Introduction to the Nervous System and Nervous Tissue – Study Notes

Chapter 11: Introduction to the Nervous System and Nervous Tissue

Module 11.1 Overview of the Nervous System

The nervous system is a complex network responsible for regulating and coordinating body activities. It is divided into structural and functional components, each with specialized roles.

• Major Functions of the Nervous System:

  • Sensory Input: The process of gathering information from sensory receptors about internal and external changes.

  • Integration: The interpretation and processing of sensory input, leading to decision-making.

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

• Structural Divisions:

  • Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for integration and control.

  • Peripheral Nervous System (PNS): Composed of nerves (bundles of axons) and ganglia (clusters of cell bodies) outside the CNS.

• Nerves of the PNS:

  • Spinal Nerves: Emerge from the spinal cord; carry impulses to and from the spinal cord.

  • Cranial Nerves: Emerge from the brain; carry impulses to and from the brain.

• Functional Divisions of the PNS:

  • Sensory (Afferent) Division: Transmits sensory information to the CNS.

    • Somatic Sensory: Carries signals from skin, muscles, and joints.

    • Visceral Sensory: Carries signals from organs (viscera).

  • Motor (Efferent) Division: Transmits commands from the CNS to effectors.

    • Somatic Motor: Controls voluntary movements of skeletal muscles.

    • Autonomic Nervous System (ANS): Controls involuntary actions (smooth muscle, cardiac muscle, glands).

Example: Touching a hot surface activates sensory receptors (sensory input), the CNS processes the information (integration), and the hand is withdrawn (motor output).

Module 11.2 Nervous Tissue

Nervous tissue consists of neurons and neuroglia, each with specialized structures and functions.

• Neuron Structure and Function:

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

  • Dendrites: Receive incoming signals and convey them toward the cell body.

  • Nissl Bodies: Rough endoplasmic reticulum; involved in protein synthesis.

  • Axon Hillock: Cone-shaped region where the axon originates; site of action potential initiation.

  • Telodendria: Fine extensions at the end of the axon; form synapses with other cells.

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

• Functional Regions of a Neuron:

  • Receptive Region: Dendrites and cell body; receive stimuli.

  • Conducting Region: Axon; transmits action potentials.

  • Secretory Region: Axon terminals; release neurotransmitters.

• Types of Neurons:

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

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

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

• Functional Classes of Neurons:

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

  • Interneurons: Integrate information within the CNS; most abundant.

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

• Groupings of Cell Bodies and Axons:

  • CNS: Cell bodies = nuclei; axons = tracts.

  • PNS: Cell bodies = ganglia; axons = nerves.

• Neuroglia (Glial Cells): Support, protect, and nourish neurons.

  • CNS Neuroglia:

    • Astrocytes: Maintain blood-brain barrier, regulate environment.

    • Oligodendrocytes: Form myelin sheaths in CNS.

    • Microglia: Act as immune cells; phagocytosis.

    • Ependymal Cells: Line ventricles; produce cerebrospinal fluid.

  • PNS Neuroglia:

    • Schwann Cells: Form myelin sheaths in PNS.

    • Satellite Cells: Support cell bodies in ganglia.

• Myelin Sheath:

  • Insulating layer around axons; increases conduction speed.

  • Formed by oligodendrocytes (CNS) and Schwann cells (PNS).

  • White Matter: Myelinated axons; transmits signals rapidly.

  • Gray Matter: Cell bodies, dendrites, unmyelinated axons; integration and processing.

Example: Oligodendrocytes can myelinate multiple axons in the CNS, while Schwann cells myelinate only one axon segment in the PNS.

Module 11.3 Electrophysiology of Neurons

Neurons communicate via electrical signals generated by ion movement across their membranes.

• Ion Channels and Resting Membrane Potential:

  • Leak Channels: Always open; allow passive ion movement.

  • Ligand-Gated Channels: Open in response to chemical signals.

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

  • Mechanically Gated Channels: Open in response to physical deformation.

  • Sodium-Potassium Pump: Actively transports 3 Na+ out and 2 K+ in, maintaining gradients.

• Resting Membrane Potential: The voltage difference across the membrane at rest (typically -70 mV).

  • Created by differences in Na+ and K+ concentrations and selective permeability.

  • Electrochemical Gradient: Combination of electrical and chemical forces driving ion movement.

• Key Terms:

  • Depolarization: Membrane potential becomes less negative (Na+ influx).

