Fundamentals of Nervous System Structure and Function: Study Notes

Nervous System Organization

Central vs. Peripheral Nervous System

The nervous system is divided into the central nervous system (CNS) and peripheral nervous system (PNS). Each division contains distinct structures and functions.

• CNS: Composed of the brain and spinal cord; responsible for processing and integrating information.

• PNS: Includes all neural tissue outside the CNS; connects the CNS to limbs and organs.

• Structures in each: CNS contains neurons, glial cells, and major tracts; PNS contains nerves and ganglia.

Functional Systems of the Nervous System

The nervous system is organized into four main functional systems, each with specialized roles.

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

• Autonomic Nervous System: Regulates involuntary functions (e.g., heart rate, digestion).

• Enteric Nervous System: Governs gastrointestinal tract activity.

• Sensory (Afferent) System: Transmits sensory information to the CNS.

Neural Cell Types and Structures

Neurons, Axons, Glia, and Myelin

Neural tissue consists of several cell types, each with unique functions.

• Neurons: The primary signaling cells; transmit electrical impulses.

• Axons: Long projections that carry signals away from the neuron cell body.

• Glia: Support cells; include astrocytes, oligodendrocytes, microglia, and Schwann cells.

• Myelin: Fatty sheath that insulates axons, increasing signal conduction speed.

Definitions:

• Axon: A long, slender projection of a neuron that conducts electrical impulses away from the cell body.

• Myelin: A lipid-rich substance formed by glial cells that wraps around axons.

• Glia: Non-neuronal cells that provide support and protection for neurons.

Dendrites, Neuron Cell Body, Axon, Myelin, and Synapse

Neurons have specialized structures for receiving and transmitting signals.

• Dendrites: Branch-like extensions that receive signals from other neurons.

• Neuron Cell Body (Soma): Contains the nucleus and organelles; integrates incoming signals.

• Axon: Transmits signals to other cells.

• Myelin: Increases speed of impulse transmission.

• Synapse: Junction where a neuron communicates with another cell via neurotransmitters.

Distinguishing Sensory, Interneuron, and Motor Neurons

Neurons are classified based on their function in the nervous system.

• Sensory Neurons: Transmit sensory information from receptors to the CNS.

• Interneurons: Connect neurons within the CNS; process and integrate information.

• Motor Neurons: Carry signals from the CNS to effectors (muscles or glands).

Glial Cells and Myelination

Types and Functions of Glial Cells

Glial cells support and protect neurons. Major types include:

• Astrocytes: Maintain the blood-brain barrier and provide nutrients.

• Microglia: Act as immune cells in the CNS.

• Oligodendrocytes: Form myelin sheaths in the CNS.

• Schwann Cells: Form myelin sheaths in the PNS.

• Satellite Cells: Support neurons in the PNS.

Oligodendrocytes vs. Schwann Cells

Both cell types produce myelin but in different locations.

• Oligodendrocytes: Myelinate axons in the CNS; each cell can myelinate multiple axons.

• Schwann Cells: Myelinate axons in the PNS; each cell myelinates a single axon segment.

Membrane Potential and Ion Channels

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the cell membrane when the neuron is not transmitting a signal.

• Typically around -70 mV in neurons.

• Maintained by ion gradients and selective permeability.

Functions for Na+/K+ Pump:

• Maintains concentration gradients of sodium and potassium.

• Active transport: 3 Na+ out, 2 K+ in per ATP hydrolyzed.

Equation:

• $V_m = \frac{RT}{F} \ln \left( \frac{[K^+]_{out}}{[K^+]_{in}} \right)$ (Nernst equation for potassium)

Ion Channels and Action Potential

Voltage-gated channels for Na+, K+, and Ca2+ are crucial for action potential generation.

• Na+ Channels: Open rapidly during depolarization.

• K+ Channels: Open during repolarization.

• Ca2+ Channels: Involved in neurotransmitter release.

Action Potential Phases:

• Depolarization: Membrane potential becomes less negative.

• Repolarization: Membrane potential returns to resting value.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Electrical Gradients and Diffusion

Ion movement is driven by electrical and chemical gradients.

• Chemical Gradient: Difference in ion concentration across the membrane.

• Electrical Gradient: Difference in charge across the membrane.

• Diffusion: Movement of ions from high to low concentration.

Action Potential Propagation

Myelinated vs. Unmyelinated Axons

Action potentials propagate differently depending on myelination.

• Myelinated Axons: Saltatory conduction; action potentials jump between nodes of Ranvier.

• Unmyelinated Axons: Continuous conduction; slower signal transmission.

Saltatory Conduction: Increases speed and efficiency of nerve impulse transmission.

Synaptic Transmission

Synapse Structure and Function

Synapses are specialized junctions for neuron-to-neuron communication.

• Presynaptic Neuron: Releases neurotransmitters into the synaptic cleft.

• Postsynaptic Neuron: Receives neurotransmitters via receptors.

• Synaptic Vesicle: Stores neurotransmitters in the presynaptic terminal.

Neurotransmitter Release: Triggered by Ca2+ influx following action potential arrival.

Neurotransmitter Categories

Neurotransmitters are classified by chemical structure and function.

Category	Examples	Function
Acetylcholine	Acetylcholine	Excitatory at neuromuscular junctions
Amino Acids	Glutamate, GABA	Glutamate: excitatory; GABA: inhibitory
Biogenic Amines	Dopamine, Serotonin, Norepinephrine	Modulate mood, arousal, and cognition
Neuropeptides	Substance P, Endorphins	Pain modulation, stress response

Postsynaptic Potentials and Summation

Excitatory vs. Inhibitory Postsynaptic Potentials (EPSP vs. IPSP)

Postsynaptic potentials determine whether a neuron will fire an action potential.

• EPSP: Depolarizes the postsynaptic membrane, increasing likelihood of action potential.

• IPSP: Hyperpolarizes the postsynaptic membrane, decreasing likelihood of action potential.

Spatial and Temporal Summation

Neurons integrate multiple signals through summation.

• Spatial Summation: Multiple synapses activate simultaneously at different locations.

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

• Summation: Determines if threshold for action potential is reached.

Action Potential Review

Action Potential Properties

Action potentials are all-or-none electrical events in neurons.

• Threshold: Minimum depolarization required to trigger an action potential.

• Depolarization: Na+ influx causes membrane potential to become positive.

• Repolarization: K+ efflux restores negative membrane potential.

• Hyperpolarization: Membrane potential becomes more negative than resting.

Excitatory vs. Inhibitory Changes: Excitatory changes (depolarization) increase likelihood of firing; inhibitory changes (hyperpolarization) decrease it.

Summary Table: Action Potential Phases

Phase	Ion Movement	Membrane Potential Change
Resting	Na+ out, K+ in	-70 mV
Depolarization	Na+ influx	Becomes positive
Repolarization	K+ efflux	Returns to negative
Hyperpolarization	Continued K+ efflux	More negative than resting

Example: At the neuromuscular junction, acetylcholine release causes EPSPs in muscle cells, leading to contraction.

Additional info: Academic context and definitions have been expanded for clarity and completeness.