Chapter 11: Fundamentals of the Nervous System and Nervous Tissue – Study Guide

Fundamentals of the Nervous System and Nervous Tissue

Major Functions of the Nervous System

The nervous system is responsible for coordinating and regulating bodily functions through rapid communication and integration of information.

• Sensory Input: Detects internal and external stimuli via sensory receptors.

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

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

• Homeostasis: Maintains internal balance by regulating physiological processes.

• Mental Activity: Enables consciousness, memory, and learning.

Organization of the Nervous System

The nervous system is divided into structural and functional components.

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integration and processing.

• Peripheral Nervous System (PNS): Composed of nerves and ganglia outside the CNS; transmits information to and from the CNS.

• Functional Divisions:

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

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

  • Somatic Nervous System: Controls voluntary movements.

  • Autonomic Nervous System: Regulates involuntary functions (e.g., heart rate, digestion).

Structure and Function of a Typical Neuron

Neurons are specialized cells for transmitting electrical signals.

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

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

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

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

• Myelin Sheath: Insulates axon, increasing conduction speed.

Structural and Functional Classification of Neurons

Neurons are classified based on structure and function.

• Structural:

  • Multipolar: Many dendrites, one axon (most common in CNS).

  • Bipolar: One dendrite, one axon (found in special senses).

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

• Functional:

  • Sensory (Afferent): Transmit impulses toward CNS.

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

  • Interneurons: Connect sensory and motor neurons within CNS.

Glial Cells of CNS and PNS

Glial cells support and protect neurons.

• CNS Glial Cells:

  • Astrocytes: Regulate environment, support neurons, form blood-brain barrier.

  • Microglia: Immune defense, phagocytize debris.

  • Ependymal Cells: Line ventricles, produce cerebrospinal fluid.

  • Oligodendrocytes: Form myelin sheaths in CNS.

• PNS Glial Cells:

  • Schwann Cells: Form myelin sheaths in PNS.

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

Myelin Sheaths in CNS and PNS

Myelin sheaths are insulating layers that speed up nerve impulse conduction.

• CNS: Myelin formed by oligodendrocytes; one cell can myelinate multiple axons.

• PNS: Myelin formed by Schwann cells; each cell myelinates a single axon segment.

• Function: Increases conduction velocity via saltatory conduction.

White Matter vs. Gray Matter

White and gray matter are distributed differently in CNS and PNS.

• White Matter: Myelinated axons; found in CNS tracts and PNS nerves.

• Gray Matter: Neuron cell bodies, dendrites, unmyelinated axons; found in CNS nuclei and cortex, PNS ganglia.

Structure of a Nerve

Nerves are bundles of axons in the PNS.

• Endoneurium: Surrounds individual axons.

• Perineurium: Surrounds groups of axons (fascicles).

• Epineurium: Surrounds entire nerve.

Neurophysiology

Chemical Synapse

Chemical synapses transmit signals via neurotransmitter release.

• Presynaptic Neuron: Releases neurotransmitter into synaptic cleft.

• Postsynaptic Neuron: Receives neurotransmitter, generating a response.

• Events:

  1. Action potential arrives at axon terminal.

  2. Ca2+ influx triggers neurotransmitter release.

  3. Neurotransmitter binds to postsynaptic receptors.

  4. Postsynaptic response generated (excitation or inhibition).

Excitatory vs. Inhibitory Synapses

Synapses can either promote or suppress postsynaptic activity.

• Excitatory Synapse: Causes depolarization, increasing likelihood of action potential.

• Inhibitory Synapse: Causes hyperpolarization, decreasing likelihood of action potential.

Membrane Potential Terms

Changes in membrane potential are key to neuron function.

• Polarization: Resting state; inside of cell is negative relative to outside.

• Depolarization: Membrane potential becomes less negative (more positive).

• Repolarization: Return to resting negative membrane potential after depolarization.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Graded vs. Action Potentials

Neurons use two types of electrical signals.

