How does a muscle work? Remember, muscles can only contract so they can only pull, not push. And it needs certain parts to do this.

Microscopic anatomy of muscle The bits and pieces Sarcolemma – plasma membrane of a muscle cell Myofibrils – long ribbon-like organelles that take up the majority of the space in a muscle cell Sarcoplasmic reticulum – a specialized smooth endoplasmic reticulum that has interconnecting tubules and sacs that surrounds every myofibril. It stores calcium and releases it when the muscle needs to contract. Sarcomeres – a tiny contractile unit of the myofibril. They align end to end along the entire length of the myofibril

Within a sacromere There are two types of myofilaments ◦ Thin filament – composed of the protein actin and some regulatory proteins (tropomyosin) that can cover the actin ◦ Thick filament – mostly made of bundled molecules of myosin as well as ATPase enzymes. Middle of thick filament is smooth while ends have tiny projecting myosin heads Cross bridges – with the right “cue”, the myosin heads of the thick filament can connect to the actin in the thin filament, making a “bridge”

Region Names (the letters) I Band – the l I ght banded area of the muscle ◦ This is where two sarcomeres meet. ◦ Contains only thin filaments ◦ Contains the Z disc – a thin disc-like membrane that the thin filament are anchored. A Band – the dArk banded area of the muscle. The thick filaments stretch the length of this band (making it dark). ◦ Contains the H zone – the bare zone. This area only has thick filaments ◦ Also contains the M line – has tiny protein rods that hold adjacent think filaments together.

Page 183 Figure 6.3

So, those are the parts, now what? Here is how a muscle is stimulated by the brain.

The Nerve Stimulus and the Action Potential A motor unit is one neuron and the several (or hundreds) of muscle cells that it stimulates. They are “connected” at neuromuscular junction Here there is a gap between them called the synaptic cleft, which is filled with interstitial fluid. When the nerve impulse reaches the neuromuscular junction, a neurotransmitter (acetylcholine, ACh) is released into the synaptic cleft.

ACh diffuses across and attaches to receptors that are part of the sarcolemma. If enough attaches, then the membrane of the sarcolemma temporary becomes more permeable to sodium and potassium ions. Na + rushes in and K + rushes out, but more Na + rushes in. This causes the inside to become more positive, which reverses the electrical conditions of the sarcolemma. The result, the sarcolemma opens more Na + channels to let in only Na +, which generates an electric current called the action potential

The Action Potential Once started, it is unstoppable It travels the entire surface of the sarcolemma This action potential signals the muscle to contract. While this potential is traveling down the entire length of the sarcolemma, the ACh is being broken down into acetic acid and choline by the enzyme acetylcholinesterase, to prevent further contraction from the same neuron signal

Coming back to rest Potassium diffuses out of the cell and the sodium potassium pump turns on to return the sodium and potassium back to their original positions so the next signal can be sent.

The signal is received, now what? Channels in the sarcoplasmic reticulum open and allow Ca +2 to escape These Ca +2 ions diffuse and attach to receptors on the thin filament This causes the actin binding sites to be revealed by the tropomyosin. The already “cocked with ATP” myosin heads attach to the now revealed actin sites and then perform a power stroke and moves the thin filament

The myosin stays connected to the actin until the molecule of ATD is replaced by a new ATP molecule. This moves it back into the ready position. If the actin sites are still revealed, the myosin heads will continue to attach, move, and detach with the help of ATP When the signals from the brain stops, the muscle relaxes because the Ca +2 ions are detached and returned to the sarcoplasmic reticulum, the binding sites are covered thus stopping the contraction.