Notes on Nervous System and Biological Clocks

Chapter 15: Nervous System Organization & Biological Clocks

Organization of the Nervous System

  • Two Major Divisions:

    1. Central Nervous System (CNS):

    • Comprises the brain and spinal cord.

    1. Peripheral Nervous System (PNS):

    • Includes all processes and soma located outside of the CNS.

    • Afferent Branch: Carries input to the CNS.

    • Efferent Branch: Carries output from the CNS.

      • Somatic Nervous System: Controls skeletal muscle.

      • Autonomic Nervous System: Controls smooth muscle, cardiac muscle, and glands.

Nervous System Evolution

  • Basic Structure:

    • Evolved from a simple reflex arc that can be divided into three parts:

    1. Stimulus (sensory reception)

    2. Processing (sensory filtering, past experiences, patterned neuronal activity)

    3. Behavior (motor output)

    • Example: Stimulation of wind receptors in cockroaches can initiate a jump.

Types of Reflex Arcs

  • Single-cell connection

  • Monosynaptic reflex arc

  • Polysynaptic reflex arc

    • Structures involved:

    • Receptor cell

    • Afferent sensory neuron

    • Interneuron (in polysynaptic arcs)

    • Efferent motor neuron

    • Effector cell

Principles of Nervous System Evolution

  1. All systems are based on one cell type, the neuron.

  2. Organization evolved through the elaboration of a fundamental pattern, the reflex arc.

  3. There is a trend toward the gathering of (inter)neurons into distinct locations (CNS).

  4. Complex organisms have more neurons than simpler organisms.

  5. As complexity increased, new structures were added to older ones; no older structures were replaced.

  6. The relative size of each brain region is related to the importance of sensory input or motor control for species survival.

  7. Many brain regions are organized into topographical maps.

Evolutionary Background

  • Embryonic Tissues of Nervous Systems:

    • Most phyla have nervous systems originating from ectoderm, mesoderm, and endoderm.

    • Exceptions: Cnidarians and sponges do not fit this model.

Specific Nervous Systems

  • Sponges:

    • Only multicellular animals without neurons.

    • Contain many neural genes found in neuroned animals.

    • Exhibit an electrical path, trabecular reticulum, made up of connected cells.

    • Movement of flagella stops upon disturbance.

  • Nerve Nets:

    • Simplest form of a nervous system, with randomly dispersed neurons.

    • No nerves or CNS.

    • Found in Cnidarians: jellyfish, anemones, Hydra.

    • Serves as a basis for more complex nervous systems.

    • Predominantly invertebrates (e.g., molluscs) and vertebrate GI tract nerve plexus.

  • Nerve Rings:

    • Found in Echinoderms and Swimming Cnidarians.

    • More complex than nerve nets, allowing for complex behavior.

    • Sensory structures provide a coverage of 360°.

    • May have two rings or ganglia groups.

Key Trends in Bilaterally Symmetric Animals

  • Bilateral Symmetry Trends:

    1. Centralization: Integration of neurons into a specific area.

    • Longitudinal nerve cords coordinate activity.

    • Peripheral nervous system containing nerves.

    1. Cephalization: Concentration of neurons at the anterior end, where many sensory receptors are typically located.

Ganglionic CNS in Arthropods

  • Ganglionic CNS:

    • Found in protostomes, particularly in arthropods.

    • CNS is ventral and solid.

    • Contains a chain of segmental ganglia linked by connectives.

    • Nerve: A bundle of processes within the PNS.

Structure of the Ganglion

  • Rind: Outer area of arthropod ganglia containing neuronal soma.

  • Inner Core:

    • Neuropile: Region of synapses between axons and dendrites.

    • Tracts: Bundles of axonal processes.

    • Commissure: A tract crossing from one side to another.

Vertebrate CNS Structure

  • Vertebrate CNS: Columnar, dorsal, and hollow, sharing features with deuterostomes and segmented structure.

  • CNS shows segmentation and cephalization similar to arthropods.

Comparative Anatomy of Nervous Systems

  • Arthropod Nervous System:

    • Anterior brain or large ganglion with segmental ganglia linked by connective fibers.

