CH 10 - Organization of the Nervous System

Nervous System Organization

• Central nervous system (CNS): brain and spinal cord; integrates information and coordinates activity (integrative functions).

• Peripheral nervous system (PNS): afferent (sensory) and efferent (motor) nerves; includes somatic and autonomic divisions.

  • Somatic nervous system (SNS): voluntary control of skeletal muscle; one-neuron system; activation of skeletal muscle is voluntary; efferent axon synapses directly on the effector cell; innervation is excitatory; neurotransmitter: acetylcholine (ACh).

  • Autonomic nervous system (ANS): involuntary control over smooth muscle, cardiac muscle, and exocrine glands; two-neuron system with pre- and post-ganglionic fibers; activation can be excitatory or inhibitory; multiple neurotransmitters: ACh, norepinephrine (NE), epinephrine (Epi).

  • Autonomic subdivisions: sympathetic (fight or flight) and parasympathetic (rest and digest).

The Neuron

• Functional unit of the brain; specialized for sending and receiving signals.

• Four distinct domains:

  • Soma (cell body)

  • Dendrites

  • Axon

  • Presynaptic terminals

• Key idea: neurons propagate electrical signals and communicate via chemical signals at synapses.

Soma (S cell body)

• Housekeeper of the neuron; synthesizes and processes proteins.

Dendrites

• Receivers of signals; membranes bear receptors that bind neurotransmitters released by nearby cells.

• Incoming message is translated into an electrical or biochemical event that changes the excitability or function of the receiving neuron.

Axon

• Messenger/Transporter; projection from the soma; can be very long (often > a meter).

• Axoplasm contains microtubules and neurofilaments for structural stability and transport; carries materials between the cell body and axon terminus.

• Oligodendrocytes contribute to axon integrity (myelination in CNS).

• Purpose: conducts an action potential to a specific target.

Presynaptic terminals

• Axon termination site; converts the action potential into a chemical signal.

• Neurotransmitters are released into the synaptic cleft and bind to receptors on the postsynaptic membrane.

Synapse

• Junction between a presynaptic terminal and the target cell (often a dendrite or the soma of a neighboring neuron).

• Postsynaptic membrane may include dendritic spines or membrane folds to increase surface area for receptors.

Axoplasmic Transport

• Fast anterograde transport: \sim 400 \text{mm/day}

• Fast retrograde transport: \sim 200-300 \text{mm/day}

• Slow anterograde transport: \sim 0.2-8 \text{mm/day}

• Functions:

  • Fast transport moves membranous organelles (vesicles, mitochondria) and other components.

  • Anterograde transport moves materials from the soma toward the axon terminal via microtubules and the motor protein kinesin (moves toward the + end of microtubules).

  • Retrograde transport moves materials from the axon terminal back to the soma, via dynein or MAP-1C; important for signaling growth factors and survival cues to the nucleus.

  • Slow transport carries cytoskeletal and other soluble proteins necessary for cell maintenance.

Structural Classifications of Neurons

• Axonal projections

  • Projection neurons: long axons connecting distant parts of the nervous system.

  • Interneurons: processes confined to a region of the brain; may be anaxonal.

• Dendritic pattern

  • Pyramidal: typically pyramidal-shaped cell bodies with apical and basal dendrites; may be spiny.

  • Stellate: star-shaped dendritic trees; may be spiny or aspiny.

• Number of processes

  • Unipolar: single process extending from the cell body; information travels away from the cell body toward the CNS.

  • Bipolar: two processes extending from opposite sides of the cell body.

  • Multipolar: multiple dendritic processes (often many) suitable for receiving numerous synaptic inputs.

Neural Modalities (Types of Information Neurons Carry)

• Direction

  • Afferent: transmit information into the CNS from sensory receptors outside the nervous system.

  • Efferent: transmit information from the CNS to muscles or secretory cells.

• Anatomical distribution

  • Visceral: information to/from internal organs or structures derived from the branchial arch (e.g., chemoreceptors of the carotid body).

  • Somatic: information to/from non-visceral structures.

• Embryological origin

  • Special: neurons for specialized visceral or somatic structures.

  • Special visceral: to/from structures derived from the branchial arch (e.g., pharyngeal muscles).

  • Special somatic: sensory information from special sense organs (retina, taste receptors, cochlea).

  • General: neurons transmitting information to/from non-special visceral or somatic structures.

Nervous System Development

• Gastrulation produces three primitive tissue layers: endoderm, mesoderm, ectoderm.

• Notochord: a cord of specialized cells directing overlying ectoderm to form the neural tube.

• Ectoderm gives rise to the nervous system and the skin.

Embryonic Development of the Brain

• 28 days: Primary vesicles form from the neural tube: Prosencephalon (forebrain), Mesencephalon (midbrain), Rhombencephalon (hindbrain).

