The Integumentary System

Overview and Functions

The integumentary system is the body's largest organ system, primarily composed of the skin and its derivatives. It serves several essential functions that contribute to homeostasis and protection.

• Protection: Acts as a physical barrier against mechanical injury, pathogens, and harmful substances.

• Thermoregulation: Regulates body temperature through sweat production and blood flow adjustments.

• Vitamin D Synthesis: Initiates the synthesis of vitamin D when exposed to UV light.

• Sensation: Contains sensory receptors for touch, pain, temperature, and pressure.

Structure of the Skin

• Two Major Layers:

  • Epidermis: Outermost, avascular layer composed of stratified squamous epithelium.

  • Dermis: Deeper, vascular layer containing connective tissue, nerves, and blood vessels.

• Epidermal Layers (from deepest to outermost):

  • Stratum basale

  • Stratum spinosum

  • Stratum granulosum

  • Stratum corneum

  • Stratum lucidum (only in thick skin, e.g., palms and soles)

• Dermal Layers:

  • Papillary layer: Superficial, contains dermal papillae and capillaries.

  • Reticular layer: Deeper, dense irregular connective tissue with collagen and elastin fibers.

Cells and Pigmentation

• Keratinocytes: Main cell type in the epidermis; produce and store keratin, a protein that provides strength and waterproofing. Found in all layers of the epidermis.

• Melanin: Pigment responsible for skin color, produced by melanocytes in the stratum basale. Melanin protects against UV radiation.

Hair and Glands

• Hair Structure: Grows from a hair follicle (invagination of the epidermis into the dermis). The hair shaft is the visible part; the root is embedded in the follicle.

• Functions of Hair: Protection, sensory input, and thermoregulation.

• Phases of Hair Growth:

  1. Anagen: Active growth phase.

  2. Catagen: Transitional phase.

  3. Telogen: Resting phase.

• Glands:

  • Sudoriferous (Sweat) Glands: Produce sweat for thermoregulation and excretion.

  • Sebaceous Glands: Secrete sebum (oil) for lubrication and waterproofing.

Thermoregulation

• Definition: The process of maintaining internal body temperature.

• Mechanism: The integumentary system works with the sympathetic nervous system to regulate temperature. Blood vessels constrict (vasoconstriction) to conserve heat or dilate (vasodilation) to release heat.

Nervous System Basics: Neurons, Action Potentials, Synapses, & Neurotransmitters

General Functions and Structure

The nervous system coordinates body activities by transmitting electrical and chemical signals. The primary cell type is the neuron.

• Neuron Structure:

  • Cell body (soma): Contains the nucleus and organelles.

  • Dendrites: Receive incoming signals.

  • Axon: Conducts impulses away from the cell body.

  • Axon hillock: Initiates action potentials.

  • Axon terminal: Releases neurotransmitters.

  • Myelin: Insulating layer that increases impulse speed.

Membrane Potentials and Ion Movement

• Resting Membrane Potential (RMP): The voltage across the neuron's membrane at rest; typically .

• Major Ions: Sodium (Na+) is higher outside the cell; Potassium (K+) is higher inside.

• Changes in Membrane Potential:

  • Altered permeability to ions (e.g., opening/closing of ion channels).

  • Changes in ion concentrations across the membrane.

Depolarization, Hyperpolarization, and Repolarization

• Depolarization: Membrane potential becomes less negative (Na+ enters the cell).

• Repolarization: Return to resting potential (K+ exits the cell).

• Hyperpolarization: Membrane potential becomes more negative than resting (excess K+ leaves).

Graded and Action Potentials

• Graded Potentials: Small, localized changes in membrane potential, usually at dendrites or cell body; can lead to action potentials if threshold is reached.

• Action Potentials: Rapid, all-or-none electrical impulses that travel down the axon. Key events include:

• Resting potential:

• Threshold:

• Peak depolarization: to

• Phases: Depolarization, repolarization, hyperpolarization.

Refractory Periods

• Absolute Refractory Period: No new action potential can be initiated.

• Relative Refractory Period: A stronger stimulus can initiate another action potential.

Impulse Speed and Synapses

• Factors Affecting Speed: Myelination and axon diameter increase conduction velocity.

• Synapse: Junction between neurons; can be chemical (uses neurotransmitters) or electrical (direct ion flow).

• Chemical Synapse Components: Presynaptic neuron, synaptic cleft, postsynaptic neuron, receptors, neurotransmitters.

• Neurotransmitter Release Events: Action potential arrives, Ca2+ influx, vesicle fusion, neurotransmitter release, receptor binding.

Neurotransmitter Systems

• Cholinergic: Acetylcholine; binds to muscarinic or nicotinic receptors.

• Biogenic Amines: Norepinephrine/Epinephrine (bind to alpha/beta adrenergic receptors), Dopamine, Serotonin (binds to 5-HT receptors).

