The Nervous System I: Neuron Structure, Function, and Action Potentials

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The Nervous System: Structure and Function

Introduction to Neurons and Nervous System Organization

The nervous system is a complex network responsible for processing and transmitting information throughout the body. It is composed of specialized cells called neurons and supporting cells known as glia. Neurons communicate using both electrical and chemical signals, enabling rapid and coordinated responses to internal and external stimuli.

Neuron: A nerve cell that transfers information within the body.
Brain: The central organ of the nervous system responsible for integration and processing of information.
Ganglia: Clusters of neuron cell bodies in the peripheral nervous system.
Sensory Neurons: Detect external or internal stimuli and transmit signals to the central nervous system.
Interneurons: Integrate sensory input and motor output, found mainly in the brain and spinal cord.
Motorneurons: Transmit signals from the central nervous system to effector cells (muscles or glands).
Cell Body (Soma): Contains the nucleus and organelles; metabolic center of the neuron.
Dendrites: Branch-like extensions that receive signals from other neurons.
Axons: Long extensions that transmit signals to other cells.

Central and Peripheral Nervous Systems

The nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, while the PNS includes all neural tissue outside the CNS.

CNS: Brain and spinal cord; site of integration and processing.
PNS: Nerves and ganglia outside the CNS; transmits sensory and motor signals.
Ganglia: Simple clusters of neurons in the PNS.

Information Processing in the Nervous System

Nervous systems process information in three main stages: sensory input, integration, and motor output.

Sensory Input: Detection of stimuli by sensory neurons.
Integration: Processing and interpretation of sensory input by interneurons in the CNS.
Motor Output: Transmission of signals from motorneurons to effector cells (muscles or glands).
Example: Touching a hot surface activates sensory neurons, which send signals to the CNS for integration, resulting in motor output that causes muscle contraction to withdraw the hand.

Neuronal Communication and Synapses

How Neurons Transmit Information

Neurons receive information, transmit it along their axons, and communicate with other cells at specialized junctions called synapses. The synaptic terminal of one axon releases chemical messengers called neurotransmitters to the postsynaptic cell.

Synapse: Junction between an axon and another cell (neuron, muscle, or gland).
Presynaptic Cell: The neuron sending the signal.
Postsynaptic Cell: The cell receiving the signal.
Neurotransmitters: Chemical messengers that transmit signals across the synaptic cleft.

Types of Neurons

Neurons are classified based on their function and structure:

Sensory Neurons: Carry information from sensory receptors to the CNS.
Interneurons: Connect neurons within the CNS and integrate information.
Motor Neurons: Transmit signals from the CNS to muscles or glands.
Example: The knee-jerk reflex involves all three types: sensory neurons detect the tap, interneurons process the signal, and motor neurons cause the leg to kick.

Glia: Supporting Cells of the Nervous System

Functions of Glial Cells

Glia are non-neuronal cells that provide support, protection, and insulation for neurons. They are essential for maintaining homeostasis and proper functioning of the nervous system.

Astrocytes: Support neurons and form the blood-brain barrier.
Ependymal Cells: Promote circulation of cerebrospinal fluid.
Microglia: Protect the nervous system from microorganisms.
Oligodendrocytes (CNS) and Schwann Cells (PNS): Form myelin sheaths around axons, increasing the speed of signal transmission.

Membrane Potential and Ion Channels

Resting Membrane Potential

Every cell has a voltage difference across its plasma membrane, known as the membrane potential. In neurons, the resting potential is the membrane potential of a neuron not sending signals, typically around -70 mV.

Membrane Potential: Difference in electrical charge across the plasma membrane.
Resting Potential: The stable, negative charge inside the neuron when not active.
Ion Channels: Proteins that allow specific ions to cross the membrane, contributing to changes in membrane potential.

Ion Concentrations in Neurons

The distribution of ions across the neuronal membrane is critical for establishing the resting potential.

Ion	Intracellular Concentration (mM)	Extracellular Concentration (mM)
Potassium (K+)	140	5
Sodium (Na+)	15	150
Chloride (Cl-)	10	120
Large anions (A-, e.g., proteins)	100	Not applicable

Formation of the Resting Potential

The sodium-potassium pump uses ATP to maintain high K+ inside and high Na+ outside the cell, creating concentration gradients that represent chemical potential energy.

Sodium-Potassium Pump: Actively transports 3 Na+ out and 2 K+ into the cell per ATP hydrolyzed.
Equation:

K+ Leak Channels: Allow K+ to diffuse out, making the inside of the cell more negative.
Large Anions: Negatively charged proteins trapped inside the cell contribute to the negative resting potential.

Action Potentials and Ion Channel Dynamics

Voltage-Gated Ion Channels

Neurons contain voltage-gated ion channels that open or close in response to changes in membrane potential, allowing rapid changes in ion flow during signaling.

Leak Potassium Channels: Always open, help set the resting potential.
Voltage-Gated Potassium Channels: Open slowly during action potentials, restoring membrane potential after depolarization.
Voltage-Gated Sodium Channels: Open rapidly during depolarization, allowing Na+ influx and initiating action potentials.
Voltage-Gated Calcium Channels: Open during depolarization, allowing Ca2+ influx, important for neurotransmitter release at synapses.

Generation of Action Potentials

An action potential is a rapid, transient change in membrane potential that propagates along the axon. It is triggered when depolarization reaches a threshold (about -55 mV), resulting in an all-or-none response.

Depolarization: Gated Na+ channels open, Na+ enters the cell, membrane potential becomes less negative.
Threshold: The critical level of depolarization required to trigger an action potential (about -55 mV).
All-or-None Response: Action potentials either occur fully or not at all; no graded response.
Propagation: The action potential travels along the axon, transmitting the nerve impulse.
Frequency: The number of action potentials per second can reflect the strength of a stimulus.

Stages of an Action Potential:

Resting State: Most voltage-gated channels are closed.
Depolarization: Voltage-gated Na+ channels open, Na+ influx.
Rising Phase: Rapid Na+ entry causes membrane potential to become positive.
Falling Phase: Na+ channels close, voltage-gated K+ channels open, K+ efflux restores negative potential.
Undershoot: K+ channels remain open briefly, causing hyperpolarization.
Return to Resting Potential: All channels reset to original state.

Graphical Representation: The action potential is visualized as a sharp spike in membrane potential, followed by a return to baseline.

Equation for Membrane Potential (Nernst Equation):

Where is the equilibrium potential for a given ion, is the gas constant, is temperature, is the charge of the ion, and is Faraday's constant.

Summary Table: Key Ion Concentrations in Neurons

Ion	Intracellular (mM)	Extracellular (mM)
K+	140	5
Na+	15	150
Cl-	10	120
Large Anions (A-)	100	Not applicable

Additional info:

Hyperpolarization occurs when the membrane potential becomes more negative than the resting potential, often due to continued K+ efflux.
Action potentials are fundamental to neural communication and underlie all nervous system functions, including sensation, movement, and cognition.