The Nervous System: Structure, Function, and Neurophysiology

The Nervous System

1.) Major Functions of the Nervous System

The nervous system is responsible for coordinating and regulating bodily activities by transmitting signals between different parts of the body. It enables rapid communication, integration of sensory input, and initiation of motor output.

• Sensory Function: Detects changes in the internal and external environment (stimuli) and transmits this information to the central nervous system (CNS).

• Integrative Function: Processes and interprets sensory input, making decisions for appropriate responses.

• Motor Function: Initiates responses by activating effectors such as muscles and glands.

• Homeostasis and Higher Functions: Maintains homeostasis and supports higher mental functions such as learning, memory, and emotion.

2.) Organization of the Nervous System

The nervous system is organized into two main divisions, each with distinct structures and functions.

• Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Composed of cranial and spinal nerves; connects the CNS to limbs and organs.

  • Somatic Nervous System: Controls voluntary movements of skeletal muscles.

  • Autonomic Nervous System: Regulates involuntary functions (e.g., heart rate, digestion); subdivided into sympathetic and parasympathetic divisions.

Neurons and Supporting Cells

3.) Structure of a Typical Neuron

Neurons are the functional units of the nervous system, specialized for transmitting electrical and chemical signals.

• Cell Body (Soma): Contains the nucleus and organelles; site of metabolic activity.

• Dendrites: Short, branched extensions that receive signals from other neurons.

• Axon: Long, singular process that transmits impulses away from the cell body.

• Axon Terminals (Synaptic Boutons): Endings where neurotransmitters are released to communicate with other cells.

• Myelin Sheath: Insulating layer around some axons, increasing conduction speed.

• Nodes of Ranvier: Gaps in the myelin sheath where action potentials are regenerated.

4.) Structural and Functional Classification of Neurons

• Structural Classification:

  • Multipolar Neurons: Many dendrites, one axon (most common in CNS).

  • Bipolar Neurons: One dendrite, one axon (found in retina, olfactory epithelium).

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

• Functional Classification:

  • Sensory (Afferent) Neurons: Transmit impulses from receptors to CNS.

  • Motor (Efferent) Neurons: Carry impulses from CNS to effectors.

  • Interneurons (Association Neurons): Connect neurons within CNS; involved in integration.

5.) Glial Cells (Supporting Cells)

Glial cells provide structural and functional support to neurons. They are more numerous than neurons and are essential for nervous system health.

• Central Nervous System (CNS):

  • Astrocytes: Maintain blood-brain barrier, regulate ion balance, support neurons.

  • Oligodendrocytes: Form myelin sheaths around CNS axons.

  • Microglia: Act as immune cells, removing debris and pathogens.

  • Ependymal Cells: Line ventricles, produce and circulate cerebrospinal fluid (CSF).

• Peripheral Nervous System (PNS):

  • Schwann Cells: Form myelin sheaths around PNS axons; aid in regeneration.

  • Satellite Cells: Support neuron cell bodies in ganglia.

6.) Myelin Sheaths in CNS and PNS

• Structure: Myelin is a lipid-rich insulating layer formed by oligodendrocytes (CNS) or Schwann cells (PNS).

• Formation: Oligodendrocytes can myelinate multiple axons; Schwann cells myelinate one axon segment each.

• Function: Increases speed of action potential conduction via saltatory conduction.

• Comparison: CNS myelin inhibits axon regeneration; PNS myelin supports regeneration.

7.) White Matter vs. Gray Matter

• White Matter: Composed mainly of myelinated axons; found in CNS tracts and PNS nerves.

• Gray Matter: Contains neuron cell bodies, dendrites, and unmyelinated axons; forms cortex and nuclei in CNS, ganglia in PNS.

• Distribution: In the brain, gray matter is superficial (cortex), white matter is deeper; in the spinal cord, the arrangement is reversed.

8.) Structure of a Nerve

Nerves are bundles of axons in the PNS, surrounded by connective tissue layers.

• Endoneurium: Surrounds individual axons.

• Perineurium: Encloses bundles of axons (fascicles).

• Epineurium: Outermost layer, encases the entire nerve.

Neurophysiology

9.) Chemical Synapses

A chemical synapse is a specialized junction where a neuron communicates with another cell via neurotransmitter release.

• Events at a Chemical Synapse:

  1. Action potential arrives at axon terminal.

  2. Voltage-gated Ca2+ channels open; Ca2+ enters terminal.

  3. Synaptic vesicles fuse with membrane, releasing neurotransmitter into synaptic cleft.

  4. Neurotransmitter binds to receptors on postsynaptic membrane, causing a response.

10.) Excitatory vs. Inhibitory Synapses

• Excitatory Synapses: Cause depolarization of postsynaptic membrane (e.g., Na+ influx).

