Fundamentals of the Nervous System and Nervous Tissue: Structured Study Notes

Fundamentals of the Nervous System and Nervous Tissue

Overview of Nervous System Functions

The nervous system is responsible for rapid communication and control within the body. It detects changes in the environment, processes sensory information, and initiates appropriate responses.

• Sensory Input: Receives information from sensory receptors.

• Integration: Processes and interprets sensory input.

• Motor Output: Activates effector organs (muscles/glands) in response to stimuli.

Organization of the Nervous System

Central Nervous System (CNS)

• Components: Brain and spinal cord.

• Function: Integration and command center for the nervous system.

Peripheral Nervous System (PNS)

• Nerves: Bundles of axons extending from the CNS.

• Functions: Carries impulses to and from the brain and spinal cord.

• Functional Subdivisions:

  • Sensory (Afferent) Division: Transmits sensory fibers from skin, skeletal muscle, and joints to the CNS.

  • Motor (Efferent) Division: Transmits motor fibers from CNS to effector organs.

    • Somatic Nervous System: Voluntary control of skeletal muscles.

    • Autonomic Nervous System: Involuntary control of smooth muscle, cardiac muscle, and glands.

Histology of Nervous Tissue

Supporting Cells (Neuroglia)

• Astrocytes: Most abundant glial cell; support neurons, regulate blood supply, and control chemical environment.

• Microglia: Phagocytic cells; act as macrophages in CNS.

• Ependymal Cells: Line cavities of brain and spinal cord; form barrier between cerebrospinal fluid (CSF) and nervous tissue.

• Oligodendrocytes: Wrap axons in CNS with myelin sheath.

• Schwann Cells: Myelinate axons in PNS; aid in regeneration of peripheral nerve fibers.

Neurons (Nerve Cells)

• Characteristics: Structural unit of the nervous system; specialized for communication.

• Structure:

  • Cell Body (Perikaryon/Soma): Contains nucleus and organelles.

  • Processes: Extensions from cell body; include dendrites (receive signals) and axons (transmit signals).

  • Nuclei: Clusters of cell bodies in CNS.

  • Ganglia: Clusters of cell bodies in PNS.

  • Tracts: Bundles of axons in CNS.

  • Nerves: Bundles of axons in PNS.

• Axon Hillock: Site where axon arises from cell body; initiates action potentials.

• Synaptic Knobs: Release neurotransmitters to communicate with other cells.

Myelin Sheath and Neurilemma

• Function: Insulates axons, increases speed of impulse transmission.

• Formation in PNS:

  • Schwann cells wrap axons, forming myelin sheath.

  • Nodes of Ranvier: Gaps between Schwann cells; facilitate rapid conduction (saltatory conduction).

  • Unmyelinated fibers: Schwann cells may enclose several axons without forming myelin.

Classification of Neurons

• By Structure:

  • Multipolar: 3+ processes (most common).

  • Bipolar: 1 axon, 1 dendrite (rare; found in special senses).

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

• By Function:

  • Sensory (Afferent): Transmit impulses toward CNS.

  • Motor (Efferent): Transmit impulses away from CNS.

  • Association (Interneurons): Connect sensory and motor neurons; most abundant in CNS.

Neurophysiology

Basic Principles of Electricity

• Potential (Voltage): Difference in electrical charge between two points.

• Current: Flow of electrical charge.

• Resistance: Hindrance to charge flow.

• Ohm's Law:

  • $I = \frac{V}{R}$

  • Where $I$ = current, $V$ = voltage, $R$ = resistance.

• Ion Channels: Proteins that allow passage of specific ions; types include leakage (always open) and gated (chemically, voltage, or mechanically gated).

Resting Membrane Potential

• Definition: The voltage difference across the membrane of a resting neuron.

• Typical Value: -70 mV (inside negative relative to outside).

• Maintained by:

  • Selective permeability of membrane to ions.

  • Sodium-potassium pump ($3 \text{Na}^+$ out, $2 \text{K}^+$ in).