    • Nerve cords are lateral, solid, and double in their origin.

    • Ganglia exist as discrete entities.

  • Vertebrate Nervous System:

    • Anterior brain with spinal cord exhibiting segmental branching.

    • Central nerve cord is dorsal, hollow, from a single neural tube.

    • Ganglia are collections of neuronal cell bodies outside the CNS.

Functional Organization of the Mammalian Brain

  • Five Principles of Functional Organization:

    1. Localization of Brain Function:

    • Example: Phrenology – the belief areas correlate with specific functions (e.g., kindness, language).

    1. Presence of Maps:

    • e.g., motor and sensory homunculus illustrating body part representation on the brain.

    1. Size Matters:

    • Neuron count varies with complexity:

      • Invertebrates (n = 300), Cephalopods (n = 10^8), Large mammals (n = 10^{11}), Humans (n = 9 x 10^{10}), Elephants (n = 1.8 x 10^{11}).

    • Intelligence does not correlate directly with neuron quantity but with connections and brain-to-body ratio.

    1. Evolution of Vertebrate Brains:

    • Involves repeated expansion of forebrain areas.

    1. Neural Circuit Plasticity.

Organization of the Vertebrate Nervous System

  • Structure:

    • Sensory stimuli enter the CNS through the afferent division, while outputs are made through the efferent division.

  • This system separates into:

    • Somatic Nervous System: Governs skeletal muscle control.

    • Autonomic Nervous System: Regulates smooth muscles and glands, further divided into sympathetic and parasympathetic subcomponents.

Autonomic Nervous System

  • Divisions:

    • Sympathetic:

      • Dominates during physical activity and stress.

      • Preganglionic neurons extend from the thoracic and lumbar regions.

      • Synapse onto postganglionic neurons in sympathetic ganglia.

    • Parasympathetic:

      • Dominates during restful states.

      • Preganglionic neurons originate in the brain and sacral spinal cord, synapsing on postganglionic neurons near or within target organs.

Neurotransmitter Functions

  • Primary Neurotransmitters:

    • Nicotinic:

      • Agonist: Nicotine

      • Antagonist: Curare

    • Muscarinic:

      • Agonist: Muscarine

      • Antagonist: Atropine

Autonomic vs. Somatic Nervous System

  • Comparison Table:

    • Site of Origin:

    • Autonomic: Brain or lateral horn; Somatic: Ventral horn.

    • Neurons from CNS to Effector:

    • Autonomic: Two-neuron chain; Somatic: Single neuron.

    • Innervated Organs:

    • Autonomic: Cardiac/smooth muscle, glands; Somatic: Skeletal muscle.

    • Type of Innervation:

    • Autonomic: Dual by sympathetic and parasympathetic; Somatic: Only motor neurons.

    • Neurotransmitter at Effector:

    • Autonomic: ACh or norepinephrine; Somatic: Only ACh.

    • Effects:

    • Autonomic: Either stimulation or inhibition; Somatic: Stimulation.

    • Control:

    • Autonomic: Involuntary control (with potential voluntary elements); Somatic: Voluntary control.

Biological Clocks

  • Definition: Endogenous physiological timing mechanisms providing temporal organization.

  • Key Terms:

    • Period: The amount of time between one part of the cycle and the same part in the next cycle.

    • Circadian Rhythm: A cycle that lasts about a day.

Free-Running Rhythms

  • Illustrative Example:

    • A squirrel exposed to a light-dark cycle exhibits a patterned running beginning near dark onset. When in constant darkness, activity periods drift later each day.

Cellular Mechanisms of Circadian Clocks

  • Master Clock Location: Suprachiasmatic nucleus

  • Mechanism:

    • Involves clock genes (e.g., CLOCK, BMAL1, per, cry) regulating neuronal firing and expression of genes that influence circadian rhythms.

    • REV-ERBα protein interferes with BMAL1 production, impacting overall timing regulation.

Effects of SCN Destruction

  • Without the suprachiasmatic nucleus, free-running rhythms can be disrupted, altering normal behavioral and physiological cycles across multiple days.

End of Chapter 15