Nervous System Overview Quiz

Nervous System Overview

• Comprises two main parts: central nervous system (CNS) and peripheral nervous system (PNS).
• CNS includes the spinal cord and brain; PNS consists of nerves extending from the CNS, including cranial nerves and peripheral nerves.

Peripheral Nervous System Organization

• Peripheral nerves categorized into sensory, motor, and mixed types.
• Mixed nerves contain both sensory and motor neurons; they facilitate both input and output functions.
• PNS further divided into afferent (sensory information to CNS) and efferent (motor signals from CNS to body) neurons.

Neuron Structure

• Neurons vary significantly in shape and size, tailored to their function.
• Each neuron typically has a cell body with a nucleus, numerous dendrites, and a single long axon.
• The axon may branch and is covered by a myelin sheath, forming the nerve fiber.

Synaptic Communication

• Communication between neurons occurs at synapses involving neurotransmitters.
• The motor unit, the functional unit of muscle contraction, includes the alpha motor nerve and the muscle fibers it innervates.
• Most innervating neurons for skeletal muscle are classified as alpha motor neurons.

Muscle Contraction Mechanism

• Each muscle can contain multiple motor units, varying in the number of muscle fibers and motor units.
• The nervous system regulates muscle fiber activity through action potentials.
• Nerve endings form a neuromuscular junction (mio-neural junction) that closely adheres to muscle fiber membranes without penetrating them.

Action Potential and Resting Potential

• Electrical potential differences exist across membranes of all living cells, with varying ion concentrations inside (predominantly negative) and outside (predominantly positive).
• Resting potential ranges from -60 to -90 millivolts; typical resting potential for neurons is -70 millivolts.
• The capacity to respond to stimuli, known as irritability, allows cells to experience changes in membrane potential.

Depolarization Process

• Upon stimulation, membrane potential becomes more positive through depolarization.
• Continuous depolarization leads to transmission of electrochemical impulses along the cell membrane, facilitating communication in both nerve and muscle cells.

Muscle Structure and Function

• Muscle is enveloped by epimysium, a connective tissue that maintains separation between adjacent muscles.
• Perimysium further divides the muscle into smaller sections known as fasciculi.
• Each fasciculus comprises numerous muscle fibers, which are the fundamental units of the muscle structure.
• Muscle fibers contain rod-shaped myofibrils, extending the full length of the fiber.
• Myofibrils are encased in the sarcolemma and consist of a gelatinous substance called sarcoplasm, housing mitochondria and the sarcoplasmic reticulum.
• Mitochondria play a vital role in metabolic processes within muscle cells.
• Muscle fibers exhibit variation in length and diameter and contain multiple nuclei.
• Myofibrils house bundles of myofilaments—actin (thin filament) and myosin (thick filament)—integral for muscle contraction.

Sarcomere Organization

• A myofibril is structured into units called sarcomeres, which are delineated by Z-lines.
• Actin and myosin filaments create striations in skeletal muscles: light areas (I bands) and darker areas (A bands).
• A bands incorporate myosin filaments and regions of overlap with actin.
• H band is the central area of the sarcomere containing only myosin filaments, while I bands consist solely of actin anchored to Z lines.
• During contraction, the A band remains unchanged, while I bands shorten and H bands disappear as actin filaments slide inward.

Muscle Contraction Mechanism

• The contraction of muscles involves the sliding of actin past myosin, bringing Z lines closer together and shortening the muscle fiber.
• Myosin filaments have globular heads that form cross bridges with actin during muscle activation.
• The Sliding Filament Theory explains how actin and myosin filaments interact to create muscle contraction.
• Multiple cycles of cross-bridge formation are necessary for a strong contraction.

Cross Bridge Cycle

• ATP is crucial for muscle contraction; it binds to the myosin heads near the cross bridge.
• Calcium ions released from the sarcoplasmic reticulum allow myosin heads to attach to actin binding sites.
• The binding triggers the breakdown of ATP into ADP and energy, facilitating movement and flexion of the myosin head.
• Myosin pulls actin filaments toward each other, resulting in muscle contraction and Z lines moving closer together.
• The cycle of coupling, flexion, uncoupling, and recharging occurs rapidly, allowing continuous muscle contraction.