Physiology: Muscle Contraction & Neuron Function

• Physiology studies how living organisms or their parts function.

Muscle Contraction Mechanisms

• Muscle contraction occurs through a complex interplay of electrical and chemical events.
• Action potentials trigger the release of calcium ions from the sarcoplasmic reticulum.
• Calcium ions bind to troponin, causing tropomyosin to shift and expose the myosin-binding sites on actin.
• Myosin heads then bind to actin, forming cross-bridges.
• ATP hydrolysis provides the energy for the myosin heads to pull the actin filaments, shortening the sarcomere and causing muscle contraction.

Neuron Structure and Function

• Neurons are the fundamental units of the nervous system, responsible for transmitting information.
• The neuron consists of the cell body (soma), dendrites, and an axon.
• Dendrites receive signals from other neurons, while the axon transmits signals to other neurons or target cells.
• Neurons communicate through electrical and chemical signals.
• Electrical signals, or action potentials, travel along the axon.
• Chemical signals, or neurotransmitters, are released at synapses to communicate with other neurons.

Electrophysiology of Nerves

• Electrophysiology studies the electrical properties of cells and tissues.
• Nerves have a resting membrane potential, typically around -70 mV, maintained by ion gradients and selective permeability of the membrane.
• Action potentials are rapid changes in membrane potential that propagate along the nerve axon.
• They are caused by the opening and closing of voltage-gated ion channels, allowing ions like sodium and potassium to flow across the membrane.
• The action potential is an "all-or-none" event, meaning it either occurs fully or not at all.

Muscle Fiber Types

• Skeletal muscle fibers are classified into different types based on their contractile properties and metabolic characteristics.
• Type I fibers are slow-twitch fibers that are fatigue-resistant and rely on aerobic metabolism.
• Type IIa fibers are fast-twitch fibers with intermediate properties, using both aerobic and anaerobic metabolism.
• Type IIb fibers are fast-twitch fibers that generate high force but fatigue quickly, relying primarily on anaerobic metabolism.

Reflex Arc Pathways

• Reflex arcs are neural pathways that mediate rapid, involuntary responses to stimuli.
• A typical reflex arc consists of a sensory receptor, sensory neuron, integration center, motor neuron, and effector.
• When a stimulus is detected, the sensory receptor generates an action potential that travels along the sensory neuron to the spinal cord or brainstem.
• In the integration center, the sensory neuron synapses with an interneuron, which in turn synapses with a motor neuron.
• The motor neuron carries the action potential to the effector, such as a muscle, causing a response.

Cell Death

• Cell death is a fundamental biological process that plays a critical role in development, tissue homeostasis, and disease.
• Apoptosis is a programmed form of cell death characterized by distinct morphological and biochemical changes, such as cell shrinkage, DNA fragmentation, and the formation of apoptotic bodies.
• Necrosis is a form of cell death caused by external factors such as injury or infection.
• Unlike apoptosis, necrosis is characterized by cell swelling, membrane rupture, and inflammation.

Cell Division

• Cell division is the process by which a cell divides into two or more daughter cells.
• Mitosis is a type of cell division that results in two identical daughter cells.
• Meiosis is a type of cell division that results in four genetically different daughter cells, each with half the number of chromosomes as the parent cell.
• Cytokinesis is the process of dividing the cytoplasm of a cell to form two separate daughter cells.

Parts of the Cell

• The cell is the basic unit of life and consists of various organelles, each with specific functions.
• The nucleus contains the cell's genetic material, DNA, and controls cell growth and reproduction.
• The endoplasmic reticulum (ER) is involved in protein synthesis, folding, and lipid metabolism.
• The Golgi apparatus processes and packages proteins and lipids for transport to other locations in the cell.
• Mitochondria are the powerhouses of the cell, generating energy through cellular respiration.
• Lysosomes contain enzymes that break down cellular waste and debris.

Membrane Potential

• The membrane potential is the difference in electrical potential between the inside and outside of a cell.
• It is created by the unequal distribution of ions across the cell membrane.
• The resting membrane potential is typically negative, due to the higher concentration of potassium ions inside the cell and sodium ions outside the cell.
• Ion channels and pumps in the cell membrane regulate the movement of ions and maintain the membrane potential.