Comprehensive Study Notes: Nervous System, Sensory Organs, and Action Potentials

Nervous System Overview

Cranial Nerves

The cranial nerves are twelve pairs of nerves that emerge directly from the brain, each with specific sensory, motor, or mixed functions.

• Names & Functions: Each cranial nerve has a unique name and function, such as olfactory (smell), optic (vision), oculomotor (eye movement), etc.

• Sensory, Motor, or Both: Cranial nerves can be classified as sensory (e.g., optic), motor (e.g., oculomotor), or both (e.g., trigeminal).

• Parasympathetic Information: Some cranial nerves (e.g., facial, glossopharyngeal, vagus) carry parasympathetic fibers that regulate involuntary functions.

• Entry/Exit Points: Cranial nerves enter or exit the skull through specific foramina and connect to the brainstem.

• Ganglion Names: Ganglia are clusters of neuron cell bodies associated with cranial nerves, such as the trigeminal ganglion.

Additional info: The cranial nerves are numbered I-XII and are essential for sensory perception, motor control, and autonomic regulation.

Autonomic Nervous System (ANS)

Structure and Function

The ANS regulates involuntary physiological functions and is divided into sympathetic and parasympathetic divisions.

• Sympathetic vs. Parasympathetic: The sympathetic division prepares the body for 'fight or flight,' while the parasympathetic division promotes 'rest and digest.'

• Structural Differences:

  • Sympathetic: Originates from thoracolumbar spinal cord, has short preganglionic and long postganglionic fibers, ganglia located near the spinal cord.

  • Parasympathetic: Originates from craniosacral regions, has long preganglionic and short postganglionic fibers, ganglia located near or within target organs.

• Organ Innervation: Some organs receive input from both divisions, leading to opposing effects (e.g., heart rate).

• Physiological Responses:

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

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

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar	Craniosacral
Fiber Lengths	Short pre-, long postganglionic	Long pre-, short postganglionic
Ganglia Location	Near spinal cord	Near/within organs
Main Neurotransmitter	Norepinephrine	Acetylcholine

Central Nervous System (CNS)

Brain Regions & Functions

The brain consists of specialized regions responsible for various functions, including cognition, emotion, and language.

• Limbic System: Regulates emotions and memory.

• Broca's Area: Controls speech production.

• Wernicke's Area: Involved in language comprehension.

Structure of Spinal Cord

The spinal cord is a long, cylindrical structure that transmits neural signals between the brain and body.

• Functional Areas: Contains sensory, motor, and mixed regions.

• Travel in Vertebral Column: The spinal cord extends from the foramen magnum to the lumbar region (typically L1-L2).

• Spinal Nerve Exit: Nerves exit superiorly or inferiorly relative to their vertebrae.

Meninges

The meninges are three protective membranes surrounding the brain and spinal cord.

• Dura Mater: Tough outer layer.

• Arachnoid Mater: Web-like middle layer.

• Pia Mater: Delicate inner layer.

• Spaces: Epidural, subdural, and subarachnoid spaces contain blood vessels and cerebrospinal fluid (CSF).

Spinal Cord Tracts

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

• Names & Types: Examples include spinothalamic (sensory) and corticospinal (motor) tracts.

• Termination & Synapse: Tracts terminate and synapse at specific regions; decussation refers to crossing over to the opposite side.

Neuroglia of CNS

Neuroglia are supporting cells in the CNS.

• Types: Astrocytes, oligodendrocytes, microglia, ependymal cells.

• Functions: Support neurons, form myelin, maintain homeostasis.

Neuron Types

Neurons are classified by structure and function.

• Unipolar: One process, typically sensory.

• Bipolar: Two processes, found in sensory organs.

• Multipolar: Multiple processes, most common type.

• Pseudounipolar: Single process that splits, common in sensory neurons.

Ventricular System

The ventricular system consists of interconnected cavities filled with CSF.

