Transcript Lecture 6- Neuromuscular transmission and muscle contraction.ppt

Neuromuscular transmission
Motor Unit
•Motor Unit :
is the motor neuron
and all the muscle
fibers it supplies
all of these fibers will have
the same type (either fast
twitch or slow twitch).
When a motor unit is
activated
The Neuromuscular junction consists of
Axon Terminal : contains
around 300,000 vesicles which contain the
neurotransmitter acetylcholine (Ach).
Synaptic Cleft :
20 – 30 nm ( nanometers ) space
between the axon terminal & the
muscle cell membrane. It contains
the enzyme cholinesterase which
can destroy Ach .
Synaptic transmission ***
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Synapse is the junction between two
neurones where electrical activity of
one neurone is transmitted to the other
Acetylcholine (1)
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Ach is synthesized locally in the cytoplasm of the
nerve terminal, from active acetate (acetylcoenzyme
A) and choline.
Then it is rapidly absorbed into the synaptic vesicles
and stored there.
The synaptic vesicles themselves are made by the
Golgi Apparatus in the nerve soma ( cell-body).
Then they are carried by Axoplasmic Transport to the
nerve terminal , which contains around 300,000
vesicles .
Each vesicle is then filled with around 10,000 Ach
molecules .
Steps involved:
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AP at the synaptic knob -----» Ca channels
open (increase Ca permeability) -----»
release of neurotransmitter (NT) from
synaptic knob to synaptic cleft -----»
NT combines with specific receptors on the
other membrane -----» postsynaptic potential
-----» AP will result
Neuromuscular transmission
Neuromuscular transmission ***
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Transmission of impulse from nerve to
muscle (neuromuscular junction)
Steps:
AP at nerve knob -----» increase Ca permeability (Ca inter
synaptic knob) -----» release of Acetylcholine (Ach) ----» Ach combine with receptors on motor end plate -----»
Na permeability increase -----» end plate potential
develop -----» AP spread on the membrane -----» muscle
contraction
Molecular basis of muscle
contraction ***
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Anatomical consideration:
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Muscle fibre
Sarcomere
Myosin (thick filament):
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Cross-bridge
Actin (thin filament)
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Regulatory protein: (Troponin,Tropomyosin)
Actin
Events of muscle contraction:
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Acetylcholine released by motor nerve »»»»» EPP »»»»»
depolarization of CM (muscle AP) »»»»»
Spread of AP into sarcoplasmic reticulum »»»»»release of Ca
into the cytoplasm
»»»»» Ca combines with troponin »»»»» troponin pull
tropomyosin sideway »»»»» exposing the active site on actin
»»»»» myosin heads with ATP on them, attached to actin active
site
»»»»» Resulting in formation of high energy actin-myosin
complex »»»»» activation of ATP ase (on myosin heads) »»»»»
energy released, which is used for sliding of actin & myosin
Events of muscle contraction:
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When a new ATP occupies the vacant site on the
myosin head, this triggers detachment of myosin
from actin
The free myosin swings back to its original position,
& attached to another actin, & the cycle repeat its
self
Events of muscle contraction:
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When ca is pumped back into sarcoplasmic reticulum
»»»»» ca detached from troponin »»»»»
tropomyosin return to its original position
»»»»» covering active sit on actin »»»»» prevent
formation of cross bridge »»»»» relaxation
Muscle contraction ****
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1- simple muscle twitch:
The mechanical response (contraction) to single AP
(single stimulus)
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2- Summation of contraction:
Spatial summation:
the response of single motor unites are added
together to produce a strong muscle contraction
Temporal summation:
when frequency of stimulation increased (on the
same motor unite), the degree of summation
increased, producing stronger contraction
Types of muscle contraction:
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1- Isometric contraction:
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No change in muscle length, but increase in muscle
tension (e.g. standing)
2- Isotonic contraction:
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Constant tension, with change in muscle length (e.g.
lifting a loud)
ELECTROMYOGRAPHY AND
MOTOR NERVE CONDUCTION
VELOCITY
ELECTROMYOGRAPHY (EMG)
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It’s a recording of electrical activity of
the muscle by inserting needle
electrode in the belly of the muscles or
by applying the surface electrodes.
The potentials recorded on volitional
effort are derived from motor units of
the muscle, hence known as motor unit
potentials (MUPs).
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Electromyography (EMG) is a
technique for evaluating and recording
physiologic properties of muscles at rest
and while contracting.
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A motor unit is defined as one motor
neuron and all of the muscle fibers it
innervates.
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