Ch 11 A - Nerves and Neuroglia and Intro to Action Potentials

Fundamentals of the Nervous System and Nervous Tissue

Overview of the Nervous System

The nervous system is the master controlling and communicating system of the body. It utilizes both electrical and chemical signals to coordinate rapid and specific responses, often resulting in immediate effects.

• Communication: Cells communicate via electrical impulses and chemical messengers.

• Functions: Sensory input, integration, and motor output.

Main Functions of the Nervous System

• Sensory Input: Gathering information from sensory receptors about internal and external changes.

• Integration: Processing and interpretation of sensory input to determine appropriate responses.

• Motor Output: Activation of effector organs (muscles and glands) to produce a response.

Divisions of the Nervous System

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

• Peripheral Nervous System (PNS): Composed of nerves extending from the brain and spinal cord; connects the CNS to the rest of the body.

Functional Divisions of the PNS

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

  • Somatic sensory fibers: From skin, skeletal muscles, and joints.

  • Visceral sensory fibers: From visceral organs.

• Motor (Efferent) Division: Transmits impulses from the 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 and promotes housekeeping functions during rest.

Neuroglia and Neurons

Neuroglia (Glial Cells)

Neuroglia are supporting cells in nervous tissue that surround and protect neurons. They are essential for the maintenance, support, and protection of the nervous system.

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

• Microglial Cells (CNS): Act as phagocytes, removing microorganisms and neuronal debris.

• Ependymal Cells (CNS): Line cerebrospinal fluid-filled cavities and help circulate CSF.

• Oligodendrocytes (CNS): Form myelin sheaths around CNS nerve fibers.

• Satellite Cells (PNS): Surround neuron cell bodies in the PNS, similar to astrocytes.

• Schwann Cells (PNS): Surround all peripheral nerve fibers and form myelin sheaths; aid in regeneration of damaged nerve fibers.

Neurons (Nerve Cells)

Neurons are the structural and functional units of the nervous system, specialized for conducting electrical impulses.

• Extreme Longevity: Can last a person's lifetime.

• Amitotic: Generally do not divide (with some exceptions for regeneration).

• High Metabolic Rate: Require continuous supply of oxygen and glucose.

• Structure: Consist of a cell body (soma) and one or more processes (dendrites and axon).

Neuron Cell Body (Soma)

• Perikaryon: Biosynthetic center; contains nucleus and organelles.

• Nuclei: Clusters of neuron cell bodies in the CNS.

• Ganglia: Clusters of neuron cell bodies in the PNS.

Neuron Processes

• Dendrites: Short, branched processes; receptive region; convey graded potentials toward the cell body.

• Axon: Long process; conducting region; generates and transmits action potentials away from the cell body.

• Axon Terminals: Distal endings; secretory region where neurotransmitters are released.

• Axolemma: Neuron cell membrane.

Axonal Transport

• Anterograde: Movement away from the cell body (e.g., mitochondria, enzymes).

• Retrograde: Movement toward the cell body (e.g., degraded organelles, viruses).

Classification of Neurons

Neurons are classified functionally by the direction in which they transmit impulses relative to the CNS.

• Sensory (Afferent) Neurons: Transmit impulses from sensory receptors toward the CNS; cell bodies in PNS ganglia.

• Motor (Efferent) Neurons: Carry impulses from the CNS to effectors; most cell bodies in the CNS.

• Interneurons (Association Neurons): Shuttle signals through CNS pathways; most are entirely within the CNS; comprise 99% of neurons.

Myelin Sheath

The myelin sheath is a whitish, protein-lipid covering that insulates axons and increases the speed of nerve impulse transmission.

• Myelinated Fibers: Have segmented sheaths; conduct impulses rapidly.

• Nonmyelinated Fibers: Lack myelin; conduct impulses slowly.

• Schwann Cells (PNS): Form myelin by wrapping around axons; each cell forms one segment.

• Oligodendrocytes (CNS): Can myelinate multiple axons simultaneously.

• Myelin Sheath Gaps (Nodes of Ranvier): Gaps between adjacent Schwann cells; sites for axon collateral emergence.

