The Nervous System: Structure, Function, and Organization

The Nervous System

Functions of the Nervous System

The nervous system is responsible for coordinating and regulating bodily functions through rapid communication and integration of sensory input and motor output.

• Sensory Input: Detects changes in the internal and external environment.

• Integration: Processes and interprets sensory information to determine appropriate responses.

• Motor Output: Initiates responses by activating effector organs (muscles and glands).

• Homeostasis: Maintains internal balance by regulating physiological processes.

• Mental Activity: Enables consciousness, memory, and learning.

Organization of the Nervous System

The nervous system is divided into structural and functional components.

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

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS; transmits information to and from the CNS.

• 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; carry information toward the CNS.

• Efferent Division: Motor pathways; carry information away from the CNS.

• Autonomic Nervous System (ANS): Controls involuntary functions; includes sympathetic and parasympathetic divisions.

Supporting Cells of the Nervous System (Neuroglia)

Neuroglia provide structural and functional support for neurons.

• Astrocytes: Maintain blood-brain barrier, regulate ion concentrations, and support neurons.

• Microglia: Act as immune cells; remove debris and pathogens.

• Ependymal Cells: Line ventricles; produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Schwann Cells: Form myelin sheaths in the PNS.

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

Neurons: Properties, Structure, Function, and Classification

Neurons are the functional units of the nervous system, specialized for transmitting electrical signals.

• Characteristic Properties: Excitability, conductivity, and secretion.

• Structure: Consists of cell body (soma), dendrites (receive signals), and axon (transmits signals).

• Function: Sensory, motor, or interneuron roles.

• Classification:

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

  • Bipolar: One dendrite, one axon (sensory organs).

  • Unipolar: Single process (sensory neurons).

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 surrounded by connective tissue.

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds fascicles (bundles of axons).

• Epineurium: Surrounds the entire nerve.

Electrophysiology of Neurons

Neurons generate and transmit electrical signals through changes in membrane potential.

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

• Action Potential: Rapid, transient change in membrane potential; all-or-none response.

• Graded Potential: Local changes in membrane potential; vary in magnitude and decay with distance.

Key Equation:

$V_m = V_{inside} - V_{outside}$

Example: Action potentials are used to transmit signals along axons to communicate with other neurons or effectors.

Propagation of Action Potentials

Action potentials travel along axons by depolarizing adjacent regions.

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

• Saltatory Conduction: In myelinated axons, action potentials jump between nodes of Ranvier.

Synapses: Electrical vs. Chemical

Synapses are junctions where neurons communicate.

• Electrical Synapses: Direct transmission via gap junctions; fast, bidirectional.

• Chemical Synapses: Use neurotransmitters; can be excitatory or inhibitory.

Example: Chemical synapses are predominant in the human nervous system.

Integration of Synaptic Events

Neurons integrate multiple synaptic inputs to determine their response.

• Temporal Summation: Multiple signals in quick succession from one synapse.

• Spatial Summation: Signals from multiple synapses at the same time.

• Presynaptic Inhibition: Reduction of neurotransmitter release by another neuron.

• Neuromodulation: Regulation of synaptic activity by chemicals.

• Convergence: Multiple neurons synapse onto one neuron.

• Divergence: One neuron synapses onto multiple neurons.

Classification of Neurotransmitters

Neurotransmitters are classified by their chemical structure and function.

• Acetylcholine: Excitatory at neuromuscular junctions.

• Amino Acids: Glutamate (excitatory), GABA (inhibitory).

• Monoamines: Dopamine, serotonin, norepinephrine.

• Peptides: Substance P, endorphins.

Divisions of the Brain

The brain is divided into four major regions, each with distinct functions.

• Cerebrum: Higher functions; sensory perception, voluntary movement, reasoning.

• Diencephalon: Includes thalamus and hypothalamus; relay and regulation.

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

• Cerebellum: Coordination and balance.

Cranial Meninges

Meninges are protective membranes surrounding the brain and spinal cord.

• Dura Mater: Tough, outer layer.

• Arachnoid Mater: Middle, web-like layer.

• Pia Mater: Delicate, inner layer.

Function: Protect CNS, contain CSF, support blood vessels.

Cerebrospinal Fluid (CSF)

CSF cushions the brain and spinal cord, provides nutrients, and removes waste.

