Neuromuscular Junctions

Physiology of Muscle Fibers

Action Potentials: The Role of Motor Neurons

Motor neurons produce motion by interacting with skeletal muscle.

Electric signals called action potentials are sent along nerve cells to muscle fibers.

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Polarization of a neuron:

Occurs when there is a charge differential across each plasma membrane.

Ion channels create polarization and the Na+/K+ pump restores the neuron to its resting state

This allows nerve signaling

How an impulse travels:

Phases of a neuron signaling

Ligand-gated Ion Channels help neurons and muscle fibers

communicate: Ligand-gated ion Channels have

cellular receptors that open in response to a signal molecule (called a ligand) binding to the receptor.

In neuromuscular junctions, the neurotransmitter (signal molecule from neurons) is Acetylcholine (Ach).

When Acetylcholine binds tothe receptor, the channel opens and Na+ enters the muscle cell.

This will eventually lead to a muscle contraction if the stimulus is above the threshold.

What do we have in common?

Puffer fish

Poison Arrow Frog

Blue-ringed Octopus

Tetrodotoxin: A neurotoxin found in puffer fish and other

toxic animals paralyzes victims by blocking sodium

ion channels

Ion channels at work:

If you are exposed to tetrodotoxin. . .What happens to your nervous

system?Your nerves will not be able to send

signals because they won’t be able to move Na+ ions

What happens at a neuromuscular junction that leads to a muscle

contraction?In response to a stimulus, a motor

neuron releases Ach into the synaptic cleft.

Ach binds to the Na+ ion channels on the muscle fiber.

Continued…The change in Na+ concentration

causes depolarization.Depolarization leads to muscle

contraction.

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Acetylcholinesterase???Acetylcholinesterase is an enzyme

released into the synaptic cleft that breaks down acetylcholine.

This prevents constant muscle stimulation.

Botox??

Neuromuscular Junction - The Real Thing!

Botulinum toxin binds to pre-synaptic membranes at neuromuscular junctions, enters the neuron, and then a component of the toxin interferes with the release of neurotransmitter.Release of acetylcholine is blocked.No acetylcholine?  No muscular contraction.(flaccid paralysis)

All or None Principle:An action potential will occur or

not occurA threshold stimulus is the

minimum stimulus strength to cause an action potential

A subthreshold stimulus is too weak to cause an action potential

How do the muscle fibers contract?Calcium is important!

The sarcoplasmic reticulum is a specialized Smooth ER that stores Ca2+ ions

The Transverse (T) Tubules work with the sarcoplasmic reticulum to rapidly concentrate and move calcium ions throughout the sarcoplasm

The role of Ca2+

Calcium ions are stored in a specialized smooth ER (in muscle fibers) called the Sarcoplasmic Reticulum.

When released, calcium makes the actin binding sites on myosin available by removing tropomyosin

When actin and myosin bind, the sliding filament theory of muscle contraction can take place

Ca2+ binds the troponin molecules on the thin actin myofilaments.

This leads to the ability of the thick myosin filaments to bind the actin.

The myosin heads pull the actin molecules inward, causing the whole fiber to shorten.

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Is ATP needed for muscles to contract or to relax?

What is happening when ATP binds the myosin head?

The cross-bridge is broken and the muscle relaxes!

What is rigor mortis?It literally means “stiffness of death”When a person stops making ATP, the

muscles cannot relax and they maintain contraction until the tissue starts to breakdown.

A pathologist can use rigor mortis to estimate time of death . . . .

A

Body Temperature

Rigor Mortis Time since death

Warm Not yet stiff Dead not more than 3 hours

Warm Stiffness starting in head

Dead 3 to 8 hours

Cold Stiff Dead 9 to 36 hours

Cold No longer stiff Dead more than 36 hours

What happens at a neuromuscular junction?

http://www.youtube.com/watch?v=9FF6UKvDgeE

http://www.youtube.com/watch?v=pcayZjTKsm8&feature=related