The Nervous System: Structure, Function, and Organization

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

The Nervous System: Overview

Introduction to the Nervous System

The nervous system is the master controlling and communicating system of the body. It utilizes electrical and chemical signals to coordinate rapid and specific responses, ensuring immediate reactions to internal and external stimuli.

Sensory Input: Information gathered by sensory receptors about changes inside and outside the body.
Integration: Processing and interpretation of sensory input to determine an appropriate response.
Motor Output: Activation of effector organs (muscles and glands) to produce a response.

Divisions of the Nervous System

Central and Peripheral Nervous Systems

The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS), each with distinct structures and functions.

Central Nervous System (CNS): Consists of the brain and spinal cord; serves as the integration and control center.
Peripheral Nervous System (PNS): Composed of nerves extending from the brain and spinal cord, including cranial and spinal nerves. It acts as communication lines between the CNS and the rest of the body.

Diagram of nervous system divisions and pathways

Functional Divisions of the PNS

Sensory (Afferent) Division: Transmits impulses from sensory receptors to the CNS. Includes somatic sensory fibers (from skin, muscles, joints) and visceral sensory fibers (from organs).
Motor (Efferent) Division: Transmits impulses from the CNS to effector organs. Subdivided into:
- Somatic Nervous System: Voluntary control of skeletal muscles.
- Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands. Includes sympathetic and parasympathetic divisions, which generally have opposing effects.

Histology of Nervous Tissue

Neuroglia (Glial Cells)

Nervous tissue is highly cellular, with two principal cell types: neuroglia and neurons. Neuroglia support and protect neurons, while neurons transmit electrical signals.

Astrocytes: Most abundant glial cells in the CNS; support neurons, regulate the chemical environment, and participate in information processing.

Astrocyte structure and relationship to neurons and capillaries

Microglial Cells: Small, defensive cells that monitor neuron health and can phagocytize debris and pathogens.
Ependymal Cells: Line the central cavities of the brain and spinal cord; may be ciliated to help circulate cerebrospinal fluid (CSF).

Ependymal cells lining brain or spinal cord cavities

Oligodendrocytes: Form myelin sheaths around CNS nerve fibers, providing electrical insulation.

Oligodendrocyte forming myelin sheaths around CNS nerve fibers

Satellite Cells (PNS): Surround neuron cell bodies in the PNS, similar in function to astrocytes.
Schwann Cells (PNS): Form myelin sheaths around peripheral nerve fibers and assist in nerve regeneration.

Neurons (Nerve Cells)

Neurons are the structural and functional units of the nervous system. They are specialized for impulse conduction, have extreme longevity, are mostly amitotic, and have a high metabolic rate.

Cell Body (Soma): Contains the nucleus and organelles; biosynthetic center of the neuron.
Dendrites: Short, branched processes that receive input and convey signals toward the cell body.
Axon: Single, long process that transmits impulses away from the cell body to other neurons or effectors.

Myelin Sheath

Structure and Function

The myelin sheath is a segmented, whitish, protein-lipoid covering that insulates axons, increasing the speed of nerve impulse transmission. Myelinated fibers conduct impulses rapidly, while nonmyelinated fibers conduct more slowly.

PNS Myelination: Schwann cells wrap around axons, forming the myelin sheath. Gaps between Schwann cells are called nodes of Ranvier.
CNS Myelination: Oligodendrocytes form myelin sheaths around multiple axons. White matter consists of myelinated fibers; gray matter contains neuron cell bodies and unmyelinated fibers.

Cross-sectional view of a myelinated axon

Classification of Neurons

Structural Classification

Neurons are classified by the number of processes extending from the cell body:

Type	Structure	Location
Multipolar	Many dendrites, one axon	Most abundant; CNS
Bipolar	One dendrite, one axon	Special senses (retina, olfactory mucosa)
Unipolar	Single process splits into peripheral and central branches	PNS (dorsal root ganglia, sensory ganglia of cranial nerves)

Table comparing structural classes of neurons Table showing neuron types and structural variations

Functional Classification

Sensory (Afferent) Neurons: Transmit impulses from sensory receptors toward the CNS; mostly unipolar.
Motor (Efferent) Neurons: Carry impulses from the CNS to effectors; multipolar.
Interneurons: Lie between sensory and motor neurons; shuttle signals through CNS pathways; most abundant.

Table showing functional classification of neurons

Membrane Potentials and Neural Signaling

Membrane Potential Changes

Neurons use changes in membrane potential as communication signals. These changes occur when ion concentrations or membrane permeability change.

Graded Potentials: Short-distance signals, usually in dendrites or cell bodies.
Action Potentials: Long-distance signals generated in axons; involve a rapid reversal of membrane potential (~100 mV).
Depolarization: Membrane potential becomes less negative, increasing the likelihood of a nerve impulse.
Hyperpolarization: Membrane potential becomes more negative, reducing the likelihood of a nerve impulse.

