Neuromuscular transmission
Jan 29, 2016
Neuromuscular transmission
Synaptic transmission ***
Synapse is the junction between two neurones where electrical activity of one neurone is transmitted to the other
Motor UnitMotor Unit
•Motor Unit : Motor Unit : is the motor neuronis the motor neuronand all the muscle and all the muscle fibers it suppliesfibers it suppliesall of these fibers will have all of these fibers will have the same type (either fast the same type (either fast twitch or slow twitch). twitch or slow twitch). When a motor unit is When a motor unit is activatedactivated
The Neuromuscular junction consists The Neuromuscular junction consists ofof
Axon Terminal :Axon Terminal : contains containsaround 300,000 vesicles which contain the around 300,000 vesicles which contain the
neurotransmitter acetylcholine (Ach).neurotransmitter acetylcholine (Ach).Synaptic Cleft :Synaptic Cleft :20 20 –– 30 nm ( nanometers ) space 30 nm ( nanometers ) spacebetween the axon terminal & thebetween the axon terminal & themuscle cell membrane. It containsmuscle cell membrane. It containsthe enzyme cholinesterase whichthe enzyme cholinesterase which
can destroy Achcan destroy Ach . .
The Neuromuscular junction consists The Neuromuscular junction consists ofof
Ach is synthesized locally in the Ach is synthesized locally in the cytoplasm of the nerve terminal , from cytoplasm of the nerve terminal , from active acetate (acetylcoenzyme A) and active acetate (acetylcoenzyme A) and choline.choline.
Then it is rapidly absorbed into the Then it is rapidly absorbed into the synaptic vesicles and stored there. synaptic vesicles and stored there.
The synaptic vesicles themselves are The synaptic vesicles themselves are made by the Golgi Apparatus in the nerve made by the Golgi Apparatus in the nerve soma ( cell-body). soma ( cell-body).
Then they are carried by Axoplasmic Then they are carried by Axoplasmic Transport to the nerve terminal , which Transport to the nerve terminal , which contains around 300,000 vesicles . contains around 300,000 vesicles .
Acetylcholine (1)Acetylcholine (1)
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:
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 ***
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 ***
Anatomical consideration: Muscle fibre Sarcomere Myosin (thick filament):
Cross-bridge Actin (thin filament)
Regulatory protein: (Troponin,Tropomyosin) Actin
Events of muscle contraction: *** 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: 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: 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 **** 1- simple muscle twitch:
The mechanical response (contraction) to single AP (single stimulus)
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: 1- Isometric contraction:
No change in muscle length, but increase in muscle tension (e.g. standing)
2- Isotonic contraction:
Constant tension, with change in muscle length (e.g. lifting a loud)