3.2 The Nervous System & Neural Processing

Functional Organization of the Nervous System

The nervous system is the body's primary high-speed communication and control network. It senses internal and external environmental changes, integrates sensory information, and coordinates rapid physiological responses. Anatomically and functionally, the nervous system is divided into two primary branches:

• Central Nervous System (CNS): Composed of the brain and spinal cord. The CNS serves as the command center, processing sensory input, executing complex cognitive functions, storing memories, and generating motor commands.
• Peripheral Nervous System (PNS): Composed of all neural tissue outside the CNS, including 12 pairs of cranial nerves and 31 pairs of spinal nerves. The PNS connects the CNS to the rest of the body's organs, sensory receptors, and muscles.

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Subdivisions of the Peripheral Nervous System

The PNS is functionally partitioned into sensory (afferent) and motor (efferent) divisions. The motor division is further divided based on conscious control:

1. Somatic Nervous System (SNS): Mediates voluntary control over skeletal muscle contractions. Sensory fibers transmit somatic inputs from skin, joints, and skeletal muscles to the CNS, while motor fibers convey conscious commands from the CNS back to effector skeletal muscles.
2. Autonomic Nervous System (ANS): Regulates involuntary physiological activities controlling cardiac muscle, smooth muscle in internal organs, and glandular secretions. The ANS is divided into two antagonistically balanced branches:
   • Sympathetic Nervous System: Mediates the "fight-or-flight" response during stress, danger, or physical exertion. Activation causes pupillary dilation (mydriasis), increased heart rate and myocardial contractility, bronchodilation (airway widening to increase oxygen intake), glycogenolysis in the liver, and inhibition of gastrointestinal motility and digestive secretions.
   • Parasympathetic Nervous System: Mediates the "rest-and-digest" or "feed-and-breed" state during calm conditions. Activation slows the heart rate (via the vagus nerve), constricts respiratory bronchioles, stimulates salivary gland secretion and gastrointestinal peristalsis, and promotes urinary bladder contraction.

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Neuron Anatomy & Structural Components

The neuron is the fundamental structural and functional unit of the nervous system, specialized for generating and conducting electrical signals. A standard motor neuron consists of four major regions:

• Dendrites: Branching, tree-like receptive processes that receive incoming biochemical signals from neighboring neurons or sensory receptors and convey graded potentials toward the cell body.
• Soma (Cell Body): Contains the nucleus, cytoplasm, mitochondria, and Nissl bodies (rough endoplasmic reticulum). It synthesizes cellular proteins and integrates incoming neural inputs.
• Axon: A single, elongated cylindrical process extending from a cone-shaped region of the soma called the axon hillock. The axon conducts action potentials away from the cell body toward target cells over short or long distances.
• Myelin Sheath & Nodes of Ranvier: A multi-layered insulating lipid wrapping around many axons. In the PNS, myelin is produced by Schwann cells; in the CNS, it is produced by oligodendrocytes. Interspersed between adjacent myelin segments are unmyelinated gaps called Nodes of Ranvier. Myelin drastically increases action potential propagation velocity by forcing electrical current to jump from node to node, a mechanism known as saltatory conduction.
• Axon Terminals (Synaptic Knobs): Distal branches at the end of the axon that store membrane-bound vesicles containing chemical neurotransmitters.

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Action Potentials & Neurotransmission

Neurons communicate using electrical action potentials along their axons and chemical signals across synaptic junctions.

Electrophysiology of the Action Potential

1. Resting Membrane Potential: Unstimulated neurons maintain a resting voltage of approximately $-70\text{ mV}$ inside the cell relative to the extracellular space. This electrical polarization is maintained primarily by the sodium-potassium pump ($Na^+/K^+$ ATPase), which actively pumps $3\text{ Na}^+$ ions out of the neuron for every $2\text{ K}^+$ ions pumped in, powered by ATP hydrolysis.
2. Depolarization: When excitatory signals shift the membrane potential from $-70\text{ mV}$ up to the critical threshold potential (approximately $-55\text{ mV}$), voltage-gated $Na^+$ channels snap open. Sodium ions rush down their electrochemical gradient into the intracellular fluid, causing a rapid positive reversal of membrane potential up to roughly $+30\text{ mV}$.
3. Repolarization: At peak voltage (+30 mV), voltage-gated $Na^+$ channels close and become inactivated. Simultaneously, voltage-gated $K^+$ channels open, allowing $K^+$ ions to efflux out of the cell. This outward positive current restores the negative resting potential inside the cell.
4. Hyperpolarization & Refractory Period: Sluggish closing of $K^+$ channels briefly dips the membrane potential below $-70\text{ mV}$ (hyperpolarization) before the $Na^+/K^+$ ATPase re-establishes normal ion gradients. During the refractory period, the neuron cannot fire another action potential, ensuring one-way signal propagation.

Synaptic Transmission

When an action potential reaches the axon terminal, it opens voltage-gated calcium ($Ca^{2+}$) channels. Calcium influx triggers synaptic vesicles to undergo exocytosis, releasing neurotransmitters into the narrow synaptic cleft. Neurotransmitters diffuse across the cleft and bind to specific receptors on the postsynaptic membrane:

• Acetylcholine (ACh): Excitatory neurotransmitter at the neuromuscular junction; also essential for parasympathetic signaling and memory.
• Norepinephrine: Key neurotransmitter of the sympathetic nervous system, modulating alertness and stress responses.
• GABA ($\gamma$-aminobutyric acid): Major inhibitory neurotransmitter in the CNS, hyperpolarizing postsynaptic membranes to prevent over-excitation.
• Dopamine: Regulates motor control, motivation, reward pathways, and emotional regulation.

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The Reflex Arc Mechanics

A reflex is a rapid, automatic, involuntary motor response to a specific sensory stimulus designed to protect the body from injury. The pathway traversed by nerve impulses producing a reflex is called a reflex arc, which functions independently of conscious brain thought. A basic reflex arc contains five essential anatomical elements:

1. Sensory Receptor: Detects a stimulus (e.g., pain, heat, stretch).
2. Sensory (Afferent) Neuron: Transmits the nerve impulse from the receptor into the spinal cord CNS via the dorsal root.
3. Integration Center: Processing area within the spinal cord gray matter. In a monosynaptic reflex (such as the patellar knee-jerk reflex), the sensory neuron synapses directly onto a motor neuron. In a polysynaptic reflex (such as the withdrawal reflex from touching a hot stove), one or more interneurons process the signal.
4. Motor (Efferent) Neuron: Conveys the motor impulse out of the spinal cord via the ventral root to the effector organ.
5. Effector: A skeletal muscle or gland that carries out the protective physical response (e.g., muscle contraction).