Unit 3 Part 3 Flashcards
chemical components and reactions that
occur when a muscle is stimulated by a __
motor nerve
sliding of each myofibril within a muscle
fiber cause __
muscle fiber to shorten
the result when many muscle fibers shorten
contraction of the skeletal muscle.
Physiology of Contraction
– 1st – synaptic transmission at the
neuromuscular junction
– 2nd – excitation-contraction coupling
– 3rd – contraction-relaxation cycle
Capable of self-generating electrical impulses
at their membranes
excitable
Electrical potentials
exist across the membranes of essentially all cells of the body
Concentration difference of ions across a selectively permeable membrane can produce a
membrane potential
The difference in potential ( voltage ) between
the inner side & outer side of the membrane
(nerve or muscle membranes)
What Is a Membrane Potential
more negative and more K+
Inside cell
more positive and more Na+
Outside cell
– Must exist for action potential to occur
– The value for Vm in inactive muscle cells is typically btwn –80 and
–90 millivolts.
– Cells that exhibit a Vm are said to be polarized.
– Vm can be changed by influx or efflux of charge.
Membrane Potentials
States of a Membrane Potential
- (1) Resting Membrane Potential ( RMP)
- (2) Graded Potential (Local Response)
- (3) Action Potential
It ranges between -70 and -90 mV in different
excitable tissue cells
Resting Membrane Potential ( RMP)
It ranges between -70 and -90 mV in different
excitable tissue cells
Resting Membrane Potential ( RMP)
Voltage difference is the result of
-positive charge is constantly leaking out of the cell
-Contribution of Na+-K+ pump
-protein anions
-MP in a stimulated cell that is producing a local , non-propagated potential;
-an electrical change which is measurable only in
the immediate vicinity of the cell but not far
from it.
Graded Potential (Local Response)
MP in case of a nerve/muscle that is generating a propagated electrical potential after stimulation by effective stimulus
Action potential(AP)
– Large changes in cell membrane potential (charge)
– Inside of the cell becomes more positive relative to the outside of the cell
– Function to transmit information over long distances
– Electrical signal that travels along the nerve axon and ends at the synaptic terminal
– All-or-none principle
– RESULTS IN: Releases neurotransmitter
(acetylcholine or ACh)
Action Potentials
level of depolarization needed to trigger an action potential. Action potential does not occur until this has been reached.
Threshold potential
-state membrane suddenly becomes permeable to Na+ ions;
-Allows tremendous numbers of (+) charged Na+ ions to flow to the interior of the axon; Potential rises rapidly in the (+) direction
Depolarization stage
Na channels begin to close; K channels open more than they normally do;
Rapid diffusion of K+ ions to the exterior re-establishes the normal negative resting membrane potential
Repolarization
Membrane potential may briefly become over negative; due to opened voltage-gated K channels
Hyperpolarization
-a motor neuron and all the muscle fibers it
supplies
-All the muscle cells controlled by one nerve cell
Motor unit
-Interface of the motor neuron and muscle fiber
-Region where the motor neuron stimulates the muscle fiber
-where the neuron meets with the muscle fiber.
Neuromuscular junction (NMJ)
Where communication occurs between a somatic
motor neuron and a muscle fiber
Synapse
Gap that separates the axonal ends and muscle
fibers ; filled with interstitial fluid
Synaptic cleft (20-30 nm wide)
The neuromuscular junction is formed by :
-End of motor neuron axon (axon terminal)/synaptic
end bulb/Pre-synaptic terminal
-The motor end plate of a muscle/Sub-synaptic or
Post-synaptic membrane
- Terminals have small membranous sacs (synaptic
vesicles) that contain the neurotransmitter
acetylcholine (ACh)
End of motor neuron axon (axon terminal)/synaptic
end bulb/Pre-synaptic terminal
- A specific part of the sarcolemma that contains ACh
receptors - Chemically-gated Na+ channels
- Acetylcholinesterase enzyme
The motor end plate of a muscle/Sub-synaptic or
Post-synaptic membrane
-The synaptic end bulb is filled w/ vesicles that
contain the neurotransmitter
-The neurotransmitter for skeletal muscle is
acetylcholine
The motor end plate is chock full of
acetylcholine receptors
Chemical released by the initial cell communicating with the second cell
Neurotransmitter
– travels across the synaptic cleft
– binds to membrane receptors on sarcolemma (motor end plate)
– Sarcolemma becomes permeable to sodium
– causes sodium–ion rush into sarcoplasm
– is quickly broken down by enzyme
(acetylcholinesterase or AChE)
Acetylcholine or ACh
enzyme that breaks down acetylcholine
(acetylcholinesterase or AChE)
-act as gated ion channels
-channels that are normally closed, but in response to a certain signal, they will open and allow specific ions to pass through them.
-When a channel is open, its specific ion(s) will
enter or exit depending on their
electrochemical gradient.
Integral Proteins
Ion Channels Important in Muscle Contraction
- Voltage-gated Ca2+ channel in Pre-synaptic
terminal - Nicotinic Acetylcholine Receptor Channel
- Chemically-gated Na+ channels in Sub-
synaptic/Post synaptic Membrane
-mediates neurotransmitter release
-At the pre-synaptic membrane of the neuron
- When an action potential spreads over these channels open and allow calcium ions to diffuse from the synaptic space to the interior of the nerve terminal.
- Causes synaptic vesicles to fuse with the neural
membrane and empty their acetylcholine into the synaptic space by the process of exocytosis.
Voltage-gated Calcium Channels
-in muscle neuromuscular junction (postsynaptic membrane, or end plate)
- mediates electrical transmission from nerve to muscle
Nicotinic Acetylcholine Receptor Channel
Events at the NMJ
- arrival of action potential at the synaptic terminal
- release ACh
- ACh binding at the motor end plate
- appearance of action potential in the sarcolemma
- return to initial state
The ACh present in the synaptic cleft is broken
down by the enzyme ___ into _____
Acetylcholinesterase; Acetyl coA+ choline
The ACh is again synthesized by the nerve cell
body and then send by ___ to the presynaptic terminal for packaging into secretory vesicles.
anterograde flow
An autoimmune neuromuscular disorder in which the
Neuromuscular junction is blocked.
MYASTHENIA GRAVIS
cause of myasthenia gravis
Auto-antibodies are formed against the
Ach receptors on the Motor End Plate.
These antibodies completely destroy the receptors.
Some myasthenia gravis patients have
other auto-immune disorders as well such as
RA, poliomyelitis.
-hallmark of Myasthenia gravis
-seen with prolonged use of the skeletal muscles.
-Muscles become progressively weaker during periods of activity and improve after periods of rest.
fatigue
Fatigue is usually more pronounced in the ___ as tongue, ___ as eye movements, ___ as swallowing, ___ as talking
proximal muscles
occulomotor
phryngeal
laryngeal muscles
Fatigue is usually more pronounced in the ___ as tongue, ___ as eye movements, ___ as swallowing, ___ as talking
proximal muscles
occulomotor
phryngeal
laryngeal muscles
drooping of the eyelids
ptosis
double vision
diplopia
DIAGNOSIS MYASTHENIA GRAVIS
- Presence of autoantibodies in the plasma
- Nerve conduction study
- Edrophonium test - Tensilon, a drug that blocks the
degradation (breakdown) of acetylcholine
TREATMENT MYASTHENIA GRAVIS
- Anti-cholinesterase drugs.e.g: Neostigmine
- Immunosuppressant drugs. E.g: glucocorticoids
- Thymectomy: removal of thymus helps rebalances
the immune system.
