Neurophysiology: Structure and Function of the Nervous System

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Neurophysiology: Structure and Function of the Nervous System

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating the body's activities by transmitting signals to and from different parts of the body. It is divided into the central nervous system (CNS) and the peripheral nervous system (PNS), each with distinct structures and functions.

Diagram of CNS and PNS divisions in the human body

Functions of the Nervous System

Sensory Input: The nervous system detects changes in the internal and external environment through sensory receptors.
Integration: The CNS processes and interprets sensory input, making decisions about appropriate responses.
Motor Output: The nervous system activates effector organs (muscles and glands) to cause a response.

Diagram showing sensory input, integration, and motor output in a soccer player

Divisions of the Nervous System

The nervous system is organized into several divisions based on structure and function:

Central Nervous System (CNS): Consists of the brain and spinal cord; responsible for integrating and processing information.
Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS; subdivided into sensory (afferent) and motor (efferent) divisions.
Motor Division: Further divided into the somatic nervous system (controls skeletal muscles) and the autonomic nervous system (controls smooth muscle, cardiac muscle, and glands).

Diagram of CNS and PNS functional divisions

Neurons: Structure and Classification

Anatomy of a Neuron

Neurons are the functional units of the nervous system, specialized for the transmission of electrical signals. Each neuron consists of several key anatomical features:

Cell Body (Soma): Contains the nucleus and organelles; responsible for metabolic activities.
Dendrites: Receive incoming signals from other neurons.
Axon: Conducts electrical impulses away from the cell body toward other neurons or effectors.
Axon Hillock: The region where the axon originates; important for action potential initiation.
Myelin Sheath: Insulating layer that increases the speed of signal transmission.

Labeled diagram of a neuron Micrograph of neurons showing dendrites, axons, and neuroglia

Functional Regions of Neurons

Neurons can be divided into three functional regions based on their roles in signal transmission:

Receptive Region: Includes dendrites and cell body; receives input.
Conducting Region: The axon; transmits action potentials.
Secretory Region: Axon terminals; release neurotransmitters to communicate with other cells.

Diagram of neuron functional regions

Classification of Neurons

Neurons are classified based on structure and function:

Sensory (Afferent) Neurons: Transmit sensory information toward the CNS.
Motor (Efferent) Neurons: Carry commands from the CNS to muscles and glands.
Interneurons: Connect neurons within the CNS and integrate information.

Structural Type	Features	Function	Location
Multipolar	Many dendrites, one axon	Motor, interneurons	CNS, motor neurons in PNS
Bipolar	One dendrite, one axon	Special sensory (e.g., retina)	Special sense organs
Pseudounipolar	Single process splits into two	Sensory (afferent)	Sensory neurons in PNS

Table comparing structural types of neurons

Terminology

Nuclei: Clusters of neuron cell bodies in the CNS.
Ganglia: Clusters of neuron cell bodies in the PNS.
Tracts: Bundles of axons in the CNS.
Nerves: Bundles of axons in the PNS.

Neuroglia (Glial Cells)

Types and Functions of Glial Cells

Glial cells support, protect, and nourish neurons. They are more numerous than neurons and can divide throughout life.

Astrocytes (CNS): Anchor neurons, regulate the extracellular environment, form the blood-brain barrier, and repair tissue.
Oligodendrocytes (CNS): Myelinate axons in the CNS.
Microglia (CNS): Act as phagocytes, removing debris and pathogens.
Ependymal Cells (CNS): Line ventricles and produce cerebrospinal fluid.
Schwann Cells (PNS): Myelinate axons in the PNS.
Satellite Cells (PNS): Support cell bodies in ganglia.

Diagram of glial cells in the CNS

Myelin Sheath and Myelination

The myelin sheath is a multilayered covering formed by glial cells (oligodendrocytes in the CNS, Schwann cells in the PNS) that insulates axons and increases the speed of electrical signal transmission. Myelinated regions appear as white matter, while unmyelinated regions (cell bodies, dendrites) are gray matter.

Diagram of myelination in the PNS

Membrane Potentials and Ion Channels

Resting Membrane Potential

The resting membrane potential is the electrical potential difference across the plasma membrane of a neuron at rest, typically about -70 mV. This is maintained by leak channels (especially potassium) and the sodium-potassium pump, resulting in a polarized membrane.

Diagram of resting membrane potential across a neuron membrane

Ion Channels in Neurons

Neurons contain several types of ion channels that contribute to their excitability:

Type of Channel	Structure	Stimulus for Opening/Closing
Leak Channel	Always open	None
Ligand-Gated Channel	Opens when a chemical binds	Neurotransmitter binding
Voltage-Gated Channel	Opens with voltage changes	Membrane potential changes
Mechanically Gated Channel	Opens with mechanical force	Pressure, stretch, etc.

