Study Guide: The Nervous System – Structure, Function, and Organization

The Nervous System: Overview and Organization

General Functions of the Nervous System

The nervous system is a complex network responsible for regulating and coordinating body activities. It enables rapid communication between different body parts and the environment.

• Sensory Input: Detects changes inside and outside the body using sensory receptors.

• Integration: Processes and interprets sensory input, deciding what action is needed.

• Motor Output: Responds to integrated stimuli by activating muscles or glands (effectors).

Structural and Functional Classifications

The nervous system can be classified based on structure and function:

• Structural Classification: Divides the system into the Central Nervous System (CNS) and Peripheral Nervous System (PNS).

• Functional Classification: Divides the PNS into sensory (afferent) and motor (efferent) divisions. The motor division is further subdivided into the somatic (voluntary) and autonomic (involuntary) systems.

Central and Peripheral Nervous Systems

• Central Nervous System (CNS): Consists of the brain and spinal cord. It is the main control center for processing information.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS, such as cranial nerves and spinal nerves. It connects the CNS to limbs and organs.

Neural Tissue: Cells and Structure

Neurons and Neuroglia

Neurons are the primary signaling cells, while neuroglia (glial cells) support and protect neurons.

• Neurons: Specialized for transmitting electrical impulses.

• Neuroglia: Include astrocytes, oligodendrocytes, microglia, and ependymal cells in the CNS; Schwann cells and satellite cells in the PNS.

General Structure of a Neuron

• Cell Body (Soma): Contains the nucleus and organelles.

• Dendrites: Receive incoming signals.

• Axon: Conducts impulses away from the cell body.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

Gray Matter and White Matter

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons.

• White Matter: Composed mainly of myelinated axons, which facilitate rapid signal transmission.

Classification of Neurons

• Structural: Multipolar, bipolar, and unipolar neurons, based on the number of processes extending from the cell body.

• Functional: Sensory (afferent), motor (efferent), and interneurons (association neurons).

General Sensory Receptors

• Mechanoreceptors: Detect mechanical forces (touch, pressure, vibration).

• Thermoreceptors: Respond to temperature changes.

• Photoreceptors: Detect light (in the retina).

• Chemoreceptors: Respond to chemical stimuli (taste, smell).

• Nociceptors: Detect pain from tissue damage.

Neural Communication and Impulse Transmission

Generation and Conduction of Nerve Impulses

Nerve impulses (action potentials) are electrical signals that travel along neurons.

• Resting Membrane Potential: The difference in electrical charge across the neuron's membrane at rest.

• Depolarization: Sodium ions enter the cell, making the inside more positive.

• Repolarization: Potassium ions exit the cell, restoring the negative charge inside.

• Propagation: The action potential moves along the axon.

• Synaptic Transmission: Neurotransmitters are released at the synapse to stimulate the next neuron.

Key Equation:

$V_m = V_{in} - V_{out}$

where $V_m$ is the membrane potential.

Major Regions of the Brain

Cerebral Hemispheres, Diencephalon, Brain Stem, and Cerebellum

• Cerebral Hemispheres: Responsible for higher brain functions such as thought, memory, and voluntary movement.

• Diencephalon:

• Brain Stem: Controls basic life functions (breathing, heart rate); includes midbrain, pons, and medulla oblongata.

• Cerebellum: Coordinates movement and balance.

Protective Structures of the CNS

Meningeal Layers

• Dura Mater: Tough, outermost layer; protects the CNS.

• Arachnoid Mater: Middle, web-like layer; contains cerebrospinal fluid (CSF).

• Pia Mater: Delicate, innermost layer; adheres to the surface of the brain and spinal cord.

Cerebrospinal Fluid and Blood-Brain Barrier

• Cerebrospinal Fluid (CSF): Cushions the brain and spinal cord, removes waste, and provides nutrients.

• Blood-Brain Barrier: Selectively allows substances to pass from the blood into the CNS, protecting it from toxins and pathogens.

Spinal Cord and Nerves

Functions of the Spinal Cord

• Conduction: Transmits nerve impulses between the brain and the rest of the body.

• Reflex Center: Integrates and processes reflexes independently of the brain.

Spinal Cord Structure

General Structure of a Nerve

• Endoneurium: Surrounds individual nerve fibers (axons).

• Perineurium: Bundles groups of fibers into fascicles.

• Epineurium: Encloses the entire nerve.

Autonomic Nervous System (ANS)

Sympathetic and Parasympathetic Divisions

• Sympathetic Division: Originates from the thoracic and lumbar spinal cord; prepares the body for 'fight or flight' responses.

• Parasympathetic Division: Originates from the brainstem and sacral spinal cord; promotes 'rest and digest' activities.

Effects of Sympathetic vs. Parasympathetic Divisions

The two divisions have opposite effects on many organs:

Organ	Sympathetic Effect	Parasympathetic Effect
Heart	Increases heart rate	Decreases heart rate
Lungs	Bronchodilation (opens airways)	Bronchoconstriction (narrows airways)
Digestive System	Inhibits digestion	Stimulates digestion
Blood Vessels	Constriction (increases blood pressure)	Dilation (decreases blood pressure)