  • Repolarization: Return to resting potential (K+ efflux).

  • Hyperpolarization: Membrane potential becomes more negative than resting.

  • Local Potential: Small, localized change in membrane potential.

• Action Potential:

  • Rapid, large change in membrane potential that propagates along the axon.

  • Involves voltage-gated Na+ (three states: closed, open, inactivated) and K+ channels (two states: open, closed).

• Phases of Action Potential:

  • Depolarization: Na+ channels open, Na+ enters.

  • Repolarization: Na+ channels inactivate, K+ channels open, K+ exits.

  • Hyperpolarization: K+ channels remain open briefly.

• Refractory Periods:

  • Absolute Refractory Period: No new action potential possible; ensures one-way transmission (all-or-none principle).

  • Relative Refractory Period: Stronger stimulus needed for another action potential.

• Propagation: Movement of action potential along the axon (axolemma).

• Conduction:

  • Continuous Conduction: Unmyelinated axons; slower.

  • Saltatory Conduction: Myelinated axons; action potential jumps between nodes of Ranvier; faster.

  • Conduction speed increases with axon diameter and myelination.

Example: The sodium-potassium pump maintains the resting membrane potential by moving ions against their gradients:

$\text{3 Na}^+ \text{ (out)} + \text{2 K}^+ \text{ (in)}$

Module 11.4 Neuronal Synapses

Synapses are specialized junctions where neurons communicate with other cells.

• Types of Synapses:

  • Electrical Synapses: Direct cytoplasmic connections (gap junctions); allow rapid, bidirectional communication; found in some brain regions and cardiac muscle.

  • Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft; most common in the nervous system.

• Structures of a Chemical Synapse:

  • Presynaptic Membrane: Releases neurotransmitter.

  • Synaptic Cleft: Small gap between neurons.

  • Postsynaptic Membrane: Receives neurotransmitter.

  • Synaptic Vesicles: Store neurotransmitters.

  • Neurotransmitter: Chemical messenger.

  • Receptor: Protein that binds neurotransmitter.

• Events of Chemical Synaptic Transmission:

  1. Action potential arrives at axon terminal.

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

  3. Neurotransmitter released into synaptic cleft.

  4. Neurotransmitter binds to postsynaptic receptors, causing a response.

• Postsynaptic Potentials:

  • Excitatory Postsynaptic Potential (EPSP): Depolarizes membrane; increases chance of action potential.

  • Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes membrane; decreases chance of action potential.

• Summation:

  • Neural Integration: Process of combining multiple synaptic inputs.

  • Temporal Summation: Multiple signals from one neuron over time.

  • Spatial Summation: Signals from multiple neurons at the same time.

• Termination of Synaptic Transmission:

  • Diffusion: Neurotransmitter moves away from synapse.

  • Degradation: Enzymes break down neurotransmitter.

  • Reuptake: Neurotransmitter is taken back into presynaptic neuron.

Example: Acetylcholine is released at neuromuscular junctions, causing muscle contraction (EPSP).

Module 11.5 Neurotransmitters

Neurotransmitters are chemicals that transmit signals across synapses. Their effect depends on the receptor they bind to.

• Neurotransmitter Effects:

  • The same neurotransmitter can be excitatory or inhibitory depending on the postsynaptic receptor.

• Major Classes of Neurotransmitters:

  • Cholinergic Synapses: Use acetylcholine; can be excitatory (e.g., neuromuscular junction) or inhibitory (e.g., cardiac muscle).

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

  • Biogenic Amines:

    • Norepinephrine, Epinephrine, Dopamine: Modulate mood, alertness, and movement.

    • Serotonin, Histamine: Affect mood, sleep, and arousal.

Example: Dopamine is excitatory in some brain pathways (reward) and inhibitory in others (motor control).

Module 11.6 Functional Groups of Neurons

Neurons are organized into functional groups and circuits to process information efficiently.

• Neuronal Pool: Group of interconnected neurons that process specific types of information; allow for complex activities like breathing or walking.

• Neural Circuits:

  • Diverging Circuit: One neuron branches to activate many; useful for amplifying signals (e.g., motor pathways).

  • Converging Circuit: Many neurons converge on one; useful for integrating information (e.g., sensory pathways).

Example: A diverging circuit allows one motor neuron to stimulate multiple muscle fibers for coordinated movement.

Additional info: Where the original notes listed only learning objectives, academic explanations and examples have been added for clarity and completeness.