Fundamentals of the Nervous System: Structure and Function

Nervous System Overview

Divisions of the Nervous System

The nervous system is divided into two main parts: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). Each division has distinct structures and functions essential for body regulation and response.

• Central Nervous System (CNS): Consists of the brain and spinal cord. Responsible for processing and integrating information.

• Peripheral Nervous System (PNS): Includes all neural tissue outside the CNS. Divided into sensory (afferent)motor (efferent)

PNS Subdivisions

• Sensory Division: Transmits sensory information from receptors to the CNS.

  • Visceral sensory division: Conveys information from internal organs.

  • Somatic sensory division: Conveys information from skin, muscles, and joints.

• Motor Division: Transmits commands from the CNS to effectors.

  • Visceral motor division (Autonomic Nervous System): Controls involuntary functions.

    • Sympathetic division: Prepares the body for 'fight or flight' responses.

    • Parasympathetic division: Promotes 'rest and digest' activities.

  • Somatic motor division: Controls voluntary movements of skeletal muscles.

Functional Organization: Sensory and Motor Pathways

Sensory (afferent) neurons carry signals toward the CNS, while motor (efferent) neurons carry signals away from the CNS to muscles and glands.

• Efferent Pathways: Motor signals travel from the spinal cord to effectors such as skin and muscle.

• Afferent Pathways: Sensory signals travel from receptors in the skin and muscle to the spinal cord.

Neurons: Structure and Function

Parts of a Neuron

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

• Dendrites: Receive incoming signals from other neurons.

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

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

• Axon terminals: Release neurotransmitters to communicate with other cells.

• Myelin sheath: Insulates the axon, increasing the speed of impulse transmission.

Neuroglia (Glial Cells)

Neuroglia are supporting cells in the nervous system, each with specialized functions.

Type	Function
Astrocyte	Support neurons, maintain blood-brain barrier, regulate ion balance.
Microglia	Act as immune cells, remove debris and pathogens.
Ependymal cell	Line ventricles, produce and circulate cerebrospinal fluid.
Oligodendrocyte	Form myelin sheaths in the CNS.
Schwann cell	Form myelin sheaths in the PNS.
Satellite cell	Support neurons in peripheral ganglia.
Myelin	Insulating layer that increases conduction speed of action potentials.

Neuronal Electrical Signaling

Resting Membrane Potential

Neurons maintain a polarized state at rest, with a difference in charge across the membrane.

• Na+ (Sodium): Higher concentration outside the cell.

• K+ (Potassium): Higher concentration inside the cell.

The resting membrane potential is typically around $-70\ \text{mV}$.

Action Potential Phases

• Depolarization: Na+ channels open, Na+ enters the cell, making the inside more positive.

• Repolarization: K+ channels open, K+ leaves the cell, restoring negative charge inside.

• Hyperpolarization: The membrane potential becomes more negative than the resting potential.

Propagation of Action Potentials

Action potentials travel 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' between nodes of Ranvier, increasing speed.

Types of Neurons

• A neurons: Large diameter, myelinated, fast conduction.

• B neurons: Medium diameter, myelinated, moderate speed.

• C neurons: Small diameter, unmyelinated, slow conduction.

Neuronal Communication

Synapses and Neurotransmitters

Neurons communicate at specialized junctions called synapses. The presynaptic neuron releases chemical messengers called neurotransmitters that bind to receptors on the postsynaptic cell.

• Synapse: Site of communication between two neurons or a neuron and an effector cell.

• Presynaptic neuron: Releases neurotransmitter.

• Postsynaptic neuron: Receives neurotransmitter.

Types of Neurotransmitters

• Biogenic amines: Includes dopamine, norepinephrine, serotonin.

• Amino acids: Includes glutamate, GABA, glycine.

• Peptides: Includes substance P, endorphins.

Summary Table: Neuroglia Types and Functions

Neuroglia Type	Location	Main Function
Astrocyte	CNS	Support, blood-brain barrier, ion regulation
Microglia	CNS	Immune defense, phagocytosis
Ependymal cell	CNS	CSF production and circulation
Oligodendrocyte	CNS	Myelination of CNS axons
Schwann cell	PNS	Myelination of PNS axons
Satellite cell	PNS	Support neurons in ganglia

Key Equations

• Resting Membrane Potential: $V_m = V_{inside} - V_{outside}$

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

Example

When you touch a hot surface, sensory neurons in your skin send signals to the spinal cord (afferent pathway). The spinal cord processes the information and sends a command via motor neurons (efferent pathway) to your muscles to withdraw your hand.

Additional info: Expanded explanations and tables were added for completeness and clarity.