Membrane Potentials That Act as Signals

• Depolarization: Membrane potential becomes less negative.

• Hyperpolarization: Membrane potential becomes more negative.

Action Potentials

• Definition: Rapid, self-propagating wave of depolarization along axon.

• Phases:

  1. Resting State: Voltage-gated channels closed; small leak of K+ and Na+.

  2. Depolarizing Phase: Na+ channels open, Na+ influx; threshold reached ($\geq$ 15 mV change).

  3. Repolarizing Phase: Na+ channels close, K+ channels open, K+ efflux.

  4. Hyperpolarization: K+ channels remain open longer than needed; membrane potential drops below resting.

• Threshold: Minimum depolarization required to trigger action potential.

• All-or-None Phenomenon: Action potential either occurs fully or not at all.

• Absolute and Relative Refractory Periods: Time during which neuron cannot or is less likely to fire another action potential.

• Conduction Velocity: Influenced by axon diameter and myelination (myelinated axons conduct faster via saltatory conduction).

The Synapse

• Presynaptic Neuron: Sends signal.

• Postsynaptic Neuron: Receives signal.

• Types:

  • Electrical Synapses: Direct flow of ions via gap junctions.

  • Chemical Synapses: Use neurotransmitters to transmit signals.

Chemical Synapse Transmission

• Action potential arrives at axon terminal.

• Ca2+ influx triggers neurotransmitter release via exocytosis.

• Neurotransmitter binds to postsynaptic receptors, opening ion channels.

• Response depends on neurotransmitter type and receptor.

• Termination by enzyme degradation, reuptake, or diffusion away from synapse.

Postsynaptic Potentials and Synaptic Integration

• Excitatory Postsynaptic Potentials (EPSPs): Depolarize postsynaptic membrane, increasing likelihood of action potential.

• Inhibitory Postsynaptic Potentials (IPSPs): Hyperpolarize postsynaptic membrane, decreasing likelihood of action potential.

• Summation: Multiple EPSPs/IPSPs can combine temporally or spatially to influence neuron firing.

• Synaptic Potentiation: Repeated stimulation increases efficiency of synaptic transmission.

• Presynaptic Inhibition: Reduces neurotransmitter release from presynaptic neuron.

Neurotransmitters

• Acetylcholine: Stimulates skeletal muscles; degraded by acetylcholinesterase.

• Biogenic Amines: Include catecholamines (dopamine, norepinephrine, epinephrine) and indolamines (serotonin, histamine).

• Amino Acids: GABA, glycine, aspartate, glutamate.

• Neuropeptides: Endorphins, enkephalins; modulate pain and emotion.

• ATP and Nitric Oxide: Act as neurotransmitters in CNS and PNS.

Classification and Mechanism of Neurotransmitter Action

• Effects: Excitatory or inhibitory.

• Mechanism: Direct (channel-linked receptors) or indirect (G protein-linked receptors).

Neural Integration

Basic Concepts

• Neurons function in groups to process information.

Patterns of Neural Processing

• Serial Processing: Information travels along one pathway to a specific destination.

• Parallel Processing: Information is processed simultaneously along multiple pathways.

Table: Comparison of CNS and PNS Glial Cells

Glial Cell Type	Location	Main Function
Astrocyte	CNS	Support neurons, regulate blood supply, maintain environment
Microglia	CNS	Phagocytosis, immune defense
Ependymal Cell	CNS	Line cavities, produce and circulate CSF
Oligodendrocyte	CNS	Form myelin sheath around axons
Schwann Cell	PNS	Form myelin sheath, aid in axon regeneration

Example: Saltatory Conduction

In myelinated axons, action potentials "jump" from one node of Ranvier to the next, greatly increasing conduction speed compared to unmyelinated axons.

Additional info: These notes expand on the original outline by providing definitions, examples, and context for key terms and processes in nervous tissue and neurophysiology, suitable for college-level Anatomy & Physiology students.