Introduction to the Nervous System

The nervous system is the master controlling and coordinating system of the body. It conducts nerve impulses between various body structures and regulates all physiological functions, both voluntary and involuntary. Its primary role is to maintain homeostasis by adjusting the body's activities in response to changes in the internal and external environment.

Organization of the Nervous System

The nervous system is structurally and functionally divided into major components:

1. Central Nervous System (CNS)

Components: Brain and Spinal Cord.

Function: Acts as the integrative and control center. It interprets incoming sensory information and dictates motor responses.

2. Peripheral Nervous System (PNS)

Components: All neural structures outside the CNS, including cranial nerves, spinal nerves, ganglia, and nerve plexuses.

Function: Serves as the communication lines linking the CNS to the rest of the body.

• Somatic (Cerebrospinal) Nervous System: Carries sensory information from skin, muscles, and joints to the CNS and motor commands from the CNS to skeletal muscles (voluntary control).
• Autonomic Nervous System (ANS): Regulates involuntary functions of glands, cardiac muscle, and smooth muscle. It has two antagonistic subdivisions:
  • Sympathetic Division: Prepares the body for "fight-or-flight" responses (e.g., increases heart rate, dilates pupils).
  • Parasympathetic Division: Promotes "rest-and-digest" activities (e.g., slows heart rate, stimulates digestion).

Nervous Tissue: Cellular Composition

Nervous tissue is composed of two principal cell types:

1. Neurons (Nerve Cells)

The excitable, impulse-conducting units of the nervous system. They are specialized for rapid communication.

Structure of a Typical Neuron:

• Cell Body (Perikaryon or Soma): Contains the nucleus, cytoplasm, and organelles. Size ranges from 5 µm to 120 µm. The site of most metabolic activity.
• Dendrites: Short, branching, afferent processes that receive signals from other neurons or receptors. They contain Nissl granules (rough endoplasmic reticulum).
• Axon: A single, long, efferent process of uniform diameter that generates and transmits action potentials away from the cell body. It is devoid of Nissl granules and is often myelinated by Schwann cells (PNS) or oligodendrocytes (CNS). It terminates in telodendria (axon terminals) that form synapses.
• Axon Hillock: The cone-shaped region of the cell body where the axon originates; it is the site of action potential initiation.

Functional Classification of Neurons:

2. Neuroglia (Glial Cells)

These are the non-excitable, supportive cells that outnumber neurons. They maintain homeostasis, form myelin, and provide support and protection.

Major Types of Neuroglia:

• Astrocytes (CNS): Star-shaped cells that regulate the extracellular environment. They are involved in neurotransmitter metabolism, potassium ion balance, contribute to the blood-brain barrier (BBB), and assist in brain development.
• Oligodendrocytes (CNS): Produce the myelin sheath that insulates axons, increasing the speed of nerve impulse conduction. One oligodendrocyte can myelinate multiple axons.
• Microglia (CNS): The resident immune cells (phagocytes) of the CNS. They become activated and migrate to sites of injury or infection.
• Ependymal Cells (CNS): Line the ventricles of the brain and the central canal of the spinal cord. They produce and assist in the circulation of cerebrospinal fluid (CSF).
• Schwann Cells (PNS): Equivalent to oligodendrocytes in the PNS. Each Schwann cell myelinates a single segment of one axon. They are crucial for nerve regeneration.
• Satellite Cells (PNS): Flattened cells that surround neuronal cell bodies in ganglia, providing structural and metabolic support.

Central Nervous System: Brain and Spinal Cord

Brain

The brain is housed within the cranial cavity and is protected by the meninges (dura mater, arachnoid mater, pia mater) and cerebrospinal fluid (CSF).

Major Divisions:

• Cerebrum: The largest part, responsible for higher functions (thought, memory, sensation, voluntary movement).
• Cerebellum: Coordinates movement, balance, and posture.
• Brainstem: Connects the cerebrum and cerebellum to the spinal cord. It consists of:
  • Midbrain
  • Pons
  • Medulla Oblongata (controls vital autonomic functions like respiration and heart rate).

Ventricles: A system of interconnected cavities within the brain (lateral, third, fourth) filled with CSF produced by the choroid plexus.

Spinal Cord

Contained within the vertebral canal, extending from the foramen magnum to the lower border of the L1 vertebra. It is also covered by meninges.

Internal Organization:

• Grey Matter: Located centrally in an "H" shape. Contains neuronal cell bodies, dendrites, and synapses. Divided into anterior (ventral) horns (motor neurons), posterior (dorsal) horns (sensory processing), and lateral horns (autonomic neurons in thoracic/lumbar regions).
• White Matter: Surrounds the grey matter. Contains bundles of myelinated axons (tracts) running longitudinally. Divided into posterior, lateral, and anterior columns (funiculi).

Key Landmark: The central canal runs through the center of the grey matter, containing CSF.

Peripheral Nervous System: Nerves and Ganglia

Structure of a Peripheral Nerve

A peripheral nerve is a cord-like bundle of nerve fibers (axons) with associated connective tissue sheaths.

Connective Tissue Coverings (from outside inwards):

• Epineurium: The dense irregular connective tissue sheath surrounding the entire nerve trunk.
• Perineurium: Surrounds each fascicle (bundle of nerve fibers). It forms a diffusion barrier important for maintaining the microenvironment.
• Endoneurium: Delicate connective tissue surrounding each individual nerve fiber (axon and its myelin sheath).

Spinal Nerves

There are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal.

Each spinal nerve is formed by the union of a dorsal (sensory) root and a ventral (motor) root at the intervertebral foramen. Immediately after formation, the nerve divides into a dorsal ramus (supplying the back muscles and skin) and a ventral ramus (supplying the anterior/lateral trunk and limbs).

Dermatome: A specific area of skin supplied by the sensory fibers of a single spinal nerve root. Knowledge of dermatomes is essential for localizing neurological lesions.

Cranial Nerves

There are 12 pairs of cranial nerves (CN I-XII) that emerge from the brainstem and primarily innervate structures in the head and neck (except CN X - Vagus). They have sensory, motor, or mixed functions. A key mnemonic for pharmacy students to remember their names is: "On Old Olympus's Towering Top, A Finn And German Viewed Some Hops" (Olfactory, Optic, Oculomotor, Trochlear, Trigeminal, Abducens, Facial, Vestibulocochlear, Glossopharyngeal, Vagus, Accessory, Hypoglossal).

Summary and Take-Home Points for Pharmacy

• The nervous system is the master regulator, integrating sensory input and coordinating motor output.
• Neurons are the functional units; their structure dictates function and drug targets (receptors, ion channels, transporters).
• Neuroglia are essential for support, myelination, immune defense, and maintaining the BBB – a major hurdle and target for CNS drug delivery.
• The CNS (brain and spinal cord) is the integrative center. Its protection (BBB, meninges, CSF) is crucial for normal function and drug distribution.
• The PNS connects the CNS to the body. Understanding nerve structure (roots, rami, dermatomes) is vital for diagnosing neurological conditions and understanding side effects of medications (e.g., peripheral neuropathies).
• The ANS (sympathetic and parasympathetic) controls involuntary functions. Most organ systems are dually innervated, providing a balance that is frequently targeted by drugs (e.g., beta-blockers, anticholinergics).