NMJ, Steps of Skeletal Muscle Contraction, Energy Needs and Aging
Objectives
- Students will be able to investigate the origination of muscle action potentials at the neuromuscular junction and the steps involved in the sliding filament mechanism of muscle contraction, and apply these to the mechanisms of skeletal muscle fiber contraction and relaxation.
- Students will be able to list and explain the steps of the Sliding Filament Theory & muscle relaxation
- Students will be able to compare anaerobic and aerobic cellular respiration processes by which muscle fibers produce ATP, and the factors that contribute to muscle fatigue.
Overview of the Skeletal Muscle Signal
If you remember back to the beginning of this unit, we talked about the 3 different types of muscle and their controls. Skeletal muscle is voluntarily controlled which means that you cause the movement of your skeletal muscles with thought/intention. We can then infer that the signal to begin a contraction originates from your brain. This signal is electrical in nature and called an action potential. The electricity that powers your computer and flows through the wires of your house is simply energy that is caused by the presence of charged particles. Your body contains many of these charged particles in the form of ions. As a review, remember that muscle cells are said to be polarized meaning that there is a charge difference between the inner and outer face of the cell membrane. This polarization results because of positively charged ions, cations, such as calcium (Ca2+) and sodium (Na+). Inside the cell there are cations, potassium (K+), but the presence of this cation is not enough to overcome the negative charge of the much larger and more common molecules like proteins, etc. Already you can see that the presence of these positively charged particles on either side of the membrane create the opportunity for a charge difference and therefore an electric signal. The electric signal allows voltage gated channels to open. Voltage gated channels are exactly how they sound - the gate opens in response to a particular voltage and closes at a particular voltage. This is important because the positively charged ions that exist on either side of the cell membrane are not at equilibrium, and will diffuse toward the side of the membrane that has fewer of that particular cation when gates are open. Imagine that the ions are claustrophobic. If there are a lot of sodium ions in one particular place they become claustrophobic and desperately want to spread out. When the opportunity presents itself in the form of an open channel the claustrophobic ions seize that opportunity and move from a highly concentrated area to an area with a lower concentration spreading out and easing the claustrophobia. In excitable tissues when voltage gated sodium channels open the sodium ions that were kept outside of the cell begin to flow into the cell diffusing down its concentration gradient. When voltage gated potassium channels open, the potassium inside the cell diffused out of the cell down its gradient. This movement of ions creates a particular type of electricity that we call an action potential. So from here on out when you hear action potential, think electric signal caused because of these cations on either side of the cell membrane that diffuse down their concentration gradients when the opportunity to do so is presented. You will discuss the specifics of an action potential in the nervous system chapter.
