Lower Motor Neurons Flashcards
Lower motor neurons
- coming from the motor cortex, neurons project through the corticospinal tract and synapse within the spinal cord and project the secondary neuron out of the spinal cord
- synapses in the grey area of the spinal cord (cell body neurons)
- lower motor neurons can have their cell bodies in different levels of the nervous system (coming out form the spinal cord, or coming out from the brain stem)
Nervous control of movement
- electrical stimulation of muscles, and the nerves that supply muscles causes muscular contraction
- this demonstrates the electrical basis for control of movement by nervous system
- external electrical stimulation can be used as a therapeutic tool to manage conditions such as drop foot
drop foot
condition that happens after strokes, where people will be paralysed on one side of the body
people relearn to walk, but a complication is that you cannot stimulate the muscles in the front of your leg
electrical stimulating can be used to overcome footdrop
pads are placed on the front of the anterior muscle of the leg, than the electrical stimulation passed through those causes your foot to rise up when you walk
brain computer inferface implanted in the brain
- closed loop control over movement
- the person thinks about moving their arm, and the thought turns into a signal that causes their arm to contract
nervous control of movement
- electrical stimulation of muscles, and the nerves that supply muscles causes muscular contraction
- demonstrates the electrical basis for control of movement by nervous system
Neuromuscular junction
Action potential is able to propagate along neurons
Gates along the myelin sheath allow for the electrical signal to flow along the inside
Axon and the terminal part of the axon –> the neuromuscular junction
In the end of the cell - vesicles contain a neurotransmitter (acetylcholine)
as the action potential propagates along the axon, it reaches the neuromuscular junction, the axon terminal depolarises, and this allows for calcium from the outside to go in –> this allows the vesicles to bind to the membrane, allowing the acetylcholine to diffuse across the membrane and binds to the receptors on the other side, allowing for depolarisation of the muscle fibre on the other side
what encourages the contraction of the muscle?
depolarisation of the sarcolemma encourages calcium to flow in and initiate contraction
the process of contraction
calcium attaching to the troponin-tropomyosin complex
actin filament
myosin filament: myosin bridges across the opposing actin filaments, and the binding of the myosin and flexing of the myosin is what causes the actin filaments to pull together
calcium fluxing in allows for the movement of the dark blue helix, exposes the binding side for the myosin to adhere to the actin filament
the sarcolemma reduces its length, muscle contraction
What is the neuromuscular junction? What is the neurotransmitter?
Junction: the synapse formed between an alpha motor neuron axon and a muscle fibre
ACh is the neuromuscular junction transmitter
Release of ACh produces a large endplate potential (which opens Ca channels)
Calcium entry triggers myosin-actin interaction, which shortens muscle fibre
Development of muscle tension
- SINGLE MUSCLE TWITCHES: at relatively slow stimulation freq, the force drops right back to resting state
- TEMPORAL SUMMATION: if we start to increase the rate at which we stimulate the muscle, we see that the muscle is unable to fully relax and go back to resting state, therefore summation of force
- UNFUSED TETANUS: increase it even more, we get sustained contraction, where there is almost no drop off of force before the next simulation comes
- FUSED TETANUS: rapid build up of force and the force remains constant and smooth
Process of muscle tension development
Myoplasmic Ca concentration creates this
Apply a single electrical stimulation, you get a single twitch and an influx of calcium which is rapidly removed and pumped into cytoplasmic reticulum
At higher forces, the mechanisms in the cell that reduce the calcium concentration stays high over the entire duration of stimulus
The motor unit
Any alpha motor neuron combined with all of those muscle fibres it supplies
Dealing with different force demands
- different sized motor units reflect a tradeoff between force and precise control
- there is a further division of motor unit types based on their relative fatigability
Types of motor units
Each axon can form synapses with several muscle fibres (forming a motor unit)
The precision of the muscle is related to motor unit size.
Small: precise movements of hte hand (e.g. fingers 1:<10)
Large: movements of the leg (e.g. 1:>300)
3 types of motor unit
Fast fatigable
Fast fatigue resistent
Slow
Types of motor units, force and fatigue
Rate of ofrce development for fast fatigable is very high, whereas the fast fatigue resistant and slow units produce a much smaller force in the same amount of time.
In terms of time, slow motor units can be much more sustained.
Medial gastrocnemius muscle under different behavioural conditions
muscles inclined for postural tasks have higher concentrations of slow fibres, whereas muscles that produce power and explosive actions have higher concentrations of fast fatigable fibres
recruitment: the first 25% of the motor neuron recruitment for any given task are slow fibres - while they reflect 25% of motor pool, they produce only 5% of the force
motor unit plasticity
motor units can change over time through training
Turning on muscle controls
The impetus for contraction comes from different places:
- Can come from voluntary commands descending form upper motor neurons
- local circuits that control stereotypical movement patterns are able to sustain themselves (CPGs)
- low level activations - afferent inputs that synapse directly or indirectly with interneurons onto alpha motor neurons (example being the knee jerk reflex)
Stretch reflex circuitry
- a monosynaptic reflex that provides negative feedback for changes in muscle length
- is mediated by teh muscle spindle which is a stretch receptor
- both rapid changes in length and sustained stretch are transduced by the muscle spindle
- ## 1a afferents relay stretch info back to spinal cord
gamma motor neurons
- gain in the stretch reflex is controlled by tension in the extrafusal muscle fibre
- tension in the intrafusal muscle fibre is controlled by gamma motor neuron activity
golgi tendon organs
- transduce tension within the musculotendinous unit
- provides negative feedback, acting to reduce tension by inhibiting alpha motor neuron activity
- important for reducing risk of damage due to high load
Muscle spindle is only sensitive to changes in length or stretch, not to changes in force
negative feedback loop
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