Study Notes on Nervous System and Action Potentials

Neuron

  • Neurons are the functional units of the nervous system.

  • Neuroglial cells (glia, meaning glue) are supporting cells that are non-neuronal and not electrically excitable.

Nervous Tissue

Chapter 9

I. Nervous System: CNS and PNS

A. Functions of Nervous System

  • Integration: The nervous system processes sensory information, controls all body responses, and stores information.

  • Sensory Input: Detecting and monitoring internal and external stimuli.

  • Motor Output: Causes a response in effectors like muscles and glands.

  • Neurology: The study of the nervous system.

B. Two Main Nervous System Divisions

  1. Central Nervous System (CNS)

    • Composed of the brain and spinal cord.

    • Detailed in Chapter 10.

  2. Peripheral Nervous System (PNS)

    • Contains cranial (12 pairs) and spinal nerves (31 pairs).

    • Approximately 85 billion neurons in total.

    • Contains about 100 million neurons in spinal nerves.

    • Comprises sensory receptors that monitor for specific internal and external stimuli.

    • More details in Chapters 11 and 12.

C. Activity of the PNS

  • The PNS operates in a two-directional manner.

1. Sensory/Afferent Division

  • Composed of receptors that detect stimuli and neurons that carry that information to the CNS.

2. Motor/Efferent Division

  • Neurons that carry impulses from the CNS to various targets.

II. Nervous System: Categories of Cells

A. Neurons

  • Functional unit of the nervous system, cannot replicate if damaged.

  • Variability in shapes, with the most common shape being multipolar.

  • Neurons respond to physical and chemical stimuli and create electrical signals known as Action Potentials (AP).

  • Neurons also release chemicals (neurotransmitters) for regulation and communication at synapses.

B. Classification of Neurons

  • Structurally: Classified by the number of processes extending from the cell body.

  • Functionally: Classified by whether the impulse travels toward (Sensory neurons) or away (Motor and Interneurons) from the CNS.

C. Structure of Neurons

  1. Cell Body

    • Contains a single nucleus, rough ER, and ribosomes for protein synthesis.

  2. Axon Hillock

    • The thick base of the cell body where AP arise.

  3. Processes

    • Dendrites: Carry impulses to the cell body, specialized for contact with other neurons.

    • Axons: Carry impulses away from the cell body, typically myelinated and branched, ending in terminal buds (synapse).

D. Axon vs Dendrite

  • Axon: Long thread-like part; conducts impulses away from the neuron.

  • Dendrite: Short, branched extension; transmits impulses toward the cell body.

III. Groupings of Neurons

A. Terms and Definitions

  • In the PNS:

    • Ganglion/Ganglia: Cluster of cell bodies.

  • In the CNS:

    • Nucleus/Nuclei: Cluster of cell bodies.

  • Nerve: Bundle of axons in the PNS.

  • Roots: Bundle of axons in the CNS.

B. White and Gray Matter

  • White Matter: Composed mostly of myelinated axons.

  • Gray Matter: Contains neuron cell bodies, dendrites, unmyelinated axons, and neuroglial cells.

IV. Neuroglial Cells

A. Overview

  • Neuroglial cells (glial cells) support and protect neurons and can be classified into types in CNS and PNS.

B. Types (4 in CNS, 2 in PNS)

CNS:

  1. Astrocytes

    • Star-shaped, form the blood-brain barrier (BBB), provide structural support.

  2. Oligodendrocytes

    • Most numerous, form the myelin sheath around multiple axons.

  3. Ependymal Cells

    • Form the epithelial lining of cavities in the brain and spinal cord; produce cerebrospinal fluid (CSF).

  4. Microglia

    • Phagocytic cells that clear away dead cells/debris.

PNS:

  1. Satellite Cells

    • Surround neuron cell bodies in ganglia, providing support.

  2. Schwann Cells

    • Wrap around axons, responsible for myelination.

V. Myelination

A. Myelination in the Nervous System

  • Most axons in the PNS are myelinated by Schwann cells and some in the CNS by oligodendrocytes.

B. Nodes of Ranvier

  • Gaps between segments of myelin, allowing ions to diffuse in and out of neurons, propagating electrical signals faster; myelinated axons conduct at up to 150 m/s compared to unmyelinated at 0.5 to 10 m/s.

VI. PNS Nerve Fiber Regeneration

A. Reasons for Regeneration Differences

  • Damaged nerve fibers in the CNS cannot regenerate due to inhibitory factors while PNS can regenerate when the environment is favorable.

Wallerian Degeneration

  • A process following peripheral nerve injury, allowing axon regeneration along the path provided by Schwann cells.

VII. Disorders Related to Nerves

A. Multiple Sclerosis

  • Chronic autoimmune disorder causing demyelination, with various symptoms including vision issues, tingling, and weakness.

B. Guillain-Barre Syndrome

  • Rare disorder attacking peripheral nerves; symptoms include weakness and paralysis.

C. Diabetic Neuropathy

  • Associated with high blood glucose levels leading to peripheral nerve damage and symptoms like numbness and pain.

VIII. Action Potential: Communication in Neurons

A. Definition

  • An action potential is an electrical event that represents neuron 'activation', involving the movement of Na+ and K+.

B. Phases of Action Potential

  1. Initiation: Creates a new potential due to a stimulus.

  2. Depolarization: Neuron becomes more positive as Na+ floods in.

  3. Repolarization: Neuron becomes negatively charged again as K+ exits.

  4. Hyperpolarization: Briefly, neuron becomes even more negative as K+ continues to leave.

C. Components of Action Potential

  • Gated Ion Channels: Responsible for the changes in membrane potential during AP, including:

    • Voltage-Gated Channels: Open/shut based on membrane potential.

    • Chemically-Gated Channels: Open based on binding of neurotransmitters.

D. Propagation of Action Potential

  • Continuous Conduction: Occurs in unmyelinated axons; AP stimulates the next segment.

  • Saltatory Conduction: Occurs in myelinated axons; AP jumps between Nodes of Ranvier, which is faster by 10-50x.

IX. The Synapse

A. Definition

  • Synapse: Junction allowing communication between neurons and other cells.

B. Components of Synapse

  1. Presynaptic Ending: Contains neurotransmitters.

  2. Synaptic Cleft: Gap between neurons.

  3. Postsynaptic Ending: Contains receptors for neurotransmitters.

C. Neurotransmitter Release

  • Sequence from AP arrival to neurotransmitter release and action on postsynaptic receptors:

    • Includes Ca2+ influx, vesicle fusion, and binding to receptors.

X. Neurotransmitters

A. Importance

  • Neurotransmitters are essential for normal nervous system function and have roles in numerous conditions.

B. Common Types

  1. Acetylcholine (ACh): Function in both CNS and PNS, can be excitatory or inhibitory.

  2. Monoamines: Include dopamine, norepinephrine, serotonin, and more; have diverse functions and clinical relevance.

  3. GABA: Major inhibitory neurotransmitter.

C. Effects of Drugs

  • Neuropharmacology: Studies the effects of drugs impacting neurotransmitter action, which varies between individuals.

  • Drugs can be agonistic (enhancing) or antagonistic (inhibiting) regarding neurotransmitter effects.