Nervous and Endocrine Systems

Nervous System

One of the body's major communication systems that produces fast, typically short-term responses primarily using electrical signaling via saltatory conduction.

Endocrine System

One of the body's major communication systems that produces slow, typically longer responses using hormonal (chemical) signaling distributed primarily through the blood.

Homeostasis

The maintenance of a stable equilibrium in the internal environment by self-regulating physiological processes, despite a changing external environment.

Negative Feedback

The primary mechanism of maintaining homeostasis that negates or reverses changes occurring to a physiological system.

Positive Feedback

A mechanism that amplifies or perpetuates changes occurring within a system, such as suckling stimulating increased or continued lactation.

Central Nervous System (CNS)

The division of the nervous system consisting of the brain and spinal cord, made up primarily of interneurons, that acts as the main processing centre of information.

Peripheral Nervous System (PNS)

The division of the nervous system extending to and from the CNS throughout the body, consisting of afferent (sensory) neurons and efferent (motor) neurons.

Sensory Neurons

Afferent neurons that transmit information into the CNS from the external environment; structurally characterized as long and myelinated with a cell body in the middle.

Motor Neurons

Efferent neurons that send signals from the CNS to muscles and other effectors; structurally characterized as long and myelinated with a cell body located near the dendrite end.

Interneurons

Neurons found primarily within the CNS that have many branches on dendrites and axons, are often shorter in length, and may or may not be myelinated.

Somatic Nervous System

The division of the nervous system that controls voluntary movements, such as directing the movements of a hand and wrist when writing.

Autonomic Nervous System

The division of the nervous system that controls involuntary physiological responses, such as pupil constriction and dilation.

Sympathetic Division

The subdivision of the autonomic nervous system responsible for the 'Fight or Flight' response; increases heart rate, dilates pupils and bronchi, inhibits digestion and bladder contraction, and releases epinephrine and norepinephrine.

Parasympathetic Division

The subdivision of the autonomic nervous system responsible for the 'Rest and Digest' response; decreases heart rate, constricts pupils and bronchi, and stimulates digestion and bladder contraction.

Neurons

Specialized cells in the nervous system responsible for transmitting electrical and chemical signals or impulses.

Nerves

Bundles of neurons along with their supporting tissues and cells.

Dendrites

Branching structures of a neuron that receive incoming signals and conduct them toward the cell body.

Axon

The elongated neuronal fiber that transmits nerve impulses away from the cell body toward the axon terminals.

Axon Terminals

Structures at the terminus of an axon that deliver signals across a synapse to the next neuron or effector.

Myelin Sheath

Fatty, insulating layers surrounding the axons of certain neurons that increase the conduction speed of nerve impulses.

Nodes of Ranvier

Unmyelinated gaps situated between sections of myelin sheath along an axon.

Glial Cells

Supporting cells in the nervous system, often nicknamed 'nurse cells', that protect, nourish, and provide structural support to neurons.

Schwann Cells

A specific type of glial cell in the PNS that forms the myelin sheath to speed transmission and produces the neurilemma to assist in neuron repair.

Neurilemma

An outer membrane layer produced by Schwann cells in the PNS that aids in the repair and regeneration of damaged neurons.

Reflex Arc

A rapid, involuntary neural circuit in which processing bypasses the brain and occurs directly in the spinal cord to minimize injury from harmful stimuli.

Resting Potential

The baseline electrical polarization of an axon membrane where the interior is negatively charged relative to the exterior fluid.

Action Potential

A rapid wave of membrane depolarization and repolarization caused by the influx of $\text{Na}^+$ into the axon followed by the delayed efflux of $\text{K}^+$.

Saltatory Conduction

The rapid transmission of nerve impulses along myelinated axons achieved by 'skipping' from one Node of Ranvier to the next.

All or Nothing Response

The physiological principle stating that an action potential occurs at full strength or not at all, meaning sensation intensity is encoded by frequency and number of potentials rather than individual amplitude.

Neurotransmitters

Chemical messengers released from presynaptic vesicles that cross the synaptic cleft to excite or inhibit a postsynaptic neuron or effector.

Summation

The additive integration of multiple excitatory and inhibitory neural inputs received by a neuron to determine whether an action potential is generated.

Acetylcholine

An excitatory model neurotransmitter that stimulates muscle contraction and transfers signals across synaptic junctions.

Cholinesterase

An enzyme in the synapse that rapidly hydrolyzes acetylcholine to terminate synaptic transmission and permit muscular relaxation.

White Matter

CNS tissue composed predominantly of myelinated axons, specialized for conducting electrical signals rapidly and efficiently across pathways.

