Fundamentals of the Nervous System and Nervous Tissue (Chapter 11) – Study Notes

Fundamentals of the Nervous System and Nervous Tissue

Basic Functions of the Nervous System

The nervous system is responsible for controlling and integrating all body activities. It uses electrical and chemical signals to rapidly communicate information throughout the body.

• Sensory Input: Detects changes (stimuli) inside and outside the body via sensory receptors.

• Integration: Processes and interprets sensory input and determines the appropriate response.

• Motor Output: Activates effector organs (muscles and glands) to cause a response.

• Example: Touching a hot surface triggers sensory input, integration in the brain/spinal cord, and motor output to withdraw the hand.

Structural and Functional Divisions of the Nervous System

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

• Structural Divisions:

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

  • Peripheral Nervous System (PNS): Cranial and spinal nerves; communication lines between CNS and the rest of the body.

• Functional Divisions:

  • Sensory (Afferent) Division: Transmits impulses from receptors to the CNS.

  • Motor (Efferent) Division: Transmits impulses from CNS to effector organs.

    • Somatic Nervous System: Voluntary control of skeletal muscles.

    • Autonomic Nervous System (ANS): Involuntary control of smooth muscle, cardiac muscle, and glands.

      • Sympathetic Division: Mobilizes body systems during activity.

      • Parasympathetic Division: Conserves energy, promotes housekeeping functions during rest.

Structural Components of a Neuron and Their Functional Roles

Neurons are the functional units of the nervous system, specialized for rapid communication.

• Cell Body (Soma): Contains the nucleus and organelles; metabolic center.

• Dendrites: Short, branched processes that receive signals from other neurons.

• Axon: Long process that transmits impulses away from the cell body.

• Axon Hillock: Cone-shaped region where the axon originates; action potentials are initiated here.

• Axon Terminals: Endings that release neurotransmitters to communicate with other cells.

• Myelin Sheath: Insulating layer that increases the speed of impulse conduction.

Classification of Neurons by Structure and Function

Neurons can be classified based on the number of processes and their functional roles.

• Structural Classification:

  • Multipolar: One axon, multiple dendrites (most common in CNS).

  • Bipolar: One axon, one dendrite (found in retina, olfactory mucosa).

  • Unipolar (Pseudounipolar): Single process that splits into two branches (sensory neurons in PNS).

• Functional Classification:

  • Sensory (Afferent) Neurons: Transmit impulses toward the CNS.

  • Motor (Efferent) Neurons: Carry impulses away from the CNS to effectors.

  • Interneurons (Association Neurons): Connect sensory and motor neurons within the CNS.

Functions of Neuroglia (Glial Cells)

Neuroglia are supporting cells that provide structural and functional support to neurons.

• Astrocytes (CNS): Support neurons, regulate the blood-brain barrier, control the chemical environment.

• Microglia (CNS): Act as phagocytes, removing debris and pathogens.

• Ependymal Cells (CNS): Line ventricles, produce and circulate cerebrospinal fluid (CSF).

• Oligodendrocytes (CNS): Form myelin sheaths around CNS axons.

• Satellite Cells (PNS): Surround neuron cell bodies in ganglia, regulate environment.

• Schwann Cells (PNS): Form myelin sheaths around PNS axons, aid in regeneration.

Structure and Function of the Myelin Sheath

The myelin sheath is a multilayered lipid and protein covering that insulates axons and increases the speed of nerve impulse conduction.

• In the CNS: Formed by oligodendrocytes; one cell can myelinate multiple axons.

• In the PNS: Formed by Schwann cells; each cell myelinates a single axon segment.

• Function: Increases conduction velocity, prevents signal loss, and aids in axon regeneration (mainly in PNS).

Nucleus vs. Ganglion; Nerve vs. Tract

These terms distinguish between structures in the CNS and PNS.

• Nucleus: Cluster of neuron cell bodies in the CNS.

• Ganglion: Cluster of neuron cell bodies in the PNS.

• Nerve: Bundle of axons in the PNS.

• Tract: Bundle of axons in the CNS.

Types of Membrane Ion Channels

Ion channels are proteins that allow specific ions to move across the neuronal membrane, crucial for generating electrical signals.

• Leak Channels: Always open; maintain resting membrane potential.

• Ligand-Gated (Chemically Gated) Channels: Open in response to binding of a chemical messenger (e.g., neurotransmitter).

• Voltage-Gated Channels: Open or close in response to changes in membrane potential.

• Mechanically Gated Channels: Open in response to physical deformation of the membrane (e.g., touch receptors).

Resting Membrane Potential and Its Electrochemical Basis

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

• Established by: Unequal distribution of ions (mainly Na+ and K+) and selective permeability of the membrane.

• Maintained by: Sodium-potassium pump (Na+/K+ ATPase) and leak channels.

• Electrochemical Gradient: Combination of concentration and electrical gradients driving ion movement.

• Equation: $V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out} + p_{Na}[Na^+]_{out}}{[K^+]_{in} + p_{Na}[Na^+]_{in}} \right)$ (Goldman-Hodgkin-Katz equation; simplified for main ions)

Graded Potentials

Graded potentials are short-lived, localized changes in membrane potential that occur in dendrites and cell bodies.

• Characteristics: Vary in amplitude, decrease with distance, can be depolarizing or hyperpolarizing.

• Examples: Receptor potentials (sensory receptors), postsynaptic potentials (synapses).

Comparison: Graded Potentials vs. Action Potentials

Graded and action potentials are two types of electrical signals in neurons.

Feature	Graded Potentials	Action Potentials
Location	Dendrites, cell body	Axon hillock, axon
Amplitude	Variable, decreases with distance	All-or-none, constant
Propagation	Local, not self-propagating	Self-propagating, long-distance
Summation	Can summate	Cannot summate

Generation and Propagation of Action Potentials

Action potentials are rapid, large changes in membrane potential that travel along axons.

• Generation: Triggered when depolarization reaches threshold at the axon hillock.

• Phases:

  1. Depolarization: Voltage-gated Na+ channels open, Na+ influx.

  2. Repolarization: Na+ channels inactivate, K+ channels open, K+ efflux.

  3. Hyperpolarization: K+ channels remain open briefly.

• Propagation: Action potential moves along the axon as local currents depolarize adjacent regions.

• Equation (Nernst for a single ion): $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{out}}{[ion]_{in}} \right)$

Absolute and Relative Refractory Periods

Refractory periods ensure unidirectional propagation and limit firing frequency.

• Absolute Refractory Period: No new action potential can be generated, regardless of stimulus strength (Na+ channels inactivated).

• Relative Refractory Period: A stronger-than-normal stimulus can initiate another action potential (some Na+ channels reset, K+ channels still open).

Saltatory vs. Continuous Conduction

Action potentials travel differently in myelinated and unmyelinated axons.

• Saltatory Conduction: In myelinated axons, action potentials "jump" from one node of Ranvier to the next, greatly increasing conduction speed.

• Continuous Conduction: In unmyelinated axons, action potentials propagate along every segment of the membrane, which is slower.

• Example: Motor neurons controlling skeletal muscles use saltatory conduction for rapid response.