Comprehensive Study Notes: The Nervous System (Structure, Function, and Physiology)

Study Guide - Smart Notes

Tailored notes based on your materials, expanded with key definitions, examples, and context.

The Nervous System

Overview and Organization

The nervous system is responsible for coordinating sensory data, motor commands, and higher functions such as thought and emotion. It is divided into the central nervous system (CNS) and the peripheral nervous system (PNS).

CNS: Consists of the brain and spinal cord; processes and integrates information.
PNS: Delivers sensory information to the CNS and carries motor commands to peripheral tissues and systems.

Functional Divisions

Somatic Nervous System (SNS): Controls voluntary skeletal muscle contractions.
Autonomic Nervous System (ANS): Controls involuntary actions, such as smooth muscle, cardiac muscle, and glandular activity. Includes sympathetic and parasympathetic divisions.

Neural Tissue: Structure and Function

Neurons

Neurons are the functional units of the nervous system, specialized for the transmission of electrical impulses.

Cell Body (Soma): Contains the nucleus and organelles.
Dendrites: Receive incoming signals.
Axon: Conducts electrical impulses (action potentials) away from the cell body.
Synapse: Site where a neuron communicates with another cell.

Neuroglia (Glial Cells)

Neuroglia support and protect neurons. Types include:

Astrocytes: Maintain the blood-brain barrier, regulate ion/nutrient concentrations, and provide structural support.
Ependymal Cells: Line ventricles of the brain and central canal of the spinal cord; produce and circulate cerebrospinal fluid (CSF).
Oligodendrocytes: Myelinate axons in the CNS.
Schwann Cells: Myelinate axons in the PNS.
Microglia: Remove cell debris, waste, and pathogens by phagocytosis.

Membrane Potentials and Neural Signaling

Resting Membrane Potential

The resting membrane potential is the electrical charge difference across the neuron's plasma membrane, typically around -70 mV.

Maintained by the sodium-potassium pump and selective permeability of the membrane.
Electrochemical Gradient: Difference in ion concentration and charge across the membrane.

Ion Channels

Leak Channels: Always open; contribute to resting potential.
Gated Channels: Open or close in response to stimuli (chemical, voltage, or mechanical).

Graded Potentials vs. Action Potentials

Graded (Local) Potential	Action Potential (Nerve Impulse)
Small, localized changes in membrane potential	Large, rapid, all-or-none electrical signal
Decreases with distance from stimulus	Propagates along the entire axon without decrement
Can be depolarizing or hyperpolarizing	Always depolarizing (followed by repolarization)

Generation of an Action Potential

Resting State: All voltage-gated channels closed.
Depolarization: Na+ channels open, Na+ enters cell, membrane potential becomes positive.
Repolarization: Na+ channels inactivate, K+ channels open, K+ leaves cell, membrane potential returns to negative.
Hyperpolarization: K+ channels remain open briefly, membrane potential becomes more negative than resting.

Refractory Period: Time during which a neuron cannot fire another action potential or requires a stronger stimulus.

Propagation of Action Potentials

Continuous Conduction: Occurs in unmyelinated axons; slower.
Saltatory Conduction: Occurs in myelinated axons; action potential "jumps" from node to node (Nodes of Ranvier), increasing speed and efficiency.

Synapses and Neurotransmitters

Types of Synapses

Electrical Synapse: Direct physical connection via gap junctions; rare in adult nervous system.
Chemical Synapse: Most common; neurotransmitter released from presynaptic neuron binds to receptors on postsynaptic cell.

Neurotransmitters

Acetylcholine (ACh): Common in both CNS and PNS; can be excitatory or inhibitory.
Other Neurotransmitters: Norepinephrine, dopamine, serotonin, GABA, glycine.

Types of Receptors

Nicotinic ACh Receptors: Ligand-gated ion channels; found at neuromuscular junctions.
Muscarinic ACh Receptors: G-protein coupled receptors; found in parasympathetic target organs.

Sensory Pathways and the Somatic Nervous System

Sensory Receptors

Mechanoreceptors: Detect touch, pressure, vibration, stretch.
Thermoreceptors: Detect temperature changes.
Nociceptors: Detect pain (tissue damage).
Proprioceptors: Monitor position of joints and muscles.

Somatic Motor Pathways

Upper Motor Neurons: Originate in the CNS; control lower motor neurons.
Lower Motor Neurons: Originate in the spinal cord or brainstem; directly innervate skeletal muscle.

