Rucete ✏ Campbell Biology In a Nutshell
Unit 7 ANIMAL FORM AND FUNCTION — Concept 49.1 Nervous Systems Consist of Circuits of Neurons and Supporting Cells
Nervous systems evolved to detect and respond to environmental changes, allowing animals to survive and reproduce more effectively. Their complexity ranges from simple nerve nets to highly organized central and peripheral systems with specialized functions.
1. Evolution of Nervous Systems
- First appeared in prokaryotes as simple response systems
- Complex nervous systems evolved during the Cambrian explosion
- Cnidarians have nerve nets, the simplest form
- More complex animals evolved nerves and ganglia
2. Nervous System Organization Across Animals
- Echinoderms: radial nerves and nerve rings
- Planarians: cephalization, brain, and longitudinal nerve cords
- Segmented worms & arthropods: ventral nerve cords and segmental ganglia
- Molluscs: nervous complexity depends on mobility
- Vertebrates: dorsal nerve cord becomes spinal cord and brain (CNS)
3. Central vs. Peripheral Nervous System
- CNS: brain and spinal cord
- PNS: nerves and ganglia outside the CNS
- Gray matter: neuron cell bodies; white matter: bundled axons
- Cerebrospinal fluid: nourishes CNS and removes waste
4. Reflexes and the Spinal Cord
- Spinal cord relays signals and controls simple movement patterns
- Reflexes: automatic responses bypassing the brain
- Reflex arc: includes sensory neurons, interneurons, and motor neurons
5. Peripheral Nervous System Function
- Afferent neurons: carry sensory input to CNS
- Efferent neurons: deliver motor output from CNS to effectors
- Motor system: controls skeletal muscles
- Autonomic system: regulates internal organs
6. Autonomic Nervous System Divisions
- Sympathetic: "fight or flight" (e.g. speeds heart, inhibits digestion)
- Parasympathetic: "rest and digest" (e.g. slows heart, promotes digestion)
- Enteric system: controls digestive tract independently
- Both use preganglionic and postganglionic neurons
- Neurotransmitters:
- Sympathetic: ACh → norepinephrine
- Parasympathetic: ACh → ACh
7. Glia and Nervous System Support
- Schwann cells (PNS) and oligodendrocytes (CNS): form myelin
- Astrocytes: regulate ions, blood flow, and form blood-brain barrier
- Microglia: CNS immune defense
- Radial glia: guide neuron development and serve as stem cells
In a Nutshell
Nervous systems evolved from simple nerve nets to complex, highly coordinated systems involving the brain, spinal cord, and peripheral nerves. Their structure supports rapid signal transmission, integration, and reflex control. Neurons are assisted by specialized glia, which nourish, protect, and regulate the nervous system for optimal function.