1. The Muscular System: Physiology
How do muscles contract?

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

3. 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

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

5. 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

6. The Nervous System Meets the Muscular System

7. 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

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

10. 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

11. 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

12. 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

13. 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

15. 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.

16. 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!

17. 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

18. 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?

19. 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)

20.
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”

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

23. 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!

25. 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

26. 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!

27. 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

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