• Inhibitory Synapses: Cause hyperpolarization (e.g., Cl- influx or K+ efflux).

11.) Membrane Potential Terms

• Polarization: The resting state; inside of the membrane is negatively charged relative to outside.

• Depolarization: Membrane potential becomes less negative (more positive).

• Repolarization: Return to resting membrane potential after depolarization.

• Hyperpolarization: Membrane potential becomes more negative than resting potential.

12.) Graded Potentials vs. Action Potentials

Graded Potentials: Local changes in membrane potential; vary in size; decremental; occur in dendrites/cell body.

• Action Potentials: All-or-none electrical impulses; propagate along axons without decrement.

13.) Action Potential Phases and Diagram

• Phases:

  1. Resting State

  2. Depolarization

  3. Repolarization

  4. Hyperpolarization

• Diagram: (Students should be able to label these phases on a graph of membrane potential vs. time.)

14.) Describe Ionic Mechanisms of Action Potential

• Threshold: Minimum depolarization needed to trigger an action potential (usually around -55 mV).

• Depolarization: Voltage-gated Na+ channels open; Na+ influx.

• Repolarization: Na+ channels inactivate; voltage-gated K+ channels open; K+ efflux.

• Hyperpolarization: K+ channels remain open briefly, causing membrane potential to become more negative than resting.

16.) Compare Threshold and Stimuli

• Threshold Stimulus: Strong enough to initiate an action potential.

• Subthreshold Stimulus: Too weak to trigger an action potential.

16Refractory Periods

• Absolute Refractory Period: No new action potential can be generated, regardless of stimulus strength (Na+ channels inactivated).

• Relative Refractory Period: A stronger-than-normal stimulus can initiate another action potential (some Na+ channels reset, K+ channels still open).

Propagation of Action Potentials

• Unmyelinated Fibers: Action potential propagates continuously along the axon.

• Myelinated Fibers: Action potential jumps between nodes of Ranvier (saltatory conduction), increasing speed.

Factors Affecting Conduction Velocity

• Axon Diameter: Larger diameter = faster conduction.

• Myelination: Myelinated axons conduct faster than unmyelinated.

• Temperature: Higher temperature increases conduction speed (within physiological limits).

Neurotransmitters

• Definition: Chemical messengers released by neurons to transmit signals across synapses.

• Release: Triggered by Ca2+ influx at axon terminal.

• Receptor Types:

  • Ionotropic: Ligand-gated ion channels; fast response.

  • Metabotropic: G-protein coupled receptors; slower, modulatory effects.

• Inactivation/Removal: Enzymatic degradation, reuptake, or diffusion away from synapse.

• Examples:

  • Acetylcholine (ACh): Excitatory at neuromuscular junction; inactivated by acetylcholinesterase.

  • Norepinephrine (NE): Excitatory or inhibitory; removed by reuptake and enzymatic breakdown.

  • Glutamate: Major excitatory neurotransmitter in CNS.

  • GABA (Gamma-aminobutyric acid): Major inhibitory neurotransmitter in CNS.

Postsynaptic Potentials: EPSPs and IPSPs

• Postsynaptic Potential: Change in membrane potential of the postsynaptic neuron due to neurotransmitter binding.

• Excitatory Postsynaptic Potential (EPSP): Depolarizes membrane, increasing likelihood of action potential.

• Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes membrane, decreasing likelihood of action potential.

• Summation:

  • Spatial Summation: Multiple synapses activated simultaneously.

  • Temporal Summation: Rapid, repeated activation of a single synapse.

  • Integration (GPSP): The net effect of all EPSPs and IPSPs determines whether an action potential is generated.

Example: Saltatory Conduction

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

Table: Comparison of CNS and PNS Glial Cells

Glial Cell Type	Location	Main Function
Astrocyte	CNS	Support neurons, maintain blood-brain barrier
Oligodendrocyte	CNS	Form myelin sheaths
Microglia	CNS	Immune defense, phagocytosis
Ependymal cell	CNS	Produce and circulate CSF
Schwann cell	PNS	Form myelin sheaths, aid regeneration
Satellite cell	PNS	Support neuron cell bodies in ganglia

Additional info: Academic context and examples have been added to expand upon the original outline and ensure the notes are self-contained and suitable for exam preparation.