Comprehensive Study Guide: Nervous and Endocrine Systems, Sensory Physiology, and Homeostasis

Nervous System Physiology

Graded Potentials vs. Action Potentials

Neurons communicate via electrical signals known as graded potentials and action potentials. Understanding their differences is essential for grasping neural signaling.

• Graded Potentials: Local changes in membrane potential that vary in magnitude and decay with distance from the stimulus site. They occur in dendrites and cell bodies.

• Action Potentials: All-or-none electrical impulses that travel along axons without decreasing in strength. They are triggered when the membrane potential reaches a threshold.

• Key Differences:

  • Graded potentials are decremental (decrease with distance), while action potentials are non-decremental.

  • Graded potentials can be summed; action potentials cannot.

  • Action potentials have a refractory period; graded potentials do not.

• Example: A postsynaptic potential is a graded potential, while the nerve impulse along an axon is an action potential.

Absolute Refractory Period of Action Potentials

The absolute refractory period is a phase during which a neuron cannot initiate another action potential, regardless of stimulus strength.

• Cause: Inactivation of voltage-gated sodium channels following depolarization.

• Significance: Ensures unidirectional propagation of action potentials and limits the frequency of firing.

• Duration: Lasts from the initiation of the action potential until the membrane repolarizes and sodium channels reset.

Neurotransmitter Release Mechanisms

Neurotransmitter release is a critical process at synapses, enabling communication between neurons.

• Key Steps:

  • Arrival of action potential at axon terminal opens voltage-gated calcium channels.

  • Calcium influx triggers synaptic vesicle fusion with the presynaptic membrane.

  • Neurotransmitters are released into the synaptic cleft via exocytosis.

  • Neurotransmitters bind to receptors on the postsynaptic cell, initiating a response.

• Example: Acetylcholine release at the neuromuscular junction.

Autonomic Nervous System (ANS) Divisions

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

• Sympathetic Division: Prepares the body for 'fight or flight' responses (e.g., increases heart rate, dilates pupils).

• Parasympathetic Division: Promotes 'rest and digest' activities (e.g., slows heart rate, stimulates digestion).

• Comparison Table:

Feature	Sympathetic	Parasympathetic
Origin	Thoracolumbar	Craniosacral
Main Neurotransmitter	Norepinephrine	Acetylcholine
Effect on Heart Rate	Increases	Decreases
Pupil Response	Dilates	Constricts

Sensory Physiology

Pathway of Sound from External Ear to Brain

Sound perception involves the transmission of sound waves through the ear to the auditory cortex.

• Sound waves enter the external auditory canal and vibrate the tympanic membrane.

• Vibrations are transmitted via the ossicles (malleus, incus, stapes) to the oval window of the cochlea.

• Movement of fluid in the cochlea stimulates hair cells in the organ of Corti.

• Hair cells convert mechanical energy into electrical signals, which travel via the cochlear nerve to the auditory cortex in the brain.

Hearing Loss and Nasal Cavity Inflammation

Severe nasal cavity inflammation can affect hearing by impacting the Eustachian tube.

• Mechanism: Inflammation can block the Eustachian tube, leading to pressure imbalance and fluid accumulation in the middle ear.

• Result: Conductive hearing loss due to impaired vibration transmission.

Dark Adaptation in Vision

Dark adaptation is the process by which eyes increase their sensitivity in low-light conditions.

• Process: Regeneration of rhodopsin in rods, increased sensitivity of photoreceptors.

• Loss of Dark Adaptation: May occur due to vitamin A deficiency, retinal disease, or damage to rods.

• Consequences: Difficulty seeing in dim light, night blindness.

Photoreceptor Activity in Light and Dark

Photoreceptors (rods and cones) respond differently in the presence and absence of light.

• In the Absence of Light: Photoreceptors are depolarized, continuously release neurotransmitter (glutamate).

• In the Presence of Light: Photoreceptors hyperpolarize, reducing neurotransmitter release.

Gustation vs. Olfaction: Sensory System Comparison

Gustation (taste) and olfaction (smell) are chemical senses with both similarities and differences.

• Similarities: Both detect chemical stimuli, use chemoreceptors, and contribute to flavor perception.

• Differences: Gustation uses taste buds on the tongue; olfaction uses olfactory receptors in the nasal cavity. Olfaction can detect a wider range of chemicals.

Endocrine System and Homeostasis

Abnormal Movement, Thirst, and Abnormal Body Size: Endocrine Causes

Symptoms such as abnormal movement, excessive thirst, and abnormal body size may indicate endocrine disorders.

• Possible Causes: Diabetes insipidus (ADH deficiency), pituitary disorders (growth hormone imbalance), thyroid dysfunction.

• Example: Excessive thirst and urination may be due to diabetes insipidus; abnormal body size may result from growth hormone excess or deficiency.

Vitamin D, Plasma Calcium, and Bone Composition

Vitamin D and calcium homeostasis are crucial for bone health.

• 1,25-(OH)2D (Calcitriol): The active form of vitamin D, increases intestinal absorption of calcium and phosphate.

• Plasma Ca2+ Levels: Regulated by parathyroid hormone (PTH), calcitonin, and vitamin D.

• Bone Composition: Composed of organic matrix (collagen) and inorganic minerals (hydroxyapatite: Ca10(PO4)6(OH)2).

Role of PTH in Epinephrine and Norepinephrine Secretion

Parathyroid hormone (PTH) primarily regulates calcium homeostasis, but the adrenal medulla controls epinephrine and norepinephrine secretion.

• PTH: Increases blood calcium by stimulating bone resorption, increasing renal reabsorption, and activating vitamin D.

• Epinephrine/Norepinephrine Secretion: Controlled by the sympathetic nervous system via the adrenal medulla, not directly by PTH.

• Additional info: Stress stimulates the adrenal medulla to release catecholamines (epinephrine and norepinephrine).