Flashcards in NEURO: MOTOR CONTROL Deck (44)
how is a skeletal muscle attached to bone?
what is a skeletal muscle comprised of?
fasciculi are comprised of several muscle fibres which are made of myofibrils - contain actin and myosin
what is a motor unit?
an alpha motor neuron and all the extrafusal skeletal muscle fibres it innervates
where are the muscle fibres in a single motor unit located?
they are all in the same muscle - may be spread out
what is the relationship between the number of muscle fibres and variation of movement?
less muscle fibres = greater variation of movement
what is the effect of more motor neurons firing?
more power is produced
what do more laterally located alpha motor neurons control?
more distal muscles
what do more medially located alpha motor neurons control?
more proximal muscles
what is muscle tone?
the degree of contraction of a muscle or proportion of motor units that are active at any one time
what does high muscle tone feel like?
firm or rigid
it resists passive stretch
what does low muscle tone feel like?
soft or flaccid
it offers little resistance to passive stretch
how does alpha motor neurons contribute to resistance to muscle stretch when relaxed?
it does not make a significant contribution
how does alpha motor neurons contribute to resistance to muscle stretch when alert?
there is more activation of alpha motor neurons occur and muscle tone increases
what is the motor end plate?
the region of the muscle fibre plasma membranes directly under the axon terminal
what is the neuromuscular junction?
it is the junction of an axon terminal with the motor end plate
what is an end plate potential (EPP)?
it is comparable to EPSPs but is larger than a single EPSP
occurs at neuromuscular junctions
what is an end plate potential in comparison to an EPSP?
a single EPP is larger than a single EPSP as the neurotransmitter is released over a larger surface area- therefore more ion channels open
how are action potentials propagated at a neuromuscular junction?
the action potential is propagated over the surface of the muscle fibre and into the T-tubules to spread throughout the muscle
the neuromuscular junction is typically located in the middle of a muscle fibre so action potentials are propagated in both directions
what happens at a neuromuscular junction?
1. action potential in motor neuron arrives at the axon terminal and depolarises into axon terminal
2. voltage gated Ca2+ channels open and Ca2+ diffuse into axon terminal
3. Ca2+ binds to proteins that allow ACh vesicles to fuse with neuronal plasma membrane
4. ACh diffuses to motor end plate and binds to cholinergic nicotinic receptors
5. this opens ion channel in receptor protein resulting in more Na+ moving in than K+ moving out - an EPP is produced
are neuromuscular junctions excitatory or inhibitory?
they are all excitatory only
which neurotransmitter is present in neuromuscular junctions?
what are upper motor neurons?
they are neurons whose cell boy originates in the cerebral cortex or brainstem and terminates within the brainstem or spinal cord
they exert their effects via lower motor neurons
which neurotransmitter is typically involved in upper motor neurons?
what happens if there is damage to an upper motor neuron?
it causes weakness or paralysis of movement for the group of muscles it innervates
what are the signs of upper motor neuron damage?
- hypertonia - high muscle tone
- hyperreflexia - brisk reflexes
- spasticity - muscle is tight
- positive babinski sign
- clonus - muscle suddenly stretched and stays there
what is the positive babinski sin?
the large toe extends (instead of flexes) in response to a blunt object stroked on the plantar surface
what is a common cause of upper motor neuron damage?
a stroke - when it affects the cerebral cortex of internal capsule
what are lower motor neurons?
multipolar neurons which connect with upper motor neurons and the skeletal muscle it innervates
the cell body lies within the ventral horn of the spinal cord or the brainstem motor nuclei
how many muscle fibres can be innervated by one lower motor neurm?
one lower motor neuron can innervate several muscle fibres but a single muscle fibre is innervated by only on LMN