Spinal Nerve Anatomy: Prerequisites & Nervous System Overview

The central nervous system (CNS) consists of the brain and spinal cord, with the brain containing the cerebrum (responsible for higher functions like memory, emotion, and reasoning), cerebellum (movement and balance), and brainstem (regulatory processes like breathing and blood pressure), while the peripheral nervous system (PNS) includes all nerves outside the CNS and is divided into the somatic nervous system (controlling voluntary movement and sensation) and the autonomic nervous system (controlling automatic functions like heart rate and digestion), which itself has two branches—the sympathetic branch that increases organ activity and the parasympathetic branch that decreases organ activity.

The nervous system is organized into the Central Nervous System (CNS) and Peripheral Nervous System (PNS). The CNS (brain and spinal cord) handles high-level information processing and decision-making, controlling homeostatic functions and physiology. The PNS includes all nerve branches extending to limbs. The PNS has two major subdivisions: the afferent (sensory) system carrying sensory information into the CNS via receptor cells, and the efferent (motor) system carrying commands to effectors. Sensory pathways typically involve first-order neurons (receptors), second-order neurons to the thalamus, and third-order neurons to the brain.

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). The CNS includes all neurons entirely within the brain and spinal cord. The PNS includes all neurons not in the brain or spinal cord, even if they have parts that touch or enter the CNS. A bundle of axons in the PNS is called a nerve, while the same structure in the CNS is called a tract. A bundle of cell bodies in the PNS is called a ganglion, while the same structure in the CNS is called a nucleus.

The nervous system is divided into two main parts: (1) Central Nervous System (CNS) - consists of the brain and spinal cord, serving as the main information processing center where sensory information is received, interpreted, and decisions are made; (2) Peripheral Nervous System (PNS) - includes all nerves outside the brain and spinal cord, responsible for bringing sensory information to the CNS and carrying out motor commands. The PNS has two divisions: (1) Sensory/Afferent division - brings information into the CNS; (2) Motor/Efferent division - carries information out from the CNS. The motor division further divides into: (1) Somatic Nervous System (SNS) - controls skeletal muscle for voluntary movements; (2) Autonomic Nervous System (ANS) - controls involuntary functions like smooth muscle, cardiac muscle, and glands. The ANS has two branches: (1) Sympathetic division - prepares the body for stress (fight or flight response); (2) Parasympathetic division - promotes relaxation and conservation of energy (rest and digest response).

The central nervous system (CNS) consists of the brain and spinal cord, while the peripheral nervous system (PNS) includes all nerves outside the CNS. In the CNS, oligodendrocytes form myelin sheaths around axons. In the PNS, Schwann cells perform this function. This distinction is important for understanding where different types of nerve damage or diseases may occur and how they affect bodily functions.

The anatomical position is a standardized reference where the body stands upright with head and eyes forward, arms at sides with palms facing forward, and feet flat on the ground; directional terms describe body parts relative to this position: medial/lateral (toward/away from midline), superior/inferior (toward head/floor, also cranial/caudal), anterior/posterior (front/back, also ventral/dorsal), proximal/distal (toward trunk/away from trunk), superficial/deep (near surface/far from surface), unilateral/bilateral (one side/both sides), and ipsilateral/contralateral (same side/opposite sides).

Anatomical directional terms are standardized language used to describe the location and position of body structures relative to each other, including anterior/ventral (front), posterior/dorsal (back), superior/cranial (toward head), inferior/caudal (toward feet), midline (central dividing line), medial (toward midline), lateral (away from midline), proximal (closer to origin), distal (farther from origin), superficial (closer to surface), deep (farther from surface), unilateral (one side), bilateral (both sides), ipsilateral (same side), and contralateral (opposite side). These terms are essential for precise anatomical communication and are used consistently regardless of body position, with embryological terminology sometimes differing (e.g., ventral/dorsal instead of anterior/posterior).

