Animal Nervous Systems: Evolution, Structure, and Function
Evolutionary Features of the Nervous System
As animals evolve, their nervous systems become increasingly complex, exhibiting several key characteristics:
- Increased number of nerve cells.
- Concentration of nerve cells, particularly at the anterior (head) end.
- Increased number of interconnections between nerve cells.
- Specialization of nerve cells to perform different functions.
Nervous Systems in Invertebrates
The complexity of an animal’s nervous system is closely related to its activity and lifestyle. Several organizational models exist:
- Cnidarians (e.g., Jellyfish): Possess a nerve net, a diffuse network of interconnected nerve cells.
- Echinoderms (e.g., Starfish): Exhibit a radial nerve structure consisting of an interconnected nerve network.
- Nematodes and Platyhelminthes (e.g., Flatworms): Display bilateral symmetry and possess cerebral ganglia, forming a primitive brain.
- Annelids and Arthropods (e.g., Earthworms and Insects): Have a more developed brain and peripheral ganglia.
- Molluscs (e.g., Clams and Squids): Show a clear relationship between nervous system complexity and lifestyle. Sessile or slow-moving molluscs like clams exhibit minimal cephalization (concentration of nerve cells in a head). Cephalopods, such as squids, possess the most sophisticated nervous systems among invertebrates.
Nervous Systems in Vertebrates
Vertebrate nervous systems share a common basic structure. The nervous system originates from the ectoderm, which forms the neural tube. The brain develops from the anterior portion of the neural tube, and the spinal cord develops from the posterior portion.
Diencephalon: Thalamus and Hypothalamus
- Thalamus: Serves as a central relay station for motor and sensory information. In mammals (except for olfactory information), all sensory input is routed through the thalamus before being transmitted to the sensory areas of the cerebral cortex.
- Hypothalamus: Acts as the primary integration center for regulating visceral functions. Located ventral to the thalamus, it sends signals to the brainstem and spinal cord and also connects the nervous and endocrine systems. In reptiles, birds, and mammals, the hypothalamus regulates body temperature, water balance, and sexual and emotional responses.
Midbrain
The midbrain receives and integrates sensory information and sends instructions to appropriate motor nerves. The optic lobes, in particular, specialize in visual processing.
Hindbrain
The hindbrain contains the cerebellum, which coordinates muscle activity. The development of the cerebellum is related to the extent and complexity of muscular activity, so its size and shape vary among different vertebrate classes.
Although vertebrates share the same basic brain structure, the cerebellum and cerebrum are more complex and specialized in higher vertebrates. The telencephalon gives rise to the cerebrum and olfactory bulbs.
The majority of the brain consists of myelinated axons, forming white matter, while the cerebral cortex consists of neuronal cell bodies and dendrites without myelin, forming gray matter.
In mammals, the cerebral cortex assumes responsibility for many functions performed by other brain regions in lower vertebrates. Furthermore, more complex functions, such as sensory-motor association and learning, develop within the cortex.
In mammals, the cerebral cortex can be smooth, but in more complex brains, the surface area is increased by numerous folds called convolutions.
Vertebrate Nervous System Structure
- Central Nervous System (CNS): Comprises the brain and spinal cord. It serves as the control center, integrating incoming information and generating appropriate responses.
- Peripheral Nervous System (PNS): Consists of nerves (cranial and spinal) and peripheral ganglia. The PNS is further divided into two systems:
- Somatic Nervous System: Formed by nerves connecting the CNS to skeletal muscles. It is responsible for voluntary body movements and daily activities.
- Autonomic Nervous System: Controls involuntary vital functions (e.g., breathing, heartbeat). It has two branches:
- Sympathetic: Dominates during stress, releasing adrenaline and accelerating bodily functions.
- Parasympathetic: Operates during rest and relaxation, promoting calm and restorative processes.
The Neuron and Synapse
The neuron is a specialized cell designed to generate and propagate nerve impulses, which are changes in electrical polarity across the neuronal membrane.
When a neuron is not stimulated, it is at rest, and its membrane is polarized, with a higher concentration of positive charges outside the cell. This difference in charge is maintained by the sodium-potassium pump, which actively transports 3 sodium ions (Na+) out of the cell for every 2 potassium ions (K+) it transports in.