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

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

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

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.

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

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

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.

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

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.

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

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

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

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

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

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