Table of ion channel types and their properties

Depolarization and Hyperpolarization

Depolarization occurs when the membrane potential becomes less negative (closer to zero), while hyperpolarization is when it becomes more negative. These changes are critical for the generation and propagation of action potentials.

Diagram showing depolarization and hyperpolarization

Action Potentials

Generation of Action Potentials

Action potentials are rapid, all-or-nothing electrical impulses that travel along axons. They are triggered when local potentials reach a threshold at the axon hillock, causing voltage-gated sodium and potassium channels to open in sequence.

Diagram of action potential generation Diagram of voltage-gated sodium and potassium channels

Events of an Action Potential

Resting state: All voltage-gated channels are closed.
Depolarization: Voltage-gated Na+ channels open, Na+ enters the cell.
Repolarization: Na+ channels inactivate, K+ channels open, K+ exits the cell.
Hyperpolarization: K+ channels remain open briefly, membrane potential becomes more negative than resting.
Return to resting state: Channels reset, resting membrane potential restored.

Diagram of action potential phases

Sodium-Potassium Pump

The sodium-potassium pump (Na+/K+ ATPase) actively transports 3 Na+ ions out of and 2 K+ ions into the neuron, maintaining the resting membrane potential.

Diagram of sodium-potassium pump cycle

Refractory Periods

The refractory period ensures unidirectional propagation of action potentials and limits their frequency:

Absolute Refractory Period: No new action potential can be generated, regardless of stimulus strength.
Relative Refractory Period: A stronger-than-normal stimulus can initiate another action potential.

$Graph of absolute and relative refractory periods$

Local Potentials vs. Action Potentials

Local Potentials: Graded, decremental, reversible, occur in dendrites/cell bodies, can be depolarizing or hyperpolarizing.
Action Potentials: All-or-nothing, nondecremental, irreversible, occur along axons, always depolarizing then repolarizing.

Propagation of Action Potentials

Action potentials propagate along axons by depolarizing adjacent membrane segments. This process is self-propagating and unidirectional due to the refractory period.

Diagram of action potential propagation steps

Saltatory vs. Continuous Conduction

Saltatory Conduction: Occurs in myelinated axons; action potentials jump between nodes of Ranvier, increasing speed.
Continuous Conduction: Occurs in unmyelinated axons; action potentials propagate along every segment of the membrane.

Diagram comparing saltatory and continuous conduction

Factors Affecting Conduction Speed

Axon Diameter: Larger diameter = faster conduction due to lower resistance.
Myelination: Myelinated axons conduct signals faster than unmyelinated axons.

Synapses and Neurotransmission

Synapse Structure and Types

A synapse is the junction where a neuron communicates with its target cell. Types include:

Electrical Synapses: Direct ion flow via gap junctions; rapid and bidirectional.
Chemical Synapses: Use neurotransmitters to transmit signals across a synaptic cleft; unidirectional and modifiable.

Diagram of synaptic connections Diagram comparing electrical and chemical synapses

Chemical Synaptic Transmission

An action potential arrives at the axon terminal.
Voltage-gated Ca2+ channels open, Ca2+ enters the terminal.
Neurotransmitter vesicles fuse with the membrane and release contents into the synaptic cleft.
Neurotransmitters bind to receptors on the postsynaptic membrane, generating a postsynaptic potential.

Diagram of chemical synaptic transmission

Postsynaptic Potentials

Excitatory Postsynaptic Potential (EPSP): Depolarizes the postsynaptic membrane, increasing the likelihood of an action potential.
Inhibitory Postsynaptic Potential (IPSP): Hyperpolarizes the postsynaptic membrane, decreasing the likelihood of an action potential.

Diagram of EPSPs and IPSPs

Neural Integration: Temporal and Spatial Summation

Neural integration is the process by which multiple synaptic inputs combine to influence the postsynaptic neuron's activity:

Temporal Summation: Repeated stimulation from a single presynaptic neuron over time.
Spatial Summation: Simultaneous stimulation from multiple presynaptic neurons.

Diagram of temporal and spatial summation

Termination of Synaptic Transmission

Synaptic transmission is terminated by:

Diffusion: Neurotransmitters diffuse away from the synaptic cleft.
Degradation: Enzymes break down neurotransmitters in the cleft.
Reuptake: Neurotransmitters are taken back into the presynaptic neuron for reuse.

Diagram of neurotransmitter termination mechanisms

Major Neurotransmitters and Their Functions

Acetylcholine: Involved in muscle contraction at the neuromuscular junction.
Norepinephrine: Regulates heart rate, blood pressure, and alertness.
Epinephrine: Similar functions to norepinephrine.
Dopamine: Modulates movement, emotion, and motivation.
Serotonin: Influences mood, attention, and cognition.

Additional info: For a more comprehensive overview, see Table 11.3 in your textbook.