M6. 1-4

Functional Introduction to the Nervous System

• Core Responsibilities of the Nervous System: The biological system functions through a three-step cycle:     * Receiving Information: It gathers data through sensory receptors.     * Processing Information: It integrates incoming signals to determine significance.     * Responding: It sends out signals to effector organs, such as muscles and glands, to create an appropriate response.

• Key Coding Acronym (S.A.M.E.): A foundational mnemonic used to remember the direction of neural signals:     * Sensory is Afferent: These are incoming signals moving toward the central control.     * Motor is Efferent: These are outgoing signals moving away from the central control to the body.

• The Three-Step Process of Coordination:     * Sensory Input: Sensory receptors detect stimuli from the external environment or internal bodily conditions.     * Integration: Occurs within the Central Nervous System (CNS). The system processes data and decides if an action is required. Most of these processes occur subconsciously without the need for intent.     * Motor Output: The production of a response in an effector organ, typically a muscle or a gland.

Major Divisions of the Nervous System

• The Central Nervous System (CNS): Consists exclusively of the brain and the spinal cord. It acts as the command and control center.

• The Peripheral Nervous System (PNS): Encompasses all neural structures outside the brain and spinal cord, including cranial nerves (arising from the brainstem) and spinal nerves.

• Specific PNS Sensory (Afferent) Receptors:     * Somatic Sense Receptors: Located in the skin, muscles, and joints. They detect position in space (proprioception), touch, pressure, pain, and temperature.     * Visceral Receptors: Located in internal organs (viscera). They monitor internal conditions like blood pressure, chemical presence, and mechanical stretch.     * Special Sensory Receptors: Dedicated to the "Special Senses": vision, hearing, taste, smell, and balance.

• Specific PNS Motor (Efferent) Divisions:     * Somatic Nervous System: Under voluntary control; it innervates skeletal muscles.     * Autonomic Nervous System (ANS): Described as the "autopilot" or involuntary nervous system. It innervates glands, smooth muscle, cardiac muscle, and adipose tissue. It is subdivided into:         * Sympathetic Nervous System: Responsible for activation during stress or emergencies.         * Parasympathetic Nervous System: Operates during normal, everyday situations to permit digestion and conserve energy.

Nervous System Organization and Vocabulary

• Structural Comparison:     * CNS: Functions as the control and integration center. Protected by the hard bony structures of the skull and vertebrae.     * PNS: Functions as the communication lines between the CNS and the rest of the body. Lacks bony protection; instead, it is anchored and held in place by dense connective tissue.

• Terminological Distinctions:     * Groups of Cell Bodies: Known as Nuclei in the CNS; known as Ganglia in the PNS.     * Groups of Axons: Known as Tracts in the CNS; known as Nerves in the PNS.

• Damage and Regeneration:     * CNS Damage: Usually causes global effects because the CNS processes information for multiple body areas. CNS axons lack the capacity for repair, making such injuries permanent and serious.     * PNS Damage: Axons in the PNS may undergo regeneration provided the cell body remains intact and alive.

Neuron Structure and Nervous Tissue

• Composition of Nervous Tissue: Consists of neurons (electrical conducting cells) and neuroglia (specialized support cells). Neurons are referred to as "king cells" that require a highly specialized environment maintained by neuroglia.

• Neuron Characteristics: Specialized for rapid communication, likened to the "wiring" of the body. They are formed early in embryonic development and lack centrioles, meaning they are incapable of mitosis.

• Primary Components of a Neuron:     * Dendrites: Numerous short extensions that conduct information in toward the cell body. They receive signals from other neurons.     * Cell Body (Soma): Contains the large nucleus and standard organelles. It functions as a manufacturing plant for neurotransmitters and contains many mitochondria for ATP production.     * Axon: A single extension that conducts information out away from the cell body toward axon terminals. It lacks rough endoplasmic reticulum and relies on the cell body for protein supply.     * Axon Hillock: The specialized region where the axon arises from the cell body.     * Axon Terminals: The ends of the axon where neurotransmitters are stored in synaptic vesicles.

• The Synapse: The connection point between neurons. Because neurons do not physically touch, there is a gap called the synaptic cleft.     * Presynaptic Neuron: The neuron located before the synaptic cleft.     * Postsynaptic Neuron: The neuron located after the synaptic cleft.

