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

Nervous System: Structure, Function, and Organization

General Functions of the Nervous System

The nervous system is a complex network responsible for regulating and coordinating body activities. It enables organisms to perceive, interpret, and respond to internal and external stimuli.

• 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: Activates effector organs (muscles and glands) to elicit a response.

Structural and Functional Classifications of the Nervous System

• Structural Classification:

  • Central Nervous System (CNS): Brain and spinal cord.

  • Peripheral Nervous System (PNS): Cranial and spinal nerves, ganglia.

• Functional Classification:

  • Somatic Nervous System: Voluntary control of skeletal muscles.

  • Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands. Subdivided into sympathetic and parasympathetic divisions.

Central and Peripheral Nervous Systems

• Central Nervous System (CNS):

  • Major Parts: Brain (cerebrum, diencephalon, brain stem, cerebellum), spinal cord.

• Peripheral Nervous System (PNS):

  • Major Parts: Cranial nerves, spinal nerves, ganglia, sensory receptors.

Neurons and Neuroglia: Structure and Function

• Neurons: Excitable cells that transmit electrical signals.

• Neuroglia (Glial Cells): Support, protect, and nourish neurons. Types include astrocytes, oligodendrocytes, microglia, ependymal cells (CNS), Schwann cells, and satellite cells (PNS).

General Structure of a Neuron

• Cell Body (Soma): Contains 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, responsible for communication between different CNS regions.

Classification of Neurons

• Structural Classification:

  • Multipolar: Many dendrites, one axon (most common in CNS).

  • Bipolar: One dendrite, one axon (found in retina, olfactory mucosa).

  • Unipolar (Pseudounipolar): Single process that splits into two branches (sensory neurons in PNS).

• Functional Classification:

  • Sensory (Afferent) Neurons: Transmit impulses toward CNS.

  • Motor (Efferent) Neurons: Carry impulses away from CNS to effectors.

  • Interneurons (Association Neurons): Connect sensory and motor neurons within CNS.

General Sensory Receptors

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

• Thermoreceptors: Detect temperature changes.

• Photoreceptors: Detect light (in retina).

• Chemoreceptors: Detect chemical changes (taste, smell, blood chemistry).

• Nociceptors: Detect pain from tissue damage.

Generation and Conduction of Nerve Impulses

• Resting Membrane Potential: Maintained by sodium-potassium pump; inside of neuron is negative relative to outside.

• Action Potential: Rapid depolarization and repolarization of membrane potential.

• Propagation: Action potential travels along axon to axon terminals.

• Synaptic Transmission: Neurotransmitters released at synapse transmit signal to next neuron.

Equation:

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

Where $V_m$ is the membrane potential.

• Cerebral Hemispheres: Higher brain functions (thought, memory, voluntary movement).

• Diencephalon: Includes thalamus (sensory relay), hypothalamus (homeostasis), epithalamus.

• Brain Stem: Midbrain, pons, medulla oblongata; controls vital functions (breathing, heart rate).

• Cerebellum: Coordinates movement and balance.

Meningeal Layers

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

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

• Pia Mater: Delicate, innermost layer; adheres to CNS surface.

Cerebrospinal Fluid and Blood-Brain Barrier

• Cerebrospinal Fluid (CSF): Formed by choroid plexuses; cushions brain and spinal cord, removes waste.

• Blood-Brain Barrier: Selective barrier formed by endothelial cells; protects brain from harmful substances.

Functions and Structure of the Spinal Cord

• Functions:

  • Conducts sensory and motor information to and from the brain.

  • Serves as a center for spinal reflexes.

• Structure: Cylindrical, extends from foramen magnum to L1-L2 vertebra; consists of central gray matter (H-shaped) surrounded by white matter.

General Structure of a Nerve

• Endoneurium: Surrounds individual axons.

• Perineurium: Bundles axons into fascicles.

• Epineurium: Encloses entire nerve.

Autonomic Nervous System: Sympathetic and Parasympathetic Divisions

• Sympathetic Division: Originates from thoracolumbar region; prepares body for "fight or flight" (increases heart rate, dilates bronchi, inhibits digestion).

• Parasympathetic Division: Originates from craniosacral region; promotes "rest and digest" (slows heart rate, constricts bronchi, stimulates digestion).

Effects of Sympathetic vs. Parasympathetic Divisions

The following table summarizes the contrasting effects of the two divisions on major organs:

Organ	Sympathetic Effect	Parasympathetic Effect
Heart	Increases rate and force	Decreases rate
Lungs (Bronchi)	Dilates bronchi	Constricts bronchi
Digestive System	Inhibits activity	Stimulates activity
Blood Vessels	Constriction (most vessels)	Little or no effect