
What are gamma motor neurons responsible for?
maintaining sensitivity of of intrafusal muscle fibres (spindle) during contraction (change in length, rate of change in length)
What does proprioception rely on?
- muscle spindles
- joint receptors
- golgi tendon organs
What are reflex movements controlled by?
Spinal cord
- cervical-brachial S.C. (head, neck, forelimb)
- lumbar S.C. (lower limb, axial muscles)
What are inter-neurons and what do they do?
- connection between afferent inputs
- function to co-ordinate the activities of muscle groups and their afferent inputs
What inputs and outputs does the brainstem receive and send?
Inputs (sensory):
- proprioception
- vestibular
- visuals
Outputs:
- to modify the reflexes without higher order processing
What are the four centres in the brainstem important for posture-movement?
- Vestibular Nuclei
- Reticular formation
- Superior colliculus
- Red nucleus
What does the vestibular nuclei do?
- receives afferent signals from vestibular apparatus regarding position of head (with regards to gravity)
What is reticular formation, and where does it output to?
- receives connection from pre-motor cortical regions and coordinates lower motor neuron pools by outputting to S.C motor neurons
What input does the superior colliculus receive?
input for sensorimotor integration (eyes, vestibular, proprioceptors)
What is the role of the red nucleus?
- plays role in motor learning
Why are posture adjustments needed?
- balance
- stabalise
- alignment of head and body with regards to gravity
- protective
What are the two mechanisms by which postural adjustments are achieved?
- Feedback (compensatory reflexes) - rapid, stereotyped and show a space-time organisation
- Feed forward (anticipatory) - pre-programmed experiences (unlike reflexes, scale of response refined with experience)
What are the functions, inputs and outputs of the cerebrocerebellum?
Functions:
- initiation
- planning of movements
- timing of movements
Inputs:
- cortical afferents
Outputs:
- dentate nucleus to motor and pre-motor cortices
What are the functions, inputs and outputs of the spinocerebellar a) vermis and b) intermediate part of hemisphere?
a) Vermis
Functions:
- control of axial and proximal muscles
- ongoing execution of movements
Input:
- auditory, visual and vestibular
Output:
- fastigial nucleus
b) Intermediate part of hemisphere
Functions:
- ongoing execution of movements
- control of distal muscles
Input:
- spinal afferent (distal parts)
Output:
- interposed nucleus (outputs via the reticular formation - to motor execution in the spinal cord)
What are the functions, inputs and outputs of the vestibulocerebellum?
Functions:
- axial control and vestibular reflexes (balance and eye movements
Input:
- vestibular apparatus
Output:
- later vestibular
What is the basal ganglia, and what is important for?
- collection of subcortical nuclei deep in white matter
- important for control of movement and learning and motivation
Which of the basal ganglia nuclei receive excitatory input from the motor cortex, thalamus and brainstem?
- caudate
- putamen
True or false: the input from the globus pallidus to the thalamus is inhibitory, thus exciting the cortex.
True!
Despite the basal ganglia containing many neurotransmitters (GABA, dopamine, acetylcholine) the input from the globus pallidus to the thalamus is inhibitory, thus producing the ‘brake’ hypothesis (smooth coordinate movement involves selective inhibition of motor output)
What are some unique features of the neuromuscular junction?
- biggest chemical synapse
- motor axons can successfully reinnervate muscle fibres at the same site
Explain the steps of an AP being propagated at a NMJ
- AP arrives at motor nerve terminal
- AP triggers the opening of voltage gated calcium channels
- Ca2+ dependent release by exocytosis of acetylcholine from synaptic vesicles - at active zones
- Acetylcholine transverses the synaptic cleft to bind to its receptor - a ligand gated ion channel
- This binding brings about the opening of this ion channel to cause a large movement of Na+ in and a small amount of K+ out of the muscle cell
- The result is depolarisation. This sets up a local current flow between the depolarised post-synaptic membrane and adjacent membrane of muscle
- This local current flow opens voltage gated Na+ channels in the adjacent membrane.
- The resulting entry of Na+ causes the resting membrane to rise from -70mV to -60mV, which triggers the generation of a muscle action potential
- Acetylcholine is subsequently destroyed by acetylcholinesterase, and enzyme located in the synaptic cleft
Describe Henneman’s size principle
For a given synaptic input, motor neurons with the smallest cell bodies are activated first and large motor neurons are activated last
What determines the size of a given synaptic input?
The frequency of APs that are delivered to the terminal ending of the pre-synaptic neurons onto the motor neuron, because as the frequency increases, large motor neurons are able to raise their resting membrane potential to threshold, thus firing off an AP.
Explain the mechanism behind why the size principle exists.
- smaller neurons have higher membrane resistance and require lower depolarising current to reach spike threshold
Define fibre type grouping.
- chronic denervation re-innervation events lead to type 1 fibre predominance: seen in MND and aging, where fast motor neurons die first/become more venerable with age
How does grouping of slow fibres occur?
- re-innervation of fast twitch fibres by a small motor neuron, converting the fibres to slow twitch
Compare and contrast lesions to LMNs and UMNs.
- both result in weakness or paralysis
- LMN lesions result in reduced/absent muscle tone, whereas UMN results in increased muscle tone (except acutely)
- LMN lesions result in lost muscle mass, whereas muscle mass is maintained in UMN lesions
- the myostatic reflexes are reduced/absent in LMN lesions, whereas they are increased (except acutely) in UMN lesions
- Babinski sign after UMN lesions
