The Nervous System: Structure, Function, and Organization

The Nervous System

Overview of Nervous System Functions

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts of the body. It enables rapid communication, integration, and response to internal and external stimuli.

• Sensory Input: Detects changes in the environment (internal and external) via sensory receptors.

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

• Motor Output: Initiates responses by activating muscles or glands (effectors).

Organization of the Nervous System

The nervous system is organized into central and peripheral divisions, each with specialized structures and functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and command.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; connects the CNS to limbs and organs.

• Spinal Nerves: 31 pairs; connect the spinal cord to the body.

• Cranial Nerves: 12 pairs; connect the brain to the head and neck.

• Afferent Division: Sensory pathways carrying information to the CNS.

• Efferent Division: Motor pathways carrying commands from the CNS to effectors.

• Autonomic Nervous System (ANS): Subdivision of the efferent division; controls involuntary functions (e.g., heart rate, digestion).

Supporting Cells of the Nervous System (Neuroglia)

Neuroglia are non-neuronal cells that support, protect, and nourish neurons.

• Astrocytes: Maintain the blood-brain barrier, regulate ion/nutrient balance, and support neurons (CNS).

• Microglia: Act as phagocytes, removing debris and pathogens (CNS).

• Ependymal Cells: Line ventricles of the brain and central canal of the spinal cord; produce and circulate cerebrospinal fluid (CNS).

• Oligodendrocytes: Form myelin sheaths around CNS axons, increasing conduction speed.

• Schwann Cells: Form myelin sheaths around PNS axons; aid in regeneration.

• Satellite Cells: Surround neuron cell bodies in PNS ganglia; regulate environment.

Neurons: Structure, Function, and Classification

Neurons are the functional units of the nervous system, specialized for communication.

• Properties: Excitability, conductivity, secretion, longevity, and amitotic nature.

• Structure: Cell body (soma), dendrites (receive input), axon (transmits impulses), axon terminals (release neurotransmitters).

• Function: Transmit electrical and chemical signals.

• Classification: Based on structure (multipolar, bipolar, unipolar) and function (sensory, motor, interneurons).

Example: Motor neurons transmit impulses from the CNS to muscles, causing contraction.

Key Terms in Nervous System Anatomy

• Nucleus: Cluster of neuron cell bodies within the CNS.

• Ganglia: Cluster of neuron cell bodies in the PNS.

• Tract (Fascicle): Bundle of axons in the CNS.

• Nerve: Bundle of axons in the PNS.

Anatomy of a Peripheral Nerve

Peripheral nerves are composed of bundles of axons (fascicles) surrounded by connective tissue layers: endoneurium (around individual axons), perineurium (around fascicles), and epineurium (around the entire nerve).

Electrophysiology of Neurons

Neurons communicate via electrical signals generated by ion movement across membranes.

• Resting Membrane Potential: The voltage difference across the membrane at rest, typically -70 mV.

• Action Potential: Rapid, transient change in membrane potential that propagates along the axon.

• Graded Potential: Local changes in membrane potential; decrease with distance from stimulus.

Equation: Nernst equation for equilibrium potential: $E_{ion} = \frac{RT}{zF} \ln \frac{[ion]_{out}}{[ion]_{in}}$

Propagation of Action Potentials

• Continuous Conduction: Occurs in unmyelinated axons; slower.

• Saltatory Conduction: Occurs in myelinated axons; action potentials jump between nodes of Ranvier, increasing speed.

• Factors Affecting Velocity: Axon diameter (larger = faster), myelination (myelinated = faster).

Synapses: Electrical vs. Chemical

• Electrical Synapses: Direct cytoplasmic connections (gap junctions); allow rapid, bidirectional flow.

• Chemical Synapses: Use neurotransmitters to transmit signals across synaptic cleft; can be excitatory or inhibitory.

Integration of Synaptic Events

• Temporal Summation: Multiple signals from one presynaptic neuron over time.

• Spatial Summation: Simultaneous signals from multiple presynaptic neurons.

• Presynaptic Inhibition: Decreased neurotransmitter release from presynaptic neuron.

• Neuromodulation: Regulation of synaptic transmission by other chemicals.

• Convergence: Multiple inputs to a single neuron.

• Divergence: One neuron sends output to multiple targets.

Classification of Neurotransmitters

• By Chemistry: Acetylcholine, biogenic amines (e.g., norepinephrine, dopamine), amino acids (e.g., GABA, glutamate), peptides (e.g., endorphins).

