Neurobiology and Muscle Physiology Quiz

Nerve and Muscle Physiology

• The functional unit of the central nervous system (CNS) is the neuron.
• Neurons transmit electrical signals to each other.
• There are approximately 14 billion neurons in the CNS, with 75% located in the cerebral cortex.
• A neuron typically has a cell body (soma), dendrites, and an axon.
• The cell body contains the nucleus and is the metabolic center of the neuron.
• Dendrites extend outwards from the soma and branch extensively.
• Axons are long, fibrous structures originating from a thickened area called the axon hillock.
• Terminal buttons arise from the axon and store synaptic transmitters.
• Myelin sheath forms from Schwann cells and surrounds the axon.

Types of Neurons

• Multipolar neurons: Have many poles. One pole gives rise to the axon, and others give rise to dendrites. Found in the brain and spinal cord.
• Bipolar neurons: Have one axon and one dendrite. Examples include olfactory cells in the nasal cavity, some retinal neurons, and sensory neurons in the inner ear.
• Unipolar neurons: Have only a single process leading away from the soma. Represent neurons that carry sensory signals to the spinal cord.
• Anaxonic neurons: Have multiple dendrites but no axon. Found in the brain and retina.

Organization of Nerve Fibers

• The cell bodies of neurons are often grouped into nuclei or laminae within the grey matter of the CNS, or in ganglia of the peripheral nervous system.
• Nerve fibers run within the white matter of the CNS or along peripheral nerves.
• Groups of nerve fibers in the same direction are typically bundled to form tracts, peduncles, or brachia (pathways).

Nerve Fiber Types

• Peripheral nerves are composed of many axons bundled together within a fibrous envelope.
• A (Alpha, Beta, Gamma, Delta) nerve fibers are myelinated, and carry sensory and motor information. Diameter ranges from 5-20 μm.
• B nerve fibers are myelinated, carrying primarily preganglionic autonomic signals. Diameter of <3 μm.
• C nerve fibers are unmyelinated; they carry primarily postganglionic autonomic signals and other sensory information. Diameter < 1.2 μm.

Excitation and Conduction

• Nerves respond to various stimuli (electrical or chemical).
• Stimulation in a nerve can produce a local potential and action potential.
• Nerve response is dependent on conduction of ions across the cell membrane.
• Neurons have a resting membrane potential of approximately -70 mV,. this is due to a separation of charges across the membrane.

Skeletal Muscle

• A skeletal muscle is an organ composed of various tissues: skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue.
• Each skeletal muscle has three layers of connective tissue: epimysium, perimysium, and endomysium.
• Epimysium surrounds the entire muscle and allows it to move independently, maintaining its structural integrity.
• Perimysium surrounds bundles of muscle fibers (fascicles).
• Endomysium surrounds each individual muscle fiber (cell) and plays a role in transferring force from the muscle fiber to the tendon.
• Skeletal muscle cells are called myofibers and are cylindrical and elongated.
• Myofibers are composed of numerous nuclei for the production of the proteins necessary for cell function.

Molecular Structure of Muscle

• Sarcomere is the basic functional unit of the myofibril.
• Sarcomeres are composed of contractile, regulatory, and structural proteins.
• The shortening of sarcomeres is responsible for muscle contraction.
• Thin filaments (actin) and thick filaments (myosin) are the main components within the sarcomere.
• The arrangement of these filaments creates the striated appearance of skeletal muscle.
• Thick filaments contain myosin, anchored to the M line.
• Thin filaments are anchored to the Z discs and extends towards the center of the sarcomere.

Sliding Filament Model of Contraction

• Muscle contraction involves the sliding of thin filaments past thick filaments.
• The sliding utilizes ATP, generating force during the power stroke.
• When stimulated, myosin heads bind to the actin, dragging the actin filament along and shortening the sarcomere.
• The cross-bridges then detach along the myosin cycle.
• Repetition of this process results in continuous muscle contraction.
• Relaxation occurs when Ca2+ levels decrease, and myosin heads detach from actin.

