Nervous and Endocrine Systems Diploma Review

Neuron Structure and Nerve Cell Components

• Definition of a Neuron: A neuron is a specialized nerve cell designed for the conduction of nerve signals over extensive distances.

• Primary Structural Components:

  • Dendrites: These are elongated cell extensions that receive electrochemical impulses from adjacent neurons.

  • Cell Body: Impulses travel from the dendrites to the cell body, which houses the nucleus of the neuron.

  • Axon: The pathway following the cell body that leads to the axon terminal.

  • Axon Terminal: The end branch of the axon.

  • Synaptic Knob: The final point of the neuron structure where signals are passed to the next cell.

• Protective and Insulating Layers:

  • Neurolemma: A membrane covering some neurons that facilitates the repair of the neuron following an injury.

  • Myelin Sheath: A white, fatty insulating layer that covers the axon. It is composed of individual Schwann cells.

  • Nodes ring ring VA: These are the exposed spaces on the axon located between individual Schwann cells (also referred to as the node of Rainbearer).

• Functional Categories of Neurons:

  • Sensory Neurons: Responsible for carrying signals from sensory organs to the Central Nervous System (CNS).

  • Motor Neurons: Responsible for carrying signals from the CNS to the muscles.

  • Interneurons: Facilitate the travel of signals within the CNS.

The Physics of the Nerve Impulse: Resting and Action Potentials

• Electrical Charge Production: Neurons generate electrical charges by manipulating ion concentrations across the neuron membrane.

• Resting Potential State:

  • Ion Distribution: Sodium ions ($Na^+$) are concentrated in the extracellular fluid (ECF) outside the axon, while potassium ions ($K^+$) are concentrated in the intracellular fluid (ICF) within the axon.

  • Membrane Permeability: The resting membrane is impermeable to sodium but slightly permeable to potassium, allowing some $K^+$ to leak into the ECF.

  • Polarization: This creates a state where the ECF is more positive than the ICF, making the ICF relatively negative.

  • Voltage Measurement: The resting membrane potential is defined from the perspective of the ICF as $-70\,\text{millivolts}$. This indicates the inside is $70\,\text{millivolts}$ more negative than the outside.

• The Action Potential (Nerve Impulse):

  • Stimulation and Threshold: If a dendrite is stimulated strongly enough to reach the electrical threshold, an action potential occurs.

  • Depolarization: The membrane becomes permeable to sodium, causing a "sodium rush" into the ICF. This continues until the ICF reaches a potential of $+30\,\text{millivolts}$. At this stage, the membrane is completely depolarized.

  • Repolarization: To return to the resting state, sodium channels close, and positive ions are actively transported back into the ECF. This restores the ICF to a negative state and the ECF to a positive state.

  • Duration: The cycle of depolarization and repolarization at a single point takes approximately $\frac{1}{500}\,\text{of a second}$.

• Conduction and the All-or-None Principle:

  • Conduction Process: Increased sodium permeability at one point triggers increased permeability at the adjacent point, creating a wave of depolarization.

  • Saltatory Conduction: In myelinated neurons, the insulation prevents the entire axon from depolarizing; instead, the action potential skips rapidly between the exposed nodes of Rainbearer, resulting in significantly faster impulse conduction.

  • All-or-None Principle: Not every stimulus causes an impulse. If a stimulus does not reach the excitation threshold, the potential will simply fade out.

Synaptic Transmission and Neurotransmitters

• The Synapse: This is the junction established between the synaptic knob of a presynaptic neuron and the dendrite of a postsynaptic neuron.

• Mechanism of Transmission:

  • 1. An action potential reaches the axon terminal.

  • 2. Calcium ions ($Ca^{2+}$) enter the axon, stimulating synaptic vesicles to release neurotransmitter molecules into the synapse.

  • 3. Neurotransmitters diffuse across the gap and bind to receptor sites on the postsynaptic dendrite, inducing depolarization.

• Primary Neurotransmitters: All neurotransmitters are proteins produced by neurons.

  • Acetylcholine: The most common neurotransmitter.

  • Norepinephrine.

  • Serotonin.

  • Dopamine.

• Regulation via Enzymes: To prevent continuous depolarization, enzymes inactivate neurotransmitters. For example, a split second after acetylcholine is released, the enzyme cholinesterolase (or cholinesterase) is released to inactivate it.

