Nervous System Composition

Overview of the Nervous System

The nervous system is a complex network responsible for coordinating bodily functions and enabling communication between different parts of the body. It is divided into the central nervous system (CNS) and peripheral nervous system (PNS), each with distinct roles and components.

• CNS: Composed of the brain and spinal cord; processes and integrates information.

• PNS: Consists of nerves and ganglia outside the CNS; transmits signals to and from the CNS.

• Enteric Nervous System: Controls gastrointestinal function independently but communicates with CNS.

Branches and Pathways of the Peripheral Nervous System

The PNS is further divided into afferent (sensory) and efferent (motor) pathways, which are responsible for transmitting information to and from the CNS.

• Afferent Pathways: Carry sensory information from receptors to the CNS.

• Efferent Pathways: Transmit motor commands from the CNS to effectors (muscles and glands).

• Somatic Nervous System: Controls voluntary movements via skeletal muscles.

• Autonomic Nervous System: Regulates involuntary functions; divided into sympathetic and parasympathetic divisions.

Anatomy of a Peripheral Neuron

Neurons are the fundamental units of the nervous system, specialized for transmitting electrical and chemical signals.

• Soma: Cell body containing the nucleus and most organelles.

• Dendrites: Receive incoming signals.

• Axon: Conducts electrical impulses (action potentials).

• Axon Hillock: Initiates action potentials.

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

Axoplasmic Transport

Axoplasmic transport is the process by which materials are moved along the axon, essential for neuronal function and survival.

• Anterograde Transport: Moves materials away from the soma toward axon terminals (mediated by kinesin).

• Retrograde Transport: Moves materials toward the soma (mediated by dynein).

• Synaptic Vesicles: Carry neurotransmitters for release at synapses.

Cells of the Nervous System

Neurons and Glial Cells

The nervous system contains two main cell types: neurons (excitable cells) and glial cells (support cells).

• Neurons: Make up about 10% of nervous tissue; responsible for signal transmission.

• Glial Cells: Compose 75–90% of CNS; provide structural and functional support.

Types of Glial Cells

Glial cells are essential for maintaining homeostasis, protecting neurons, and facilitating signal transmission.

• Astrocytes: Regulate homeostasis, form the blood-brain barrier, modulate synaptic activity, and protect neurons.

• Ependymal Cells: Line CNS cavities and produce cerebrospinal fluid (CSF).

• Microglia: Act as resident immune cells, performing phagocytosis and protecting against oxidative stress.

• Oligodendrocytes: Form myelin in the CNS.

• Schwann Cells: Form myelin in the PNS.

Myelin Sheath-Forming Glia

Myelin is a lipid-rich substance that insulates axons, increasing the speed of electrical signal transmission.

• Oligodendrocytes: Myelinate multiple axons in the CNS.

• Schwann Cells: Myelinate single axons in the PNS.

• Node of Ranvier: Gaps in myelin sheath where action potentials are regenerated.

Protective Structures of the CNS

Meninges

The CNS is protected by three layers of connective tissue called meninges.

• Dura Mater: Tough, outermost layer.

• Arachnoid Mater: Middle, web-like layer.

• Pia Mater: Delicate, innermost layer.

Cerebrospinal Fluid (CSF) and Ventricular System

CSF is a clear fluid that bathes the CNS, providing physical and metabolic support.

• Produced by: Ependymal cells in the choroid plexus.

• Circulation: Flows through ventricles and subarachnoid space; reabsorbed by arachnoid villi.

• Functions: Cushions the brain, maintains stable interstitial environment.

Comparison of Plasma and CSF Composition

Plasma and CSF differ in their chemical composition, reflecting their distinct roles in the body.

Blood-Brain Barrier and Neurovascular Unit

Blood-Brain Barrier (BBB)

The BBB is a selective barrier formed by endothelial cells with tight junctions, astrocyte end-feet, and pericytes. It regulates the exchange of substances between blood and CNS, maintaining homeostasis and protecting against toxins and pathogens.

• Capillaries: Non-fenestrated, contain tight junctions.

• Functions: Restricts passage of pathogens, toxins, and large molecules; selectively transports nutrients; clears waste.

Blood Supply to the CNS

Metabolic Demands and Stroke

The CNS has high metabolic demands, requiring continuous blood supply for oxygen and glucose. Disruption of blood flow can lead to stroke, resulting in neuronal injury or death.

• Brain Weight: 3–4 pounds (~2% of body weight).

• Blood Supply: Receives 15% of cardiac output.

• Oxygen and Glucose Use: 20% and 50% of body consumption at rest, respectively.

• Stroke: Caused by occlusion or hemorrhage of cerebral blood vessels.

• Transient Ischemic Attack (TIA): Temporary reduction in blood supply (“mini-stroke”).

Types of Stroke

Stroke can be classified based on its underlying mechanism.

• Atherosclerotic Stroke: Blockage due to plaque buildup.

• Hemorrhagic Stroke: Rupture and bleeding of blood vessels.

• Ischemic Stroke: Blockage due to thrombus formation.

Ischemic Cascade and Brain Injury

The ischemic cascade describes the sequence of events following reduced blood flow to the brain, leading to excitotoxicity, oxidative stress, inflammation, and apoptosis.

• Ischemic Core: Area of irreversible damage.

• Penumbra: Area of potentially reversible injury.

• Oligemia: Reduced blood flow but not severe enough to cause cell death.

Summary Table: Nervous System Components

Key Equations

Action Potential Propagation

• Membrane potential change:

• Nernst equation for ion equilibrium:

Additional info:

• Some context and explanations were expanded for clarity and completeness.

• Tables and images were recreated and placed only where directly relevant to the content.