explain the primary motor cortex
- primary motor cortex contains cell bodies of upper motor neurons
- upper motor neurons are part of CNS
- axons project down through brainstem and spinal cord to innervate lower motor neurons
- lower motor neurons are part of peripheral nervous system project to and innervate skeletal muscle
describe the organization of the somatic motor division of the peripheral nervous system
- peripheral pathway consists of 1 lower motor neuron
-> neuron may be long and originates in the brain (cranial nerve nuclei) or spinal cord (cell bodies in the ventral horn) - lower motor neurons innervate skeletal muscles
- axon collaterals branch out at their distal end (near the muscle targets)
- each collateral will synapse with 1 muscle fiber
explain the anatomy of the neuromuscular junction
- neuromuscular junction consists of axon terminals, motor end plates on the muscle membrane, and schwann cell sheaths
- somatic motor neuron branches at its distal end
- Schwann cells secrete chemical signals required for formation and maintenance of NMJ
- mitochondrion
- the motor end plate is specialized region of muscle membrane that contains high concentration of nicotinic ACh receptors
- the membrane surface has many invaginations to increase the surface area
explain synaptic transmission at the neuromuscular junction
- motor neuron carries an action potential (electrical signal) down its axon to the axon terminal at the neuromuscular junction.
- lies very close to the motor end plate
- causes voltage gated Ca2+ channels to open and influx of Ca2+ ions
- increase in Ca2+ triggers synaptic vesicles filled with ACh to fuse with presynaptic membrane and release ACh neurotransmitters via exocytosis into the synaptic cleft
- post synaptic membrane is modified into a motor end plate, ACh binds to nicotinic receptors (ligand gated ion channels)
- ion channel opens Na+ goes in K+ leaves = depolarization
- ACh is metabolized by acetylcholinesterase
explain how the nicotinic ACh receptor is non selective cation channel
- skeletal muscle nAChR are different from those found in autonomic nervous system but general function is the same
- when the channel open Na+ and Ca2+ flow in, K+ flows out
- net cation influx results in depolarizing end plate potential
- neuromuscular transmission is always excitatory
what is the organization of skeletal muscle and muscle terminology
skeletal muscle -> muscle fascicle (bundle of fibers) -> muscle fiber
muscle cell = muscle fiber
cell membrane = sarcolemma
cytoplasm = sarcoplasm
modified ER = sarcoplasmic reticulum
what are the components of the muscle fiber ultrastructure
- SR
- nucleus
- T tubules
- sarcolemma
- mitochondria
- myofibril
- thick and thin filament
what is the function of T tubules and sarcoplasmic reticulum
- t tubules are extensions of the cell membrane (sarcolemma) that associate with the ends (terminal cisternae) of the sarcoplasmic reticulum
- t tubule brings action potentials into interior of muscle fiber (into the triad)
explain the components of the myofibril ultrastructure
- take up the bulk of intracellular space >1000/ muscle fiber
- contain the contractile elements of a muscle fiber (myosin and actin)
- other proteins within myofibrils
-> regulatory proteins (troponin and tropomyosin)
-> giant accessory proteins (titin and nebulin) - sarcomere = a unit of myofibril between 2 Z disks
- I band (only actin) shortens
- H band (only myosin) shortens or disappears
- A band (myosin and actin) doesn’t change size
explain how skeletal muscle fibers contain “organized sarcomeres”
- fibers are large, multinucleate cells that appear striped or striated under the microscope
- can visualize striations under a light microscope
- > these reflect the alternations between A bands a I bands arranges within a sarcomere
explain the sarcomere anatomy (myosin)
myosin = motor protein that creates movement
- myosin molecule = 2 intertwined protein chains
-> stiff rod like tail, two mobile heads
- each myosin head has one actin binding site and one ATP binding site
- thick filament = 250 myosin molecules (500 myosin heads)
explain the sarcomere anatomy (actin)
- one actin molecule = globular G actin
- polymerize to form F actin (filamentous)
- each G actin has a single myosin binding site
- thin filaments formed by two chains of F actin intertwining like a double strand of beads
what is troponin
- trimeric protein complex that binds to tropomyosin and has 2 Ca2+ binding sites
what is tropomyosin
- long protein dimer complex that attaches to actin, covering up the myosin binding sites so that myosin can not securely attach
what is the function of titin and nebulin
- titin spans the distance from one Z disk to the neighboring M line
-> elastic molecule, returns stretch muscle to resting length - provides elasticity and stabilizes myosin
- nebulin lies along the thin filaments, attaches to a Z disk but does not reach M line
-> inelastic molecule - helps align actin
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