lecture 12 Flashcards
The lower motorneuron
What is the bit that actually does the movement?
- skeletal muscles - many muscles each with compartments of possible control
- graded capacity
From what do skeletal muscles receive their innervation?
Excitatory neurons called motorneurons - sit in the spinal cord or brain stem
‘motorneuron pools’ because each muscle has its own pool of many neurons that innervate it.
What is a lower motorneuron?
- the neuron that sends its axon out and excites muscle
What is a lower motorneuron sign?
a positive (something that is happening that shouldn’t be) or negative (something that isn’t happening that should be) sign that is used in clinical medicine to diagnose a problem in the action of the lower motorneuron (could include a problem that affects the receptor on the muscle - technically not a problem with the lower motor neuron itself but a problem in the pathway)
What activates the lower motorneuron?
- whatever synaptic inputs they have that
- come from neurons within the spinal cord (local circuit neurons/interneurons)
- descending inputs: axons that come down from the brain through the spinal cord and synapse on the lower motorneuron
- also sensory inputs that act on local circuit neurons that act bypassing the brain
What is the neurotransmitter used by lower motorneurons?
- acetylcholine
Where do lower motorneurons begin?
- in the ventral part of the spinal cord
Where do descending inputs come from?
areas of the brain whose role is control of movement
brain stem
critical but don't act directly: cerebellum basal ganglia their role is in feedback with the brainstem and the cortex interact laterally
What does the statement that the motorneuron is the final common pathway mean?
- if you are going to have any affect on movement, if the muscle is going to do anything at all, it must ultimately come from signals through the motorneuron
What is something that can directly excite a lower motor neuron?
- sensory input e.g. muscle spindles
Can muscle be inhibited?
- no, there are no inhibitory neurons going to muscle, just these excitatory lower motor neurons
Where do lower motorneurons live?
- in the ventral horn of the grey matter
- exist in constellations or clusters related to the muscle they innervate
- nerves innervating a particular muscle will lie in a sort of column over many spinal segments
What’s the relationship to where a pool is to where a muscle is?
- proximal muscle pools are located more medially while distal muscle pools are located more laterally
- similarly for ventral/dorsal
- somatotopic
What do the interneurons indicate about muscles in the body?
- little neurons in the spinal cord
- ones that connect between the medial motorneurons (i.e. close to the midline) tend to be long, to lick up big spans i.e. medial muscles tend to be a lot more interconnected across the body than the neurons out laterally that control things like digits meaning the muscles don’t have a lot of interconnection
- this reflects the precision needed in distal muscles whereas midline muscles need coordinated activity to maintain things such as balance
- lateral muscles tend to be for voluntary dextrous skill movements and this is reflected beautifully in the interneurons
What is a motor unit?
An element of muscle control
- how many fibres in the muscle are activated by each individual motor neuron
- not one
- one to many relationship
- eyes maybe one to two or three
- big muscles maybe one to a hundred
- there will be a spectrum of motor unit sizes in one muscle
- the fibres will be distributed around the muscle