Grey Matter

CNS tissue comprised mainly of unmyelinated neuronal cell bodies, dendrites, and short branched interneurons, specialized for information processing.

Meninges

A three-layered protective membrane structure enclosing both the brain and the spinal cord, consisting of the dura mater, arachnoid mater, and pia mater.

Dura Mater

The tough, fibrous outermost layer of the meninges adhered directly to the surrounding bone of the skull and vertebral canal.

Arachnoid Mater

The middle web-like layer of the meninges that contains spaces filled with protective cerebrospinal fluid.

Pia Mater

The delicate, highly vascularized innermost layer of the meninges adhered closely to the contours of the brain and spinal cord tissue.

Pons

A swelling at the top of the spinal cord (meaning 'bridge') that relays incoming signals from the spinal cord to other regions of the brain.

Medulla Oblongata

A lower brainstem structure situated beneath the pons that regulates autonomic vital reflexes including heart rate, respiration, and digestion.

Cerebellum

A convoluted, cauliflower-like structure located at the base of the brain that coordinates posture, voluntary muscle movement, balance, and equilibrium.

Midbrain

A central brain region involved in reflex processing, sleep-wake cycles (alertness), relaying auditory and visual signals, and producing dopamine.

Cerebrum

The largest and most prominent part of the brain, divided into two hemispheres, whose convoluted surface (cerebral cortex) coordinates higher-order mental and sensory functions.

Thalamus

A central forebrain relay station that directs incoming sensory signals (except olfaction) to the appropriate sensory areas of the cerebral cortex.

Corpus Callosum

A prominent band of white matter tracts connecting the left and right cerebral hemispheres, allowing interhemispheric communication.

Hypothalamus

A forebrain region situated below the thalamus that functions as the coordinating centre for homeostatic processes and regulates the pituitary gland via releasing factors.

Pituitary Gland

An endocrine gland located beneath the hypothalamus, often termed the 'master gland', that secretes multiple hormones regulating other glands and body processes.

Olfactory Bulb

A forebrain structure located beneath the frontal lobe that processes sensory signals related to smell directly from the nasal cavity.

Hippocampus

A seahorse-shaped forebrain structure essential for the consolidation of short-term memories into long-term storage.

Amygdala

A limbic system structure heavily involved in processing emotions such as fear and aggression, as well as emotional memory consolidation.

Frontal Lobe

The anterior lobe of the cerebral cortex responsible for motor control, reasoning, decision making, concentration, and aspects of personality.

Parietal Lobe

The cerebral cortex lobe situated between the frontal and occipital lobes that processes somatosensory and tactile information (touch, temperature, and pain).

Temporal Lobe

The lateral lobe of the cerebral cortex primarily responsible for processing auditory information, speech comprehension, and advanced visual patterns.

Occipital Lobe

The posterior lobe of the cerebral cortex dedicated to receiving and processing visual sensory input.

Ganglion

A cluster or collection of neuronal cell bodies situated outside the central nervous system that functions primarily as a local relay centre.

Sclera

The tough, white, opaque outer protective layer of the eyeball providing structural support.

Choroid

The vascular intermediate coat of the eyeball situated between the retina and sclera that provides nutrients and absorbs stray light.

Retina

The innermost, light-sensitive layer of the eye containing photoreceptor cells (rods and cones) and neural circuits that convert light into nerve impulses.

Cornea

The transparent, curved anterior portion of the fibrous coat of the eye that refracts light onto the lens and retina.

Fovea Centralis

A tiny pit located in the central retina containing a high density of cone photoreceptors, providing the highest visual acuity and detail.

Optic Chiasm

The X-shaped anatomical junction where optic nerve fibers partially decussate, sorting visual fields from both eyes to contralateral visual cortices.

Rods

Photoreceptors that operate across a wide range of wavelengths to provide low-detail, monochromatic vision in low-light environments with minimal cognitive demand.

Cones

Photoreceptor cells existing in three variations sensitive to specific wavelengths of light, allowing high-acuity color vision.

Ossicles

The three tiny bones of the middle ear—the malleus (hammer), incus (anvil), and stapes (stirrup)—that amplify and conduct vibrations to the oval window.

Eustachian Tube

A canal connecting the middle ear cavity to the nasopharynx, functioning to equalize atmospheric air pressure across the tympanic membrane.

Cochlea

A spiral, snail-shell-shaped fluid-filled structure of the inner ear containing three chambers that transform mechanical vibrations into nerve signals.

Organ of Corti

The sensory receptor organ for hearing, situated along the basilar membrane in the cochlea, which uses hair cells deflected by the tectorial membrane to initiate action potentials.

Vestibular System

The sensory apparatus of the inner ear that detects head orientation, acceleration, tilt, and rotational motion to maintain equilibrium.

