A&PII Midterm Study Guide: Nervous System, Special Senses, and Related Topics

Nervous System Fundamentals

Action Potential Graph: Four Steps

The action potential is a rapid electrical signal that travels along neurons. Understanding its four steps is crucial for grasping neural communication.

• Resting State: The neuron maintains a resting membrane potential, typically around -70 mV. Voltage-gated channels are closed.

• Depolarization: Sodium (Na+) channels open, allowing Na+ influx, causing the membrane potential to become more positive.

• Repolarization: Potassium (K+) channels open, K+ exits the cell, restoring the negative membrane potential.

• Hyperpolarization: K+ channels remain open briefly, causing the membrane potential to dip below resting level before returning to baseline.

Example: The action potential enables rapid communication between neurons and muscle cells.

Equation:

$\Delta V = V_{final} - V_{initial}$

Main Areas of the Cortex, Limbic System, and RAS

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

• Cerebral Cortex: Responsible for higher-order functions such as sensory perception, voluntary movement, and reasoning.

• Limbic System: Controls emotions, memory, and motivation. Includes structures like the hippocampus and amygdala.

• Reticular Activating System (RAS): Regulates wakefulness and alertness.

Example: The limbic system is activated during emotional responses.

Brain Waves and Sleep Stages

Brain waves reflect electrical activity and are associated with different states of consciousness.

• Alpha Waves: Relaxed, awake state.

• Beta Waves: Alert, active thinking.

• Delta Waves: Deep sleep.

• Theta Waves: Light sleep or drowsiness.

Sleep Stages: Progress from light (Stage 1) to deep (Stage 3/4) sleep, followed by REM (Rapid Eye Movement) sleep.

Tracts and Plexi

Major Nerves and Functions of Each Plexus

Plexuses are networks of nerves that supply specific regions.

• Cervical Plexus: Supplies neck and diaphragm (phrenic nerve).

• Brachial Plexus: Supplies upper limb (median, ulnar, radial nerves).

• Lumbar Plexus: Supplies anterior thigh (femoral nerve).

• Sacral Plexus: Supplies posterior thigh and lower leg (sciatic nerve).

Major Tracts: Items Carried and Direction

Tracts are bundles of axons in the CNS, carrying specific information.

• Corticospinal Tract: Motor signals from cortex to spinal cord.

• Spinothalamic Tract: Sensory signals (pain, temperature) from spinal cord to thalamus.

• Dorsal Column: Sensory signals (touch, proprioception) to brain.

• Spinocerebellar Tract: Proprioceptive information to cerebellum.

• Additional info: Tracts are classified as ascending (sensory) or descending (motor).

Cranial Nerves: Basic Function

Cranial nerves are twelve pairs, each with specific sensory or motor functions.

• Optic (II): Vision

• Facial (VII): Facial expression, taste

• Vagus (X): Parasympathetic control of heart, lungs, digestive tract

• Additional info: Review all twelve for exam reference.

Peripheral Nervous System (PNS)

Reflex Arc Steps

The reflex arc is the neural pathway for reflex actions.

• Receptor: Detects stimulus.

• Sensory Neuron: Transmits signal to CNS.

• Integration Center: Processes information (spinal cord or brain).

• Motor Neuron: Sends response to effector.

• Effector: Muscle or gland responds.

Types of Reflexes

• Stretch Reflex: Maintains muscle tone (e.g., knee-jerk).

• Flexor Reflex: Withdrawal from painful stimulus.

• Crossed Extensor Reflex: Balances body during withdrawal.

Receptors: Extero-, Intero-, and Proprioceptors

Receptors detect different types of stimuli.

• Exteroceptors: Respond to external stimuli (touch, temperature).

• Interoceptors: Monitor internal environment (viscera).

• Proprioceptors: Sense body position and movement (muscles, joints).

Autonomic Nervous System (ANS)

Parasympathetic vs Sympathetic Systems

The ANS regulates involuntary functions via two divisions.