• Amino Acids: Glutamate (excitatory, ionotropic receptors), GABA and Glycine (inhibitory, ionotropic/metabotropic receptors).

The Central Nervous System: Brain and Spinal Cord

Major Structures and Protection

• CNS Components: Brain and spinal cord.

• Ventricles: Four fluid-filled chambers in the brain.

• Protective Coverings: Cranial/vertebral bones, cerebrospinal fluid (CSF), and meninges (dura mater, arachnoid mater, pia mater).

• CSF: Produced by the choroid plexus, circulates in ventricles and subarachnoid space, cushions and nourishes CNS.

• Blood Supply: Internal carotid and vertebral arteries supply the brain.

Gray and White Matter

• Gray Matter: Neuron cell bodies, dendrites, and unmyelinated axons; site of processing.

• White Matter: Myelinated axons; transmits signals.

Brain Anatomy

• Gyri: Ridges on the brain surface.

• Sulci: Grooves between gyri.

• Longitudinal Fissure: Divides right and left hemispheres (contralateral organization).

• Cerebrum/Cerebral Cortex: Made of gray matter; divided into lobes:

  • Frontal: Motor control, decision-making.

  • Parietal: Sensory processing.

  • Temporal: Auditory and memory processing.

  • Occipital: Visual processing.

• Subcortical Structures:

  • Hippocampus: Memory formation.

  • Amygdala: Emotion processing.

  • Basal ganglia/nuclei: Movement regulation.

• Diencephalon:

  • Thalamus: Sensory relay station.

  • Hypothalamus: Homeostasis, autonomic and endocrine control.

• Brainstem: Midbrain, pons, medulla oblongata; controls vital functions (breathing, heart rate).

• Cerebellum: Coordination and balance.

• EEG: Measures electrical activity of the brain; useful for diagnosing neurological disorders.

Spinal Cord Anatomy

• Functions: Transmits signals between brain and body; processes reflexes.

• Regions: Cervical, thoracic, lumbar, sacral.

• Internal Anatomy: Central gray matter (processing), surrounding white matter (signal transmission).

• Dorsal Horn: Sensory neuron cell bodies/dendrites.

• Ventral Horn: Motor neuron cell bodies/dendrites.

The Peripheral Nervous System (PNS)

Organization and Structure

• PNS Components: Nerves outside the CNS.

• Divisions: Somatic (voluntary) and autonomic (involuntary).

• Nerve Structure:

  • Fascicle: Bundle of nerve fibers.

  • Epineurium: Outermost connective tissue layer.

  • Perineurium: Surrounds each fascicle.

  • Endoneurium: Surrounds individual nerve fibers.

Neuron Types and Pathways

• Neuron Types: Classified by structure (multipolar, bipolar, unipolar) and function (sensory, motor, interneurons).

• Somatic Nervous System: Controls voluntary movements.

• Ascending Pathways: Sensory information from periphery to brain; three-neuron chain (first, second, third order neurons).

• Descending Pathways: Motor commands from brain to muscles; two-neuron chain (upper and lower motor neurons).

Reflexes and the Autonomic Nervous System (ANS)

• Reflex: Automatic response to stimuli.

• Reflex Arc Components: Receptor, sensory neuron, integration center, motor neuron, effector.

• Types of Reflexes: Stretch, tendon (contraction/relaxation), withdrawal (e.g., hand from hot object).

• ANS: Controls involuntary responses (heart rate, digestion, etc.).

• Autonomic Motor Pathways: Preganglionic neuron, autonomic ganglion, postganglionic neuron. Fiber lengths differ between sympathetic (short pre-, long post-) and parasympathetic (long pre-, short post-) divisions.

• Brain Regulation: Hypothalamus is the master controller of the ANS.

• Adrenergic vs. Cholinergic Neurons:

  • Cholinergic: Release acetylcholine.

  • Adrenergic: Release norepinephrine.

  • Preganglionic neurons in both divisions release acetylcholine.

• Sympathetic vs. Parasympathetic Responses: Sympathetic prepares for "fight or flight"; parasympathetic promotes "rest and digest" functions.

Neurotransmitters & Pharmacology, Special Senses

Major Neurotransmitters and Receptors

• Acetylcholine: Binds to nicotinic (ionotropic) or muscarinic (metabotropic) receptors; part of the cholinergic system.

• Norepinephrine/Epinephrine: Bind to alpha or beta adrenergic receptors; part of the adrenergic system.

• Serotonin: Binds to 5-HT receptors; serotonergic system.

• Glutamate: Main excitatory neurotransmitter; binds to ionotropic receptors, increases action potentials.

• GABA: Main inhibitory neurotransmitter; binds to ionotropic receptors, decreases action potentials.

Clinical Applications

• Understanding neurotransmitter systems and receptor types is essential for pharmacology and clinical scenarios (e.g., drug effects, neurological disorders).

Table: Comparison of Nervous System Divisions

Additional info: Some explanations and table entries were expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.