White Matter vs. Gray Matter

• White Matter: Regions with dense collections of myelinated fibers (fiber tracts).

• Gray Matter: Mostly neuron cell bodies and nonmyelinated fibers.

Membrane Potentials and Electrical Principles

Resting Membrane Potential

All cells have a resting membrane potential, but neurons can rapidly change this potential, making them highly excitable.

• Resting Membrane Potential: Approximately -70 mV; inside of the membrane is negatively charged relative to the outside.

• Polarized Membrane: The voltage difference across the membrane.

Basic Principles of Electricity

• Voltage (V): Measure of potential energy generated by separated charge; measured in volts (V) or millivolts (mV).

• Current (I): Flow of electrical charge (ions) between two points.

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

• Ohm's Law:

$I = \frac{V}{R}$

• Current is directly proportional to voltage and inversely proportional to resistance.

Membrane Ion Channels

• Leakage (Nongated) Channels: Always open; allow ions to move across the membrane.

• Gated Channels: Open or close in response to specific signals.

  • Chemically Gated: Open with binding of a specific chemical (e.g., neurotransmitter).

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

  • Mechanically Gated: Open and close in response to physical deformation of receptors.

Electrochemical Gradient

• Combined effect of electrical and chemical gradients; determines the direction of ion flow.

• Ion flow creates electrical current and voltage changes across the membrane.

Generating the Resting Membrane Potential

• Ion Distribution: Extracellular fluid (ECF) has higher Na+; intracellular fluid (ICF) has higher K+.

• Permeability: Membrane is more permeable to K+ than Na+ due to more K+ leakage channels.

• Negative Proteins: Large, negatively charged proteins inside the cell contribute to the negative resting potential.

Changes in Membrane Potential

• Depolarization: Decrease in membrane potential (moves toward zero and above); inside becomes more positive.

• Hyperpolarization: Increase in membrane potential (moves away from zero); inside becomes more negative.

• Changes in membrane potential are used as signals to receive, integrate, and send information.

Graded Potentials

Characteristics of Graded Potentials

Graded potentials are short-lived, localized changes in membrane potential. They are triggered by stimuli that open gated ion channels and can result in depolarization or hyperpolarization.

• Strength: Stronger stimulus causes greater voltage change and farther current flow.

• Types: Receptor potentials (in sensory neurons) and postsynaptic potentials (in neurons).

• Decay: Current dissolve quickly; signals are only effective over short distances.

Action Potentials

Characteristics of Action Potentials

Action potentials are the principal means by which neurons send long-distance signals. They involve a rapid reversal of membrane potential and do not decay over distance.

• Occurrence: Only in muscle cells and axons of neurons.

• Magnitude: Large depolarization (>100 mV).

• All-or-None: Once threshold is reached, the action potential is generated and propagated.

Phases of the Action Potential

1. Resting State: All gated Na+ and K+ channels are closed.

   • Na+ channels have activation and inactivation gates.

   • K+ channels have one voltage-sensitive gate.

2. Depolarization: Na+ channels open; Na+ rushes into the cell, making the inside more positive.

   • Threshold: ~ -55 to -50 mV; all Na+ channels open.

   • Membrane polarity jumps to +30 mV.

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

4. Hyperpolarization: Some K+ channels remain open, causing the membrane potential to dip below resting value.

Restoration of Ionic Conditions

• After repolarization, the Na+/K+ pumps restore ionic conditions.

• Electrical conditions are reset during repolarization, but ionic gradients are restored by active transport.

Comparison of Structural Classes of Neurons

Structural Class	Description	Location	Function
Multipolar	Many processes; one axon, many dendrites	Most neurons in CNS	Motor and interneurons
Bipolar	Two processes; one axon, one dendrite	Retina of eye, olfactory mucosa	Sensory (special senses)
Unipolar	Single short process; divides into peripheral and central branches	Mainly in PNS	Sensory neurons

Example: Sensory neurons in the skin are unipolar, motor neurons in the spinal cord are multipolar.

Additional info: Some details about the molecular mechanisms of ion channel gating and the specific roles of glial cells were inferred for completeness.