• Ventricles: Lateral, third, and fourth ventricles.

• Openings/Canals: Allow CSF flow between ventricles and subarachnoid space.

• CSF Flow: CSF is produced by choroid plexus and circulates to cushion the CNS.

Peripheral Nervous System (PNS)

Brachial Plexus

The brachial plexus is a network of nerves supplying the upper limb.

• Structure: Roots, trunks, divisions, cords, and terminal branches.

• Regions: Each region contributes to specific terminal nerves (e.g., musculocutaneous, median, ulnar).

• Sensory/Motor Info: Different nerves provide sensory and motor innervation to the arm and hand.

Neuroglia of PNS

• Schwann Cells: Form myelin in the PNS.

• Satellite Cells: Support neuron cell bodies in ganglia.

Primary Neurotransmitters

• Acetylcholine: Used at neuromuscular junctions and by parasympathetic neurons.

• Norepinephrine: Used by sympathetic neurons.

Types of Receptors

• Encapsulated: Surrounded by connective tissue (e.g., Meissner's corpuscles).

• Nonencapsulated: Free nerve endings, detect pain and temperature.

• Location & Function: Found in skin, muscles, and organs, each type detects specific stimuli.

Nerves & Action Potentials

Creation of Action Potentials

Action potentials are rapid electrical signals generated by neurons.

• Steps:

  1. Resting potential maintained by ion gradients.

  2. Depolarization: Sodium channels open, Na+ enters.

  3. Repolarization: Potassium channels open, K+ exits.

  4. Return to resting potential.

• Equation: $V = IR$

Characteristics of Action Potentials

• All-or-none response

• Unidirectional propagation

• Refractory periods

Factors Influencing Impulse Propagation

• Myelination increases speed

• Axon diameter

• Temperature

Postsynaptic Potentials

• Excitatory (EPSP): Depolarizes postsynaptic membrane, increases likelihood of action potential.

• Inhibitory (IPSP): Hyperpolarizes membrane, decreases likelihood of action potential.

Nerve Fiber Types

Type	Diameter	Conduction Speed	Function
A	Large	Fast	Motor/sensory
B	Medium	Moderate	Autonomic
C	Small	Slow	Pain/temperature

Neurotransmitter Classification

• Acetylcholine: Found in both CNS and PNS.

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

• Monoamines: Dopamine, norepinephrine, serotonin.

• Peptides: Substance P, endorphins.

Additional info: Neurotransmitters are classified by chemical structure and function; their location determines their physiological effect.

Ear

Structure of the Ear

The ear is divided into three regions: external, middle, and inner ear.

• External Ear: Auricle, external auditory canal.

• Middle Ear: Tympanic membrane, ossicles (malleus, incus, stapes).

• Inner Ear: Cochlea, vestibule, semicircular canals.

Cochlear Sound Reception

• Different regions of the cochlea receive different sound frequencies: base (high frequency), apex (low frequency).

Auditory Pathway Steps

1. Sound waves enter external ear.

2. Vibrate tympanic membrane and ossicles.

3. Transmit to cochlea, where hair cells convert vibrations to electrical signals.

4. Signals travel via cochlear nerve to brainstem and auditory cortex.

Eye

Photoreceptors

• Rods: Detect low light, provide night vision.

• Cones: Detect color, provide sharp vision.

Humors of the Eye

• Vitreous Humor: Gel-like, fills posterior segment, maintains eye shape.

• Aqueous Humor: Watery, fills anterior segment, nourishes lens and cornea.

Tongue

Taste Bud Structure

Taste buds are sensory organs for gustation, located on papillae of the tongue.

• Location: Distributed on fungiform, foliate, and circumvallate papillae.

• Structure: Each taste bud contains gustatory cells, supporting cells, and basal cells.

• Taste Regions: Different regions of the tongue are sensitive to sweet, salty, sour, bitter, and umami tastes.