15 - Motor Units And Excitation-contraction Coupling Flashcards
Pathway from stimulus to response
Stimulus -> sensory neurons -> interneurons interneurons -> motor neurons -> effectors -> response
T/F — stimulus can be internal/external and response can be detected internally/externally
True
T/F - spinal cord arranged same as cortex w/ grey matter outside and white matter on inside
FALSE — arranged inside out to cortex
What is white matter of spinal cord?
Myelinated axons
Descending axons head…
Away from brain (ascending head towards)
Does ventral or dorsal root carry sensory info into spinal cord?
Dorsal (ventral carries motor info out)
Lower motor neurons
- involved in all movements
- directly innervate muscle
- cell bodies in spinal cord
Mixed spinal nerve
Combines axons of motor and sensory fibres and synapses into single muscle fibre
What neurotransmitter is involved in muscle contraction?
Acetylcholine (ACh)
How many muscle fibres does individual motor neuron innervate?
One or more
Motor unit
1 alpha motor neuron + muscle fibres it innervates
Small motor units involve….
<10 muscle fibres
Large motor units involve…
> 1000 muscle fibres
Motor neuron pool
Collection of alpha motor neurons that innervates a single muscle
Following each successive AP, there’s an increase in…
Force within muscle fibres
As freq of APs increases ________ occurs
Force summation
Unfused tetanus
Oscillations in summed force generation
Fused tetanus
Smooth force summation
Small motor unit
Motor neuron w/ small cell body that innervates few muscle fibres
Large motor unit
Motor neuron with large cell body that innervates many muscle fibres
To generate graded forces…
Small motor units (small forces) recruited 1st, followed by progressively larger motor units (larger forces)
What takes care of grading motor recruitment automatically?
Cell body size
Small motor neurons have a ___________ membrane resistance and reach threshold _______ easily
Higher, more easily
An incoming spike causes a ________ change in mempot in a smaller neuron
Larger (because V = IR, so when R⬆️, V⬆️)
T/F larger motor neurons reach spiking threshold more easily
False - smaller
Inverse relationship between no. Motor units in a muscle and …
Their force generating capacity
Many small motor units needed to generate small amount of force whereas
Fewer larger motor units needed to generate large amount of force
Types of muscle
Cardiac, skeletal and smooth
Skeletal muscle
- striated and under voluntary control
- each cell has multiple peripheral nuclei
- orderly structure -> contractions in one direction
Cardiac muscle
- striated and under involuntary control
- single central nucleus per cell
- branching and unordered striations -> contraction in many directions
Smooth muscle
- no striations and under involuntary control
- single nucleus per cell
Functional structure of skeletal muscle (hierarchy)
Whole muscle -> muscle fibre (single muscle cell) -> myofibril -> myofilaments
Excitation
- AP depolarises axon terminal of motor neuron
- Ca2+ influx in pre-synaptic neuron through Cav channels
- ACh release, which binds to post-synaptic ACh receptors
- Na+ entry through ion channels post-synaptically
- Nav channels open
- AP propagates along post-synaptic plasma membrane
NMJ has probability of vesicle release of…
1, so AP leads to guaranteed neurotransmitter release
AP propagates along muscle fibre plasma membrane and down…
Transverse tubules
T-tubules are…
Electrically continuous w/ surface of plasma membrane
Dihydropyridine receptor
Voltage-gated Ca2+ channel that opens in response to AP
Both dihydropyridine and ryanodine receptors are
Physically coupled ca2+ channels
AP ________cytosolic Ca2+ levels because…
Increases, because Ca2+ released from SR
Contraction — cross-bridge cycling steps
- Myosin head initially unbound but energised
- Actin binding site blocked by tropomyosin
- Ca2+ binds to troponin, moving tropomyosin
- Myosin cross-bridges bind to actin
- Power stroke -> myosin head rotates (+ release of ADP and Pi)
- ATP binding to myosin breaks actin-myosin linkage
- Bound ATP split, re-energising myosin head
Hydrolysis of ATP by myosin energises cross-bridges by providing energy for
Force generation
ATP binding to myosin dissociates…
Cross-bridges bound to actin, allowing repeated cycling
What’s the role of the Ca2+-ATPase?
Hydrolyses ATP and actively transports Ca2+ back into SR
Relaxation
- Ca2+-ATPase actively sequesters Ca2+ back into SR
2. Ca2+ removal leads to covering of myosin binding sites by troponin, ceasing cross-bridge cycle