• 35 days: Secondary vesicles form: Telencephalon and Diencephalon from the Prosencephalon; Mesencephalon remains; Metencephalon and Myelencephalon form from Rhombencephalon.

• Adult derivatives (illustrative mapping):

  • Prosencephalon → Telencephalon (cerebral hemispheres) and Diencephalon (thalamus, subthalamus, hypothalamus, neuropituitary)

  • Mesencephalon → Midbrain

  • Rhombencephalon → Metencephalon (pons, cerebellum) and Myelencephalon (medulla)

• Cavities: lateral ventricles within the cerebral hemispheres; most of the third ventriclelies in the diencephalon; cerebral aqueduct in the midbrain; fourth ventricle in the brainstem/caudal hindbrain region.

Neural Tube Development (Detailed Structures)

• Neuroepithelial layer lines the neural tube with distinct plates:

  • Roof plate; Alar plate (sensory regions)

  • Central canal

  • Basal plate; Floor plate (motor regions)

  • Marginal layer

• Spinal cord organization (early to mature):

  • Dorsal root ganglion forms from neural crest cells.

  • Mature spinal cord sections show dorsal (posterior) and ventral (anterior) horns, central canal, dorsal horn, ventral horn, and various root structures.

• Brainstem organization: caudal medulla; rostral medulla; sulcus limitans separates alar/basal regions; choroid plexus present.

• Spinal cord and brainstem also exhibit various cranial nerve nuclei and associated pathways.

Development Defects of the Neural Tube

• Congenital malformations commonly arise from neural tube defects:

  • Anencephaly: absence of cerebral hemispheres.

  • Cephalocele: herniation of brain tissue through a skull defect (cranium bifidum).

  • Meningocele: herniation of meninges through skull or spinal defect.

  • Spina bifida: vertebral arch defect with varying severity.

  • Occulta: vertebral arch defect only, no external sac.

  • Cystica: herniation of dura and arachnoid through vertebral defect.

  • Myelomeningocele: herniation of spinal cord and meninges through vertebral defect.

Neural Development (Neurons and Glia)

• Neurons and glial cells originate from neuroepithelial cells and are initially organized into ventricular zone (VZ) and subventricular zone (SVZ).

• Neuroepithelial cells generate neural precursor cells; migration and differentiation follow along radial glial cells.

• Radial glia can differentiate into:

  • Astrocytes

  • Oligodendrocytes

  • Multipotent progenitors

• Neurons migrate along radial glia to reach their final destinations; radial cells serve as scaffolding for neuronal migration.

Neural Degeneration and Regeneration

• Neurons are mature cells with limited mitotic activity; SVZ may retain some regenerative potential.

• Lesions can cause distal degeneration of pathways and may lead to retrograde degeneration.

• Cell death may occur via apoptosis or necrosis depending on injury type and context.

Subdivisions of the Nervous System (Major Components and Orientation)

• Axes of the CNS:

  • Telencephalon → Cerebral hemispheres

  • Diencephalon → Thalamus, subthalamus, hypothalamus, neuropituitary

  • Mesencephalon → Midbrain

  • Metencephalon → Pons and Cerebellum

  • Myelencephalon → Medulla

  • Spinal cord

• Surface anatomy of the cerebral cortex:

  • Frontal, Parietal, Temporal, Occipital lobes

• Telencephalon (Cerebral Cortex):

  • Contains N \,\approx 1.5\times 10^{10} to 2.0\times 10^{10} neurons

  • Functions: thinking, learning, memory, consciousness

• Cerebellum:

  • Located immediately dorsal to brainstem; ~10% of CNS by volume

  • Contains ~50% of all neurons

  • Receives information from nearly every receptor type; integrates motor and sensory information

• Diencephalon (Thalamus, Subthalamus, Hypothalamus):

  • Thalamus: main integrating station for sensory information destined for conscious perception; involvement in arousal

  • Subthalamus: receives input from basal ganglia projections; important for motor control

  • Hypothalamus: endocrine interactions; regulates body temperature, hunger, thirst, cardiovascular function

Major Brain Regions (Brain Stem and Related Structures)

• Brain stem components: Midbrain, Pons, Medulla

• Reticular formation: essential for levels of consciousness and arousal

• Midbrain: somatic motor control for eyes, hearing, and vision

• Pons: somatic motor control for mastication, eye movement, facial muscles; processes information related to hearing and balance

• Medulla: somatic motor neurons to neck and tongue; cardiovascular and respiratory control; auditory and equilibrium processing

Anatomy of the Spinal Cord and Nerve Roots

• Spinal cord anatomy includes:

  • Dorsal root ganglion

  • Dorsal horn (sensory processing)

  • Ventral horn (motor neurons)

  • Lateral funiculus, dorsal/ventral funiculi, anterior median fissure

  • Ascending tracts and descending tracts (e.g., fasciculus gracilis, fasciculus cuneatus, spinothalamic tract, spinocerebellar tract, corticospinal tract, reticulospinal tracts, vestibulospinal tract, rubrospinal tract, tectospinal tract)

• Spinal Reflex Arc: a simple circuit including

  • Primary sensory neuron -> sensory axon -> interneuron (in many pathways) -> dorsal root -> dorsal horn -> ventral root -> motor neuron -> motor end plate of skeletal muscle

• Ascending and descending pathways are organized in tracts within the cord, linking peripheral receptors to brain and efferent outputs back to muscles and glands.

