Neuronal Signaling and the Structure of the Nervous System: Study Guide

exam 2 study guide

Neuronal Signaling and the Structure of the Nervous System

I. Functions and Divisions of the Nervous System

The nervous system is responsible for coordinating the body's response to internal and external stimuli. It is divided into central and peripheral components, each with specialized functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord. It processes information and is the control center for the body.

• Peripheral Nervous System (PNS): Composed of nerves outside the CNS. It is subdivided into:

  • Sensory Division: Transmits sensory information from receptors to the CNS.

  • Motor Division: Transmits commands from the CNS to effectors (muscles and glands). Includes:

    • Somatic Nervous System: Controls voluntary movements of skeletal muscles.

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

II. Histology of Nervous Tissue

Nervous tissue is composed of neurons and supporting cells called neuroglia. Each type has distinct roles in maintaining nervous system function.

• Neuroglia: Support, protect, and nourish neurons. Types include:

  • In CNS:

    • Astrocytes: Maintain the blood-brain barrier and provide structural support.

    • Microglial Cells: Act as immune defense cells in the CNS.

    • Ependymal Cells: Line ventricles and produce cerebrospinal fluid.

    • Oligodendrocytes: Form myelin sheaths around CNS axons.

  • In PNS:

    • Satellite Cells: Support neuron cell bodies in ganglia.

    • Schwann Cells: Form myelin sheaths around PNS axons.

• Neurons: The functional units of the nervous system, specialized for signal transmission.

  • Neuron Cell Body: Contains the nucleus and organelles.

  • Neuron Processes: Extensions from the cell body, including dendrites (receive signals) and axons (transmit signals).

  • Structural Classification:

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

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

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

  • Functional Classification:

    • Sensory Neurons: Transmit impulses from receptors to CNS.

    • Motor Neurons: Carry impulses from CNS to effectors.

    • Interneurons: Connect sensory and motor neurons within the CNS.

III. Membrane Potentials

Neurons communicate via electrical signals, which depend on membrane potentials created by ion gradients across the plasma membrane.

• The Resting Membrane Potential:

  • Difference in Ionic Composition: The inside of the neuron is negatively charged relative to the outside, mainly due to differences in Na+ and K+ concentrations.

  • Difference in Plasma Membrane Permeability: Selective permeability to ions helps maintain the resting potential.

  • Typical Resting Membrane Potential: $-70\ \text{mV}$

• Membrane Potentials That Act as Signals:

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

  • Hyperpolarization: Membrane potential becomes more negative.

  • Graded Potentials: Local changes in membrane potential that vary in size and decay with distance.

  • Action Potentials: All-or-none electrical impulses that travel along axons.

    • Generation of an Action Potential:

      1. Resting State: All voltage-gated channels closed.

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

      3. Repolarization: K+ channels open, K+ leaves cell.

      4. Hyperpolarization: K+ channels remain open briefly, membrane potential becomes more negative than resting.

    • Propagation of an Action Potential: The action potential moves along the axon as a wave of depolarization.

    • Refractory Periods:

      • Absolute Refractory Period: No new action potential can be initiated.

      • Relative Refractory Period: A stronger-than-normal stimulus is required to initiate another action potential.

IV. Synapse

Synapses are specialized junctions where neurons communicate with other neurons or effectors.

• Chemical Synapses:

  • Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft.

  • Synaptic Cleft: The small gap between the presynaptic and postsynaptic cells.

• Information Transfer Across Chemical Synapses: Neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic cell, causing a response.

• Excitatory Synapses and EPSPs: Cause depolarization and increase the likelihood of an action potential.

• Inhibitory Synapses and IPSPs: Cause hyperpolarization and decrease the likelihood of an action potential.

V. Neurotransmitters and Their Receptors

Neurotransmitters are chemical messengers that transmit signals across synapses. They are classified by their chemical structure and mechanism of action.

• Classification of Neurotransmitters by Chemical Structure:

  • Acetylcholine: Involved in muscle activation and autonomic functions.

  • Biogenic Amines: Includes dopamine, norepinephrine, and serotonin; involved in mood, arousal, and cognition.

  • Gases: Such as nitric oxide; act as neuromodulators.

• Neurotransmitter Receptors:

  • Channel-Linked Receptors: Ligand-gated ion channels that mediate fast synaptic transmission.

  • G Protein-Linked Receptors: Activate intracellular signaling cascades, leading to slower but longer-lasting effects.

Additional info: This study guide is based on an outline and has been expanded with academic context for clarity and completeness.