The Nervous System: Structure and Function

The Nervous System

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts of the body. It is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

• Central Nervous System (CNS): Consists of the brain and spinal cord; processes information and determines responses.

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS; transmits signals between the CNS and the rest of the body.

Neurons and Neuroglial Cells

Structure and Types of Neurons

Neurons are the primary functional units of the nervous system, specialized for the transmission of electrical impulses.

• Cell Body (Soma): Contains the nucleus and organelles; responsible for metabolic activities.

• Dendrites: Receive incoming signals from other neurons.

• Axon: Conducts electrical impulses away from the cell body toward other neurons or effectors.

Neuroglial Cells

Neuroglia (glial cells) support, protect, and nourish neurons. Major types include:

• Astrocytes: Maintain the blood-brain barrier and provide structural support.

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Schwann Cells: Form myelin sheaths in the PNS.

• Microglia: Act as immune cells, removing debris and pathogens.

• Ependymal Cells: Line fluid-filled cavities in the CNS and help circulate cerebrospinal fluid.

Myelinated vs. Unmyelinated Nerve Fibers

Structure and Function

Nerve fibers can be myelinated or unmyelinated, affecting the speed of nerve impulse conduction.

• Myelinated Fibers: Axons covered with a myelin sheath, which increases the speed of impulse transmission.

• Unmyelinated Fibers: Axons without a myelin sheath; impulses travel more slowly.

Saltatory Conduction

In myelinated axons, action potentials jump from one node of Ranvier to the next, greatly increasing conduction speed.

• Saltatory Conduction: The process by which action potentials leap between nodes of Ranvier in myelinated fibers.

• Continuous Conduction: Occurs in unmyelinated fibers, where the action potential travels along every part of the membrane.

Information Flow in Neurons

Direction of Signal Transmission

Neurons transmit information in a unidirectional manner:

• Dendrites: Receive signals.

• Cell Body: Integrates incoming signals.

• Axon: Conducts the action potential to the axon terminals.

Electrical Signals: Graded and Action Potentials

Graded Potentials

Graded potentials are changes in membrane potential that vary in size and decay with distance.

• Occur in dendrites and cell bodies.

• Can summate to trigger an action potential if threshold is reached.

Action Potentials

Action potentials are rapid, all-or-none electrical impulses that travel along axons.

• Triggered when membrane potential reaches threshold.

• Exhibit a refractory period and do not decay over distance.

Graded Potentials	Action Potentials
Proportional to stimulus intensity	All-or-none for adequate stimuli
No threshold	Threshold required
No refractory period	Refractory period present
Decay over distance	Regenerative, do not decay
Anywhere on neuron	Axon only

Action Potential Generation and Propagation

The action potential is generated at the axon hillock and propagates along the axon.

• Depolarization: Na+ influx causes the membrane potential to become more positive.

• Repolarization: K+ efflux restores the negative membrane potential.

• Hyperpolarization: Membrane potential becomes more negative than resting potential before stabilizing.

Synaptic Transmission

Chemical Synapses

At chemical synapses, neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic cell, initiating a new electrical signal.

• Presynaptic Terminal: Releases neurotransmitter in response to an action potential.

• Synaptic Cleft: The gap between neurons where neurotransmitters diffuse.

• Postsynaptic Membrane: Contains receptors that bind neurotransmitters and generate a response.

Autonomic vs. Somatic Motor Systems

Comparison of Motor Pathways

The somatic and autonomic nervous systems differ in structure, neurotransmitters, and effectors.

• Somatic Motor System: Controls voluntary movements via skeletal muscle; uses a single neuron pathway and acetylcholine (ACh) as the neurotransmitter.

• Autonomic Motor System: Controls involuntary functions (e.g., heart, glands, smooth muscle); uses a two-neuron chain and various neurotransmitters (ACh, norepinephrine).

System	Neural Pathway	Neurotransmitter	Effector	Effect
Somatic	Single, heavily myelinated axon	ACh	Skeletal muscle	Stimulatory
Autonomic (Sympathetic)	Two-neuron chain	ACh, NE	Cardiac/smooth muscle, glands	Stimulatory or inhibitory
Autonomic (Parasympathetic)	Two-neuron chain	ACh	Cardiac/smooth muscle, glands	Stimulatory or inhibitory

Functional Areas of the Brain

Cerebral Cortex Regions

The brain contains specialized regions responsible for different functions.

• Primary Motor Cortex: Controls voluntary movements.

• Premotor Cortex: Involved in planning and selecting movements in response to stimuli.

• Prefrontal Cortex: Responsible for complex behaviors, decision-making, and personality.

• Somatosensory Cortex: Processes sensory input from the body.

• Cerebellum: Coordinates movement and balance.

Summary Table: Key Concepts

Concept	Description
Neuron	Basic functional unit of the nervous system
Neuroglia	Support cells for neurons
Myelin	Insulating sheath that increases conduction speed
Action Potential	All-or-none electrical impulse along axon
Synapse	Junction between two neurons
Somatic System	Controls voluntary muscles
Autonomic System	Controls involuntary functions

Additional info: This guide integrates foundational concepts in nervous system anatomy and physiology, including neuron structure, neuroglia, electrical signaling, synaptic transmission, and functional brain regions, as relevant to introductory college-level ANP courses.