Meditaliano IMAT Prep

Part 1: The Nervous System - Rapid Communication

This section details the structure of neurons, the generation of electrical signals (action potentials), and how these signals are transmitted between cells (synapses).

1.1 Neuron Structure and Resting Potential

The neuron is the functional unit of the nervous system. Key parts include the dendrites (receive signals), the cell body (soma) (contains the nucleus), the axon (transmits signals), and the axon terminal (sends signals to the next cell). Many axons are insulated by a myelin sheath (produced by oligodendrocytes in the CNS, Schwann cells in the PNS), which speeds up conduction. Gaps in the myelin are called Nodes of Ranvier, where the signal "jumps" in a process called saltatory conduction.

At rest, a neuron maintains a resting membrane potential of about -70mV. This is established and maintained primarily by:

1. The Na⁺/K⁺ pump: Actively transports 3 Na⁺ ions out for every 2 K⁺ ions in, creating concentration gradients.
2. Potassium (K⁺) leak channels: The membrane is much more permeable to K⁺ at rest. K⁺ flows out down its concentration gradient, making the inside of the cell negative.

1.2 The Action Potential (All-or-None Signal)

An action potential is a rapid, temporary, and self-propagating reversal of the membrane potential, used for long-distance communication. It is an "all-or-none" event: if the membrane at the axon hillock depolarizes to a threshold potential (around -55mV), a full action potential is fired. If not, nothing happens.

• 1. Depolarization: A stimulus causes voltage-gated Na⁺ channels to open. Na⁺ rushes into the cell, making the membrane potential rapidly positive (up to +30mV).
• 2. Repolarization: At the peak, voltage-gated Na⁺ channels inactivate, and voltage-gated K⁺ channels open. K⁺ rushes out of the cell, returning the potential to negative.
• 3. Hyperpolarization: The K⁺ channels are slow to close, causing a brief "undershoot" where the membrane is more negative than resting potential. This period, along with Na⁺ channel inactivation, creates the refractory period, ensuring the signal travels in only one direction.

1.3 Synaptic Transmission

A synapse is the junction where a neuron communicates with another cell.

• Chemical Synapse (Most Common): 1. The action potential arrives at the presynaptic terminal. 2. This depolarizes the terminal, opening voltage-gated Ca²⁺ channels. 3. Ca²⁺ influx causes synaptic vesicles (filled with neurotransmitters) to fuse with the presynaptic membrane. 4. Neurotransmitters are released into the synaptic cleft. 5. They bind to ligand-gated ion channels (receptors) on the postsynaptic membrane. 6. This binding causes an EPSP (Excitatory Postsynaptic Potential, e.g., Na⁺ entry) or an IPSP (Inhibitory Postsynaptic Potential, e.g., Cl⁻ entry).
• Electrical Synapse: Cells are directly connected by gap junctions, allowing ions to flow directly. This is much faster but allows for less regulation.

1.4 CNS vs PNS Organization

The nervous system is broadly divided into two main parts:

• Central Nervous System (CNS): The "command center," consisting of the brain and spinal cord.
• Peripheral Nervous System (PNS): All the nerves outside the CNS. It connects the CNS to the rest of the body. The PNS is further divided:
  • Somatic Nervous System: Controls voluntary movements (skeletal muscles).
  • Autonomic Nervous System: Controls involuntary processes (heart rate, digestion, breathing). This is split into:
    • Sympathetic: "Fight-or-flight" (e.g., increases heart rate, dilates pupils, inhibits digestion).
    • Parasympathetic: "Rest-and-digest" (e.g., slows heart rate, stimulates digestion).

1.5 Glial Cells: The Support System

Neurons are not alone; they are outnumbered by glial cells (or neuroglia), which support, nourish, and protect them.

1.6 Key Neurotransmitters

These are the chemical messengers of the nervous system. Their balance is critical for all brain functions.

• Acetylcholine (ACh): Used at the neuromuscular junction (to trigger muscle contraction) and in the autonomic nervous system.
• Glutamate: The primary excitatory neurotransmitter in the CNS.
• GABA: The primary inhibitory neurotransmitter in the CNS.
• Dopamine: Involved in reward, motivation, and motor control (degeneration causes Parkinson's).
• Serotonin (5-HT): Regulates mood, sleep, and appetite.

Part 2: The Endocrine System - Widespread Regulation

This section covers the major types of hormones, the glands that secrete them, and the concept of feedback control, exemplified by glucose homeostasis.

2.1 Types of Hormones

Hormones are chemical messengers that travel through the bloodstream to act on distant target cells.

