Fundamentals of the Nervous System and Nervous Tissue: Study Notes

Fundamentals of the Nervous System and Nervous Tissue

11.1 Functions of the Nervous System

The nervous system is the master controlling and communicating system of the body. It uses electrical and chemical signals to coordinate rapid and specific responses.

• Functions:

  • Sensory Input: Information gathered by sensory receptors about internal and external changes.

  • Integration: Processing and interpretation of sensory input.

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

• Divisions:

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

  • Peripheral Nervous System (PNS): Outside CNS; consists mainly of nerves that extend from brain and spinal cord.

• PNS Functional Divisions:

  • Sensory (Afferent) Division: Somatic and visceral sensory fibers; conducts impulses from receptors to CNS.

  • Motor (Efferent) Division: Transmits impulses from CNS to effector organs; divided into somatic and autonomic nervous systems.

11.2 Neuroglia

Neuroglia are supporting cells in nervous tissue, providing structural and functional support for neurons.

• Types of Neuroglia in CNS:

  • Astrocytes: Most abundant; support neurons, regulate exchanges, guide migration, and influence neuronal functioning.

  • Microglial Cells: Small, ovoid; monitor neuron health, can transform into phagocytes.

  • Ependymal Cells: Line central cavities of brain and spinal cord; circulate cerebrospinal fluid (CSF).

  • Oligodendrocytes: Branched cells; form myelin sheaths in CNS.

• Types of Neuroglia in PNS:

  • Satellite Cells: Surround neuron cell bodies in PNS; similar function to astrocytes.

  • Schwann Cells: Form myelin sheaths in PNS; vital to regeneration of damaged peripheral nerve fibers.

11.3 Neurons

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

• Characteristics:

  • Long-lived, amitotic, high metabolic rate.

  • Require continuous supply of oxygen and glucose.

• Neuron Cell Body (Soma):

  • Contains nucleus and organelles.

  • Most located in CNS; clusters called nuclei (CNS) and ganglia (PNS).

• Neuron Processes:

  • Dendrites: Short, tapering, branched; receive input.

  • Axon: Conducting region; transmits impulses away from cell body.

  • Axon Terminals: Secretory region; release neurotransmitters.

Myelination

• Myelin Sheath: Whitish, protein-lipid substance; insulates axons, increases speed of transmission.

• Myelination in PNS: Formed by Schwann cells; wraps around axon in jelly roll fashion.

• Myelination in CNS: Formed by oligodendrocytes; can coil around multiple axons.

• Nodes of Ranvier: Gaps between myelin sheaths; facilitate rapid conduction.

Classification of Neurons

• Structural:

  • Multipolar: Three or more processes; most common.

  • Bipolar: Two processes; rare (retina, olfactory mucosa).

  • Unipolar: Single short process; mainly in PNS.

• Functional:

  • Sensory (Afferent): Transmit impulses toward CNS.

  • Motor (Efferent): Carry impulses away from CNS.

  • Interneurons: Shuttle signals through CNS pathways; most are multipolar.

11.4 Membrane Potentials

Neurons use changes in membrane potential for communication. The resting membrane potential is typically -70 mV.

• Basic Principles:

  • Energy is required to keep charges separated across a membrane.

  • Voltage: Measure of potential energy generated by separated charge.

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

  • Resistance: Hindrance to charge flow.

• Ohm's Law:

  • $I = \frac{V}{R}$

  • Where I is current, V is voltage, R is resistance.

Role of Membrane Ion Channels

• Types: Chemically gated, voltage-gated, mechanically gated.

• Electrochemical Gradient: Electrical and chemical gradients combined; ions move along gradients.

Resting Membrane Potential

• Generated by differences in ionic composition of ICF and ECF.

• Maintained by sodium-potassium pump ($Na^+/K^+$ ATPase).

11.5 Graded Potentials

Graded potentials are short-lived, localized changes in membrane potential, triggered by stimulus that opens gated ion channels.

• Can result in depolarization or hyperpolarization.

• Current dissipates and decays with distance.

