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).

Example: Touching a hot surface triggers sensory input (pain), integration (processing in the brain), and motor output (withdrawal of the hand).

Structural and Functional Divisions of the Nervous System

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

• Central Nervous System (CNS): Brain and spinal cord; integration and command center.

• Peripheral Nervous System (PNS): 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: 3 or more processes; most common; primary neuron in CNS.

  • Bipolar: 2 processes; rare, found in special sense organs.

  • Pseudounipolar (Unipolar): 1 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.

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 cerebrospinal fluid.

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

Additional info: In the PNS, Schwann cells produce myelin sheaths, and satellite cells support neuron cell bodies.

Myelin Sheath

The myelin sheath is a whitish, fatty, segmented covering around most long axons, increasing the speed of nerve impulse transmission.

• Functions:

  • Protects the axon

  • Electrically insulates fibers

  • Increases speed of impulse transmission

• Formation:

  • In the PNS: Formed by Schwann cells; each cell wraps around a segment of the axon.

  • In the CNS: Formed by oligodendrocytes; both myelinated and unmyelinated fibers are present.

  • Nodes of Ranvier: Gaps between adjacent Schwann cells or oligodendrocytes; important for saltatory conduction.

Ganglia, Nuclei, Nerves, and Tracts

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

	Central Nervous System	Peripheral Nervous System
Cell bodies	Nuclei	Ganglia
Cell projections/processes	Tracts	Nerves

Electrical Properties of Neurons

Neurons transmit information via electrical signals, which depend on the flow 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; allow ions to move down their concentration gradients.

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

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

Electrochemical Gradient: Combination of electrical and chemical gradients; determines the direction of ion flow.

Resting Membrane Potential (RMP)

The resting membrane potential is the voltage difference across the membrane of a resting neuron, typically -70 mV.

• Membrane is polarized: negative inside relative to outside.

• Maintained by differences in ion concentrations and selective permeability.

• Na+/K+ pump: 3 Na+ pumped out, 2 K+ pumped in; maintains concentration gradient.

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

Changes in Membrane Potential

Neurons communicate by changing their membrane potential.

• Depolarization: Membrane potential becomes less negative (closer to zero).

• Hyperpolarization: Membrane potential becomes more negative.

These changes can produce:

• Graded Potentials: Short-lived, localized changes; strength varies with stimulus.

• Action Potentials: Brief reversal of membrane potential with a total amplitude of ~100 mV; only generated by muscle cells and neurons.

Action Potential: Generation and Propagation

Action potentials are rapid, self-propagating electrical signals generated by neurons.

• Involves three steps:

  1. Increase in Na+ permeability (depolarization)

  2. Restoration of K+ permeability (repolarization)

  3. Restoration of ion concentrations (hyperpolarization, short-lived)

Details:

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

2. Depolarization Phase: Na+ channels open, Na+ enters the cell, membrane potential becomes less negative.

3. Repolarization Phase: Na+ channels close, K+ channels open, K+ exits the cell, membrane potential returns to resting value.

Action Potential Equation:

$I = g(V_m - E_{ion})$

Where $I$ is ionic current, $g$ is conductance, $V_m$ is membrane potential, and $E_{ion}$ is equilibrium potential for the ion.

Propagation: Action potentials are propagated along the axon as a nerve impulse; each segment depolarizes the next.

Synapses and Neural Communication

Synapses are junctions between neurons where information is transmitted from one cell to another.

• Types of Synapses:

  • Electrical Synapses: Direct flow of ions between cells via gap junctions.

  • Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft.

• Structure of Chemical Synapse:

  1. Action potential arrives at axon terminal.

  2. Voltage-gated Ca2+ channels open; Ca2+ enters terminal.

  3. Ca2+ triggers neurotransmitter release into synaptic cleft.

  4. Neurotransmitter binds to receptors on postsynaptic membrane.

  5. Postsynaptic potential generated; may be excitatory or inhibitory.

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

Postsynaptic Potentials

Postsynaptic potentials are changes in membrane potential of the postsynaptic neuron.

• Excitatory Postsynaptic Potentials (EPSPs): Depolarize the postsynaptic membrane, increasing likelihood of action potential.

• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize the postsynaptic membrane, decreasing likelihood of action potential.

Summation: Multiple EPSPs or IPSPs can combine to influence whether an action potential is generated.

• Temporal Summation: One synapse is stimulated repeatedly in quick succession.

• Spatial Summation: Multiple synapses are stimulated simultaneously.

Example: Rapid firing of a presynaptic neuron can cause temporal summation, leading to an action potential in the postsynaptic neuron.

Additional info: These notes cover the major competencies for the college-level Anatomy & Physiology nervous system unit, including structure, function, electrical properties, and synaptic transmission.