1. Skeletal Muscle Activity Pg. 168 - 175
October 4, 2012

2. A little vocabulary…
Muscle cells have two very important functional properties that allow them to do what they do:
Irritability: ability to receive and respond to a stimulus
Contractility: ability to shorten (forcibly) when an adequate stimulus is received

3. The Nerve Stimulus and the Action Potential
Skeletal muscle cells will not move until they are stimulated by a nerve impulse
One motor neuron (nerve cell) may stimulate a few muscle cells or hundreds of them
Motor unit: one neuron and all the muscle cells it stimulates
Neuromuscular junction: the location where the neuron forms a junction with the muscle cells
Synaptic cleft: tiny space between the neuron and the muscle cell

4. What really happens… Nerve impulse reaches the end of the neuron
Neurotransmitter (chemical) is released
For skeletal muscles, the neurotransmitter is acetylcholine (Ach)
Acetylcholine travels across the cleft and attaches to the muscle
Sodium ions rush into the muscle cell and upsets the electrical charge of the cell
This is called the action potential
In an attempt to balance the charge of the cell, potassium ions rush out.
When the cell is “happy” again, a sodium/potassium ion pump restores the original concentrations of ions in and outside of the cell.

5. Sliding Filament Theory
Filaments = actin and myosin
After the muscle is stimulated, as described in the previous slide…
Calcium “unlocks” the actin filament, making it available for myosin to attach to
Myosin attaches at specific sites (called myosin binding sites, whodathunkit?) and PULLS on the actin
This causes the entire muscle to shorten

6. Graded Responses – Contraction of a Skeletal Muscle
A muscle cell either contracts completely or not at all. However, it is possible for some parts of a whole muscle to contract and others to stay relaxed.
Graded responses: different degrees of shortening
Graded muscle contractions can be produced by…
Changing the FREQUENCY of muscle stimulation
Changing the NUMBER of muscle cells stimulated

7. FREQUENCY of muscle stimulation
Muscle twitches sometimes occur as a result of nervous system problems
It is NOT the way our muscles normally operate
Nerve impulses are delivered at a very rapid rate
With each impulse, the contractions are stronger and smoother – when you contract your muscles and HOLD it, that is complete tetanus.
Until the muscle gets to this point, it is considered incomplete tetanus.

8. NUMBER of muscle cells stimulated
Purpose of tetanus: produce stronger muscle contractions AND to produce smooth and prolonged muscle contractions
More muscle cells stimulated = stronger contraction
When a few cells are stimulated, the muscle is only slightly contracted
When most or all cells are stimulated, the muscle contraction is as strong as it can get

9. Energy for Muscle Contraction
Muscles only store enough ATP for 4-6 seconds of contraction
Ways they get energy:
Direct phosphorylation of ADP by creatine phosphate: creatine phosphate is ONLY found in muscle cells and is very effective at regenerating ATP
Aerobic respiration: provides 95% of ATP to muscles; uses glucose and oxygen to make ATP
Anaerobic glycolysis and lactic acid formation: occurs when muscles are working so hard that your body cannot keep up with the oxygen demand; uses glucose and NO oxygen to make ATP. Lactic acid is responsible for the “burn” you feel during exercise. This only makes a few molecules of ATP, but it is much faster than aerobic respiration.

10. Muscle Fatigue
Fatigue occurs when we exercise strenuously for a long time.
Fatigue = muscle cannot contract even though it is being stimulated
Believed to be the result of oxygen debt – a person is not able to take in oxygen fast enough to keep the muscles supplied
Lactic acid begins to accumulate in the muscle
ATP supply runs low
True muscle fatigue (complete loss of function) rarely occurs because we stop before that point
You continue to breathe heavily after exercise to “repay” the oxygen debt and to break down the lactic acid build up

11. Types of Muscle Contractions
Isotonic
Myofilaments are successful in their sliding movements
Muscle shortens and movement occurs
Ex: bending your knee, rotating your arms, and smiling
Isometric
Myofilaments are “skidding their wheels” – tension in the muscle keeps increasing
Filaments try to slide, but the muscle is against a heavy or immovable object
Ex: lifting a 400 lb. dresser by yourself

12. Muscle Tone Continuous, partial contraction of muscles
Contraction isn’t visible because it is partial
Result of different SCATTERED motor units being stimulated by the nervous system in a systematic way

13. Effect of Exercise
Inactivity leads to muscle weakening and wasting away – USE IT OR LOSE IT!
Regular exercise increases muscle size, strength, and endurance
Two types
Aerobic
Anaerobic

14. Aerobic (Endurance) Exercise
Isotonic exercises
Aerobic exercise improves and increases blood supply to the muscles – leads muscles to create more mitochondria and store more oxygen
Results in stronger, more flexible muscles with greater resistance to fatigue
Also improves metabolism, digestion (and elimination), enhances neuromuscular coordination, and makes the skeleton stronger
Heart enlarges and becomes more efficient, lungs become more efficient, fat deposits are cleared from vessels
Does NOT cause skeletal muscle size to increase

15. Anaerobic (Resistance) Exercise
Isometric exercises
Anaerobic exercise causes skeletal muscles to grow larger
Can be done a few minutes every other day and STILL can be effective!
Requires little to no special equipment
Key: forcing muscles to contract AS MUCH AS POSSIBLE
Amount of reinforcing connective tissue can also be increased by this type of exercise