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

Introduction to 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 essential for sensation, movement, cognition, and homeostasis.

Divisions of the Nervous System

• Central Nervous System (CNS): Consists of the brain and spinal cord. It processes information and is the main control center.

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

Functional Divisions:

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

• Efferent (Motor) Division: Sends commands from the CNS to effectors (muscles, glands).

Somatic and Autonomic Nervous Systems

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

• Autonomic Nervous System: Controls involuntary actions by sending signals to smooth muscle, cardiac muscle, and glands.

Structure of a Neuron

Neurons are the basic functional units of the nervous system. They are specialized cells that transmit electrical and chemical signals.

• Dendrites: Receive incoming signals from other neurons.

• Cell Body (Soma): Contains the nucleus and organelles.

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

• Axon Terminals: Transmit signals to other neurons or effectors.

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

• Myelin Sheath: Fatty layer produced by Schwann cells (PNS) or oligodendrocytes (CNS) that insulates axons and increases signal speed.

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

Neuroglia (Glial Cells)

Neuroglia are supporting cells in the nervous system. They do not conduct nerve impulses but provide structural and functional support.

• Astrocytes: Star-shaped cells that form the blood-brain barrier and support neurons.

• Oligodendrocytes: Myelinate axons in the CNS.

• Schwann Cells: Myelinate axons in the PNS.

• Microglia: Phagocytic cells that remove debris and pathogens.

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

Electrical Properties of Neurons

Neurons communicate via electrical signals known as action potentials. These are rapid changes in membrane potential that travel along the axon.

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

• Depolarization: Influx of Na+ ions makes the inside of the neuron less negative.

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

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

• 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 threshold is reached, an action potential is generated and propagated without decrement.

Key Equation:

• Resting membrane potential is determined by the Nernst equation: $E = \frac{RT}{zF} \ln \left( \frac{[\text{ion outside}]}{[\text{ion inside}]} \right)$ where E is the equilibrium potential, R is the gas constant, T is temperature, z is the charge of the ion, F is Faraday's constant.

Propagation of Action Potentials

• 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 at synapses, where neurotransmitters are released from the presynaptic neuron and bind to receptors on the postsynaptic neuron.

• 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.

• If EPSPs outnumber IPSPs, the neuron will fire an action potential; if IPSPs outnumber EPSPs, the neuron is inhibited.

Summary Table: Types of Neuroglia and Their Functions

Neuroglia Type	Location	Function
Astrocytes	CNS	Form blood-brain barrier, support neurons
Oligodendrocytes	CNS	Myelinate CNS axons
Schwann Cells	PNS	Myelinate PNS axons
Microglia	CNS	Phagocytosis of debris and pathogens
Ependymal Cells	CNS	Produce and circulate cerebrospinal fluid

Additional info:

• Action potentials are unidirectional due to the refractory period, which prevents the signal from traveling backward.

• Neurotransmitters can be excitatory (e.g., glutamate) or inhibitory (e.g., GABA).

• Disorders of the nervous system can result from demyelination (e.g., multiple sclerosis) or neurotransmitter imbalances (e.g., Parkinson's disease).