1. Overview and Organization of the Nervous System

The nervous system is the body's primary control and communication network. Together with the endocrine system, it maintains homeostasis, integrates sensory input, controls voluntary and involuntary motor functions, and enables higher cognitive abilities such as perception, behavior, learning, and memory.

1.1 Structural Subdivisions

• Structural
  Central Nervous System (CNS)

  Comprises the brain (located within the cranium) and the spinal cord (located within the vertebral canal). Functions as the primary integrating and command center for processing incoming sensory information and initiating motor responses.

• Structural
  Peripheral Nervous System (PNS)

  Consists of all nervous tissue outside the CNS, including 12 pairs of cranial nerves, 31 pairs of spinal nerves, peripheral ganglia, and sensory receptors. Connects the CNS to limbs, organs, and peripheral tissues.

Figure 1.1: Organization of the Nervous System. The CNS (brain and spinal cord) exchanges signals with the PNS — afferent (sensory) signals travel toward the CNS, efferent (motor) signals travel away from it. The PNS afferent division carries information from somatic and visceral receptors; the efferent division splits into the somatic nervous system (voluntary, skeletal muscle) and the autonomic nervous system (involuntary), which is further divided into the antagonistic sympathetic and parasympathetic divisions acting on smooth muscle, cardiac muscle, and glands.

1.2 Functional Subdivisions of the PNS

• Functional
  Afferent (Sensory) Division

  Carries sensory information from peripheral receptors (somatic and visceral) toward the CNS.

• Functional
  Efferent (Motor) Division

  Transmits motor commands from the CNS to peripheral effector organs (muscles and glands). Subdivided into the Somatic Nervous System (SNS) and the Autonomic Nervous System (ANS).

• Efferent — SNS
  Somatic Nervous System

  Voluntary control; transmits impulses from the CNS directly to skeletal muscles.

• Efferent — ANS
  Autonomic Nervous System

  Involuntary control; transmits impulses to cardiac muscle, smooth muscle, and exocrine/endocrine glands. Subdivided into the sympathetic and parasympathetic divisions, described below.

• ANS Branch
  Sympathetic Division

  "Fight-or-flight" responses mobilized during stress or exercise — increases heart rate, redirects blood flow to skeletal muscle, and dilates airways.

• ANS Branch
  Parasympathetic Division

  "Rest-and-digest" activities that promote conservation and restoration of energy — slows heart rate and stimulates digestive and glandular activity.

1.3 Enteric Nervous System (ENS)

An extensive intrinsic neural network embedded within the walls of the gastrointestinal (GI) tract. Capable of operating independently (autonomous reflex arcs) or being modulated by the ANS (sympathetic and parasympathetic inputs).

2. Histology of Nervous Tissue

Nervous tissue consists of two major functional cell types: neurons (excitable signaling cells) and neuroglia or glial cells (non-excitable supporting cells).

2.1 Neurons

Neurons are the structural and functional units of the nervous system. They possess electrical excitability — the capacity to respond to physical or chemical stimuli, convert them into action potentials (nerve impulses), and propagate these signals at speeds ranging from 0.5 to 130 meters per second.

Structural Components of a Neuron

Figure 2.1: Structural Components of a Neuron. Dendrites receive incoming signals and conduct them toward the cell body (perikaryon), which houses the nucleus and Nissl bodies (rough ER and free ribosomes). Signals are triggered at the axon hillock/initial segment and propagate along the axon. In the PNS, Schwann cells wrap the axon in a segmented myelin sheath, with exposed nodes of Ranvier between segments enabling saltatory conduction, terminating in axon terminals (telodendria) with synaptic knobs.

• Component
  Cell Body (Perikaryon)

  Contains a prominent central nucleus with a conspicuous nucleolus. Cytoplasm contains typical organelles (mitochondria, lysosomes, Golgi complex) and dense clusters of rough endoplasmic reticulum and free ribosomes known as Nissl bodies (responsible for high protein synthesis). Clusters of neuronal cell bodies in the PNS are called ganglia; in the CNS, they are termed nuclei.

• Component
  Dendrites

  Short, highly branched, tapering processes extending from the cell body. Serve as the primary receiving or input region for incoming signals from other neurons.

• Component
  Axon

  A long, thin cylindrical projection that conducts action potentials away from the cell body toward another neuron, muscle fiber, or gland cell.

• Axon Part
  Axon Hillock

  Cone-shaped region connecting the cell body to the axon; the initial segment of the axon serves as the trigger zone where action potentials originate.

• Axon Part
  Axoplasm & Axolemma

  Axoplasm is the cytoplasm of the axon, enclosed by the axolemma (plasma membrane). Lacks rough ER; thus protein synthesis does not occur in the axon.

• Axon Part
  Axon Collaterals

  Side branches emerging at right angles along the length of the axon.

• Axon Part
  Axon Terminals (Telodendria)

  Fine terminal arborizations at the distal end, ending in swollen structures termed synaptic knobs (terminal buttons) containing synaptic vesicles loaded with neurotransmitters.

2.2 Classification of Neurons

Structural Classification

Based on the number of processes extending from the cell body.

