Muscle Contraction

A voluntary skeletal muscle contraction begins as a conscious effort in the frontal lobe, which activates the primary motor cortex. This signal travels down an alpha motor neuron in the spinal cord to a specific muscle fiber. The action potential terminates at the motor endplate, where the neuron establishes synaptic contact with the muscle fiber, initiating the contraction cascade.

At the motor endplate, acetylcholine is released by the motor neuron and diffuses across the synaptic cleft to bind acetylcholine receptors on the sarcolemma. This binding increases sodium ion permeability, triggering an action potential in the muscle cell. Subsequently, acetylcholinesterase breaks down acetylcholine to prevent excessive muscle stimulation.

Sodium ion entry through acetylcholine receptors generates action potentials that spread along the sarcolemma and penetrate into the muscle fiber via T-tubules. This electrical signal triggers calcium ion release from the sarcoplasmic reticulum into the myofibrils. Calcium initiates actin-myosin cross-bridge activity, causing the muscle to shorten and contract.

In myasthenia gravis, individuals develop antibodies against acetylcholine receptors, preventing neurotransmitter binding and blocking electrical signal transmission between motor neurons and muscle fibers. Treatment involves drugs that inhibit acetylcholinesterase, allowing more opportunities for acetylcholine to stimulate remaining receptors, or immunosuppressants to prevent antibody formation.

Smooth muscles in internal organs are innervated by the autonomic nervous system and undergo involuntary contractions, unlike skeletal muscles. Smooth muscle contraction is mediated by actin-myosin interaction and depends on intracellular calcium concentration. Calcium binds to calmodulin, activating myosin light chain kinase, which phosphorylates myosin to enable actin interaction.

In smooth muscle, calcium enters through calcium channels on the sarcolemma or is released from the sarcoplasmic reticulum in response to neurotransmitter, hormone signals, or muscle stretching. Inside the cell, calcium binds to the regulatory protein calmodulin, forming a complex that activates myosin light chain kinase to initiate contraction.

Calcium is essential because it directly initiates actin-myosin cross-bridge activity in skeletal muscle and regulates this interaction through calmodulin in smooth muscle. In skeletal muscle, calcium released from the sarcoplasmic reticulum triggers the observation of muscle shortening. The excitatory and inhibitory effects of neurotransmitters ultimately control calcium availability to regulate contraction.