Chapter 10.1-10.3

Chapter 10: Nervous System I

Basic Structure and Function

10.1 General Characteristics of the Nervous System

• Master regulatory system: Responsible for sending and receiving information.

  • Sensory receptors: Detect stimuli and send input to the brain and spinal cord.

  • Motor output: Activates effectors such as muscles and glands.

  • Integration: Process and interpret sensory input to determine the appropriate motor output.

Overview of Nervous System Structure

• Central Nervous System (CNS): Comprises the brain and spinal cord.

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

  • Cranial nerves: Emanate from the brain.

  • Spinal nerves: Emanate from the spinal cord.

• Divisions of the Nervous System:

  • Sensory (Afferent) Division: Carries signals from sensory receptors to the CNS.

    • Sensory receptors: End of neurons in PNS that monitor external and internal environments.

    • Functions: Transduce information into nerve impulses conducted along peripheral nerves to the CNS.

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

    • Somatic Nervous System: Controls voluntary movements (e.g., skeletal muscles).

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

10.2 Nervous Tissue Cells: Neurons

• Neurons: Fundamental cells responsible for communication through nerve impulses (electrochemical signaling).

• Structure of Neurons:

  • Cell Body (Soma): Contains organelles typical of other cells.

  • Nissl bodies: Composed of rough endoplasmic reticulum (RER) for synthesizing neurotransmitters.

  • Neurofibrils: Fine threads supporting axons.

  • Dendrites: Highly branched extensions that receive input from other neurons.

  • Input process: Dendrites → soma → output via axon.

  • Axons: Singular long extension that transmits impulses away from the cell body.

  • Axon hillock: Region where the axon originates from the cell body.

  • Collaterals: Branches off an axon allowing for multiple signaling pathways.

  • Axon terminals: Distal end containing synaptic knobs for communication with other cells.

  • Axonal transport: Mechanism for transporting biochemicals and organelles across the neuron.

  • Classification in PNS/CNS: Bundled as nerves in PNS and tracts in CNS.

Myelin

• Myelin: Fatty insulating sheath around axons.

  • Functions:

  • Speeds up electrical signal transmission.

  • Protects axons, preventing signal loss.

  • Essential for normal nervous system function.

• Oligodendrocytes: CNS cells that produce and maintain myelin, able to myelinate multiple axons simultaneously.

• Schwann Cells: PNS cells that myelinate single segments of one axon, significantly enhancing conduction.

  • Non-myelinating Schwann cells support axons in autonomic nerves.

Myelinated vs. Unmyelinated Axons

• Myelinated axons conduct impulses rapidly and appear white, constituting white matter.

• Unmyelinated axons appear gray, representing gray matter.

Structural Classification of Neurons (by Shape)

• Multipolar: One axon plus many dendrites.

• Bipolar: One axon and one dendrite (e.g., in sensory organs).

• Unipolar: Single process bifurcated into peripheral and central processes.

Functional Classification of Neurons (by Role)

• Sensory (Afferent) Neurons: Carry information to the CNS; typically unipolar.

• Motor (Efferent) Neurons: Convey signals from CNS to muscles and glands; mostly multipolar.

• Interneurons: Connect neurons within the CNS, facilitating processing and integration of information.

Specific Types of Neurons

• Multipolar Neurons: Most common form, linking CNS to effectors.

• Bipolar Neurons: Primary sensory neurons; found in sensory organs (e.g., eyes, nose, ears).

• Unipolar Neurons: Characteristic of afferent sensory neurons; bypass cell body aiding in swift signal transmission.

Neuroglia Classification

• Neuroglia: Support cells in the nervous system crucial for house maintenance, functioning, and repair.

• CNS Neuroglia include:

  • Astrocytes: Supply nutrients and support; involved in forming the blood-brain barrier.

  • Oligodendrocytes: Myelinate multiple axons in the CNS.

  • Microglia: Immune responders in the CNS, proliferate post-injury.

  • Ependyma: Line brain ventricles; circulate cerebrospinal fluid (CSF).

• PNS Neuroglia:

  • Schwann Cells: Produce myelin around peripheral axons.

  • Satellite Cells: Surround neuronal cell bodies, providing support and regulating the chemical environment.

Neuroglia and Axonal Regeneration

• Peripheral Nervous System (PNS): Axonal regeneration is feasible; Schwann cells assist in forming a regeneration tube and clear debris.

• Central Nervous System (CNS): Limited or no regeneration occurs; oligodendrocytes do not assist regrowth, resulting in often permanent damage.

10.3 Cell Membrane Potential

• Cell Membrane Potential: Electrical charge difference across the plasma membrane, leading to polarization.

  • Ion distribution: Higher K⁺ concentration inside and higher Na⁺ concentration outside the cell.

  • Resting membrane potential: Approximately -70 mV, arising from concentration gradients and ion rarity.

Ion Channels and Membrane Potential

• Ion Channels: Allow ion diffusion across membranes; include leak channels and gated channels (voltage-gated or chemically gated).

• Diffusion Dynamics: At rest, more permeable to K⁺, leading to rapid outward diffusion and a net negative charge within the neuron.

• Na⁺/K⁺ Pump: Maintains concentration gradients critical for neuronal impulse conduction.

The Action Potential

• Action Potential: Rapid, temporary change in membrane potential that enables neuron communication.

  • All-or-none principle: Neurons either fully fire or not.

  • Initiated by: Opening of Na⁺ channels in response to neurotransmitters.

Thresholds and the Action Potential

• Threshold Level: Required for the initiation of an action potential, typically -55 mV.

• Membrane Potential Dynamics: Transition from resting to depolarization (via Na⁺ influx) to repolarization (K⁺ efflux) and finally to hyperpolarization.

Steps of an Action Potential:

1. Resting State (~ -70 mV): Na⁺ channels closed, K⁺ leak channels open.

2. Depolarization: Voltage-gated Na⁺ channels open, Na⁺ rushes in making inside positively charged.

3. Repolarization: Na⁺ channels close, K⁺ channels open, K⁺ exits, restoring negative potential.

4. Hyperpolarization: K⁺ channels slow to close, membrane becomes even more negative.

5. Return to Resting Potential: Na⁺/K⁺ pump restores ion balances.

Refractory Period

• Absolute Refractory Period: After an action potential, the neuron cannot respond to another stimulus due to closed Na⁺ channels.

• Relative Refractory Period: A strong stimulus can initiate another action potential, but only if intensity is sufficient; maximum firing rate is about 700 impulses/s, while typical is about 100/s.

• Function: Prevents backward flow of action potentials and sets limits on how quickly a neuron can fire.

Impulse Conduction

• Propagation: Action potentials move along the axon due to voltage-gated channels that open in response to preceding action potentials.

Myelination and Impulse Conduction

• Speed of Impulse Conduction: Affected by myelination and axon diameter.

  • Myelinated Axons: Conduct impulses rapidly via saltatory conduction from node to node (Nodes of Ranvier).

  • Unmyelinated Axons: Conduct impulses continuously across the entire axon length.

  • Energy Efficiency: Myelinated axons require less energy for nerve signal propagation.

Visual Aids

• Visuals such as diagrams indicating the structure of neurons, depolarization and repolarization mechanics, and the propagation of action potentials can provide clarity to the above points.

Note: Refer to images provided in the transcript for detailed visuals of neuron structure and potential changes.