Figure 2.2: Structural Classification of Neurons. Multipolar neurons (many dendrites, one axon) predominate in the brain and spinal cord. Bipolar neurons (one dendrite, one axon from opposite poles) occur in specialized sensory organs. Unipolar (pseudounipolar) neurons have a single process leaving the cell body that splits into a peripheral process (toward receptors) and a central process (toward the CNS), typical of dorsal root ganglion sensory neurons.

• Structural Type
  Multipolar Neurons

  Feature several dendrites and one single axon. Most common structural type in the brain and spinal cord (e.g., motor neurons, interneurons).

• Structural Type
  Bipolar Neurons

  Feature one main dendrite and one axon extending from opposite ends of the cell body. Found in specialized sensory organs: retina of the eye, inner ear (cochlea/vestibule), and olfactory mucosa.

• Structural Type
  Unipolar (Pseudounipolar) Neurons

  Feature a single process extending from the cell body that divides into two functional branches: a peripheral axon (extending to sensory receptors) and a central axon (entering the CNS). Primary sensory neurons with cell bodies located in the dorsal root ganglia of spinal nerves.

Functional Classification

Based on the direction of nerve impulse conduction.

• Functional Type
  Afferent (Sensory) Neurons

  Conduct nerve impulses from peripheral sensory receptors into the CNS. Most are structurally unipolar.

• Functional Type
  Efferent (Motor) Neurons

  Transmit impulses away from the CNS to effector organs (muscles and glands). Structurally multipolar.

• Functional Type
  Interneurons (Association Neurons)

  Located entirely within the CNS between sensory and motor neurons. Integrate incoming information and elicit motor responses. Account for approximately 99% of all neurons in the human body.

Figure 2.3: The Reflex/Circuit Pathway. A sensory receptor generates an impulse conducted by an afferent (sensory) neuron into the CNS, where an interneuron integrates the signal before relaying it to an efferent (motor) neuron, which carries the command to an effector organ (muscle or gland).

2.3 Neuroglia (Glial Cells)

Neuroglia constitute approximately 90% of all neural cells in the CNS and about 50% of brain volume. Unlike neurons, glial cells do not generate or conduct action potentials, but they retain the ability to divide throughout life.

Figure 2.4: Classification of Neuroglial Cells. CNS glia include astrocytes (blood-brain barrier and homeostatic support), oligodendrocytes (each myelinating segments of multiple CNS axons), microglia (resident phagocytic immune cells), and ependymal cells (line the ventricles/central canal and circulate cerebrospinal fluid). PNS glia comprise Schwann cells (each myelinating one axon segment and forming the neurolemma) and satellite cells (support ganglionic cell bodies).

Glial Cells of the CNS

• CNS Glia
  Astrocytes

  Star-shaped cells with numerous processes; largest and most abundant glial cells. Maintain the microenvironment for neuronal signaling; form foot processes around capillaries that maintain the blood-brain barrier (BBB); provide structural support; regulate extracellular K⁺ and neurotransmitter concentration.

• CNS Glia
  Oligodendrocytes

  Broad, flat processes wrap around multiple adjacent axons within the CNS to form the myelin sheath. One oligodendrocyte can myelinate segments of up to 30 or more axons.

• CNS Glia
  Microglia

  Small cells with slender spine-like processes. Act as resident macrophages (immune defense) in the CNS, phagocytosing cellular debris, microbes, and damaged nervous tissue.

• CNS Glia
  Ependymal Cells

  Cuboidal to columnar epithelial cells (often ciliated) lining the cerebral ventricles and the central canal of the spinal cord. Produce, monitor, and assist in circulating cerebrospinal fluid (CSF).

Glial Cells of the PNS

• PNS Glia
  Schwann Cells (Neurolemmocytes)

  Form the myelin sheath around single axon segments in peripheral nerves. Form the neurolemma (outer nucleated cytoplasmic layer), facilitating axon regeneration following injury.

• PNS Glia
  Satellite Cells

  Flat cells surrounding neuronal cell bodies within peripheral ganglia; provide structural support and regulate chemical exchanges between cell bodies and interstitial fluid.

2.4 Myelination and Conduction Velocity

The myelin sheath is a multilayered lipid and protein wrapping that electrically insulates axons and dramatically increases the speed of nerve impulse conduction.

Figure 2.5: Cross-Section of a Myelinated PNS Axon. From the center outward: the axoplasm (containing neurofibrils and mitochondria) is bounded by the axolemma; concentric wraps of the Schwann cell's plasma membrane form the multilayered myelin sheath; and the outermost neurolemma is the Schwann cell's nucleus-containing cytoplasmic rim.

• Structure
  Nodes of Ranvier

  Unmyelinated gaps along the axon between adjacent Schwann cells or oligodendrocyte wraps where voltage-gated ion channels are concentrated, permitting saltatory conduction.

• Velocity Factor
  Myelination

  Myelinated fibers conduct significantly faster (saltatory conduction) than unmyelinated fibers (continuous conduction).

• Velocity Factor
  Axon Diameter

  Larger diameter axons conduct impulses faster due to lower resistance to local current flow.

• Velocity Factor
  Temperature

  Higher temperatures increase ion channel kinetics and impulse speed.