The Nervous System: Structure, Function, and Organization

Nervous System Overview

Introduction

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

• CNS vs. PNS: The CNS includes the brain and spinal cord; the PNS consists of cranial and spinal nerves and ganglia.

• Major Components: Brain, spinal cord, cranial nerves, spinal nerves, ganglia.

Primary Functions of the Nervous System

Sensory, Integrative, and Motor Functions

• Sensory Input: Detects changes (stimuli) inside and outside the body via sensory receptors.

• Integration: Processes and interprets sensory input, deciding what should be done at each moment.

• Motor Output: Activates effector organs (muscles and glands) to cause a response.

Example: Seeing a bear (stimulus) triggers sensory input, the CNS integrates the information, and the body responds by running (motor output).

Neurohistology

Neurons and Neuroglia

• Neurons: Specialized cells for sensing, integrating (thinking, remembering), and effecting (controlling muscles, regulating glandular secretions).

• Neuroglia: Support, nourish, and protect neurons; maintain homeostasis in interstitial fluid.

Neuron Structure

Major Parts of a Neuron

• Dendrites: Receive incoming signals.

• Soma (Cell Body): Contains nucleus, Nissl bodies (rough ER), and neurofilaments.

• Axon Hillock: Region where action potentials are initiated.

• Axon: Conducts impulses away from the cell body.

• Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid conduction.

• Synaptic End Bulb: Releases neurotransmitters to communicate with other cells.

• Presynaptic vs. Postsynaptic: Refers to the transmitting and receiving sides of a synapse, respectively.

Myelin Sheath

Formation and Function

• Myelin Sheath: A multilayered lipid and protein covering that insulates axons, increasing the speed of nerve impulse conduction.

• Schwann Cells (PNS): Form myelin sheaths around peripheral nerves.

• Oligodendrocytes (CNS): Form myelin sheaths around central nervous system axons.

Example: Myelinated axons conduct impulses much faster than unmyelinated axons.

PNS vs. CNS

Structural and Functional Differences

• PNS: Composed of nerves and ganglia outside the CNS; Schwann cells myelinate axons.

• CNS: Composed of the brain and spinal cord; oligodendrocytes myelinate axons.

Axonal Transport

Axonal Network for Transport

• Axonal Transport: Movement of materials between the neuron cell body and axon terminals, essential for neuron function and survival.

• Types: Fast axonal transport (both directions), slow axonal transport (anterograde only).

Neuronal Classification

Structural and Functional Types

• Structural:

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

  • Bipolar: One dendrite, one axon (special senses).

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

• Functional:

  • Sensor (afferent): Transmit impulses toward CNS.

  • Integrator (interneuron): Process information within CNS.

  • Motor (effector): Transmit impulses away from CNS to effectors.

Table: Comparison of Structural Classes of Neurons

Neuron Type	Structure	Location	Function
Multipolar	Many dendrites, one axon	CNS (most abundant)	Motor neurons, interneurons
Bipolar	One dendrite, one axon	Special senses (retina, olfactory epithelium)	Sensory neurons
Unipolar	Single process splits into two branches	PNS (dorsal root ganglia, sensory ganglia of cranial nerves)	Sensory neurons

Neuroglia

Types and Functions

• Astrocytes (CNS): Support neurons, maintain blood-brain barrier, regulate ion balance.

• Oligodendrocytes (CNS): Form myelin sheaths.

• Microglia (CNS): Immune defense, phagocytosis.

• Ependymal Cells (CNS): Line ventricles, produce cerebrospinal fluid (CSF).

• Schwann Cells (PNS): Form myelin sheaths.

• Satellite Cells (PNS): Support neuron cell bodies in ganglia.

