The Muscular System: Physiology How do muscles contract?

What makes muscles contract? The muscular system is very intimately related to the nervous system Muscle cells need nerve impulses to contract

What makes muscles contract? One motor neuron (nerve cell) may stimulate a few muscle cells (aka muscle fiber) or a hundred One unit and all of the skeletal muscle cells it stimulates is called a motor unit

The Nervous System Meets the Muscular System The basic nerve cell in a nutshell:

The Nervous System Meets the Muscular System A nerve cell (neuron) can have an axon that branches into a number of axon terminals These can form junctions with the sarcolemma of different muscle cells neuromuscular junctions Although the nerve endings and the muscle cell membranes are very close, they DO NOT touch There is a little space separating the neuron from the muscle fiber called a synaptic cleft

The Nervous System Meets the Muscular System

The Contraction When the brain issues a signal for a muscle to contract, the neuron delivers the signal from the brain, to the spinal cord, to the specific nerves These nerves release a chemical messenger into the synaptic cleft known as a neurotransmitter For muscle contraction, this neurotransmitter is called acetylcholine (ACh), although we will be learning of others when we cover the nervous system

Question What would happen if the nerve cell was over stimulated? Prolonged contraction of the muscle cell—your muscles wouldn’t relax!

Tetanus Caused by a bacteria You CAN get it off a rusty nail, but also from a clean nail as well! Causes there to be excess firings of the nerve cell to make muscles contract permanently

The Contraction When acetylcholine (ACh) is released, is attaches to receptors on the sarcolemma membrane When enough ACh is released and attaches to the receptors, it causes the sarcolemma to be temporarily more permeable to sodium ions (Na+) This means that for a short while, it lets sodium INSIDE the cell This causes potassium ions (K+) to rush OUTSIDE of the cell

The Contraction This causes a slight imbalance of the electric conditions within the muscle cell which signals other sodium channels to open along other sites along the fiber This “upset” is known as an action potential Think of an action potential as a lit match: the flame/signal travels along the length of the fiber/match

The Contraction When there is increased Na+ within the cell, it stimulates the sarcoplasmic reticulum to release some calcium by going down a tubule called the T tubule

Refresher of Myofilaments The thick filaments (myosin) are supposed to bind to the thin filaments (actin). However, there are proteins bound of the thin filaments that prevent the binding so you don’t get muscle contraction at inappropriate times.

The Contraction Those proteins on the thin filaments prevent the thick filaments/myosin from binding EXCEPT when in the presence of calcium So, when the neuron sends down its signal to the muscle cell, Na+ is going to rush into the cell. This is going to cause the sarcoplasmic reticulum to release calcium. The calcium will tell the myofilaments that it is okay to bind!

Myofilament Activity When muscles contract, they shorten Experiment by bending your arm at the elbows or moving your knee around The muscles shorten BECAUSE the myofilaments shorten

Myofilament Activity The thin filament (actin) is connected to the sarcolemma in order to shorten the muscle fiber There is a specific set of proteins that bind the actin to the sarcolemma Question: What would happen if the actin was NOT bound to the sarcolemma?

Answer: Duchenne’s Muscular Dystrophy Caused when the proteins that bind the actin to the sarcolemma are missing. Symptoms Muscle weakness Inability to walk Scoliosis (curvature of the spine and back; think about muscles and posture)

http://www.youtube.com/watch?v=IpoT46EAuCU Only boys have DMD because it is passed on by the mother’s X chromosomes One way you can tell if someone has DMD is by the way they get up from a laying down position Called “Gowers’ sign”

Sliding Filament Theory The sliding filament theory explains HOW those filaments shorten Get excited!

Sliding Filament Theory The myosin head acts like a mouse trap It can be straight or “cocked” This all depends on the presence of energy (ATP) and calcium!

Why do people get “stiff” when they die? Called rigor mortis When you are dead, you are not producing any more energy in the form of ATP ATP is needed for the myosin head to let go of the actin and reposition itself Dead = no ATP = myosin cannot let go = contraction

Challenge Make a series of steps to describe muscle contraction GO! Start with “the brain tells the muscle to move” and end with “myosin binds to actin) GO!

My Steps The brain tells the muscle to move Acetylcholine is released from the neuron ACh binds to the sarcolemma and sodium (Na+) is let in, potassium (K+) let out The action potential is sent down the length of the muscle fiber

The sarcoplasmic reticulum releases calcium (Ca2+) Myosin binds to actin The sarcomere contracts