lecture 16- somatic motor system and skeletal muscle Flashcards
primary motor cortex
contains cell bodies of upper motor neurons
innervation of lower motor neurons in the primary motor cortex
- axons project down though brainstem and spinal cord to innervate lower motor
neurons - lower motor neurons are part of peripheral nervous system; project to and
innervate skeletal muscle
upper motor neurons are part of the
CNS
peripheral pathway consists of —-
1 lower motor neuron
lower motor neurons innervate —–
skeletal muscles
where does the 1 lower motor neuron originate from
- in the brain?
- in the spinal cord
in the brain= cranial nerve nuclei
in the spinal cord= cell bodies in ventral horn
where do axon collaterals branch out?
at their distal end
(near the muscle targets)
- each collateral will synapse with 1 muscle fiber
what does the neuromuscular junction consist of?
- axon terminals
- motor end plates on the muscle membrane
- Schwann cell sheaths
what do Schwann cells do in terms of the NMJ?
secrete chemical signals for the formation and maintenance of the NMJ
what is the motor end plate?
a specialized region of the muscle membrane that contains high concentrations of nicotinic ACh receptors
*membrane surface has many invaginations to increase SA
what is Acetylcholine (ACh) metabolized by?
acetylcholinesterase (AChE)
how does a nicotinic ACh receptor work?
skeletal muscle nAChR are diff from the ones in the autonomic nervous system
-when the channel opens, K+ flows out and Na+ and Ca2+ flows in
-depolarizing end plate potential
-excitatory
neuromuscular transmission is ALWAYS
excitable
muscle equivalent:
muscle cell
muscle fiber
muscle equivalent:
cell membrane
sarcolemma
muscle equivalent:
cytoplasm
sarcoplasm
muscle equivalent:
modified endoplasmic reticulum
sarcoplasmic reticulum
(SR)
T tubules are
extensions of the cell membrane (sarcolemma) that associate with the ends (terminal cisternae) of the SR
T tubule function
brings APs into the interior of a muscle fiber!!
SR function
stores Ca2+
what takes up the bulk of the intracellular space?
myofibrils
what do myofibrils contain? important for contraction
the two contractile elements of a muslce fiber!!
myosin and actin
other proteins within myofibrils
Regulatory proteins= troponin and tropomyosin
Giant accessory proteins= titin and nebulin
what is a sarcomere?
a unit of myofibril between 2 Z discs
what do skeletal muscle striations reflect in a microscope
the alternations between A bands and I bands arranged within a sarcomere
myosin
motor protein that creates movement
myosin molecule
2 intertwined protein chains
describe the two intertwined protein chains (myosin molecule) structure
stiff rod- like “tail”
two mobile “heads”
each myosin head has one actin binding site
how many myosin molecules in a thick filament?
250
one actin molecule=
G-actin (globular)
what does G-actin polymerize to form
F-actin (filamentous)
how many myosin binding sites does each G-actin have?
1
thin filaments are formed by
two chains of F-actin intertwining
(like a double strand of beads)
troponin
a trimeric protein complex that binds to tropomyosin and had 2 Ca2+ binding sites
Tropomyosin
a long protein dimer complex that attaches to actin,
covering up the myosin binding sites so that myosin can not securely attach
titin
spans the distance from one Z disc to the neighboring M line
–> elastic molecule, returns stretched muscle to resting length
Nebulin
lies along the thin filaments, attaches to a Z disc but does not reach the M line
–> inelastic molecule
which filaments: actin
thin
which filaments: myosin
thick
thin filaments: 3 things
troponin, actin, tropomyosin