• Imbalances: Hypersecretion or hyposecretion of these chemicals can profoundly impact behavior, emotion, sensation, and thought.

Organization of the Nervous System

• Central Nervous System (CNS): Composed of the brain and the spinal cord.

• Peripheral Nervous System (PNS): Composed of $31$ pairs of spinal nerves extending from the spinal cord. It branches into:

  • Stomatic Nervous System: Controls voluntary actions; contains motor and sensory nerves.

  • Autonomic Nervous System: Regulates smooth muscles, glands, and organs without conscious control. It further branches into:

    • Sympathetic System: Activated during perceived threats (fight or flight). It diverts blood from the gut, excretory, reproductive systems, and skin to the skeletal muscles, heart, lungs, and brain. It increases heart rate, blood pressure, and breathing while dilating pupils. Stimulated by norepinephrine (adrenaline).

    • Parasympathetic System: Restores the body once the threat has passed. Stimulated by acetylcholine.

Brain Structure, Lobes, and Functional Specialization

• Cerebrum: The top portion of the brain, highly convoluted to increase surface area, divided into two hemispheres.

  • Cortex: Outer layer of gray, non-myelinated tissue.

  • Medulla: Inner layer of myelinated tissue.

• The Four Major Lobes of the Cerebrum:

  • Frontal Lobe: personality and voluntary motion via the motor cortex.

  • Parietal Lobe: Contains the sensory cortex for processing physical sensations.

  • Octopical Lobe: Located at the back; contains the visual cortex.

  • Temporal Lobe: Responsible for hearing (auditory processing).

• Hemispheric Specialization:

  • Left Hemisphere: Specialized for logic, speech, language, and mathematics.

  • Right Hemisphere: Specialized for creative and non-verbal expression, such as art and music.

  • Crossover: Due to the Pons, nerve tracks cross over; the right hemisphere controls the left side of the body and vice versa.

  • Corpus call colosseum: A band of myelinated fibers that transmits information between the two hemispheres.

• Other Brain Structures:

  • Cerebellum: Sits at the top of the brainstem; coordinates muscular movements.

  • Pons: Acts as a relay center for information moving to and from the cerebrum.

  • Medulla Oblongata: Controls autonomic functions including breathing, heart rate, and blood shunting.

  • Hypothalamus: Monitors the endocrine system and controls temperature, hunger, and thirst.

  • Pituitary Gland: Secretes various hormones as directed by the hypothalamus.

Neural Pathways and Reflexes

• Reflex Arc: The simplest neural pathway, allowing for rapid, life-saving responses by bypassing the brain.

• Process of a Reflex:

  • 1. A sensory receptor sends an impulse along a sensory neuron.

  • 2. The impulse enters the spinal cord through a spinal nerve.

  • 3. The sensory neuron synapses with an interneuron in the spinal cord.

  • 4. The interneuron synapses with a specific motor neuron.

  • 5. The motor neuron carries the impulse via the spinal nerve to an effector (either a muscle or a gland).

• Example: The knee-jerk reflex, which is present at birth.

The Eye

• Three Layers of the Eye:

  • Sclera: The white, rigid outer protective layer.

  • Choroid: The middle layer containing blood vessels for retinal nourishment.

  • Retina: The inner layer containing photoreceptors.

• Photoreceptors:

  • Rods: Perceive black and white images in low light levels.

  • Cones: Absorb colors and function in high light levels. High concentration is found in the fovea at the back of the retina for acute vision.

• Optical Structures and Fluids:

  • Cornea: A convex surface that begins light focusing.

  • Aqueous Humor: Watery fluid behind the cornea; excessive pressure buildup here causes Glaucoma.

  • Iris: Colored circular muscles that dilate or constrict to control light entry through the Pupil.

  • Lens: Transparent protein that converges light rays. Its shape is altered by ciliary muscles in a process called accommodation (flatter for distant objects, more spherical for near objects).

  • Vitreous Humor: Clear gelatinous fluid behind the lens that maintains the eyeball's shape.

  • Blind Spot: Area where the optic nerve exits the eye.

• Vision Abnormalities:

  • Nearsightedness: Eyeball is too long; images focus in front of the retina.

  • Farsightedness: Eyeball is too short; images focus behind the retina.

  • Astigmatism: Uneven curvature of the cornea or lens.

