Neuromuscular Junctions Physiology of Muscle Fibers Action Potentials

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.

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 to the receptor, the channel opens and Na+ enters the muscle cell. This will eventually lead to a muscle contraction if the stimulus is above 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.

Acetylcholinesterase? ? ? Acetylcholinesterase is an enzyme released into the synaptic cleft that breaks down acetylcholine. This prevents constant muscle stimulation.

Botox? ?

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 occur A 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 Ca 2 + ions

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

The role of Ca 2+ 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

Ca 2+ 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.

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. . 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 A Cold Dead 9 to 36 hours No longer stiff Dead more than 36 hours

What happens at a neuromuscular junction? http: //www. youtube. com/watch? v=9 FF 6 UKv. Dge. E http: //www. youtube. com/watch? v=pcay Zj. TKsm 8&feature=related