Table: Glial Cells and Their Functions

Glial Cell	Location	Function
Astrocyte	CNS	Support, blood-brain barrier, ion regulation
Oligodendrocyte	CNS	Myelin sheath formation
Microglia	CNS	Immune defense, phagocytosis
Ependymal cell	CNS	CSF production, line ventricles
Schwann cell	PNS	Myelin sheath formation
Satellite cell	PNS	Support neuron cell bodies

Organization of the Nervous System

CNS and PNS Divisions

• CNS: Brain and spinal cord; integration and command center.

• PNS: Cranial and spinal nerves; communication lines between CNS and body.

• PNS Subdivisions:

  • Somatic Nervous System: Voluntary control of skeletal muscles.

  • Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands.

  • Enteric Nervous System: Controls GI tract activity.

Sensory Physiology

Pathways and Receptor Classification

• Pathways: Sensory input travels from receptors to CNS for processing.

• Integration: CNS integrates sensory input for perception and response.

• Receptor Classification: Based on stimulus type, location, and structural complexity.

• Transduction: Conversion of stimulus energy into electrical signals (coding for modality, location, intensity, duration).

Types of Senses

General and Special Senses

• General Senses: Distributed throughout the body (touch, pressure, temperature, pain, proprioception, visceral senses).

• Special Senses: Localized in specific organs (smell, taste, sight, hearing, balance).

Somatosensory System Organization

Levels of Processing

• Receptor Level: Sensory receptors detect stimuli.

• Circuit Level: Processing in ascending pathways (spinal cord, brainstem).

• Perceptual Level: Processing in cortical sensory centers (e.g., somatosensory cortex).

Maps (Homunculus)

Somatotopic Organization

• Sensory Homunculus: Map of body regions in the primary somatosensory cortex.

• Motor Homunculus: Map of body regions in the primary motor cortex.

Sensory Receptor Classification

By Stimulus Type, Location, and Complexity

• Stimulus Type:

  • Mechanoreceptors: Respond to mechanical force (touch, pressure, vibration).

  • Thermoreceptors: Detect temperature changes.

  • Chemoreceptors: Respond to chemicals (smell, taste, blood chemistry).

  • Photoreceptors: Detect light (in retina).

  • Nociceptors: Detect pain (damaging stimuli).

• Location:

  • Exteroceptors: Sensitive to stimuli outside the body.

  • Interoceptors: Monitor internal environment.

  • Proprioceptors: Monitor body position and movement.

• Structural Complexity:

  • Simple: Free nerve endings, encapsulated endings.

  • Complex: Special sense organs.

Table: Simple Sensory Receptors (Unencapsulated and Encapsulated)

Type	Location	Function
Free nerve endings	Most body tissues, especially epithelia	Pain, temperature, pressure
Modified free nerve endings (Merkel discs)	Basal layer of epidermis	Light touch
Hair follicle receptors	Surrounding hair follicles	Hair movement
Tactile (Meissner's) corpuscles	Dermal papillae of hairless skin	Light pressure, discriminative touch
Lamellar (Pacinian) corpuscles	Dermis, hypodermis	Deep pressure, vibration
Muscle spindles	Skeletal muscles	Muscle stretch
Tendon organs	Tendons	Tendon stretch

Receptive Field

Definition and Importance

• Receptive Field: The area monitored by a single sensory neuron.

• Smaller receptive fields: Allow for greater discrimination of stimuli (e.g., fingertips).

• Larger receptive fields: Less precise localization (e.g., back).

Autonomic Nervous System (ANS)

Branches and Functions

• Sympathetic Division: Prepares body for 'E' situations (emergency, exercise, excitement); increases heart rate, dilates pupils, inhibits digestion.

• Parasympathetic Division: Promotes 'rest and digest'; SLUDD responses (Salivation, Lacrimation, Urination, Digestion, Defecation); decreases heart rate, constricts pupils.

ANS Neurons and Pathways

Organization and Neurotransmitters

• Two-Neuron Pathway: Preganglionic neuron (CNS) synapses with postganglionic neuron (autonomic ganglion) which innervates target tissue.

• Sympathetic Pathways: Use acetylcholine (ACh) at ganglia, norepinephrine (NE) at effectors.

