Skeletal Muscle || Neuromuscular Junction

I started by simply explaining the skeletal muscles in the body and their function in very simple terms but as we keep delving into the topic more, we need to get into a deep understanding of the skeletal muscle system, and today I will be looking at muscle physiology and neurotransmission in the muscle.

In one of my posts, I mentioned that muscles are innervated by motor neurons and this starts with the spinal cord which has cell bodies at its anterior side which is not myelination known as the anterior grey horn of the spinal cord which houses the cell bodies of the somatic motor neuron. The somatic nervous system is responsible for the voluntary control of skeletal muscles and the skeletal system. For a muscle to move or be excited, they receive neuro stimulus and this causes a change in the potentials of the membrane.

I started with the spinal cord, doesn't mean that it doesn't have anything to do with the brain. It does have something to do with the brain. In the brain, the cerebral cortex sends impulses to the spinal cord where alpha motor neurons are sent to the muscles. The alpha motor neuron goes with action potentials which consist of voltage gates that receive Sodium (Na+) into the axons. The cell is negative and when sodium rushes into the cell of the axon, we know that Cations are rushing into the cells, making it positive and the depolarizing wave moves through the cell membrane to the synaptic bulb, they stimulate the voltage-gated channel in the bulb which is specific to calcium which is more concentrated outside the cell than inside the cell. When it reaches these voltage-gated channels, they open to allow calcium to flow in. At the synaptic bulb, there are vesicles that are made in the cell body of the anterior gray horn and are transported through anterograde transport (Axoplasmic Transport) to the synaptic bulb. In the synaptic bulb are neurotransmitters known as Acetocholyne (ACH) and it is made in the synaptic bulb composed of choline which comes from diet and acetate from the mitochondria, and it is converted into Acetocholine by Choline-Acetyl-Transferase and the Acetocholyne is sent into the synaptic vesicle.,

Acetocholine difuses across the synaptic cleft to the muscle cell membrane which possesses ligand-gated ion channel known as nicotinic receptor and that is where Acetocholine binds to. After binding, the gate is opened allowing sodium ion in more and potassium ion to flow out less, leading to a positive charge and produces the end plate potential. The positive charges moves across the membrane reaching channels, and there are activation and inactivation gates. The activation gate is stimulated by threshold potentials and the inactivation gate is also stimulated by threshold potentials but slowly. When this gates are stimulated, sodium flows in. The positive charges moves through the transverse tubule and depolarizes the transverse membrane which is close to the cytoplasmic reticulum and stimulates the Dihydropyridine receptor protein which is connected to the Ryanodine receptor type 1. In the cytoplasmic reticulum, Calcium is concentrated as a result of the protein calsequestrin, so when the Ryanodine receptor type 1 is displaced, the calcium leaves the cytoplasmic reticulum into the cytoplasm of the muscle cell. The muscle cells is made predominantly of Myofibrils and the calcium goes in. When the muscle is at rest the myosin binding site is blocked from binding to the actin active site by tropomyosin, but when it is stimulated by calcium, it binds with troponin, which pulls tropomyosin away allowing binding. ,,,. ATP is bound to the myosin head detaching it from actin. Let me state clearly that the site where the muscle receive information from the nueron is the nueromuscular junction so we are actually explaining that in this post. The myosin are anchored at the center of the sacomere which is known as the M-line, while actine is anchored to the Z-line. Contraction occurs when the myosin pulls the actin across its length in a cross-bridge. The myosin allowing contraction begins when a ATP is hydrolysed to ATP and inorganic phosphate. When ATP attaches to the myosin, it releases the actin causing it to relax the muscle. ,,,,.

When bringing the muscle from resting potential to a threshold potential, it is dependent on the frequency of the neuro stimuli and the strength of the neuro stimuli. A muscle can have a rapid contraction from one neuro stimulus, and this is often regarded as muscle twitch., I will be looking into muscle twitch and the muscle mechanics in full in my next post.

Image Reference

Image 1 || Anatomy & Physiology by CCC Online || Muscular Levels of Organization

Image 2 || Wikimedia Commons || Neuromuscular junction