The Nervous System: Overview

Introduction to the Nervous System

The nervous system is the most complex organ system in the human body, responsible for rapid communication and coordination of bodily functions. It works alongside the endocrine system to maintain homeostasis, but its responses are faster and more brief.

• Key Functions:

  • Monitors internal and external environments

  • Integrates sensory information

  • Coordinates voluntary and involuntary responses

Anatomical Divisions of the Nervous System

The nervous system is divided into two main anatomical divisions:

• Central Nervous System (CNS): Includes the brain and spinal cord; integrates and coordinates sensory processing and motor transmission; site of higher functions (intelligence, memory, emotion).

• Peripheral Nervous System (PNS): All nervous tissue outside the CNS; connects the CNS to the rest of the body.

Functional Divisions of the PNS

• Afferent Division: Brings information to the CNS from receptors in tissues and organs.

• Efferent Division: Carries information from the CNS to effectors (muscles and glands).

• Efferent Subdivisions:

  • Somatic Nervous System (SNS): Controls skeletal muscle contractions.

  • Autonomic Nervous System (ANS): Controls smooth and cardiac muscle, glands; includes sympathetic, parasympathetic, and enteric divisions.

Nervous Tissue

Cell Types in Nervous Tissue

Nervous tissue consists of two main cell types:

• Neurons: Basic functional units; communicate with other cells.

• Neuroglia (Glial Cells): Support and regulate the environment around neurons; more numerous and capable of division.

General Structure of Neurons

Neurons have specialized structures for communication:

• Cell Body: Contains nucleus, nucleolus, mitochondria, ribosomes, and rough ER (Nissl bodies).

• Axon Hillock: Origin of action potentials.

• Dendrites: Receive signals.

• Axon: Transmits signals to other cells.

Neuron Regeneration

Most CNS neurons lack centrioles and cannot divide, making them irreplaceable if destroyed. Neural stem cells are mostly inactive except in specific regions (nose, hippocampus).

Structural Classification of Neurons

• Multipolar Neurons: Two or more dendrites, one axon; most common in CNS; all motor neurons controlling skeletal muscles.

• Unipolar Neurons: Dendrites and axon are continuous; cell body off to one side; most sensory neurons in PNS.

• Bipolar Neurons: One dendrite, one axon; cell body between; rare, found in special sense organs.

Functional Classification of Neurons

• Sensory (Afferent) Neurons: Receive information from sensory receptors and relay to CNS.

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

• Interneurons: Located entirely within CNS; interconnect neurons; involved in higher functions (memory, learning).

Neuroglia (Glial Cells)

• CNS Types: Astrocytes, oligodendrocytes, microglia, ependymal cells.

• PNS Types: Satellite cells, Schwann cells.

Anatomical Organization of the Nervous System

Neuron Organization in the PNS and CNS

• PNS: Collections of cell bodies (ganglia), bundles of axons (nerves), spinal and cranial nerves.

• CNS: Collections of cell bodies (centers/nuclei), neural cortex, bundles of axons (tracts/columns), sensory (ascending) and motor (descending) pathways.

Membrane Potential and Action Potentials

The Membrane Potential

Living cells have a polarized membrane, with a resting membrane potential of –70 mV in neurons.

• Factors: Imbalance in electrical charges, differing fluid composition (ECF high in Na+ and Cl–, ICF high in K+ and proteins).

• Channels: Leak channels (always open), gated channels (open/close under specific conditions).

Maintaining Resting Membrane Potential

• Sodium–Potassium Exchange Pump: Actively exchanges 3 Na+ out for 2 K+ in, maintaining the potential difference.

Changes in Membrane Potential

• Depolarization: Opening Na+ channels shifts potential toward 0 mV.

• Hyperpolarization: Opening K+ channels shifts potential more negative.

Graded Potentials

Local changes in membrane potential that decrease with distance; can trigger action potentials in excitable cells.

Action Potentials

Propagated changes in membrane potential; follow the all-or-none principle; refractory period limits response rate.

• Threshold: Level of depolarization required to initiate an action potential.

Propagation of Action Potentials

• Continuous Propagation: Occurs in unmyelinated axons; relatively slow.

• Saltatory Propagation: Occurs in myelinated axons; much faster.

Synapses and Neurotransmitters

The Synapse

The synapse is the site of communication between neurons or between a neuron and another cell type.

• Presynaptic Neuron: Sends signal; axon terminal contains neurotransmitter vesicles.

• Postsynaptic Neuron: Receives signal; has receptors for neurotransmitters.

Acetylcholine and Other Neurotransmitters

• Acetylcholine (ACh): Released at cholinergic synapses; removed by acetylcholinesterase (AChE).

• Other Neurotransmitters: Norepinephrine (NE), dopamine, GABA, serotonin, nitric oxide (NO), carbon monoxide (CO).