The Synapse

Structure and Function

Synapses are junctions that mediate information transfer between neurons or between a neuron and an effector cell. They can be classified by their anatomical location:

Axodendritic: Between axon terminals and dendrites.
Axosomatic: Between axon terminals and neuron cell bodies.
Other types: Axoaxonic, dendrodendritic, somatodendritic.

Types of synapses: axodendritic, axosomatic, axoaxonal

Presynaptic Neuron: Conducts impulses toward the synapse.
Postsynaptic Neuron: Transmits impulses away from the synapse.

Central Nervous System: Brain Structure and Function

Major Brain Regions

Cerebral Hemispheres: Frontal, parietal, temporal, occipital, and insula lobes.
Diencephalon: Thalamus, hypothalamus, epithalamus.
Brain Stem: Midbrain, pons, medulla oblongata.
Cerebellum: Coordinates movement and balance.

Cerebral Cortex

The cerebral cortex is the site of conscious mind functions, including awareness, sensory perception, voluntary motor initiation, communication, memory, and understanding. It is organized into motor, sensory, and association areas.

Primary Motor Cortex: Controls voluntary movements; represented by the motor homunculus.

Motor and sensory homunculus maps in the cerebral cortex

Premotor Cortex: Plans and coordinates complex movements.
Prefrontal Cortex: Involved in intellect, cognition, personality, and decision-making.
Posterior Association Area: Involved in pattern recognition, spatial localization, and language comprehension.
Limbic Association Area: Provides emotional context and memory formation.

Diencephalon

Thalamus: Gateway to the cerebral cortex; sorts, edits, and relays sensory information.
Hypothalamus: Regulates autonomic functions, emotions, body temperature, hunger, thirst, sleep-wake cycles, and endocrine activity.
Epithalamus: Contains the pineal gland, which secretes melatonin to regulate sleep-wake cycles.

Sagittal section of the brain showing diencephalon and brainstem structures

Peripheral Nervous System: Structure and Function

Sensory Receptors and Nerves

The PNS links the CNS to the external environment through sensory receptors and nerves. Sensory receptors are classified by stimulus type, location, and structural complexity.

Mechanoreceptors: Respond to touch, pressure, vibration, and stretch.
Thermoreceptors: Detect temperature changes.
Photoreceptors: Respond to light (e.g., in the retina).
Chemoreceptors: Detect chemicals (e.g., taste, smell, blood chemistry).
Nociceptors: Respond to pain-causing stimuli.

Classification of Nerves

Mixed Nerves: Contain both sensory and motor fibers.
Sensory (Afferent) Nerves: Carry impulses toward the CNS.
Motor (Efferent) Nerves: Carry impulses away from the CNS.

Regeneration and Reflexes

Regeneration of Nerve Fibers

Peripheral axons can regenerate if the cell body is intact, but CNS fibers rarely regenerate due to inhibitory factors and scar tissue formation.

PNS Regeneration: Involves Wallerian degeneration, macrophage cleanup, and axon regrowth through a regeneration tube formed by Schwann cells.
CNS Regeneration: Inhibited by oligodendrocyte proteins and astrocyte scar tissue.

Reflexes and Reflex Arcs

Reflexes are rapid, involuntary responses to stimuli. Reflex arcs consist of five components: receptor, sensory neuron, integration center, motor neuron, and effector.

Inborn Reflexes: Present at birth; maintain posture and control visceral activities.
Learned Reflexes: Acquired through practice (e.g., driving skills).

Autonomic Nervous System (ANS)

Structure and Function

The ANS regulates involuntary functions by innervating smooth muscle, cardiac muscle, and glands. It operates subconsciously and is divided into sympathetic and parasympathetic divisions, which generally have antagonistic effects to maintain homeostasis.

Sympathetic Division: Mobilizes the body during activity ("fight-or-flight").
Parasympathetic Division: Promotes maintenance activities and conserves energy ("rest-and-digest").

Effects of Drugs on the ANS

Certain drugs can influence the ANS by acting on specific receptors or enzymes. These drugs are used clinically to treat various conditions.

Drug Class	Receptor Bound	Effects	Example	Clinical Application
Nicotinic agents	Nicotinic ACh receptors	Stimulate all postganglionic neurons; increase blood pressure	Nicotine	Smoking cessation products
Parasympathomimetic agents	Muscarinic ACh receptors	Enhance parasympathetic activity	Pilocarpine	Glaucoma (opens aqueous humor drainage pores)
Acetylcholinesterase inhibitors	Enzyme AChE	Indirect effect at all ACh receptors; prolong ACh effect	Neostigmine	Myasthenia gravis (increases availability of ACh)
Sympathomimetic agents	Adrenergic receptors	Enhance sympathetic activity	Albuterol	Asthma (dilates bronchioles)
Sympathetic agents	Adrenergic receptors	Decrease sympathetic activity	Propranolol	Hypertension (decreases blood pressure)

Table of drug classes influencing the autonomic nervous system

Developmental Aspects of the ANS

Aging affects the ANS, leading to conditions such as constipation, dry eyes, frequent eye infections, and orthostatic hypotension due to less responsive pressure receptors and declining cardiovascular control.