Comprehensive Study Notes: The Nervous System (Anatomy & Physiology)

The Nervous System

Overview and Functions

The nervous system is a complex network responsible for coordinating body activities and transmitting signals between different parts of the body. It is essential for sensation, movement, cognition, and homeostasis.

• Central Nervous System (CNS): Composed of the brain and spinal cord; integrates sensory information and coordinates responses.

• Peripheral Nervous System (PNS): Consists of cranial and spinal nerves; connects the CNS to limbs and organs.

• Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion).

• Functions: Sensory input, integration, motor output, and regulation of homeostasis.

Organization of the Nervous System

The nervous system is organized into several divisions based on structure and function.

• Central Nervous System: Brain and spinal cord.

• Peripheral Nervous System: Cranial nerves, spinal nerves, ganglia.

• Afferent Division: Sensory pathways to the CNS.

• Efferent Division: Motor pathways from the CNS.

• Autonomic Nervous System: Sympathetic and parasympathetic divisions.

Supporting Cells of the Nervous System

Neuroglia, or glial cells, support and protect neurons in the nervous system.

• Astrocytes: Maintain the blood-brain barrier, provide nutrients, and regulate ion balance.

• Microglia: Act as immune cells, removing debris and pathogens.

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

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Schwann Cells: Form myelin sheaths in the PNS.

• Satellite Cells: Support neurons in peripheral ganglia.

Classification and Properties of Neurons

Neurons are specialized cells for transmitting electrical and chemical signals.

• Structure: Cell body (soma), dendrites, axon.

• Types: Sensory (afferent), motor (efferent), interneurons.

• Properties: Excitability, conductivity, secretion.

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

Key Terms in Neuroanatomy

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

• Ganglion: 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 consist of bundles of axons surrounded by connective tissue layers.

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds fascicles (bundles of axons).

• Epineurium: Surrounds the entire nerve.

Electrophysiology of Neurons

Neurons communicate via electrical signals known as action potentials.

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

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

• Graded Potential: Local changes in membrane potential that vary in size.

Key Equation:

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

Action Potentials and Their Propagation

Action potentials are generated by the movement of ions across the neuronal membrane.

• Depolarization: Influx of Na+ ions.

• Repolarization: Efflux of K+ ions.

• Factors Affecting Velocity: Axon diameter, myelination.

Electrical vs. Chemical Synapses

Synapses are junctions where neurons communicate with other cells.

• Electrical Synapse: Direct passage of ions via gap junctions; rapid transmission.

• Chemical Synapse: Neurotransmitter release; slower but more versatile.

• Excitatory vs. Inhibitory: Excitatory synapses increase likelihood of action potential; inhibitory decrease it.

Integration of Synaptic Events

Neurons integrate multiple synaptic inputs to determine their response.

• Temporal Summation: Multiple signals in quick succession.

• Spatial Summation: Signals from multiple locations.

• Presynaptic Inhibition: Reduction of neurotransmitter release.

• Neuromodulation: Regulation of synaptic activity by other chemicals.

• Convergence/Divergence: Multiple inputs to one neuron (convergence); one neuron to multiple targets (divergence).

Neurotransmitters: Chemistry and Function

Neurotransmitters are chemicals that transmit signals across synapses.

• Classification: Amino acids, peptides, monoamines, acetylcholine.

• Function: Excitatory or inhibitory effects on postsynaptic cells.

Major Structures of the Brain

The brain consists of several regions, each with specialized functions.

• Cerebrum: Gyri, sulci, lobes, cortex, white matter.

• Diencephalon: Thalamus, hypothalamus, epithalamus.

• Brainstem: Midbrain, pons, medulla oblongata.

• Cerebellum: Coordination and balance.

• Limbic System: Emotion and memory.

• Reticular Formation: Arousal and consciousness.

Ventricular System and Cerebrospinal Fluid (CSF)

The ventricular system produces and circulates CSF, which cushions and nourishes the brain.

• Ventricles: Lateral, third, fourth ventricles.

• Cerebral Aqueduct: Connects third and fourth ventricles.

• CSF Pathway: Produced in choroid plexus, circulates through ventricles and subarachnoid space.

Meninges

The meninges are protective membranes covering the brain and spinal cord.

• Dura Mater: Tough outer layer.

• Arachnoid Mater: Middle, web-like layer.

• Pia Mater: Delicate inner layer.

Blood-Brain Barrier

The blood-brain barrier protects the brain from harmful substances while allowing essential nutrients to pass.

• Structure: Tight junctions between endothelial cells.

• Function: Selective permeability.

Ascending and Descending Tracts

Tracts are bundles of axons that carry information up and down the spinal cord.

• Ascending Tracts: Sensory pathways to the brain.

• Descending Tracts: Motor pathways from the brain.

Reflexes

Reflexes are rapid, involuntary responses to stimuli.

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

• Types: Stretch reflex, tendon reflex, withdrawal reflex, crossed extensor reflex.

Cranial Nerves

Cranial nerves are twelve pairs of nerves that emerge from the brain and control sensory and motor functions of the head and neck.

• Examples: Olfactory (I), Optic (II), Vagus (X).

• Functions: Smell, vision, taste, facial movement, etc.

Spinal Nerves and Plexuses

Spinal nerves connect the spinal cord to the body and form plexuses that innervate limbs.

• Plexuses: Cervical, brachial, lumbar, sacral.

• Rami: Branches of spinal nerves.

Somatic vs. Autonomic Nervous System

The somatic nervous system controls voluntary movements; the autonomic nervous system regulates involuntary functions.

• Somatic: Skeletal muscle control.

• Autonomic: Smooth muscle, cardiac muscle, glands.

Sympathetic vs. Parasympathetic Divisions

These are subdivisions of the autonomic nervous system with opposing effects.

• Sympathetic: "Fight or flight" responses; thoracolumbar origin.

• Parasympathetic: "Rest and digest" responses; craniosacral origin.

• Neurotransmitters: Acetylcholine, norepinephrine.

Cholinergic vs. Adrenergic Effects

Neurons are classified by the neurotransmitters they release.

• Cholinergic: Release acetylcholine.

• Adrenergic: Release norepinephrine or epinephrine.

• Receptors: Nicotinic, muscarinic (cholinergic); alpha, beta (adrenergic).

Effects of the SNS and PNS on Organs

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

Organ	Sympathetic Effect	Parasympathetic Effect
Iris of Eye	Dilates pupil	Constricts pupil
Digestive Glands	Decreases secretion	Increases secretion
Heart Muscle	Increases rate and force	Decreases rate
Blood Vessels	Constriction (most)	No direct effect
Bronchioles	Dilates	Constricts
Liver	Stimulates glucose release	Promotes glycogen synthesis

Additional info: Table entries inferred from standard physiology textbooks.