• Graded Potentials: Local changes in membrane potential; vary in size; decremental.

• Action Potentials: All-or-none, propagated along axon; constant amplitude.

• Function: Graded potentials initiate action potentials; action potentials transmit signals long distances.

Action Potential Phases

An action potential consists of distinct phases.

• Resting State: Voltage-gated channels closed; membrane at -70 mV.

• Depolarization: Na+ channels open; Na+ influx.

• Repolarization: Na+ channels inactivate; K+ channels open; K+ efflux.

• Hyperpolarization: K+ channels remain open; membrane potential drops below resting.

Diagram: (Students should label phases on a graph of membrane potential vs. time.)

Ionic Mechanisms of Action Potential

Ion channel conformations and triggers are crucial for action potential phases.

• Depolarization: Voltage-gated Na+ channels open.

• Repolarization: Na+ channels inactivate; K+ channels open.

• Hyperpolarization: K+ channels remain open longer.

• Triggers: Voltage changes open/close channels.

Threshold for Action Potential

Threshold is the minimum membrane potential required to trigger an action potential.

• Typical Threshold: About -55 mV.

• At Threshold: Voltage-gated Na+ channels open, initiating action potential.

Subthreshold vs. Threshold Stimuli

Stimuli are classified by their ability to trigger action potentials.

• Subthreshold: Not strong enough to reach threshold; no action potential generated.

• Threshold: Sufficient to reach threshold; action potential generated.

Absolute and Relative Refractory Periods

Refractory periods limit action potential frequency.

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

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

Propagation of Action Potential

Action potentials travel along axons by depolarizing adjacent regions.

• Unmyelinated Fibers: Continuous conduction; slower.

• Myelinated Fibers: Saltatory conduction; faster, jumps between nodes of Ranvier.

Factors Affecting Conduction Velocity

Several factors influence how fast action potentials travel.

• Axon Diameter: Larger diameter = faster conduction.

• Myelination: Myelinated axons conduct faster.

• Temperature: Higher temperature increases velocity.

Neurotransmitters

Neurotransmitters are chemical messengers released at synapses.

• Release: Triggered by Ca2+ influx at axon terminal.

• Action: Bind to postsynaptic receptors, causing excitation or inhibition.

• Inactivation/Removal: Enzymatic breakdown, reuptake, or diffusion.

• Examples:

  • Acetylcholine (Ach): Excitatory at neuromuscular junction.

  • Norepinephrine (NE): Excitatory or inhibitory; involved in alertness.

  • Glutamate: Major excitatory neurotransmitter in CNS.

  • GABA: Major inhibitory neurotransmitter in CNS.

Postsynaptic Potentials: EPSPs and IPSPs

Postsynaptic potentials determine neuron response.

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

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

• Interaction: EPSPs and IPSPs can summate.

• Spatial Summation: Multiple inputs at different locations combine.

• Temporal Summation: Rapid, repeated inputs at same location combine.

• Integration (GPSP): Net effect of all EPSPs and IPSPs determines neuron response.

Example Table: Comparison of CNS and PNS Glial Cells

Glial Cell Type	CNS	PNS	Main Function
Astrocyte	Yes	No	Support, blood-brain barrier
Microglia	Yes	No	Immune defense
Ependymal	Yes	No	CSF production
Oligodendrocyte	Yes	No	Myelination (CNS)
Schwann Cell	No	Yes	Myelination (PNS)
Satellite Cell	No	Yes	Support neuron cell bodies

Key Equations

• Nernst Equation: Used to calculate equilibrium potential for an ion. $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$

• Resting Membrane Potential: $V_{m} = \frac{g_{K}E_{K} + g_{Na}E_{Na} + g_{Cl}E_{Cl}}{g_{K} + g_{Na} + g_{Cl}}$

Additional info: Academic context and examples were added to expand brief learning outcomes into comprehensive study notes.