2.2 Major Endocrine Glands and Hormones

2.3 Feedback Control: Glucose Homeostasis

Feedback loops are the primary mechanism for regulating hormone levels. Negative feedback is the most common: a downstream product inhibits the upstream signal, ensuring stability.
Example 1: Blood Glucose Regulation

• High Blood Glucose (e.g., after a meal): 1. Pancreatic β-cells (beta-cells) detect high glucose. 2. They release insulin into the blood. 3. Insulin acts on liver, muscle, and fat cells, causing them to take up glucose from the blood. 4. The liver stores glucose as glycogen. 5. Blood glucose levels fall, which in turn reduces insulin release (negative feedback).
• Low Blood Glucose (e.g., fasting): 1. Pancreatic α-cells (alpha-cells) detect low glucose. 2. They release glucagon into the blood. 3. Glucagon acts primarily on the liver, stimulating glycogenolysis (breakdown of glycogen) and gluconeogenesis. 4. The liver releases glucose into the blood. 5. Blood glucose levels rise, which in turn reduces glucagon release (negative feedback).

2.4 The Hypothalamic-Pituitary Axis (HPA)

This is a classic example of hierarchical endocrine control. The HPA axis governs the body's stress response.

1. Hypothalamus (in response to stress) releases CRH (Corticotropin-releasing hormone).
2. Anterior Pituitary (stimulated by CRH) releases ACTH (Adrenocorticotropic hormone).
3. Adrenal Cortex (stimulated by ACTH) releases Cortisol (a steroid hormone).
4. Cortisol mediates the stress response (e.g., increases blood glucose, suppresses immune system).
5. Negative Feedback: Cortisol itself inhibits the release of CRH and ACTH, shutting down the pathway.

2.5 Homeostasis Example 2: Calcium Regulation

Blood calcium levels are tightly controlled by two opposing hormones:

• High Blood Calcium: The thyroid gland releases Calcitonin. Calcitonin *lowers* blood calcium by stimulating osteoblasts to build bone (taking Ca²⁺ from blood) and decreasing reabsorption in the kidneys.
• Low Blood Calcium: The parathyroid glands release Parathyroid Hormone (PTH). PTH *raises* blood calcium by stimulating osteoclasts to crush bone (releasing Ca²⁺ into blood) and increasing reabsorption in the kidneys.

Part 3: Comparative Control, Immunity, and Homeostasis (LO 19.0/20.0)

This section directly addresses the learning objectives by comparing the nervous and endocrine systems, introducing the immune system, and linking all three to physiological stability.

3.1 Comparison: Nervous vs. Endocrine Control

3.2 The Immune System: Defense and Stability

The immune system is a complex network of cells and proteins that defends the body against pathogens and maintains homeostasis by removing damaged cells.

• Innate Immunity (Non-specific): The first line of defense. Includes physical barriers (skin, mucus), chemical barriers (stomach acid), and specialized cells like macrophages (phagocytosis) and Natural Killer (NK) cells. It also includes the inflammatory response (redness, heat, swelling) mediated by histamine.
• Adaptive Immunity (Specific): A slower, highly specific response that creates "memory". It is mediated by lymphocytes:
  • B-cells (Humoral Immunity): When activated, they differentiate into plasma cells, which produce vast quantities of antibodies (immunoglobulins). Antibodies neutralize pathogens or "tag" them for destruction.
  • T-cells (Cell-mediated Immunity):
    • Helper T-cells (Tₕ): The "generals" of the immune system. They activate B-cells and Killer T-cells.
    • Cytotoxic (Killer) T-cells (Tₖ): Directly kill virus-infected cells or tumor cells by inducing apoptosis.

3.3 Advanced Immunity: MHC & Vaccination

• MHC (Major Histocompatibility Complex): These are proteins on the cell surface that "present" antigens to T-cells.
  • MHC Class I: Found on all nucleated cells. They present *internal* antigens (e.g., from a virus) to Cytotoxic T-cells (Tₖ). This is the "kill me" signal.
  • MHC Class II: Found only on Antigen-Presenting Cells (APCs) (like macrophages, B-cells). They present *external* antigens (from phagocytosis) to Helper T-cells (Tₕ). This is the "activate the army" signal.
• Vaccination: This works by safely exposing the body to an antigen (e.g., a dead virus or viral protein). This triggers a primary immune response, which is slow but creates memory cells. If the real pathogen enters later, these memory cells launch a secondary immune response, which is much faster and stronger, preventing disease.

3.4 Physiological Stability (Homeostasis)

Homeostasis is the dynamic process of maintaining a stable internal environment (e.g., temperature, pH, glucose concentration) despite external changes. All the systems we've discussed are crucial for this:

• Nervous System: Provides rapid detection and response (e.g., pulling your hand from a hot stove, shivering when cold).
• Endocrine System: Manages long-term adjustments and metabolic balance (e.g., glucose regulation, water balance via ADH, metabolic rate via thyroid hormone).
• Immune System: Maintains stability by defending against disruptive pathogens and clearing cellular debris, preventing disease.

Interactive Practice Quiz (30 Questions)

Rigorously test your understanding of these advanced concepts. Choose the best answer for each question and then submit to see your results.