Introduction to the Nervous System and Nervous Tissue: Structured Study Notes

Overview of the Nervous System

Major Functions of the Nervous System

The nervous system is responsible for coordinating and regulating bodily functions through rapid communication. It enables perception, integration, and response to internal and external stimuli.

• Sensory Function: Detects changes in the environment (internal and external) via sensory receptors.

• Integrative Function: Processes and interprets sensory input, deciding what action is needed.

• Motor Function: Initiates responses by activating effector organs (muscles and glands).

• Homeostasis: Maintains internal balance by regulating physiological processes.

• Mental Activity: Supports cognition, memory, and emotion.

Structures and Functions of Central and Peripheral Nervous Systems

• Central Nervous System (CNS): Consists of the brain and spinal cord. Integrates and processes information.

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS. Transmits signals between CNS and the rest of the body.

• Organs: Brain (controls higher functions), Spinal Cord (conducts signals and mediates reflexes), Nerves (bundles of axons), Ganglia (clusters of neuron cell bodies).

Functional Divisions of the Peripheral Nervous System

• Somatic Nervous System (SNS): Controls voluntary movements via skeletal muscles.

• Autonomic Nervous System (ANS): Regulates involuntary functions (e.g., heart rate, digestion). Subdivided into Sympathetic (fight or flight) and Parasympathetic (rest and digest) divisions.

Nervous Tissue

Structure and Function of Neuron Components

Neurons are specialized cells for transmitting electrical and chemical signals.

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

• Dendrites: Receive incoming signals from other neurons.

• Axon: Conducts electrical impulses away from the cell body.

• Axon Terminals: Release neurotransmitters to communicate with other cells.

Types of Neurons

• Multipolar Neurons: Many dendrites, one axon; most common in CNS.

• Bipolar Neurons: One dendrite, one axon; found in sensory organs (e.g., retina).

• Unipolar Neurons: Single process splits into two branches; common in PNS sensory neurons.

Structure-Function Relationship in Neurons

• Multipolar neurons are suited for integration and processing due to multiple dendrites.

• Bipolar neurons efficiently transmit sensory information.

• Unipolar neurons rapidly relay sensory signals from the periphery to the CNS.

Neuroglial Cells of CNS and PNS

• CNS Neuroglia:

  • Astrocytes: Support neurons, regulate environment, form blood-brain barrier.

  • Oligodendrocytes: Produce myelin in CNS.

  • Microglia: Immune defense, phagocytosis of debris.

  • Ependymal Cells: Line ventricles, produce cerebrospinal fluid.

• PNS Neuroglia:

  • Schwann Cells: Myelinate PNS axons.

  • Satellite Cells: Support and regulate environment around PNS neuron cell bodies.

Structure-Function Relationship in Neuroglial Cells

• Astrocytes' extensive processes allow regulation of ion and nutrient levels.

• Oligodendrocytes and Schwann cells wrap axons for insulation and faster conduction.

• Microglia's mobility enables immune surveillance.

• Ependymal cells' cilia help circulate cerebrospinal fluid.

Electrophysiology of Neurons

Voltage-Gated Ion Channels and Action Potential

• Voltage-Gated Sodium Channels: Open rapidly during depolarization.

• Voltage-Gated Potassium Channels: Open during repolarization.

• These channels are essential for generating and propagating action potentials.

Phases of Action Potential

• Depolarization: Membrane potential becomes less negative as Na+ enters.

• Repolarization: K+ exits, restoring negative membrane potential.

• Hyperpolarization: Membrane potential temporarily becomes more negative than resting.

Example Graph: Action potential graph shows rapid rise (depolarization), fall (repolarization), and undershoot (hyperpolarization).

Refractory Periods

• Absolute Refractory Period: No new action potential can be generated; Na+ channels are inactivated.

• Relative Refractory Period: Action potential possible with stronger stimulus; some K+ channels still open.

Conduction Types

• Continuous Conduction: Occurs in unmyelinated axons; slower.

• Saltatory Conduction: Occurs in myelinated axons; action potential jumps between nodes of Ranvier; faster.

Factors Affecting Conduction Speed

• Axon Diameter: Larger diameter = faster conduction.

• Myelination: Myelinated axons conduct faster due to saltatory conduction.

Neuronal Synapses

Electrical vs. Chemical Synapses

• Electrical Synapses: Direct cytoplasmic connection via gap junctions; rapid transmission.

• Chemical Synapses: Use neurotransmitters; slower but allow modulation.

Structure of Chemical Synapse

• Presynaptic Terminal: Contains synaptic vesicles with neurotransmitters.

• Synaptic Cleft: Gap between neurons.

• Postsynaptic Membrane: Contains receptors for neurotransmitters.

Neurotransmitter-Receptor Relationship

• Neurotransmitters bind to specific receptors, triggering postsynaptic responses.

• Response depends on receptor type, not just neurotransmitter.

Events of Chemical Synaptic Transmission

1. Action potential arrives at presynaptic terminal.

2. Ca2+ influx triggers neurotransmitter release.

3. Neurotransmitter diffuses across synaptic cleft.

4. Binds to postsynaptic receptors, causing response.

5. Neurotransmitter removed by reuptake, degradation, or diffusion.

Postsynaptic Potentials

• Excitatory Postsynaptic Potential (EPSP): Depolarizes membrane, increases likelihood of action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes membrane, decreases likelihood of action potential.

Summation of Synaptic Potentials

• Temporal Summation: Multiple signals in quick succession from one synapse.

• Spatial Summation: Signals from multiple synapses combine.

Neurotransmitters

Excitatory vs. Inhibitory Effects

• A neurotransmitter can be excitatory or inhibitory depending on the receptor it binds to.

• Example: Acetylcholine is excitatory at neuromuscular junctions, inhibitory in the heart.

Major Classes of Neurotransmitters

• Amino Acids: e.g., Glutamate (excitatory), GABA (inhibitory).

• Monoamines: e.g., Dopamine, Serotonin, Norepinephrine.

• Peptides: e.g., Endorphins.

• Others: e.g., Acetylcholine.

Common Neurotransmitters in CNS

• Excitatory: Glutamate.

• Inhibitory: GABA.

Functional Groups of Neurons

Neuronal Pools

• Neuronal Pool: Group of interconnected neurons that process specific types of information.

• Purpose: Integrate and relay signals for complex functions.

Types of Neural Circuits

• Diverging Circuit: One neuron stimulates many others; amplifies signal.

• Converging Circuit: Many neurons stimulate one; focuses signal.

Type of Circuit	Main Function	Example
Diverging	Amplifies signal	Motor pathways
Converging	Integrates input	Sensory pathways

Additional info: These notes expand on brief learning objectives by providing definitions, examples, and context for each topic, making them suitable for exam preparation in an Anatomy and Physiology (ANP) college course.