Neuromuscular Transmission

• The neuromuscular junction (NMJ) is the site where a motor neuron interacts with a muscle fiber.
• The terminal buttons of the motor neuron innervate the junctional folds of the motor end plate.
• The space between the nerve and the motor end plate is called the synaptic cleft.
• Neurotransmitter acetylcholine (ACh) is crucial in transmitting signals from the nerve to the muscle.

Sequence of Events during Transmission

• Nerve impulses arriving at the terminal buttons increase the permeability to calcium (Ca²+).
• Ca²+ triggers the release of acetylcholine (ACh) containing vesicles into the synaptic cleft.
• Acetylcholine diffuses to muscle receptors and activates them, leading to depolarization.
• The muscle action potential is generated and initiates muscle contraction.
• Acetylcholinesterase breaks down ACh, ending the signal.

Smooth Muscle

• Smooth muscle is found in various organs and differs from skeletal muscle concerning its fiber organization and signaling.
• Multi-unit smooth muscle fibers are discrete, operate independently, and are typically innervated by a single nerve fiber.
• The outer surface of the fiber is insulated by basement membrane-like substance.
• Example is ciliary muscles of the eye, the iris, and piloerector muscles.
• Unitary smooth muscle fibers are aggregated in sheets, have cell membranes that are attached, and joined by many gap junctions.
• The fiber contracts as a single unit(syncytial). The type of smooth muscle found in viscera.
• Example includes muscles of the gut, bile ducts, ureters, uterus, and many blood vessels.
• Smooth muscle contraction is regulated by calcium and ATP in a different mechanism compared to skeletal muscle contraction.
• Actin filaments, connected by dense bodies, enabling force transmission among adjacent cells.

Muscle Hypertrophy

• Hypertrophy is an increase in the total mass of a muscle.
• Occurs due to increased actin and myosin filaments in muscle fibers.
• Prolonged, strong contractions with loading are primary factors contributing to hypertrophy, causing significant development in 6-10 weeks.
• There is a significant rate of synthesis of muscle proteins during development of hypertrophy.

Muscle Atrophy

• Atrophy is a decrease in the total mass of muscle tissue.
• The rate of decay of contractile proteins exceeds the rate of replacement during inactivity (unuse).
• Continued shortening of a muscle can lead to reduced sarcomeres.
• Loss of nerve supply is a substantial cause of muscle atrophy.
• Degeneration changes appear in muscle fibers, and the associated nerve supply can recover in 3 months, while total recovery may take 1-2 years.
• Atrophy is often followed by replacement of muscle fibers by fibrous and fatty tissue.
• Resultant fibers in the later stages do not have contractile properties, and the replacement connective tissue continues shortening for months

Hyperplasia of Muscle Fibers

• Hyperplasia is the moderate increase in the number of muscle fibers which occurs due to strenuous activity and force generation.
• The mechanism is the linear splitting of predominantly enlarged fibers.

Myasthenia Gravis

• Myasthenia Gravis is a form of neuromuscular disease that results in the inability of neuromuscular junctions to transmit sufficient signals from nerve fibers to muscle fibers resulting in muscle paralysis.
• It is an autoimmune disease where the body's immune system attacks and negatively affects the body's own Acetylcholine-activated channel proteins.
• End plate potentials become too weak to stimulate muscles, leading to significant weakness.

Muscle Fatigue

• Muscle fatigue is the gradual decline in a muscle's ability to contract.
• Prolonged contractions and the depletion of glycogen are primary factors.
• Decreased ability of nerve signals to travel to the muscles can also contribute to fatigue.
• Reduced blood flow and loss of oxygen and nutrients supply also contribute to muscle fatigue.

Rigor Mortis

• Rigor Mortis is a post-death muscle tightening phenomenon that is usually caused by the depletion of ATP which is a necessary co-factor for muscle relaxation processes.
• The muscles remain in a constricted state until protein degradation processes occur.
• Degradation occurs due to the enzymes released by decaying cells called lysosomes. This process usually takes about 15-25 hours post-mortem.

Excitation-Contraction Coupling

• It refers to the sequence of events that converts an electrical stimulus to a mechanical contraction in the muscle fibers.
• Implicates the Sarcoplasmic reticulum and Transverse Tubules in transmitting and converting an electrical stimulus to a mechanical contractile action in the muscle tissues.