Table of Contents
Overview of Vertebrate Nervous Systems
Vertebrates (fish, amphibians, reptiles, birds, mammals) all share the same basic nervous system plan. Compared with invertebrates, their nervous systems are characterized by:
- A dorsal (back-side) nerve cord protected by the vertebral column
- A clearly separated central nervous system (CNS) and peripheral nervous system (PNS)
- A brain encased in a skull, usually with pronounced regional specialization
- A segmentally organized spinal cord with paired spinal nerves
This chapter focuses on these common design principles and how they are modified across major vertebrate groups. General mechanisms of excitation, conduction, and synaptic transmission are assumed knowledge from parent chapters and are not repeated here.
Basic Structural Plan
Central vs. Peripheral Nervous System
- Central nervous system (CNS)
- Brain and spinal cord
- Processes sensory information and generates coordinated responses
- Peripheral nervous system (PNS)
- Nerves and ganglia outside CNS
- Connects CNS with sense organs, muscles, and internal organs
- Includes:
- Somatic nervous system: voluntary control of skeletal muscles, conscious sensation
- Autonomic nervous system (ANS): involuntary control of internal organs, glands, smooth muscle, heart
These divisions are present in all vertebrates, though the size and complexity of particular parts differ.
Dorsal Hollow Nerve Cord and Segmentation
In vertebrates, the main nerve cord runs along the dorsal side and is:
- Hollow, with a central canal filled with cerebrospinal fluid
- Surrounded by protective bony structures (vertebrae) and connective tissue coverings (meninges in mammals)
- Segmentally organized in the spinal cord: each segment gives rise to a pair of spinal nerves supplying a specific body region
This segmentation underlies the orderly arrangement of motor and sensory innervation, visible for example in the dermatomes of mammals (skin areas supplied by a single spinal nerve).
The Vertebrate Brain: Main Regions
Despite great diversity, vertebrate brains always show the same major regions (front to back):
- Forebrain
- Midbrain
- Hindbrain
These regions specialize in different tasks.
Forebrain
The vertebrate forebrain includes:
- Telencephalon (endbrain)
- Cerebral hemispheres
- Olfactory bulbs (especially large in many fish, amphibians, and mammals that rely on smell)
- Diencephalon
- Thalamus: major relay station for sensory information to the cortex (in species with a cortex)
- Hypothalamus: controls autonomic nervous system and endocrine system (via the pituitary gland); key for homeostasis (temperature, hunger, thirst, circadian rhythms, sexual behavior)
In more derived vertebrates (especially birds and mammals), the forebrain is the main center of information integration, learning, and complex behavior.
Midbrain
The midbrain is a primary center for processing sensory information and controlling movements:
- In fish and amphibians, the midbrain (especially the optic tectum) is often the dominant visual processing center.
- In reptiles, birds, and mammals, the midbrain still integrates visual and auditory information and controls reflexive orienting movements (turning head and eyes toward stimuli), but higher processing shifts more to the forebrain.
Hindbrain
The hindbrain is composed of:
- Cerebellum
- Coordinates movements, posture, balance
- Refines motor commands and is essential for motor learning (e.g., learning to fly, swim, walk, grasp)
- Medulla oblongata (bulb) and pons (in mammals and birds)
- House vital centers for autonomic functions:
- Respiration
- Heart rate and blood pressure
- Basic reflexes like swallowing, coughing, vomiting
The cerebellum varies greatly in size: small in many fishes and amphibians, extremely well developed in birds and mammals with complex, fast movements.
The Spinal Cord and Spinal Nerves
The spinal cord is the main conduction pathway between brain and body and a center for many reflexes.
Internal Organization
- Gray matter (cell bodies, synapses) is usually central, forming an H or butterfly shape in cross section (in mammals)
- White matter (myelinated axons) surrounds the gray matter and forms ascending (sensory) and descending (motor) tracts
Two major root types emerge from each spinal cord segment:
- Dorsal root: carries sensory (afferent) fibers into the spinal cord; often has a dorsal root ganglion with the sensory neuron cell bodies
- Ventral root: carries motor (efferent) fibers from the spinal cord to muscles
The dorsal and ventral roots unite to form a mixed spinal nerve supplying a specific body segment.
Spinal Reflexes
Many rapid, stereotyped responses are organized at the level of the spinal cord without requiring conscious brain involvement, e.g.:
- Withdrawal reflexes (pulling a limb away from a painful stimulus)
- Simple stretch reflexes maintaining muscle tone
These spinal reflexes are a fundamental and conserved vertebrate feature.