11.6 Action Potentials

Action potentials are long-distance signals of neural communication, involving rapid changes in membrane potential.

• Phases:

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

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

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

  4. Hyperpolarization: Some K+ channels remain open; membrane potential below resting.

• Threshold: Minimum depolarization required to trigger AP; typically -55 to -50 mV.

• All-or-None Principle: AP either happens completely or not at all.

Propagation and Coding

• Propagation: Allows AP to be transmitted down entire axon length.

• Myelination: Saltatory conduction in myelinated axons; faster than continuous conduction.

• Coding for Stimulus Intensity: Frequency of APs encodes intensity.

Refractory Periods

• Absolute Refractory Period: Neuron cannot trigger another AP.

• Relative Refractory Period: Follows absolute; requires stronger stimulus for AP.

Conduction Velocity

• Depends on axon diameter and degree of myelination.

• Group A fibers: Large diameter, myelinated; fastest.

• Group B fibers: Intermediate diameter, lightly myelinated.

• Group C fibers: Small diameter, unmyelinated; slowest.

11.7 The Synapse

Synapses are junctions that mediate information transfer from one neuron to another or to an effector cell.

• Types:

  • Axodendritic: Between axon terminals of one neuron and dendrites of others.

  • Axosomatic: Between axon terminals and soma.

  • Chemical Synapse: Most common; specialized for release and reception of neurotransmitters.

  • Electrical Synapse: Less common; neurons connected by gap junctions.

Information Transfer Across Chemical Synapses

1. AP arrives at axon terminal of presynaptic neuron.

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

3. Ca2+ causes neurotransmitter release via exocytosis.

4. Neurotransmitter diffuses across synaptic cleft, binds to postsynaptic receptors.

5. Binding opens ion channels, creating graded potentials.

6. Neurotransmitter effects terminated by reuptake, degradation, or diffusion.

11.8 Postsynaptic Potentials

• Excitatory Postsynaptic Potentials (EPSPs): Depolarize postsynaptic membrane; can trigger AP.

• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize postsynaptic membrane; reduce likelihood of AP.

Integration and Modification of Synaptic Events

• Summation: EPSPs can add together (temporal or spatial summation) to reach threshold.

• Presynaptic Inhibition: Release of neurotransmitter by one neuron is inhibited by another.

11.9 Neurotransmitters

Neurotransmitters are chemical messengers of the nervous system. Over 50 have been identified.

• Classification by Chemical Structure:

  • Acetylcholine (ACh): Released at neuromuscular junctions; synthesized from acetic acid and choline.

  • Biogenic Amines: Catecholamines (dopamine, norepinephrine, epinephrine) and indolamines (serotonin, histamine).

  • Amino Acids: Glutamate, aspartate, glycine, GABA.

  • Peptides: Substance P, endorphins.

  • Purines: ATP, adenosine.

  • Gases and Lipids: Nitric oxide, carbon monoxide.

• Classification by Function:

  • Excitatory vs. Inhibitory: Some neurotransmitters have both effects depending on receptor type.

  • Direct vs. Indirect Action: Directly open ion channels or act through second messengers.

Neurotransmitter Receptors

• Channel-linked Receptors: Ligand-gated ion channels; mediate fast synaptic transmission.

• G Protein-linked Receptors: Indirect, complex, slow; involve second messengers (e.g., cyclic AMP).

11.10 Neural Integration

Neural integration involves neurons functioning together in groups to contribute to broader neural functions.

• Neuronal Pools: Functional groups of neurons that integrate incoming information.

• Patterns of Neural Processing:

  • Serial Processing: Input travels along one pathway to a specific destination.

  • Parallel Processing: Input travels along several pathways simultaneously.

• Types of Circuits: Diverging, converging, reverberating, parallel after-discharge.

Developmental Aspects of Neurons

• Nervous system originates from neural tube and neural crest.

• Neuroblasts become neurons; growth cone guides axon to target.

• Synapse formation requires astrocytes and chemical signals.

• Most neurons are amitotic after birth, but some populations (olfactory neurons, hippocampus) can divide.