• Parasympathetic Pathways: Use acetylcholine (ACh) at both ganglia and effectors.

Table: Comparison of Somatic and Autonomic Pathways

System	Neurons	Myelination	Neurotransmitter	Effector	Effect
Somatic	Single neuron	Heavily myelinated	ACh	Skeletal muscle	Stimulatory
Autonomic (Sympathetic)	Two neurons	Lightly myelinated preganglionic, nonmyelinated postganglionic	ACh, NE	Smooth/cardiac muscle, glands	Stimulatory or inhibitory
Autonomic (Parasympathetic)	Two neurons	Lightly myelinated preganglionic, nonmyelinated postganglionic	ACh	Smooth/cardiac muscle, glands	Stimulatory or inhibitory

CNS: Key Structures Overview

Major Brain Regions

• Cerebrum: Hemispheres, lobes, basal nuclei (ganglia), limbic system.

• Diencephalon: Thalamus, hypothalamus, pituitary gland, pineal gland.

• Brainstem: Medulla oblongata, pons, midbrain, reticular activating system (RAS).

• Cerebellum: Coordination of movement and balance.

• Spinal Cord: Conduction pathway and integration center.

Cerebral Hemispheres

Functional Specialization

• Each hemisphere controls voluntary movement and sensory function of the opposite (contralateral) side of the body.

• Motor Cortex: Initiates voluntary movements.

• Basal Nuclei: Regulate movement, part of extrapyramidal system.

• Limbic System: Emotion and memory.

• Left Hemisphere (LH): Language, logic (usually dominant).

• Right Hemisphere (RH): Creativity, emotion, spatial relationships.

Cerebral Lobes and Areas

Functional Regions

• Frontal Lobe: Motor cortex, personality, decision making.

• Parietal Lobe: Sensory cortex, spatial processing.

• Temporal Lobe: Auditory processing, memory.

• Occipital Lobe: Visual processing.

Protection for the Brain

Physical and Chemical Barriers

• Skull: Bony protection.

• Meninges: Three layers—dura mater, arachnoid, pia mater.

• Cerebrospinal Fluid (CSF): Cushions brain, maintains chemical environment; produced and reabsorbed at equal rates.

• Blood Brain Barrier: Selective permeability protects brain from harmful substances.

The Cranial Nerves

Names, Numbers, and Functions

• I – Olfactory: Sensory (smell)

• II – Optic: Sensory (vision)

• III – Oculomotor: Motor (eye movement)

• IV – Trochlear: Motor (eye movement)

• V – Trigeminal: Both (facial sensation, chewing)

• VI – Abducens: Motor (eye movement)

• VII – Facial: Both (facial expression, taste)

• VIII – Acoustic/Vestibulocochlear: Sensory (hearing, balance)

• IX – Glossopharyngeal: Both (taste, swallowing)

• X – Vagus: Both (autonomic control of viscera)

• XI – Accessory: Motor (neck muscles)

• XII – Hypoglossal: Motor (tongue movement)

Table: Cranial Nerves—Sensory, Motor, or Both

Nerve	Type
I – Olfactory	Sensory
II – Optic	Sensory
III – Oculomotor	Motor
IV – Trochlear	Motor
V – Trigeminal	Both
VI – Abducens	Motor
VII – Facial	Both
VIII – Acoustic/Vestibulocochlear	Sensory
IX – Glossopharyngeal	Both
X – Vagus	Both
XI – Accessory	Motor
XII – Hypoglossal	Motor

CNS Structures: Diencephalon

Key Components

• Thalamus: Relay station for sensory information.

• Hypothalamus: Homeostasis, controls autonomic nervous system and endocrine system.

• Pituitary Gland: Master endocrine gland, regulated by hypothalamus.

• Pineal Gland: Secretes melatonin, regulates circadian rhythms.

Additional info: These notes are based on lecture slides and include expanded academic context for clarity and completeness.