The vertebrate nervous system is the body’s command network, responsible for coordinating actions, transmitting signals between different parts of the body, and responding to the environment. This system integrates sensory information and orchestrates appropriate motor outputs, enabling everything from simple reflexes to complex thoughts and movements.
The Two Main Divisions
The vertebrate nervous system is organized into two primary components: the Central Nervous System (CNS) and the Peripheral Nervous System (PNS). The CNS, which includes the brain and spinal cord, acts as the main processing and integration center. It receives information, interprets it, and formulates a response, and is protected by the skull and vertebrae.
Connecting the CNS to the rest of the body is the PNS, a vast network of nerves and ganglia. The PNS functions as the communication line, relaying sensory information from the body to the CNS and transmitting motor commands back to the muscles and glands. These nerves are bundles of fibers that extend from the central system to the limbs, organs, and sensory structures.
The Central Nervous System
The CNS is composed of the brain, housed within the skull, and the spinal cord, which is protected by the vertebral column. The brain can be broadly divided into three main functional regions: the cerebrum, the cerebellum, and the brainstem.
The cerebrum is the largest part of the brain and is divided into two cerebral hemispheres. It is the center for higher-order functions such as thought, memory, language, and voluntary movement. Sensory information is processed in the cerebrum, which then directs conscious motor responses. The cerebellum, located at the back of the brain, coordinates movement and maintains balance.
The brainstem connects the cerebrum and cerebellum to the spinal cord and manages many of the body’s autonomic functions. It controls processes like heart rate, breathing, and blood pressure, operating without conscious thought.
The spinal cord serves a dual function. It acts as the primary conduit for nerve signals traveling between the brain and the rest of the body and also functions independently to mediate reflexes. The spinal cord contains distinct regions of gray matter and white matter.
The Peripheral Nervous System
The PNS encompasses all the nerves and ganglia outside of the brain and spinal cord. It is composed of nerves that extend from the brain (cranial nerves) and the spinal cord (spinal nerves), connecting the central system to every other part of the body.
Functionally, the PNS is separated into the somatic nervous system and the autonomic nervous system. The somatic system governs voluntary actions and the reception of external stimuli. It includes sensory neurons that carry information to the CNS and motor neurons that carry instructions from the CNS to skeletal muscles.
The autonomic nervous system regulates involuntary bodily functions to maintain a stable internal environment. This system is further divided into two opposing parts: the sympathetic and parasympathetic nervous systems. The sympathetic division prepares the body for “fight-or-flight” responses during stress. Conversely, the parasympathetic division oversees “rest-and-digest” activities, promoting calm and conserving energy.
Cellular Communication
The nervous system is constructed from specialized cells called neurons. A neuron consists of a cell body, which contains the nucleus, and two types of extensions: dendrites and an axon. Dendrites are branch-like structures that receive signals from other neurons, while the axon carries signals away from the cell body.
Communication between neurons occurs through electrical signals known as action potentials. When a neuron is sufficiently stimulated, it generates this electrical impulse, which travels down the length of its axon.
The junction where one neuron passes a signal to another is called a synapse. When an action potential reaches the end of an axon, it triggers the release of chemical messengers called neurotransmitters. These neurotransmitters travel across the gap and bind to receptors on the receiving neuron, either exciting it to fire its own action potential or inhibiting it from doing so. This process converts the electrical signal of the action potential into a chemical signal and then back into an electrical one.
Evolution of Nervous System Complexity
The structure of the vertebrate nervous system is the result of a long evolutionary history. The trend has moved from the simple, decentralized nerve nets found in some invertebrates toward the highly organized, centralized system seen in vertebrates. This path reflects adaptations for more active lifestyles.
A defining development in this process is cephalization, the evolutionary trend of concentrating nervous tissue and sensory organs at the head end of an animal. This clustering of sensory structures allows an organism to better sense its environment as it moves forward. This concentration of nerve cells at the head end ultimately led to the formation of a distinct brain.