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar	Craniosacral
Neurotransmitter	Norepinephrine	Acetylcholine
Effect	"Fight or flight" (increases HR, dilates pupils)	"Rest and digest" (decreases HR, constricts pupils)
Additional info:	Sympathetic is generally stimulatory; parasympathetic is inhibitory.

Special Senses: Eye and Vision

Anatomy and Function of Eye Structures

The eye consists of external and internal structures for vision.

• External: Eyelids, conjunctiva, lacrimal apparatus

• Internal: Cornea, lens, retina, iris, ciliary body

Path of Tears

• Lacrimal gland → Excretory ducts → Eye surface → Lacrimal puncta → Canaliculi → Lacrimal sac → Nasolacrimal duct

Rods vs Cones

Feature	Rods	Cones
Location	Peripheral retina	Central retina (fovea)
Function	Dim light, night vision	Color, sharp vision
Threshold	Low	High
Vision Type	Scotopic	Photopic

Direction of Light Through Eye

• Cornea → Aqueous humor → Lens → Vitreous humor → Retina

Path of Visual Stimulus

• Photoreceptors (rods/cones) → Bipolar cells → Ganglion cells → Optic nerve → Optic chiasm → Optic tract → Thalamus → Visual cortex

Accommodation of Eye

• Near Accommodation: Lens thickens for close objects.

• Far Accommodation: Lens flattens for distant objects.

• Additional info: Ciliary muscles adjust lens shape.

Dark and Light Adaptation

• Dark Adaptation: Increased sensitivity, more rhodopsin produced.

• Light Adaptation: Decreased sensitivity, rhodopsin bleached.

ANS Effect on Pupil Size

• Sympathetic: Dilates pupil (mydriasis).

• Parasympathetic: Constricts pupil (miosis).

Aqueous vs Vitreous Humor

Feature	Aqueous Humor	Vitreous Humor
Location	Anterior chamber	Posterior chamber
Function	Nourishes cornea/lens	Maintains eye shape
Consistency	Watery	Gel-like

Special Senses: Ear, Hearing, and Balance

Anatomy and Function of Ear Structures

• External: Auricle, external auditory canal

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

• Internal: Cochlea, vestibule, semicircular canals

Path of Sound

• Sound waves → Tympanic membrane → Ossicles → Oval window → Cochlea → Hair cells → Auditory nerve

Inner Ear Membranes

• Basilar membrane: Supports hair cells, vibrates with sound.

• Tectorial membrane: Contacts hair cells, aids in signal transduction.

• Reissner's membrane: Separates cochlear chambers.

Hair Cell Function

Hair cells convert mechanical vibrations into electrical signals for hearing.

Frequency Receptors Location

• High Frequency: Base of cochlea

• Low Frequency: Apex of cochlea

Static Equilibrium: Macula, Saccule vs Utricle

• Macula: Sensory region for balance.

• Saccule: Detects vertical movement.

• Utricle: Detects horizontal movement.

Semicircular Canals: Dynamic Equilibrium

Semicircular canals detect rotational movement via fluid displacement and hair cell activation.

Special Senses: Olfaction and Taste

Anatomy and Function of Olfactory and Gustatory Structures

• Olfactory: Nasal epithelium, olfactory bulb, olfactory tract

• Taste: Taste buds (papillae), gustatory cells

Olfactory Pathway

• Olfactory bipolar cells → Olfactory bulb → Olfactory tract → Insula (cortex)

Taste Bud Types and Mechanism

• Types: Fungiform, foliate, circumvallate papillae

• Mechanism: Direct ion channel activation or second messenger pathways for five major flavors (sweet, sour, salty, bitter, umami)

Cranial Nerves Carrying Taste Sensation

Nerve	Region Innervated
Facial (VII)	Anterior 2/3 of tongue
Glossopharyngeal (IX)	Posterior 1/3 of tongue
Vagus (X)	Epiglottis and lower pharynx

Additional info: These topics cover Ch. 11-15 of the typical A&PII curriculum, focusing on nervous system fundamentals, tracts, plexi, PNS, ANS, and special senses (eye, ear, olfaction, taste).