Organization and Functional Classification of the Nervous System

Main Divisions of the Nervous System

The nervous system is a complex network responsible for integrating and coordinating body functions. It is divided into two main subdivisions: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS).

• Central Nervous System (CNS): Consists of the brain and spinal cord. It acts as the integrating and command center for the nervous system.

• Peripheral Nervous System (PNS): Composed mainly of nerves extending from the brain and spinal cord, connecting the CNS to limbs and organs.

Functions of the Nervous System

The nervous system performs three primary functions to maintain homeostasis and respond to environmental changes:

• Sensory Input: Gathering information from sensory receptors (e.g., photoreceptors, baroreceptors, nociceptors) that detect changes in the internal and external environment.

• Integration: Processing and interpreting sensory input to determine the appropriate response.

• Motor Output: Activating effectors (muscles or glands) to cause a response based on the integrated information.

Clinical Connections

• Neurology: The branch of medicine focused on diagnosing and treating disorders of the nervous system. Neurologists perform surgery in the nervous system and manage conditions such as stroke, epilepsy, and multiple sclerosis.

• Psychiatry: Specializes in mental health, emotional, and behavioral disorders. Psychiatrists diagnose and treat psychological disorders (e.g., ADHD, bipolar disorder, depression) and may prescribe medication or psychotherapy.

• Psychology: The scientific study of mind and behavior. Psychologists perform psychotherapy and evaluate behavioral and psychological factors.

Functional Classification of the Peripheral Nervous System (PNS)

Sensory (Afferent) Division

The sensory division transmits impulses from sensory receptors to the CNS. It is responsible for detecting stimuli and relaying information for processing.

• Somatic Sensory Fibers: Carry impulses from skin, skeletal muscles, and joints.

• Visceral Sensory Fibers: Transmit impulses from visceral organs.

Motor (Efferent) Division

The motor division transmits impulses from the CNS to effector organs (muscles and glands), resulting in a response.

• Somatic (Voluntary) Nervous System (SNS): Controls voluntary movements by conveying signals from the CNS to skeletal muscles.

• Autonomic (Involuntary) Nervous System (ANS): Regulates involuntary activities by transmitting signals from the CNS to smooth muscle, cardiac muscle, and glands.

Subdivisions of the Autonomic Nervous System (ANS)

• Sympathetic Nervous System: Supports exercise or emergency "fight-or-flight" responses by increasing heart rate and energy mobilization.

• Parasympathetic Nervous System: Promotes "rest-and-digest" activities, conserving energy and supporting digestion.

• Enteric Nervous System: Regulates the activity of smooth muscle and glands in the gastrointestinal tract, functioning independently or with the ANS.

Dual Innervation: Most organs receive input from both sympathetic and parasympathetic divisions, allowing for balanced regulation.

Nervous Tissue

Neurons (Nerve Cells)

Neurons are the basic functional units of the nervous system, specialized for transmitting electrical impulses. They consist of a cell body, dendrites, and an axon.

• Cell Body: Contains the nucleus and organelles.

• Dendrites: Receive incoming signals.

• Axon: Transmits impulses away from the cell body.

Neuroglia (Glial Cells)

Neuroglia are supporting cells that protect, nourish, and insulate neurons. They do not conduct electrical impulses but are essential for nervous system function.

• Astrocytes: Maintain the blood-brain barrier (BBB), regulate ion concentrations, and support neurons.

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

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

• Oligodendrocytes: Form myelin sheaths in the CNS, increasing impulse conduction speed.

Glial Cells of the Peripheral Nervous System

• Satellite Cells: Surround neuron cell bodies in the PNS, providing protection and regulating the exchange of materials.

• Schwann Cells: Form myelin sheaths around PNS axons, aiding in rapid impulse transmission.

Myelination

Myelination is the process of forming a myelin sheath around axons, which increases the speed of nerve impulse conduction.

• Myelinated Axons: Axons covered by myelin sheath; impulses travel faster.

• Unmyelinated Axons: Axons without myelin sheath; impulses travel slower.

Nodes of Ranvier: Gaps between myelin segments that facilitate rapid signal transmission.

Cerebrospinal Fluid (CSF)

Formation and Circulation

CSF is a clear, colorless plasma ultrafiltrate that protects the brain and spinal cord from chemical and physical injuries. It is produced by the choroid plexuses in the ventricles of the brain.

• Choroid Plexus: Network of capillaries in the ventricles that filter blood plasma to produce CSF.

• CSF Circulation: CSF flows through the ventricles, central canal of the spinal cord, and subarachnoid space, providing cushioning and nutrient transport.

• CSF Volume: Total CSF volume is about 80–150 mL (3–5 oz) in an adult.

Blood-Brain Barrier (BBB)

The BBB is a selective barrier formed by tight junctions in capillary endothelial cells, astrocyte end-feet, and pericytes. It restricts the passage of substances from the blood to the brain, protecting neural tissue from toxins and pathogens.

• Function: Maintains the brain's stable environment, allowing only specific molecules to pass.

• Clinical Relevance: Disruption of the BBB can lead to neurological disorders.

Summary Table: Major Glial Cells and Their Functions

Key Equations and Concepts

• Nerve Impulse Transmission: The speed of impulse transmission is faster in myelinated axons due to saltatory conduction.

• CSF Production Rate:

• Blood-Brain Barrier:

Additional info:

• Clinical conditions such as gliomas (tumors derived from glial cells) can be highly malignant and grow rapidly.

• Disorders of myelination, such as multiple sclerosis, result in impaired nerve conduction.