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BIOL5_Ch7_NervousSys_I

The Nervous System Overview

  • Chapter 7

  • Understanding of the nervous system and its components.

Lecture Outline

  1. Overview of the Nervous System

  2. Neurons and Supporting Cells

    • Neurons and Classification of Neurons

    • Supporting Cells

  3. Axon Regeneration

  4. Electrical Activity in Axons

    • Ion Gating in Axons

    • Mechanism of Action Potential Generation

    • Conduction of Nerve Impulse: Axonal Conduction

  5. Synapses and Synaptic Transmission

Nervous System Components

  • Divisions:

    • Central Nervous System (CNS): Brain and spinal cord.

    • Peripheral Nervous System (PNS): Cranial and spinal nerves.

  • Cell Types:

    • Neurons: Functional units of the NS.

    • Glial Cells: Maintain homeostasis, 5-7x more than neurons.

Neurons

  • Function:

    • Gather and transmit information by responding to stimuli, sending electrochemical impulses, and releasing neurotransmitters.

  • Structure:

    • Composed of cell body, dendrites, and axon.

    • Groups of cell bodies are called nuclei (CNS) or ganglia (PNS).

  • Dendrites: Receive information; convey to cell body.

  • Axons: Conduct impulses away from cell body.

Functional Classification of Neurons

  • Sensory (Afferent) Neurons: Conduct impulses into CNS.

  • Motor (Efferent) Neurons: Carry impulses out of CNS.

  • Association (Interneurons): Integrate NS activity; located entirely within CNS.

Structural Classification of Neurons

  • Pseudounipolar: Cell body alongside a single process (e.g., sensory neurons).

  • Bipolar: Dendrite and axon on opposite ends (e.g., retinal neurons).

  • Multipolar: Many dendrites and one axon (e.g., motor neurons).

Supporting / Glial Cells

  • Types in PNS:

    • Schwann Cells: Myelinate PNS axons.

    • Satellite Cells: Cover sensory and autonomic ganglia.

  • Types in CNS:

    • Oligodendrocytes, Microglia, Astrocytes, Ependymal Cells.

  • Functions:

    • Support and maintain neurons; myelinate axons; participate in immune response.

Myelination

  • In PNS, each Schwann cell myelinates 1mm of one axon.

  • Myelin insulates axons and facilitates faster signal transmission via nodes of Ranvier (unmyelinated gaps).

Axon Regeneration

  • More feasible in PNS than CNS; oligodendrocytes inhibit regrowth, while astrocytes form scar tissue blocking regrowth.

  • When PNS axon is severed:

    1. Distal part degenerates.

    2. Schwann cells form a regeneration tube to guide regrowth toward synaptic site.

Electrical Activity in Axons

  • Nerve Impulse: Series of action potentials that change membrane potential.

  • Resting Membrane Potential (RMP): Internal charge is -70 mV, maintained by ion distribution and ion channels.

    • High Na+ outside and K+ inside.

Excitability and Membrane Potential Changes

  • Excitability: Rapid changes in resting membrane potential due to ion permeability.

  • Depolarization & Hyperpolarization: Changes in membrane potential.

  • Ion Channels:

    • K+ leakage channels always open; voltage-gated channels respond to voltage changes.

The Action Potential (AP)

  • A wave of change that travels along the axon induced by Na+ influx and K+ efflux.

  • All-or-None Principle: AP triggered once threshold is reached; AP amplitude remains the same.

  • Refractory Periods:

    • Absolute: No new AP can occur.

    • Relative: Harder to reach threshold due to open K+ channels.

Axonal Conduction

  • Types of Conduction:

    • Continuous conduction in unmyelinated axons (slow).

    • Saltatory conduction in myelinated axons (fast, jumps from node to node).

Factors Affecting AP Speed

  • Fiber Classification:

    • A Fibers (fast, myelinated); B Fibers (medium speed, myelinated); C Fibers (slow, unmyelinated).

Synaptic Transmission

  • Definition: Connection between presynaptic neuron and postsynaptic cell.

  • Types of Synapses:

    • Chemical (via neurotransmitters) and electrical (via gap junctions).

    • Neurons communicate using graded potentials (EPSPs and IPSPs).

Graded Potentials and Action Potentials

  • EPSPs: Depolarizing graded potentials that can summate to cause an AP.

  • IPSPs: Hyperpolarizing potentials that inhibit neuron activity.

  • Synaptic Integration: If summation of EPSPs and IPSPs reaches threshold, an AP is generated.

Comparison: Action Potentials vs. EPSPs

  • AP: All-or-none, regenerated along axon; exhibits refractoriness.

  • EPSPs: Graded, short-range communication; no refractory period.

Neurotransmitter Activity

  • Neurotransmitter receptors can be ionotropic (direct ion channel opening) or metabotropic (indirect via G-proteins).

  • Mechanism: NT binding opens specific channels, resulting in depolarization (EPSP) or hyperpolarization (IPSP).