Autonomic Nervous System in Vertebrates
The vertebrate autonomic nervous system (ANS) regulates internal organs and maintains homeostasis. It has two main branches with often opposing effects:
- Sympathetic nervous system
- Prepares the body for “fight-or-flight” situations
- Effects include:
- Increased heart rate and blood pressure
- Dilated bronchi
- Reduced digestive activity
- Parasympathetic nervous system
- Promotes “rest-and-digest” functions
- Effects include:
- Reduced heart rate
- Stimulated digestion and gland secretion
- Energy storage
The organization (locations of ganglia, neurotransmitters) is broadly conserved across vertebrates, though details vary among groups.
Comparative Features of Vertebrate Nervous Systems
Certain trends appear when comparing major vertebrate groups. The overall plan is conserved, but relative sizes and roles of brain regions shift.
Fish
- Dominant sense: often vision, lateral line (water movement), and smell
- Brain:
- Large olfactory bulbs in many species
- Well-developed optic tectum (midbrain) for visual processing
- Modest cerebellum in slow-moving bottom dwellers; much larger in active, fast-swimming fish and agile hunters
- Spinal cord:
- Controls swimming pattern generators (central pattern generators) that produce rhythmic tail movements
Amphibians
- Still largely dependent on water, with a mix of aquatic and terrestrial life stages
- Brain remains relatively small; the midbrain optic tectum and hindbrain are prominent
- Forebrain begins to take on more important roles in integrating sensory inputs and in simple learning
- Nervous system must coordinate different locomotion modes (swimming larvae, walking or jumping adults in many species)
Reptiles
- More terrestrial; behavior and sensory world more complex
- Forebrain expansion:
- Cerebral hemispheres grow, especially regions related to olfaction and simple integrative functions
- Cerebellum becomes more important for coordinated limb movements on land
- The nervous system supports more complex hunting strategies, territorial behavior, and social interactions in some species.
Birds
- High demand for coordinated, fast movements during flight
- Very large cerebellum and highly developed midbrain centers for:
- Balance
- Flight control
- Eye–head coordination
- Forebrain organization differs from mammals (less layered “cortex”, more nuclear structures), but:
- Many birds, especially corvids and parrots, show advanced learning, problem-solving, and complex vocal communication
- Specialized sensory adaptations:
- Excellent vision (large optic lobes and specialized forebrain areas)
- Complex song control circuits in many passerine birds
Mammals
- Most extreme forebrain expansion, particularly the cerebral cortex
- Cortex organized into distinct areas and layers:
- Sensory cortices (visual, auditory, somatosensory)
- Motor cortex
- Association areas important for integration, memory, and planning
- Neocortex especially enlarged in primates, cetaceans, some carnivores and ungulates
- Corpus callosum connects the two cerebral hemispheres in eutherian (placental) mammals, coordinating bilateral processing
Human Nervous System as an Extreme Mammalian Example
Within mammals, humans show:
- Very large and highly folded (gyrencephalic) neocortex
- Extensive association areas supporting:
- Complex language
- Abstract thought
- Long-term planning
- Self-reflection
These expansions build on the same vertebrate structural plan; no entirely new brain region appears, but existing ones are massively elaborated and reconnected.
Developmental and Evolutionary Aspects
Embryonic Origin
In all vertebrates:
- The nervous system develops from a neural tube formed from the embryonic ectoderm
- The anterior neural tube expands into the three primary brain vesicles:
- Forebrain (prosencephalon)
- Midbrain (mesencephalon)
- Hindbrain (rhombencephalon)
- The posterior neural tube becomes the spinal cord
Later, the forebrain and hindbrain subdivide further, creating the detailed adult brain regions. This shared development is a key indication of common evolutionary origin.
Evolutionary Trends
Across vertebrate evolution, the following tendencies are evident:
- Increasing relative size and complexity of the forebrain, particularly structures supporting learning, memory, and flexible behavior
- Increasing sophistication of sensory processing and motor control, often linked to ecological niches (e.g., aerial vs aquatic vs terrestrial)
- Strong correlation between behavioral complexity and:
- Degree of brain regionalization
- Amount of neural tissue in association areas
- Conserved basic plan: even in highly derived vertebrates, the same major divisions and many detailed features are still recognizable.
Functional Consequences for Behavior
The structure of the vertebrate nervous system shapes behavioral capabilities:
- Simple, stereotyped behaviors (basic escape responses, simple locomotion) can be managed by spinal cord and brainstem circuits across all vertebrates.
- More flexible behaviors (learning, problem-solving, social interactions, tool use) require expanded and more interconnected forebrain regions.
- Sensorimotor specializations (echolocation in some mammals, advanced color vision in many birds, electrolocation in some fish) are associated with specialized and often enlarged brain nuclei or cortical areas.
Thus, while the architectural framework is shared, differences in nervous system size, connectivity, and regional emphasis underlie the enormous diversity of vertebrate behaviors.