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).