• By Function: Excitatory (e.g., glutamate), inhibitory (e.g., GABA), modulatory.

Divisions of the Brain

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

• Diencephalon: Thalamus (relay center), hypothalamus (homeostasis), epithalamus.

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

• Cerebellum: Coordinates movement and balance.

Cranial Meninges

• Dura Mater: Tough, outermost layer.

• Arachnoid Mater: Middle, web-like layer.

• Pia Mater: Thin, innermost layer; adheres to brain surface.

• Function: Protect CNS, contain cerebrospinal fluid (CSF).

Cerebrospinal Fluid (CSF)

• Function: Cushions CNS, removes waste, provides nutrients.

• Composition: Clear, colorless fluid; low protein, few cells.

• Production: By choroid plexus in ventricles.

• Circulation Pathway: Lateral ventricles → third ventricle → cerebral aqueduct → fourth ventricle → subarachnoid space → arachnoid villi → venous circulation.

Blood Brain Barrier (BBB)

• Structure: Endothelial cells with tight junctions, astrocyte end-feet.

• Function: Regulates passage of substances from blood to CNS, protects brain from toxins.

Spinal Cord: Structure and Function

• Structure: Cylindrical, extends from foramen magnum to L1/L2 vertebra.

• Function: Conducts sensory and motor information; mediates reflexes.

Spinal Meninges

• Same three layers as cranial meninges; protect spinal cord, contain CSF.

Ascending and Descending Tracts

• Ascending Tracts: Carry sensory information to the brain (e.g., spinothalamic tract).

• Descending Tracts: Carry motor commands from the brain (e.g., corticospinal tract).

Reflexes and Reflex Arcs

• Reflex: Rapid, involuntary response to a stimulus.

• Components of Reflex Arc: Receptor, sensory neuron, integration center, motor neuron, effector.

• Types of Reflexes:

  • Cranial vs. Spinal

  • Inborn (innate) vs. Acquired

  • Somatic vs. Autonomic

  • Deep tendon, flexor (stretch), crossed extensor reflexes

Cranial Nerves

• 12 pairs; arise from brain; serve head and neck structures.

• Each has specific sensory, motor, or mixed functions.

Spinal Nerves, Plexuses, and Rami

• Spinal Nerves: 31 pairs; mixed nerves (sensory and motor fibers).

• Plexuses: Networks of nerves (cervical, brachial, lumbar, sacral) supplying limbs.

• Rami: Branches of spinal nerves; dorsal and ventral rami innervate different regions.

Somatic vs. Autonomic Nervous System

• Somatic: Voluntary control of skeletal muscles; single neuron pathway.

• Autonomic: Involuntary control of smooth/cardiac muscle, glands; two-neuron pathway (preganglionic and postganglionic).

Sympathetic vs. Parasympathetic Nervous System

Feature	Sympathetic	Parasympathetic
Overall Structure	Thoracolumbar origin	Craniosacral origin
Preganglionic Neurons	Short	Long
Postganglionic Neurons	Long	Short
Autonomic Ganglia	Sympathetic chain, collateral ganglia	Terminal ganglia (near/within target organs)
Neurotransmitters	Acetylcholine (preganglionic), norepinephrine (postganglionic)	Acetylcholine (both)

Cholinergic vs. Adrenergic Fibers

Fiber Type	Neurotransmitter Released	Location in ANS	Receptor Utilized
Cholinergic	Acetylcholine	All preganglionic neurons, parasympathetic postganglionic neurons	Nicotinic, muscarinic
Adrenergic	Norepinephrine (and epinephrine)	Sympathetic postganglionic neurons	Alpha, beta adrenergic

Effects of Sympathetic and Parasympathetic Stimulation

Organ/System	Sympathetic Effect	Parasympathetic Effect
Iris of the Eye	Dilates pupil	Constricts pupil
Digestive Glands	Decreases secretion	Increases secretion
Sweat Glands	Increases secretion	No effect
Adrenal Glands	Stimulates secretion of epinephrine/norepinephrine	No effect
Heart Muscle	Increases rate and force	Decreases rate
Blood Vessels of Heart	Dilates	No significant effect
Bronchioles	Dilates	Constricts
Digestive Tract Muscle	Decreases motility	Increases motility
Liver	Stimulates glucose release	Stimulates glycogen synthesis
Blood Vessels (General)	Constriction (most)	Little or no effect

Additional info: Some details (e.g., specific neurotransmitters, tract names, and reflex types) are expanded for academic completeness.