Excitatory vs. Inhibitory Neurotransmitters

• Excitatory: ACh and NE trigger depolarization.

• Inhibitory: Dopamine, GABA, serotonin trigger hyperpolarization.

Neuronal Pools

Groups of interconnected interneurons with specific functions; communicate in circuit patterns.

The Meninges

Three Layers of Meninges

Specialized membranes surround the brain and spinal cord, providing stability and shock absorption.

• Dura Mater: Tough, fibrous outer layer; contains dural folds and sinuses.

• Arachnoid: Squamous epithelial cells; subarachnoid space filled with CSF.

• Pia Mater: Innermost layer; highly vascularized, provides nutrients.

Spinal Cord Structure

Spinal Cord Overview

The spinal cord is the major neural pathway between the brain and PNS, integrating information and controlling reflexes.

• Segments: 31 segments, each giving rise to a pair of spinal nerves.

• Enlargements: Cervical and lumbosacral regions for limb innervation.

Gray and White Matter

• Gray Matter: Contains neuron cell bodies; forms H or butterfly shape; projections called horns.

• White Matter: Contains myelinated and unmyelinated axons; organized into columns (posterior, anterior, lateral).

Brain Structure and Function

Major Regions of the Brain

• Cerebrum: Conscious thoughts, sensations, memory.

• Diencephalon: Thalamus, hypothalamus, epithalamus; relay and processing centers.

• Brainstem: Midbrain, pons, medulla oblongata; basic life functions.

• Cerebellum: Motor coordination, balance.

Cerebral Cortex and Hemispheres

• Gray Matter: Cerebral cortex (outer layer), basal nuclei (deeper).

• White Matter: Deep to cortex, surrounds basal nuclei.

• Gyri, Sulci, Fissures: Increase surface area.

• Lobes: Frontal, parietal, temporal, occipital, insula.

Motor and Sensory Areas

• Primary Motor Cortex: Precentral gyrus of frontal lobe.

• Primary Sensory Cortex: Postcentral gyrus of parietal lobe.

• Other Areas: Visual (occipital), auditory/olfactory (temporal), gustatory (frontal).

Association Areas and Processing Centers

• Somatic Sensory Association: Recognizes touch.

• Somatic Motor Association: Coordinates learned movements.

• Wernicke’s Area: General interpretive area.

• Broca’s Area: Speech center.

Basal Nuclei and Limbic System

• Basal Nuclei: Subconscious control of muscle tone and learned movements.

• Limbic System: Emotional states, memory storage/retrieval.

Diencephalon Components

• Epithalamus: Contains pineal gland (melatonin).

• Thalamus: Relay and processing center for sensory information.

• Hypothalamus: Controls emotions, autonomic functions, hormone production.

Brainstem and Cerebellum

• Midbrain: Visual and auditory processing, motor nuclei.

• Pons: Links cerebellum with other brain regions; influences respiration.

• Cerebellum: Adjusts motor activities, balance.

• Medulla Oblongata: Connects brain to spinal cord; cardiovascular and respiratory centers.

Peripheral Nervous System (PNS)

Cranial and Spinal Nerves

• Cranial Nerves: 12 pairs; originate from the brain; sensory, motor, or mixed functions.

• Spinal Nerves: 31 pairs; connect to spinal cord; grouped by vertebral region.

Nerve Plexuses and Dermatomes

• Plexuses: Cervical, brachial, lumbar, sacral; networks of major nerve trunks.

• Dermatome: Region of skin monitored by a pair of spinal nerves; important for clinical diagnosis.

Reflexes and Pathways

Reflex Arc

Reflexes are rapid, automatic responses to stimuli, helping maintain homeostasis.

• Monosynaptic Reflex: One synapse; e.g., patellar reflex.

• Polysynaptic Reflex: Multiple synapses; e.g., withdrawal reflex.

Sensory and Motor Pathways

• Sensory (Ascending) Pathways: Posterior column, spinothalamic, spinocerebellar; sensory homunculus maps.

• Motor (Descending) Pathways: Corticospinal (pyramidal), medial/lateral (extrapyramidal); motor homunculus maps.

Autonomic Nervous System (ANS)

Structure and Function

• Sympathetic Division: "Fight-or-flight"; increases alertness, heart rate, dilates pupils, decreases digestive activity.

• Parasympathetic Division: "Rest-and-digest"; constricts pupils, increases digestive activity, decreases heart rate.

• Dual Innervation: Both divisions affect the same organ, balancing responses.

Aging and Integration

Aging Effects on the Nervous System

• Reduction in brain size and weight

• Decrease in neuron number and synaptic connections

• Decreased blood flow and neurotransmitter production

• Increase in plaques; dementia may result

Integration with Other Body Systems

The nervous system interacts with all other body systems, coordinating responses and maintaining homeostasis.

Summary Table: Structural and Functional Divisions of the Nervous System