Human Nervous System and Sensory Physiology Essentials

The Neuron and Synaptic Communication

Neurons consist of several key components:

• Cell Body (Soma): Houses the nucleus, mitochondria, and DNA; controls cell activities.
• Dendrites: Branching extensions that receive electrical signals and transmit them to the soma.
• Axon: Extends from the cell body to carry electrical signals to other neurons, muscles, or glands.

Synapses

A synapse is the junction where a neuron connects with another cell to send signals.

• Electrical Synapses: Connected by gap junctions allowing direct cytoplasmic continuity. Ions flow directly, making them very fast and often bidirectional; ideal for quick communication in large groups of cells.
• Chemical Synapses: Separated by a tiny gap (the synaptic cleft). They release neurotransmitters that bind to receptors across the cleft. These are slower than electrical synapses, unidirectional, and support learning, adaptation, and flexibility.

Synaptic Interactions

• Axodendritic: Interaction between an axon terminal and a dendrite.
• Axosomatic: Interaction between an axon terminal and the neuron's cell body (soma).
• Axoaxonal: Interaction between an axon terminal and another axon.

Neurotransmitters and Their Roles

• Acetylcholine (ACh): Controls muscle contractions, learning, and memory.
• Dopamine: Involved in movement, motivation, reward pathways, and pleasure.
• Serotonin: Regulates mood, sleep, appetite, and digestion.
• GABA: The brain's main inhibitory chemical messenger; key for reducing anxiety and neuronal excitability.
• Glutamate: The most abundant neurotransmitter; essential for cognitive functions like memory and learning.
• Norepinephrine: Increases alertness, attention, and the body's response to stress.
• Epinephrine (Adrenaline): Primarily involved in the stress response, increasing heart rate and energy.
• Endorphins: The body's natural painkillers, released during stress and exercise to induce euphoria.

Glial Cells

• Oligodendrocytes: Form myelin sheaths around neurons in the CNS, facilitating rapid signal transmission.
• Astrocytes: Provide structural support, regulate ion balance, and release growth factors.
• Microglia: Act as immune cells, removing debris and pathogens from the brain.
• Ependymal cells: Line the ventricles of the brain and produce cerebrospinal fluid.

Electrophysiology and Action Potentials

Action potentials are rapid, all-or-none electrical signals in excitable cells. They are generated when voltage-gated sodium channels open, causing an influx of sodium ions that depolarizes the membrane to a threshold, followed by potassium channel activation, which repolarizes the membrane.

Action Potential Phases

• Resting Potential: The cell is at rest with a negative membrane potential due to ion imbalance and the sodium-potassium pump.
• Stimulus & Threshold: An external stimulus depolarizes the membrane. If it reaches a threshold (e.g., -55 mV), an action potential is triggered.
• Depolarization: Voltage-gated sodium channels open, allowing Na+ to rush in, making the cell interior positive (rising phase).
• Repolarization: Sodium channels inactivate, and potassium channels open, allowing K+ to flow out, returning the membrane to its negative resting potential.
• Hyperpolarization & Refractory Period: The potential may briefly become more negative than the resting state. The absolute refractory period follows, during which no new action potential can be generated.

Propagation

In myelinated axons, the myelin sheath insulates the axon, and depolarization leaps between nodes of Ranvier (saltatory conduction), significantly increasing signal speed compared to unmyelinated axons.

Brain Anatomy

Cerebrum

Responsible for thinking, reasoning, memory, decision-making, and comprehension. It is divided into left and right hemispheres.

Cerebellum and Brainstem

• Cerebellum: Located at the back of the brain, this cauliflower-shaped structure coordinates muscle movements, balance, posture, and motor memory.
• Brainstem: Extends from the brain to the spinal cord. It consists of the midbrain, pons, and medulla oblongata. It controls vital functions like breathing, heart rate, and digestion, and houses the reticular activating system.

Spinal Cord and Nerve Roots

• Spinal Cord: The central bundle of nerves running from the brainstem down the spine. It relays sensory information, carries motor commands, and coordinates reflexes.
• Nerve Roots: Paired branches exiting the spinal cord. They transmit specific sensory and motor information. Injury to these is known as radiculopathy.

Electroencephalographic (EEG) Waveforms

• Delta (0.5-4 Hz): Deep sleep.
• Theta (4-7 Hz): Light sleep or drowsiness.
• Alpha (8-12 Hz): Relaxed wakefulness.
• Beta (13-30 Hz): Intense mental activity and alertness.

Peripheral Nervous System and Autonomic Control

The Autonomic Nervous System controls internal organs and glands, maintaining homeostasis. It is divided into the sympathetic system (stress response) and the parasympathetic system (day-to-day operations).

Neurological Disorders

• Epilepsy: Imbalance between excitatory and inhibitory signals causing seizures.
• Alzheimer's Disease: Buildup of amyloid plaques and tau protein tangles.
• Multiple Sclerosis (MS): Autoimmune damage to the myelin sheath.
• Parkinson's Disease: Progressive disorder affecting movement and coordination.
• Cerebral Palsy (CP): Movement and coordination disorders due to damage to the developing brain.
• Stroke: Interrupted blood flow causing brain cell death.
• ALS: Progressive degeneration of motor neurons.

Sensory Physiology

Vision

Key structures include the cornea (refraction/protection), lens (focusing), iris/pupil (light regulation), retina (signal conversion), and optic nerve (transmission to brain).

Hearing and Balance

The outer ear collects sound, the middle ear (ossicles) transmits vibrations, and the inner ear (cochlea) converts vibrations into nerve signals. The vestibular system manages balance.

Smell and Taste

Olfactory epithelium detects odors, while taste buds on the tongue identify nutrients and toxins via gustatory nerves.

Endocrine System

Hormone Classifications

• Steroid Hormones: Lipid-soluble (e.g., testosterone, estrogen). They pass through cell membranes to bind with intracellular receptors and alter gene expression.
• Peptide Hormones: Water-soluble (e.g., insulin). They bind to surface receptors to trigger rapid second messenger systems.
• Amine Hormones: Derived from amino acids (e.g., epinephrine). They typically bind to surface receptors for rapid responses.

Endocrine Glands

• Hypothalamus: Links the CNS and endocrine system.
• Pituitary Gland: The "master gland" at the base of the brain.
• Pineal Gland: Releases melatonin.
• Thyroid: Regulates metabolism.
• Parathyroid: Regulates blood calcium.
• Thymus: Trains T-cells for the immune system.