Introduction to the Nervous System: Structure, Function, and Physiology

Introduction to the Nervous System

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating body activities and processing sensory information. It is divided into distinct anatomical and functional components, each with specialized roles.

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

• Peripheral Nervous System (PNS): Includes all nerves outside the brain and spinal cord; connects the CNS to limbs and organs.

• Afferent (Sensory) Division: Gathers information from internal and external environments and sends it to the CNS.

• Efferent (Motor) Division: Transmits commands from the CNS to effectors such as muscles and glands.

Functional Divisions of the Nervous System

The nervous system is functionally divided to process and respond to sensory input and coordinate motor output.

• Somatic Nervous System: Controls voluntary movements by transmitting signals to skeletal muscles, joints, and skin.

• Autonomic Nervous System: Regulates involuntary functions, such as heart rate and digestion, by sending signals to smooth muscle, cardiac muscle, and glands.

• Motor Division: Further divided into somatic (voluntary) and autonomic (involuntary) motor divisions.

Structure of Neurons

Neurons are the basic functional units of the nervous system, specialized for communication.

• Dendrites: Receive electrical signals from other cells.

• Axon: Transmits electrical impulses away from the cell body.

• Axon Hillock: The region where action potentials are initiated.

• Axon Terminals: Release neurotransmitters to communicate with other neurons or effectors.

• Nissl Bodies: Aggregates of rough endoplasmic reticulum involved in protein synthesis.

• Myelin Sheath: Insulating layer formed by Schwann cells (PNS) or oligodendrocytes (CNS) that increases the speed of impulse conduction.

• Nodes of Ranvier: Gaps in the myelin sheath that facilitate rapid signal transmission via saltatory conduction.

Types of Neurons

Neurons are classified based on their function:

• Sensory Neurons: Transmit sensory information to the CNS.

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

• Motor Neurons: Carry commands from the CNS to muscles and glands.

Neuroglia (Glial Cells)

Neuroglia are supporting cells in the nervous system that provide structural and functional support to neurons.

• Astrocytes: Star-shaped cells that form the blood-brain barrier and regulate the extracellular environment.

• Oligodendrocytes: Myelinate axons in the CNS.

• Schwann Cells: Myelinate axons in the PNS.

• Microglia: Act as phagocytes, removing debris and pathogens.

• Ependymal Cells: Line the ventricles of the brain and produce cerebrospinal fluid.

Electrical Properties of Neurons

Neurons communicate via electrical signals known as action potentials, which are generated by changes in membrane potential.

• Resting Membrane Potential: The difference in electrical charge across the neuron's membrane at rest, typically around -70 mV.

• Action Potential: A rapid change in membrane potential that propagates along the axon.

• Threshold Potential: The critical level to which the membrane potential must be depolarized to initiate an action potential (usually around -40 mV).

• All-or-None Principle: Once the threshold is reached, an action potential is always generated and propagated.

• Depolarization: Influx of Na+ ions makes the inside of the neuron more positive.

• Repolarization: Efflux of K+ ions restores the negative membrane potential.

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

• Refractory Period: Time during which a neuron cannot fire another action potential.

Key Equations

• Resting Membrane Potential: $V_m = V_{inside} - V_{outside}$ Where $V_m$ is the membrane potential.

• Nernst Equation (for ion equilibrium potential): $E_{ion} = \frac{RT}{zF} \ln \left( \frac{[ion]_{outside}}{[ion]_{inside}} \right)$ Where $E_{ion}$ is the equilibrium potential for a specific ion.

Propagation of Action Potentials

Action potentials are propagated along axons by two main mechanisms:

• Continuous Conduction: Occurs in unmyelinated axons; the action potential moves along every part of the membrane.

• Saltatory Conduction: Occurs in myelinated axons; the action potential jumps from one node of Ranvier to the next, increasing conduction speed.

Synaptic Transmission

Neurons communicate with each other and with effectors at synapses, where neurotransmitters are released.

• Presynaptic Neuron: Releases neurotransmitters into the synaptic cleft.

• Postsynaptic Neuron: Receives the neurotransmitter and generates a response.

• 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.

Table: Comparison of Glial Cells

Glial Cell Type	Location	Main Function
Astrocytes	CNS	Blood-brain barrier, support, regulation
Oligodendrocytes	CNS	Myelination of axons
Schwann Cells	PNS	Myelination of axons
Microglia	CNS	Phagocytosis, immune defense
Ependymal Cells	CNS	Produce cerebrospinal fluid

Summary and Applications

• The nervous system is essential for sensation, integration, and response to stimuli.

• Neurons and glial cells work together to ensure rapid and efficient communication throughout the body.

• Understanding the structure and function of the nervous system is foundational for studying neurophysiology, pathology, and clinical applications.

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