• Composition: Clear, watery fluid; contains ions, glucose, proteins.

• Production: Produced by choroid plexus in ventricles.

• Pathway: Ventricles → subarachnoid space → absorbed into venous blood.

Blood Brain Barrier

The blood-brain barrier protects the brain from harmful substances.

• Structure: Tight junctions between endothelial cells, astrocyte end-feet.

• Function: Selectively allows passage of substances; restricts toxins and pathogens.

Spinal Cord: Structure, Function, and Location

The spinal cord is a major pathway for information between the brain and body.

• Structure: Cylindrical, extends from foramen magnum to lumbar region.

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

• Location: Within vertebral canal.

Spinal Meninges

Spinal meninges are similar to cranial meninges, providing protection and support.

• Dura Mater: Outermost layer.

• Arachnoid Mater: Middle layer.

• Pia Mater: Innermost layer; adheres to spinal cord.

Ascending and Descending Tracts

Tracts are bundles of axons that carry information up (ascending) or down (descending) the spinal cord.

• Ascending Tracts: Sensory pathways to the brain.

• Descending Tracts: Motor pathways from the brain.

Reflexes and Reflex Arcs

Reflexes are automatic responses to stimuli, mediated by reflex arcs.

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

• Cranial Reflex: Involves cranial nerves.

• Spinal Reflex: Involves spinal nerves.

• Inborn (Innate) Reflex: Present at birth.

• Acquired Reflex: Learned.

• Somatic Reflex: Involves skeletal muscles.

• Autonomic Reflex: Involves smooth muscle, cardiac muscle, or glands.

• Deep Tendon Reflex: E.g., knee-jerk.

• Flexor (Stretch) Reflex: Withdrawal from pain.

• Crossed Extensor Reflex: Opposite limb supports body during withdrawal.

Cranial Nerves: Structure, Function, and Location

Cranial nerves are twelve pairs that emerge from the brain, each with specific functions.

• Structure: Composed of axons, may be sensory, motor, or mixed.

• Function: Sensory input, motor output, or both.

• Location: Originate from brainstem or cerebrum.

Anatomy of Spinal Nerves, Plexuses, and Rami

Spinal nerves branch and form networks (plexuses) to innervate body regions.

• Plexuses: Cervical, brachial, lumbar, sacral.

• Rami: Branches of spinal nerves; dorsal and ventral.

• Major Spinal Nerves: E.g., sciatic, phrenic.

• Innervation: Supply muscles and skin.

Somatic vs. Autonomic Nervous System

The somatic and autonomic nervous systems differ in structure and function.

• Somatic: Voluntary control; skeletal muscles.

• Autonomic: Involuntary control; smooth muscle, cardiac muscle, glands.

Sympathetic vs. Parasympathetic Nervous System

The autonomic nervous system is divided into sympathetic and parasympathetic branches.

• Sympathetic: "Fight or flight"; thoracolumbar origin; short preganglionic, long postganglionic neurons; sympathetic ganglia near spinal cord; neurotransmitters: acetylcholine (preganglionic), norepinephrine (postganglionic).

• Parasympathetic: "Rest and digest"; craniosacral origin; long preganglionic, short postganglionic neurons; ganglia near or within target organs; neurotransmitter: acetylcholine.

Cholinergic vs. Adrenergic Fibers

Autonomic fibers are classified by the neurotransmitter they release.

• Cholinergic: Release acetylcholine; found in all preganglionic neurons and parasympathetic postganglionic neurons; act on muscarinic and nicotinic receptors.

• Adrenergic: Release norepinephrine; found in most sympathetic postganglionic neurons; act on alpha and beta receptors.

Effects of SNS and PNS on Body Regions

The sympathetic and parasympathetic systems have opposing effects on various organs.

Body Area	Sympathetic Effect	Parasympathetic Effect
Iris of the Eye	Dilates pupil	Constricts pupil
Digestive Glands	Inhibits secretion	Stimulates secretion
Sweat Glands	Stimulates secretion	No effect
Adrenal Glands	Stimulates secretion	No effect
Heart Muscle	Increases rate and force	Decreases rate
Blood Vessels of the Heart	Dilates	No effect
Bronchioles	Dilates	Constricts
Digestive Tract Muscle	Inhibits motility	Stimulates motility
Liver	Stimulates glucose release	No effect
Blood Vessels (General)	Constriction	No effect