Skeletal Muscle Contraction: Physiology and Structure

Posted on May 4, 2024 in Biology

PHYSIOLOGY OF MUSCLE CONTRACTION

Contraction and Relaxation of Skeletal Muscle Fibers

The contraction and relaxation of skeletal muscle fibers involve a series of steps:

  1. Muscle Contraction: Cross-bridges attach to and “walk” along the thin filaments at both ends of a sarcomere, pulling the thin filaments towards the center. This shortens the sarcomere and causes muscle contraction.
  2. Contraction Cycle: This repeating sequence of events drives filament sliding:
    1. Myosin ATPase hydrolyzes ATP and becomes energized.
    2. The myosin head attaches to actin, forming a cross-bridge.
    3. The cross-bridge generates force as it rotates toward the center of the sarcomere (power stroke).
    4. Binding of ATP to the myosin head detaches it from actin. The myosin head hydrolyzes the ATP, returns to its original position, and binds to a new site on actin as the cycle continues.
  3. Role of Calcium Ions: An increase in Ca2+ concentration initiates filament sliding, while a decrease stops it. The muscle action potential triggers the release of calcium ions from the sarcoplasmic reticulum (SR). These ions bind to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.
  4. Relaxation: Ca2+ active transport pumps remove Ca2+ from the sarcoplasm back into the SR. As calcium ion concentration decreases, tropomyosin blocks the myosin-binding sites, and the muscle fiber relaxes.
  5. Length-Tension Relationship: A muscle fiber generates its maximum force when there is an optimal overlap between thick and thin filaments.

The Neuromuscular Junction (NMJ)

The NMJ is the synapse between a somatic motor neuron and a skeletal muscle fiber. It includes the axon terminals, synaptic cleft, and the motor end plate of the muscle fiber. When a nerve impulse reaches the axon terminals, it triggers the release of acetylcholine (ACh). ACh diffuses across the synaptic cleft and binds to receptors on the motor end plate, initiating muscle contraction. An enzyme then quickly breaks down ACh.

What is Skeletal Muscle?

Skeletal muscle, also known as voluntary muscle, is under conscious control. These muscles are attached to bones via tendons and are responsible for movement. They are also called striated muscles due to their characteristic banded appearance under a microscope.

Structure of the Brain

The brain is a complex organ and the center of the nervous system. It is enclosed within the cranium and consists of several key parts:

  1. Cerebrum: The largest part of the brain, divided into two hemispheres connected by the corpus callosum. The cerebral cortex, the outer layer of the cerebrum, is responsible for higher-level functions like reasoning and planning.
  2. Brain Stem: Connects the brain to the spinal cord and includes the medulla oblongata, pons, and midbrain. It controls vital functions like breathing and heart rate.
  3. Cerebellum: Located below the cerebrum, it plays a crucial role in coordinating movement and balance.
  4. Diencephalon: Situated between the cerebrum and brain stem, it includes structures like the thalamus and hypothalamus, which are involved in sensory processing and hormone regulation.

Each part of the brain has a specific structure and function, working together to control our thoughts, actions, and bodily functions.