This section covers the ten fundamental directional terms used in anatomy. Anterior/ventral refers to structures toward the front (e.g., chest anterior to heart). Posterior/dorsal indicates structures toward the back (e.g., spine posterior to heart). Superior/cranial means above another structure (e.g., head superior to feet). Inferior/caudal means below another structure (e.g., feet inferior to head). Medial indicates proximity to the body's midline (e.g., heart medial to lungs). Lateral indicates distance from the midline (e.g., ears lateral to eyes). Ipsilateral means structures on the same side (e.g., right arm and right leg). Contralateral means structures on opposite sides (e.g., right arm and left leg). Proximal indicates closeness to the trunk or origin point (e.g., elbow proximal to wrist). Distal indicates distance from the trunk or origin point (e.g., toes distal to knee).

The anatomical position is the standard reference position: standing upright, facing forward, eyes forward, spine erect, arms at sides with palms facing forward, and feet in normal base of support. Directional terms include: anterior (front/ventral), posterior (back/dorsal), medial (toward midline), lateral (away from midline), superior (cranial/upward), inferior (caudal/downward), proximal (closer to trunk), distal (farther from trunk), superficial (closer to surface), deep (further inside), ipsilateral (same side), and contralateral (opposite sides). These terms provide a standardized language for describing body positions and movements.

The anatomical position (vertical, facing forward, palms forward, thumbs outward) serves as the reference for all directional terms. Anterior/ventral refers to the front; posterior/dorsal to the back. Superior refers toward the head; inferior toward the feet. Medial indicates proximity to the midline; lateral indicates distance from the midline. Ipsilateral means same side; contralateral means opposite side. Proximal indicates proximity to the trunk; distal indicates distance from the trunk. Superficial indicates proximity to the surface; deep indicates distance from the surface.

Neurons generate action potentials through a coordinated sequence of events: at resting potential (-70 mV), voltage-gated sodium channels open upon reaching threshold (-55 mV), causing rapid depolarization to +30 mV; subsequently, potassium channels open, leading to repolarization and hyperpolarization; the sodium-potassium pump then restores the resting potential; action potentials propagate unidirectionally along axons due to refractory periods and myelination via saltatory conduction at nodes of Ranvier, ultimately triggering neurotransmitter release at synapses.

Action potentials are rapid, all-or-nothing electrical signals that propagate along neurons. They begin with depolarization when voltage-gated sodium channels open, allowing Na+ influx and membrane potential to become positive. Repolarization follows as potassium channels open, allowing K+ efflux and restoring negative charge. Hyperpolarization briefly makes the membrane more negative than resting potential. The sodium-potassium pump then restores original ion distribution. This sequence enables rapid, long-distance signal transmission.

Action potential (also called neural signal or nerve impulse) is the electrical signal that allows nerves to communicate with each other. The instructor explains that action potentials are similar to electricity in homes but with important differences. The nervous system uses electrical signals to transmit information throughout the body, enabling functions like hearing, seeing, and memory formation. Without action potentials, the nervous system would not function, and organisms could not survive.

Action potential is an electrical signal that propagates along the neuron membrane as a non-graded depolarization, consisting of five stages: resting state (membrane polarized at -70 mV with Na+ outside and K+ inside), depolarization (stimulus opens voltage-gated Na+ channels, causing Na+ influx and membrane potential to rise toward threshold of -55 mV), rising phase (threshold reached triggers more Na+ channels to open, membrane potential rises to +35 mV), falling phase (Na+ channels close and K+ channels open, K+ efflux causes repolarization), and undershoot/hyperpolarization (K+ channels close slowly, membrane potential drops below resting level before being restored by the Na+/K+ pump).

Action potentials are brief electrical signals that travel along neurons. They represent a rapid reversal of the membrane's electrical polarity. These signals allow neurons to communicate information quickly and reliably over long distances within the nervous system. The reversal involves the inside becoming positive and the outside becoming negative, enabling rapid neural communication.