Fundamentals of the Nervous System: Study Guide for Anatomy & Physiology

Chapter 11 – Fundamentals of the Nervous System

Organization of the Nervous System

The nervous system is divided into several major components, each with distinct functions and structures. Understanding these divisions is essential for grasping how the body processes and responds to information.

• Central Nervous System (CNS): Composed of the brain and spinal cord; responsible for integrating sensory information and responding accordingly.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; subdivided into the somatic and autonomic nervous systems.

• Somatic Nervous System: Controls voluntary movements via skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion); further divided into sympathetic and parasympathetic divisions.

Example: The sympathetic division prepares the body for 'fight or flight' responses, while the parasympathetic division promotes 'rest and digest' activities.

Types of Cells in the Nervous System

The nervous system contains specialized cells that perform unique functions.

• Neurons: The primary signaling cells; transmit electrical impulses.

• Glial Cells: Support, protect, and nourish neurons. Types include astrocytes, oligodendrocytes, microglia, and Schwann cells.

Example: Astrocytes maintain the blood-brain barrier and regulate ion concentrations.

Channels, Pumps, and Gradients in Neurons

Neuronal function depends on the movement of ions across the cell membrane, which is regulated by various channels and pumps.

• K+ and Na+ Leak Channels: Allow passive movement of potassium and sodium ions, contributing to the resting membrane potential.

• Na+/K+ Pump: Actively transports 3 Na+ out and 2 K+ into the cell, maintaining concentration gradients.

Equation:

$ext{Na}^+/ ext{K}^+ ext{ATPase}: 3 ext{Na}^+_{ ext{out}} + 2 ext{K}^+_{ ext{in}}$

Action Potentials in Neurons

An action potential is a rapid change in membrane potential that allows neurons to transmit signals.

• Ion Channels: Voltage-gated Na+ and K+ channels open and close in response to changes in membrane potential.

• All-or-None Principle: An action potential either occurs fully or not at all, depending on whether the threshold is reached.

Equation:

$V_m = V_{rest} + ext{change due to ion flux}$

Absolute and Relative Refractory Periods

After an action potential, neurons experience periods during which they cannot or are less likely to fire another action potential.

• Absolute Refractory Period: No new action potential can be initiated.

• Relative Refractory Period: A stronger stimulus is required to initiate another action potential.

Channels Involved in Action Potentials

Different channels open and close during the phases of an action potential.

• Depolarization: Voltage-gated Na+ channels open.

• Repolarization: Voltage-gated K+ channels open.

• Hyperpolarization: K+ channels remain open briefly after repolarization.

Myelin Sheath and Conduction Speed

The myelin sheath is a fatty layer that insulates axons, increasing the speed of action potential conduction.

• Saltatory Conduction: Action potentials jump between nodes of Ranvier in myelinated axons.

• Demyelination: Diseases like multiple sclerosis disrupt myelin, slowing conduction.

Synapse Anatomy and Chemical Transmission

Synapses are junctions where neurons communicate with other cells.

• Presynaptic Neuron: Releases neurotransmitters.

• Postsynaptic Cell: Receives the signal via receptors.

• Chemical Transmission: Neurotransmitters cross the synaptic cleft to bind receptors.

IPSPs and EPSPs

Postsynaptic potentials can be excitatory (EPSP) or inhibitory (IPSP).

• EPSP: Depolarizes the postsynaptic membrane, increasing likelihood of action potential.

• IPSP: Hyperpolarizes the membrane, decreasing likelihood of action potential.

• Summation: Multiple EPSPs/IPSPs can combine spatially or temporally.

G Protein-Linked Receptors

These receptors initiate intracellular signaling cascades in response to neurotransmitter binding.

• Sequence of Events: Neurotransmitter binds → G protein activated → Second messenger produced → Cellular response.

Neurotransmitter Classification

Neurotransmitters are chemicals that transmit signals across synapses.

• Common Types: Acetylcholine, dopamine, serotonin, norepinephrine, GABA, glutamate.

• Classification: By chemical structure (amines, amino acids, peptides) or function (excitatory/inhibitory).

Chapter 12 – Central Nervous System (CNS)

General Organization of the Brain

The brain is organized into four major regions, each with specialized functions.

• Lobes: Frontal, parietal, temporal, occipital.

• Fissures and Gyri/Sulci: Increase surface area and separate functional regions.

Gray Matter vs. White Matter

Brain tissue is classified based on the presence of neuron cell bodies and myelinated axons.

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; involved in processing and integration.