Peripheral Nervous System

• PNS comprises cranial and spinal nerves, sensory ganglia, and sensory receptors.

• Four primary functions:

  • Transduction of physical/chemical stimuli from the external environment

  • Convey sensory information to the CNS via axons

  • Convey motor signals from the CNS to target organs (skeletal and smooth muscle)

  • Convert motor signals to chemical signals at peripheral synapses on target tissues

Structure of the Peripheral Nerve

• Nerves are bundles called fascicles; each fascicle is surrounded by perineurium.

• Peripheral nerve dysfunction (neuropathy) may present with:

  • Fibrillation: spontaneous twitching of individual muscle fibers

  • Fasciculation: small, spontaneous twitches of muscle fibers in unison within a motor unit

  • Damage to a motor neuron that has not yet lost continuity with the muscle fibers (early signs of denervation)

• Paresthesia: tingling sensations or pain in areas served by diseased nerves

Dermatomes

• Dermatomes are areas of cutaneous sensory innervation supplied by a single dorsal root and its ganglion.

• Clinically useful for localizing lesions along the spinal cord or dorsal roots based on sensory distribution.

Connections and Real-World Relevance

• Foundational principles:

  • Neurons communicate via electrical signals (action potentials) and chemical signals (neurotransmitters) at synapses.

  • Myelination and axoplasmic transport are critical for rapid signal propagation and long-distance signaling.

  • Developmental patterning (neural tube formation, neural plate derivatives) underlies the organized layout of the adult CNS.

• Practical implications:

  • Neural tube defects can lead to severe motor and sensory deficits; early detection and prevention (e.g., folate supplementation in pregnancy) are key public health concerns.

  • Peripheral nerve injuries and neuropathies affect sensation and motor control; understanding dermatomes and nerve distributions aids in diagnosis.

  • The autonomic system’s dual-neuron layout is crucial for pharmacological targeting in diseases affecting heart rate, digestion, and glandular function.

Key Equations and Numerical References (LaTeX)

• Fast axonal transport speeds:

  • v_{ ext{anterograde}} \sim 4.0\times 10^{2} \,\text{mm/day}

  • v_{ ext{retrograde}} \sim 2.0-3.0\times 10^{2} \,\text{mm/day}

• Slow axonal transport speed:

  • v_{ ext{slow}} \sim 0.2-8 \,\text{mm/day}

• Neuron counts in cerebral cortex:

  • N_{ ext{cortex}} \approx 1.5\times 10^{10} \,\text{to} \, 2.0\times 10^{10}

• Cerebellum neuron proportion:

  • Contains \approx 50\% of all neurons in the CNS, occupying about 10\% of CNS volume

• Neurotransmitters listed:

  • Acetylcholine: \text{ACh}

  • Norepinephrine: \text{NE}

  • Epinephrine: \text{Epi}

Summary of Core Concepts

• The nervous system is organized into CNS and PNS, with functional divisions into somatic and autonomic components.

• Neurons have specialized structures (soma, dendrites, axon, presynaptic terminals) that enable signal reception, integration, and transmission.

• Axoplasmic transport sustains neuron structure and signaling over long distances via fast and slow mechanisms, driven by motor proteins (kinesin for anterograde, dynein for retrograde).

• Neurons are classified by projection, dendritic pattern, and number of processes, with directional modalities (afferent/efferent) and embryological origins (special/general) shaping their roles.

• CNS development proceeds from neural tube formation through primary and secondary vesicle stages to adult brain regions; defects during development can lead to serious congenital anomalies.

• The brain and spinal cord are organized into major regions (telencephalon, diencephalon, mesencephalon, metencephalon, myelencephalon) with distinct functions and sensory-motor integration roles.

• The spinal cord contains reflex arcs and multiple ascending and descending tracts that coordinate movement and sensation.

• The PNS includes nerves, sensory ganglia, and receptors, with peripheral nerves organized into fascicles and protected by the perineurium; notable clinical signs include fibrillation, fasciculation, and paresthesias.

• Dermatomes map sensory innervation to dorsal roots, aiding localization in neurological assessment.