Classification of Neurons

• Structural Classification (Based on extensions from the cell body):     * Multipolar: Contains $3$ or more extensions ($1$ axon and many dendrites). These are typically motor neurons.     * Bipolar: Contains $2$ extensions (typically one dendrite and one axon). Found in special sensory systems.     * Unipolar (Pseudo-unipolar): A single process off the cell body branches into two. It has a peripheral process (running to the sensory receptor) and a central process (running to the CNS). These are sensory neurons.

• Functional Classification (Based on signal direction):     * Sensory (Afferent) Neurons: Carry data from the PNS to the CNS. They are unipolar and enter the spinal cord via the dorsal (posterior) root.     * Interneurons (Association Neurons): Multipolar cells found exclusively in the CNS. They connect sensory and motor neurons.     * Motor (Efferent) Neurons: Multipolar cells sending messages from the CNS to the PNS. Their cell bodies are inside the CNS, and their axons exit via the ventral (anterior) root of the spinal cord.

Membrane Potentials and Electrical Impulses

• Conductivity: Neurons and muscle cells are the only cells capable of changing their membrane potential to create an action potential.

• Resting Membrane Potential: The electrical charge difference at rest is approximately $-70\,mV$. The inside of the cell is net negative compared to the positive outside.     * Contributing Factors: Intracellular proteins have a negative charge, and ion pumps maintain unequal distribution.

• Maintaining the Potential:     * Passive (Leak) Channels: Always open. The membrane is more permeable to potassium ($K^+$) than sodium ($Na^+$). Because more $K^+$ leaks out than $Na^+$ leaks in, positive charge is lost from the cell interior.     * Active Sodium-Potassium Pump: Uses ATP to move ions against gradients. It pumps out $3\,Na^+$ ions for every $2\,K^+$ ions it pumps in.

• Gated Ion Channels (Active Gates):     * Ligand-gated Channels: Open when a specific chemical (ligand), such as a neurotransmitter, binds to them (e.g., acetylcholine in the neuromuscular junction).     * Voltage-gated Channels: Open in response to specific changes in membrane potential voltage. Found in axons and the sarcolemma.     * Mechanically-gated Channels: Open in response to physical changes or pressure; located in sensory neuron dendrites.

Protection of the Central Nervous System

• Protective Compartments: CNS tissue is highly sensitive and cannot be replaced if damaged by pressure or lack of oxygen/nutrients. Protection includes:     * Bony Protection: The skull (brain) and vertebral column (spinal cord).     * The Meninges: Three connective tissue layers:         * Dura Mater: The tough, leathery, double-layered outer membrane against the skull. It contains dural sinuses (large veins). Folds include the falx cerebri (separates cerebral hemispheres), tentorium cerebelli (separates cerebrum from cerebellum), and falx cerebelli (separates cerebellar hemispheres).         * Arachnoid Mater: The middle layer, resembling a loose spider web. The subarachnoid space below it contains blood vessels and Cerebrospinal Fluid (CSF).         * Pia Mater: Thin inner layer tightly attached to the brain's bumps and grooves.     * Blood Brain Barrier (BBB): A diffusion barrier created by astrocytes wrapping around capillaries. It maintains a stable chemical environment by blocking pathogens and large particles. Fat-soluble substances (gases, alcohol, nicotine) cross easily; nutrients (glucose) require specialized transport.

Cerebrospinal Fluid (CSF) and Ventricles

• CSF Production and Composition: Produced in the choroid plexus by ependymal cells and permeable capillaries. It is a watery fluid derived from blood plasma with a unique ion concentration.

• Functions: Bathes the brain/spinal cord, provides nutrients, and allows the brain to "float" so it does not bump against the hard skull.

• The Ventricular System: Cavities within the brain containing CSF:     * Two Lateral Ventricles: The largest, C-shaped structures.     * Third Ventricle: Located in the middle of the brain.     * Fourth Ventricle: Connected to the third ventricle and leads to the central canal of the spinal cord.

• Circulation Pathway:     1. Produced in the choroid plexus.     2. Flows into the ventricles.     3. Leaks out through the median and lateral apertures.     4. Enters the subarachnoid space to circulate around CNS tissue.     5. Absorbed into the dural venous sinuses via arachnoid villi (extensions of the arachnoid membrane).     6. Enters general venous circulation.