• White Matter: Composed of myelinated axons; responsible for communication between brain regions.

Brain Regions and Functions

Each region of the brain has distinct functions.

• Cerebral Cortex: Higher cognitive functions, sensory perception, voluntary movement.

• Brain Stem: Basic life functions (breathing, heart rate).

• Cerebellum: Coordination and balance.

Wernicke's and Broca's Areas

These areas are critical for language processing.

• Wernicke's Area: Language comprehension; damage leads to receptive aphasia.

• Broca's Area: Speech production; damage leads to expressive aphasia.

Ventricles and Cerebrospinal Fluid (CSF)

The brain contains interconnected cavities called ventricles, filled with CSF.

• Function: Protects and nourishes the brain, removes waste.

• Key Structures: Septum pellucidum, cerebral aqueduct, interventricular foramen.

Corpus Callosum

The corpus callosum is a large bundle of nerve fibers connecting the two cerebral hemispheres, enabling communication between them.

Homunculus and Somatosensory Cortex

The homunculus is a visual representation of the body mapped onto the somatosensory cortex, illustrating the relative sensory input from different body regions.

Thalamus, Hypothalamus, and Other Brain Structures

These structures are involved in sensory relay, homeostasis, and autonomic control.

• Thalamus: Sensory relay station.

• Hypothalamus: Regulates hormones, temperature, hunger.

• Midbrain, Pons, Medulla: Involved in basic life functions and reflexes.

Limbic System

The limbic system is involved in emotion, memory, and motivation.

• Key Structures: Hippocampus (memory), amygdala (emotion).

Brain Protection

The brain is protected by several physical and chemical barriers.

• Cerebrospinal Fluid (CSF): Cushions the brain.

• Meninges: Three layers (dura mater, arachnoid mater, pia mater).

• Blood-Brain Barrier: Selectively restricts passage of substances.

• Skull: Provides structural protection.

Spinal Cord Organization

The spinal cord transmits signals between the brain and body and is organized into distinct regions.

• Gray Matter: Central region; contains cell bodies.

• White Matter: Surrounds gray matter; contains myelinated axons.

• Spinal Cord vs. Spinal Column: The cord is nervous tissue; the column is bony protection.

Peripheral Nervous System (PNS)

Nomenclature and Organization of Spinal Nerves

Spinal nerves are named and organized based on their location and function.

• Dorsal Roots: Carry sensory information to the CNS.

• Ventral Roots: Carry motor information from the CNS.

• Dorsal/Ventral Rami: Branches of spinal nerves serving different body regions.

Dermatomes

A dermatome is an area of skin supplied by a single spinal nerve.

Reflex Arcs

Reflex arcs are neural pathways that mediate automatic responses to stimuli.

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

• Stretch Reflex: Example is the knee-jerk reflex.

• Withdrawal Reflex: Protects the body from harm (e.g., touching a hot object).

• Polysynaptic Activation: Involves multiple synapses and can activate muscles on the opposite side of the body.

Autonomic Nervous System (ANS)

Divisions and Anatomy

The ANS controls involuntary bodily functions and is divided into sympathetic and parasympathetic systems.

• Sympathetic Nervous System (SNS): Prepares the body for stress-related activities.

• Parasympathetic Nervous System (PSNS): Conserves energy and promotes 'rest and digest' functions.

• Cell Bodies, Projections, Neurotransmitters: SNS uses norepinephrine; PSNS uses acetylcholine.

Functions and Actions

• SNS: Increases heart rate, dilates pupils, inhibits digestion.

• PSNS: Decreases heart rate, constricts pupils, stimulates digestion.

Cholinergic and Adrenergic Receptors

Receptors mediate the effects of neurotransmitters in the ANS.

• Cholinergic Receptors: Nicotinic (N1, N2) and muscarinic types; respond to acetylcholine.

• Adrenergic Receptors: Alpha (α1, α2) and beta (β1, β2); respond to norepinephrine and epinephrine.

Receptor Type	Location	Function
Nicotinic (N1, N2)	Neuromuscular junction, autonomic ganglia	Excitatory, muscle contraction
Muscarinic	Parasympathetic target organs	Varied (e.g., slows heart rate, stimulates digestion)
Alpha 1	Blood vessels	Vasoconstriction
Alpha 2	Presynaptic terminals	Inhibits neurotransmitter release
Beta 1	Heart	Increases heart rate and force
Beta 2	Bronchioles, blood vessels	Bronchodilation, vasodilation

Example: Beta-2 agonists are used to treat asthma by dilating bronchioles.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.