Vertebrate Nervous System: Structure, Function, and Integration
Vertebrate Nervous System: Structure and Function
The vertebrate nervous system (VNS) is a highly evolved system responsible for coordinating and controlling bodily functions. Key features include a dorsal location, an enlarged anterior region (encephalon), and an elongated posterior region (spinal cord).
Central Nervous System (CNS)
The CNS is divided into two main parts:
- Encephalon (brain)
- Spinal Cord
Peripheral Nervous System (PNS)
The PNS is further divided into:
- Somatic Nervous System (spinal and cranial nerves)
- Autonomic Nervous System (sympathetic and parasympathetic)
CNS Protection and Composition
The CNS is protected by:
- Bone: Skull and vertebral column
- Membranes: Meninges
The CNS contains two types of tissue:
- White matter: Myelinated axons
- Gray matter: Cell bodies and dendrites
Encephalon (Brain)
The encephalon features interconnected internal cavities called cerebral ventricles. During development, the primary brain forms three vesicles: hindbrain, midbrain, and forebrain. The midbrain divides into five secondary vesicles, which develop into definite structures. The forebrain integrates major functions and divides into:
- Telencephalon: Includes olfactory lobes and the cerebrum. In birds and mammals, the cerebrum is large and highly convoluted (gyri and sulci).
The telencephalon centralizes and controls movement, volition, memory, and intelligence.
Spinal Cord
The spinal cord has a narrow central cavity (ependyma) containing cerebrospinal fluid. Neuron cell bodies cluster internally (gray matter), while axons form the surrounding white matter. The gray matter is butterfly-shaped and has horns (astas). Functions include transmitting impulses and controlling reflex activities.
Peripheral Nervous System (PNS) Details
The PNS connects receptors and effectors with the CNS. It consists of afferent nerves (from receptors to the CNS) and efferent nerves (from the CNS to effectors). The PNS is divided into the somatic and autonomic nervous systems.
Somatic Nervous System
Motor neurons in the somatic system innervate skeletal muscles, which are under voluntary control. The somatic system includes:
- 10 or 12 pairs of cranial nerves in the head and upper trunk
- 31 pairs of spinal nerves that exit the spinal cord and are involved in reflex actions
Autonomic Nervous System
The autonomic nervous system regulates involuntary activities. The brain has no direct control over these functions. It consists of preganglionic fibers that synapse before the postganglionic fibers. The pre- and postganglionic synapses are unmyelinated. The autonomic nervous system is composed of receptors, sensory fibers, and motor effectors and is divided into:
- Sympathetic Nervous System: Fibers originate from the spinal cord, with ganglia near the spinal cord. Postganglionic fibers are long.
- Parasympathetic Nervous System: Fibers originate from the brain and terminate near or within effector organs. Preganglionic fibers are long, and postganglionic fibers are short.
Nervous System Integration
The simplest mechanism that produces a specific response to a stimulus is the reflex arc. The effect of a reflex arc’s activity is the reflex action. Key components include:
- Spinal Cord: Conductor and center for reflex actions.
- Medulla Oblongata: Conductor and center for reflexes related to breathing, heart activity, blood pressure, and mastication.
- Cerebellum: Coordinates muscle tone, posture, and equilibrium.
- Thalamus: Processes sensory information and contributes to consciousness.
- Hypothalamus: Regulates body temperature, hunger, fluid volume, and hormonal control through connections with the pituitary gland.
- Cerebrum: The association center where information is processed and integrated before an appropriate response occurs. It is the center for higher functions, including intelligence, memory, and volition.
Key areas of the cerebrum include:
- Primary Motor Area: Issues commands for voluntary movement.
- Secondary Motor Area: Stores memories of learned movements.
- Elementary Sensory Area: Receives sensations.
- Secondary Sensory Area: Interprets sensations.