Semicircular Canals

Three fluid-filled, mutually perpendicular loops in the inner ear containing cupulae and hair cells that detect rotational acceleration.

Otoliths

Calcium carbonate granules embedded in the gelatinous matrix of the utricle and saccule maculae that shift in response to gravity and linear acceleration.

Proprioception

The body's sensory ability to detect the position and movement of limbs and trunk in space by integrating feedback from skin, joints, muscles, and the vestibular apparatus.

Electroencephalogram (EEG)

A diagnostic recording tool using surface scalp electrodes to monitor the collective electrical firing activity of cortical neurons.

Multiple Sclerosis (MS)

A progressive neurodegenerative disease characterized by the breakdown of myelin sheaths along neurons, causing impaired and slowed nerve impulse conduction.

Alzheimer’s Disease

A neurodegenerative disease caused by the progressive build-up of amyloid plaques and neurofibrillary tangles, leading to synaptic damage and neuronal death.

Parkinson’s Disease

A movement disorder caused by the loss of dopamine-producing cells in the brain, resulting in a dopamine/acetylcholine imbalance that manifests as tremors, muscle rigidity, and abnormal movement.

Huntington’s Disease

An inherited neurodegenerative disorder characterized by progressive loss of brain neurons, resulting in early dopamine excess, involuntary motor twitches, and restlessness.

Tropic Hormones

Hormones that exert their primary physiological effect by stimulating or regulating the secretion of other endocrine glands.

Prolactin (PRL)

An anterior pituitary hormone that stimulates milk production and lactation in mammary tissue.

Human Growth Hormone (hGH)

An anterior pituitary hormone that promotes overall body growth, cellular reproduction, and development, specifically targeting bones, muscles, and internal organs.

Oxytocin

A posterior pituitary hormone that promotes uterine contractions during childbirth, milk ejection during suckling, and social bonding.

Antidiuretic Hormone (ADH)

A posterior pituitary hormone, also called vasopressin, released in response to hypothalamic osmoreceptors to increase water reabsorption in renal collecting tubules.

Thyroxine ($\text{T}_4$)

An iodine-containing metabolic hormone secreted by the thyroid gland that increases basal metabolic rate and supports cellular growth and differentiation.

Calcitonin

A thyroid hormone that acts to decrease blood calcium levels by inhibiting osteoclast activity in bone and reducing renal calcium reabsorption.

Parathyroid Hormone (PTH)

A hormone produced by parathyroid glands that acts antagonistically to calcitonin, elevating serum calcium by increasing bone resorption, intestinal absorption, and renal reabsorption.

Cortisol

A glucocorticoid hormone released by the adrenal cortex during long-term stress that increases blood glucose concentrations and mobilizes amino acids.

Aldosterone

A mineralocorticoid synthesized by the adrenal cortex that promotes renal sodium ($\text{Na}^+$) and water retention to regulate blood pressure.

Islets of Langerhans

Endocrine cell clusters scattered throughout the pancreas that synthesize and secrete insulin and glucagon directly into systemic circulation.

Glucagon

A pancreatic hormone released during hypoglycemia that stimulates hepatic glycogenolysis, converting glycogen to glucose to raise blood sugar.

Insulin

A pancreatic hormone released during hyperglycemia that promotes cellular glucose uptake and stimulates glycogenesis in the liver to reduce blood glucose.

Type 1 Diabetes

An autoimmune metabolic condition characterized by the destruction of pancreatic beta cells in the islets of Langerhans, leading to an absolute insulin deficiency.

Type 2 Diabetes

A metabolic disorder caused by cellular resistance to insulin action, typically presenting in adulthood and managed via dietary changes, exercise, and oral antihyperglycemic drugs.

Diabetes Insipidus

A water-balance condition resulting from a deficit in ADH secretion or renal unresponsiveness to ADH, causing extreme polyuria and polydipsia without glycosuria.

Goiter

An enlargement of the thyroid gland, typically visible as a swelling in the neck, which often occurs due to dietary iodine deficiency disrupting $\text{T}_4$ production.

Acromegaly

A hormonal disorder resulting from excessive secretion of human growth hormone (hGH) during adulthood, leading to enlargement of facial bones, hands, and feet.

Cushing's Syndrome

A hypercortisolemic disorder characterized by upper-body and facial adipose deposition ('moon face', 'buffalo hump'), purple striae, easy bruising, and impaired wound healing.

Addison’s Disease

An endocrine deficiency resulting from adrenal cortex hypofunction, presenting with low cortisol, fatigue, weight loss, hypotension, salt cravings, and cutaneous hyperpigmentation.