1.2 - organization of the nervous system

1.2.1 Compare the General Designs of Animal Nervous Systems

Universal Features:

• Nearly all multicellular animals (except sponges) have some form of nervous system utilizing neurons and glia.

• Nervous systems vary in structure and complexity across animal groups.

Major Organizational Themes:

• Neural Nets:

  • Oldest, simplest form.

  • Seen in Cnidaria (hydra, corals, jellyfish).

  • Meshwork of interconnected neurons, not clustered or centralized into nerves/brains.

  • Provides diffuse conduction; no CNS or ganglia.

• Nerve Cords and Ganglia:

  • Seen in more complex invertebrates (flatworms, annelids, arthropods).

  • Ganglia = clusters of neuronal cell bodies.

  • Nerve cords may run longitudinally, sometimes forming a "ladder" (flatworms).

  • Multiple ganglia for local control of body regions/muscles; often have “brain-like” cerebral ganglia.

• Centralization & Cephalization:

  • Bilateral animals (with left/right mirror symmetry) show clustering of neurons (centralization) and concentration at the anterior end (cephalization = head/brain).

  • Example: flatworms—anterior ganglia + parallel nerve cords (ladder-like).

• Vertebrates:

  • Have highly centralized, dorsal CNS (brain + spinal cord, encased in bone).

  • Complex behaviors arise from a large, centralized brain.

  • Always possess both a CNS and PNS (peripheral nerves & ganglia).

Symmetry:

• Bilateral symmetry: One major nerve cord and central ganglia/brain (humans, most vertebrates, many invertebrates).

• Radial symmetry: Nervous system distributed radially around a central axis (sea stars, hydra).

1.2.2 Describe the Divisions of the Human Nervous System and Basic Anatomical Organization

Two Main Divisions:

1. Central Nervous System (CNS):

   • Brain and spinal cord.

   • Contained within bone (skull, vertebrae).

   • Organizes processing, integration, memory, learning, movement, perception.

   • Groups of neuronal cell bodies = nuclei.

   • Bundles of axons = tracts (white matter; myelinated).

   • Gray matter = neuron cell bodies, dendrites, and unmyelinated axons.

   • Brain—higher functions; spinal cord—relays information and local processing (e.g., reflexes, rhythmic walking via central pattern generators).

2. Peripheral Nervous System (PNS):

   • All nervous tissue outside CNS: peripheral nerves and ganglia.

   • Carries messages to/from CNS and all areas of the body (muscles, skin, organs).

   • Cell bodies grouped in ganglia; axon bundles called nerves.

Other Major Features:

• Meninges: Three-layered protective covering (dura mater, arachnoid, pia mater).

• Ventricular system: Four ventricles + aqueduct, filled with cerebrospinal fluid (CSF) for cushioning & chemical exchange.

• BBB: Blood-brain barrier formed by tight junctions in capillaries; protects brain from toxins/infections.

1.2.3 Describe the Basic Organization of a Simple Neural Circuit

What is a Neural Circuit?

• A set of interconnected neurons organized to process a specific type of information or produce a response.

• Range: Simple (reflex arc) to complex (entire brain circuits).

Basic Neural Circuit Components:

• Afferent (sensory) neuron: Senses stimulus, relays information to CNS.

• Interneurons: Process, integrate, or modulate information within CNS (spinal cord, brain). Can provide direct connection or regulate by excitation/inhibition.

• Efferent (motor) neuron: Sends output from CNS to the effector (muscle/gland).

Classic Example – Reflex Arc:

• Ex: Withdrawal reflex (touching something hot).

  • Stimulus → sensory neuron detects → signal to spinal cord → may synapse on interneuron or directly on motor neuron.

  • Motor neuron stimulates muscle contraction; quick, involuntary movement (before conscious awareness).

  • Brain receives message after the reflex for awareness and future learning.

• Knee-jerk reflex: Sensory neuron activates both excitatory (to extensors) and inhibitory pathways (to flexors via interneuron) for coordinated response.

Complex Circuits:

• Simple animal models (e.g., C. elegans) have completely mapped connectomes (302 neurons, ~7000 connections).

• Human brain: Ongoing connectome projects (Human Connectome Project, BRAIN Initiative) aim to map full complexity.

SUMMARY

• Animal nervous systems range from simple diffuse nerve nets (Cnidaria) to centralized systems with ganglia, nerve cords (invertebrates), and highly centralized brains (vertebrates).

• In vertebrates (including humans), the nervous system is divided into:

  • CNS (brain, spinal cord): processing, integration, control, local circuits.

  • PNS (nerves, ganglia): communication pathways to/from the entire body.

• Gray matter: cell bodies, dendrites, unmyelinated axons; white matter: myelinated axon tracts.

• Basic circuits consist of sensory, interneuron, and motor components; classic reflex arcs bypass higher brain regions for speed.

• Human neuroscience is focused on mapping these circuits at increasing levels of complexity—from simple reflexes to dynamic, whole-brain networks.

HIGH-YIELD KEY FACTS

• Neural net: Simplest, no clusters, e.g., hydra/jellyfish.

• Ganglia: Clusters of cell bodies, common in invertebrates/ladders (earthworm).

• Centralized CNS: Always found in vertebrates.

• Animal symmetry:

  • Bilateral (humans, worms): one nerve cord, centralization & cephalization (head/brain).

  • Radial (starfish, hydra): no brain, nerves radiate from center.

• CNS: Brain + spinal cord; nuclei (cell bodies), tracts (axons).

• PNS: All outside CNS; ganglia (cell bodies), nerves (axons).

• Gray matter: Outer brain, inner spinal cord (cell bodies, dendrites).

• White matter: Inner brain, outer spinal cord (myelinated axons).

• Meninges: Dura, arachnoid, pia mater—protect CNS.

• Reflex arc: Sensory → (± interneuron) → motor neuron → response; can be excitatory & inhibitory (e.g., knee-jerk).

• Connectome: Complete neuronal wiring diagram; only completed for C. elegans and fruit fly larvae so far.