Feature	Upper Motor Neuron (UMN)	Lower Motor Neuron (LMN)
Location	Brain (cerebral cortex or brainstem)	Spinal cord (anterior horn) or cranial nerve nuclei
Function	Initiate and control voluntary movement	Directly innervate skeletal muscle
Direct Muscle Contact	No	Yes

The Autonomic Nervous System (ANS)

Divisions of the ANS

Sympathetic Division: Prepares the body for "fight or flight"; increases heart rate, dilates pupils, inhibits digestion.
Parasympathetic Division: Promotes "rest and digest"; decreases heart rate, stimulates digestion, conserves energy.

Organization of the ANS

Preganglionic Neuron: Cell body in CNS; axon extends to autonomic ganglion.
Postganglionic Neuron: Cell body in ganglion; axon extends to effector organ.

Neurotransmitters and Receptors in the ANS

Sympathetic: Preganglionic neurons release ACh; postganglionic neurons release norepinephrine (NE) or ACh.
Parasympathetic: Both pre- and postganglionic neurons release ACh.

Table: Comparison of Sympathetic and Parasympathetic Divisions

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar (T1-L2)	Craniosacral (brainstem & S2-S4)
Ganglia Location	Near spinal cord (chain & collateral ganglia)	Near or in target organs
Preganglionic Fiber Length	Short	Long
Postganglionic Fiber Length	Long	Short
Main Neurotransmitter (Postganglionic)	Norepinephrine (NE)	Acetylcholine (ACh)
General Effect	"Fight or flight"	"Rest and digest"

Spinal Cord and Spinal Nerves

Structure of the Spinal Cord

Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons.
White Matter: Contains myelinated axons organized into tracts.
Central Canal: Filled with cerebrospinal fluid (CSF).

Spinal Meninges

Dura Mater: Outermost, tough layer.
Arachnoid Mater: Middle, web-like layer; subarachnoid space filled with CSF.
Pia Mater: Innermost, delicate layer adhering to the spinal cord.

Spinal Nerves

Mixed nerves containing both sensory (afferent) and motor (efferent) fibers.
Each spinal nerve is associated with a specific region of skin called a dermatome.

Reflexes and Reflex Arcs

Reflex Arc

A reflex arc is the route followed by nerve impulses to produce a reflex action. It typically involves:

Receptor
Sensory neuron
Integration center (CNS)
Motor neuron
Effector

Reflexes are rapid, automatic responses to stimuli that help maintain homeostasis and protect the body.

The Brain: Structure and Function

Major Regions of the Brain

Cerebrum: Largest part; responsible for conscious thought, memory, and voluntary movement.
Cerebellum: Coordinates movement and balance.
Diencephalon: Includes thalamus and hypothalamus; relays sensory information and controls autonomic functions.
Brainstem: Includes midbrain, pons, and medulla oblongata; controls vital functions such as heart rate and breathing.

Ventricles of the Brain

Four ventricles: two lateral, third, and fourth ventricles.
Filled with cerebrospinal fluid (CSF), which cushions and protects the brain.

Blood Supply and the Blood-Brain Barrier (BBB)

Brain receives a rich blood supply to meet high metabolic demands.
Blood-Brain Barrier: Formed by tight junctions between endothelial cells; restricts passage of substances from blood to brain tissue.

Electroencephalogram (EEG) and Brain Waves

EEG records electrical activity of the brain.
Different brain waves (alpha, beta, theta, delta) correspond to different states of brain activity.

Clinical Correlations

Concussion: Temporary loss of brain function due to trauma.
Epilepsy: Disorder characterized by recurrent seizures due to abnormal electrical activity in the brain.
Multiple Sclerosis: Demyelinating disease affecting CNS myelin.

Summary Table: Types of Neuroglia and Their Functions

Neuroglia	Location	Function
Astrocytes	CNS	Maintain BBB, structural support, regulate ions/nutrients
Oligodendrocytes	CNS	Myelinate CNS axons
Microglia	CNS	Phagocytosis of debris/pathogens
Ependymal cells	CNS	Produce/circulate CSF
Schwann cells	PNS	Myelinate PNS axons
Satellite cells	PNS	Support neuron cell bodies in ganglia

Key Equations

Nernst Equation (for equilibrium potential):

Ohm's Law (for membrane current):

Additional info:

Some explanations and context have been expanded for clarity and completeness.
Tables have been recreated and summarized for study purposes.