Study Notes: Nervous System – Structure, Function, and Physiology

Unit II: Nervous System

Basic Functions of the Nervous System

The nervous system is the master controlling and communicating system of the body. It is responsible for monitoring stimuli, interpreting information, and initiating responses.

• Sensory Input: Monitoring stimuli occurring inside and outside the body.

• Integration: Interpretation of sensory input and determination of response.

• Motor Output: Response to stimuli by activating effector organs (muscles or glands).

Structural and Functional Divisions of the Nervous System

The nervous system is divided into central and peripheral components, each with specific roles in processing and transmitting information.

• Central Nervous System (CNS): Consists of the brain and spinal cord; serves as the integration and command center.

• Peripheral Nervous System (PNS): Includes paired spinal and cranial nerves; carries messages to and from the CNS.

Functional divisions:

• Sensory (Afferent) Division:

  • Somatic Sensory Division: Carries impulses from skin, skeletal muscles, and joints to the brain.

  • Visceral Sensory Division: Transmits impulses from visceral organs to the brain.

• Motor (Efferent) Division:

  • Transmits impulses from the CNS to effector organs.

  • Subdivided into:

    • Somatic Nervous System: Conscious control of skeletal muscles.

    • Autonomic Nervous System (ANS): Regulates smooth muscle, cardiac muscle, and glands.

      • Sympathetic Division: Mobilizes body systems in emergencies/stress (fight or flight).

      • Parasympathetic Division: Conserves energy, oversees digestion, elimination, gland activity (rest and digest).

Classification of Neurons

Neurons are classified by structure and function, which determines their role in the nervous system.

• Structural Classification:

  • Multipolar: Three or more processes; most common; primary neuron in CNS.

  • Bipolar: Two processes; rare; found in special sense organs.

  • Pseudounipolar (Unipolar): One process; found primarily in the PNS.

• Functional Classification:

  • Afferent (Sensory): Transmit impulses from effectors to the CNS.

  • Efferent (Motor): Transmit impulses from CNS to effectors.

  • Interneurons (Association Neurons): Located between motor and sensory neurons; most common; primary neuron in the CNS.

Neuron anatomy includes dendrites, cell body, axon, axon hillock, myelin sheath (Schwann cells), axon terminals, nodes of Ranvier, etc.

Neuroglia (Glial Cells) and Their Functions

Neuroglia are supporting cells in the CNS and PNS that provide structural and metabolic support for neurons.

• Astrocytes: Most abundant; support and brace neurons; anchor them to nutrient supply lines; regulate extracellular environment; help repair damaged tissue.

• Microglial Cells: Small with long processes; monitor neuron health; can become macrophages to remove microorganisms and debris.

• Ependymal Cells: Ciliated; line cavities filled with cerebrospinal fluid; help circulate the fluid.

• Oligodendrocytes: Wrap around nerve fibers in the CNS, producing myelin sheaths.

Myelin Sheath

The myelin sheath is a whitish, fatty (protein-lipoid) segmented sheath around most long axons. It protects axons, electrically insulates fibers, and increases the speed of nerve impulse transmission.

• Myelin Sheath in PNS: Formed by Schwann cells; each cell forms a segment; nodes of Ranvier are gaps between Schwann cells.

• Myelin Sheath in CNS: Formed by oligodendrocytes; both myelinated and unmyelinated fibers are present; nodes of Ranvier are widely spaced.

• White Matter: Regions of CNS with dense arrays of myelinated axons.

• Gray Matter: Regions of CNS with cell bodies and unmyelinated axons.

Ganglia, Nuclei, Nerves, and Tracts

Cell bodies and projections are organized differently in the CNS and PNS.

Central Nervous System	Peripheral Nervous System
Nuclei (clusters of cell bodies)	Ganglia (clusters of cell bodies)
Tracts (bundles of neuron processes)	Nerves (bundles of neuron processes)

Electrical Properties of Neurons

Neurons transmit information via electrical signals, which depend on the movement of ions across the cell membrane.

• Current: Flow of electrical charge from one point to another.

• Resistance: Hindrance to charge flow; insulators have high resistance, conductors have low resistance.

• Ohm's Law: $I = \frac{V}{R}$, where $I$ is current, $V$ is voltage, and $R$ is resistance.

Ion Channels: Plasma membranes contain ion channels, which may be:

• Leak (nongated) channels: Always open.

• Chemically-gated channels: Open when a chemical (e.g., neurotransmitter) binds.

• Voltage-gated channels: Open/close in response to changes in membrane potential.

Electrochemical gradient: Combination of electrical and chemical gradients; the net movement of ions is determined by this gradient.

Resting Membrane Potential (RMP)

The resting membrane potential is the voltage difference across the membrane of a resting neuron, typically around -70 mV. The inside of the membrane is negatively charged relative to the outside.

• Maintained by differences in ion concentrations and permeability.

• Na+/K+ pump: 3 Na+ pumped out, 2 K+ pumped in, maintaining the gradient.

• K+ plays the most important role in generating RMP.

Changes in Membrane Potential

Neurons communicate by changing their membrane potential, which can be produced by altering ion concentrations or membrane permeability.

• Depolarization: Reduction in membrane potential; inside becomes less negative.

• Hyperpolarization: Increase in membrane potential; inside becomes more negative.

Types of electrical potential changes:

• Graded Potentials: Short-lived, localized changes; strength varies with stimulus; may be depolarizing or hyperpolarizing.

• Action Potentials: Brief reversal of membrane potential with a total amplitude of ~100 mV; only generated by muscle cells and neurons; all-or-none response; propagated along axons as a nerve impulse.

Action Potential Generation and Propagation

Action potentials are generated by changes in membrane permeability, involving three steps: increase in Na+ permeability, restoration of Na+ impermeability, and increase in K+ permeability.

1. Resting State: Voltage-gated channels closed; leakage channels maintain resting potential.

2. Depolarization: Na+ channels open; Na+ enters cell, causing membrane potential to become less negative; positive feedback leads to further depolarization.

3. Repolarization: Na+ channels close, K+ channels open; K+ exits cell, restoring negative membrane potential.

Propagation: Action potentials are self-propagating and travel along the axon, transmitting signals to other neurons or effectors.

Synapses and Neural Communication

Synapses are junctions between neurons that allow for the transmission of signals. They can be electrical or chemical.

• Chemical Synapse: Involves release of neurotransmitters from presynaptic neuron, which bind to receptors on postsynaptic neuron, causing ion channels to open and generating a new action potential.

• Termination: Neurotransmitter action is terminated by reuptake, enzymatic degradation, or diffusion away from the synapse.

Types of Summation:

• Temporal Summation: Multiple signals from one neuron in rapid succession.

• Spatial Summation: Signals from multiple neurons at the same time.

Key Equations

• Ohm's Law: $I = \frac{V}{R}$

• Resting Membrane Potential: $V_{m} \approx -70\,\text{mV}$

Example: Action Potential Sequence

1. Resting state: All voltage-gated channels closed.

2. Depolarization: Na+ channels open, Na+ enters cell.

3. Repolarization: Na+ channels close, K+ channels open, K+ exits cell.

4. Return to resting state: Ion concentrations restored by Na+/K+ pump.

Additional info: These notes expand on brief points from the original materials, providing definitions, examples, and context for key concepts in nervous system anatomy and physiology.