  • Cataract: Clouding of the lens.

The Ear

• Outer Ear:

  • Pinna: Directs sound waves into the auditory canal.

  • Auditory Canal.

• Middle Ear (Air-Filled):

  • Tympanic Membrane (Eardrum): Converts sound waves into vibrations.

  • Ossicles: Ear bones that carry vibrations across the middle ear to the oval window of the cochlea.

  • Eustachian Tube: Equalizes air pressure between the middle ear and the auditory canal; can also act as a pathway for microorganisms leading to infections.

• Inner Ear (Fluid-Filled):

  • Conchia (Cochlea): Coiled structure where vibrations produce waves in fluid.

  • Organ of Corti: Located within the cochlea, covered in hair cells that function as hearing receptors. The pitch of a sound is determined by the specific location of the deflected hair cell.

  • Round Window: Disperses compression waves.

  • Semicircular Canals: Three fluid-filled canals with hair cell receptors that respond to motion to maintain balance.

  • Saccule and Ucreule (Utrical): Detect the position of the head.

• Technological Interventions: Hearing loss can be treated with technologies like cochlear implants.

Principles of the Endocrine System and Hormonal Classification

• Mechanism: Employs endocrine glands to secrete hormones into the bloodstream to communicate with target cells.

• Chemical Types of Hormones:

  • Peptide/Protein Hormones: Bind to receptors on the surface of target cells.

  • Steroid Hormones: Pass through cell and nuclear membranes to bind with receptors in the nucleus, directly changing gene expression.

Glandular Functions, Target Tissues, and Hormonal Effects

• Hypothalamus (via Posterior Pituitary):

  • ADH: Targets kidney tubules; increases water reabsorption into the blood during dehydration.

  • Oxytocin: Targets uterine muscle (contractions for birth) and mammary glands (milk release).

• Anterior Pituitary:

  • TSH (Thyroid Stimulating Hormone): Targets the thyroid; stimulates thyroxine release.

  • ACTH (Adrenocortotropic Hormone): Targets the adrenal cortex; releases cortisol, aldosterone, and androgens in response to stress.

  • HGH (Growth Hormone): Targets bones and muscles; stimulates growth via mitosis.

• Thyroid:

  • Thyroxine: Targets all body cells; increases metabolic rate, ATP production, and heat.

  • Calcitonin: Targets bones, teeth, and gut; decreases blood calcium by depositing it in bones/teeth.

• Pancreas:

  • Insulin: Targets all cells; decreases blood glucose by increasing cell permeability to glucose.

  • Glucagon: Targets liver and muscles; increases blood glucose by converting glycogen into glucose.

• Adrenal Cortex:

  • Cortisol: Targets many tissues; converts protein into glucose.

  • Aldosterone: Targets kidney tubules; increases blood volume by reabsorbing sodium and water.

  • Disgesterone: Targets many tissues; increases muscle, bone, and hair growth in both sexes.

• Adrenal Medulla:

  • Norepinephrine: Targets heart, lungs, and blood vessels; shunts blood to core muscles and increases heart/breathing rates.

• Parathyroid:

  • Parathyroid Hormone: Targets bones and gut; increases blood calcium by dissolving bones and increasing gut absorption.

Homeostasis and Endocrine Paths

• Homeostasis: Maintenance of a stable internal condition, primarily through Negative Feedback Loops.

• Example: Thyroxine Feedback Loop:

  • 1. If the hypothalamus detects high thyroxine, it stops releasing the TSH releasing factor.

  • 2. This prevents the anterior pituitary from secreting TSH.

  • 3. The thyroid gland stops secreting thyroxine, lowering the metabolic rate.

  • 4. Conversely, low thyroxine levels feedback to cause the release of TSH factor.

• Hormone Imbalances:

  • Hypothyroidism: Lower metabolic rate.

  • Hyperthyroidism: Higher metabolic rate; can cause Graves' disease, characterized by high blood pressure, sweating, irritability, and weight loss.

  • Gaudiere: Swelling of the thyroid gland often caused by lack of iodine.

  • Type 1 Diabetes Mellitus: Cells are impermeable to glucose.

  • Type 2 Diabetes Mellitus: Usually in older, obese individuals; caused by inadequate supply of insulin receptors.

  • HGH Abnormalities: Imbalances in growth hormone secretion result in Dwarfism or Giantism.