The Human Nervous System: Structure, Function, and Organization
The Nervous System
Introduction
The nervous system, a complex network of specialized tissue, allows for communication and coordination within the body. It responds to internal and external environmental changes, integrating activities across different body parts. This system emerges around the third week of embryonic development, originating from the ectoderm.
Divisions of the Nervous System
The nervous system is divided for educational purposes into several categories based on anatomical, embryological, and functional criteria. These divisions help us understand its complex organization.
Anatomical Division
- Central Nervous System (CNS): Located within the axial skeleton, encompassing the cranial cavity and vertebral canal.
- Peripheral Nervous System (PNS): Located outside the axial skeleton.
Central Nervous System
- Brain: Encased within the skull, the brain consists of the cerebrum, cerebellum, and brainstem (midbrain, pons, and medulla oblongata).
- Spinal Cord: Housed within the spinal canal.
Peripheral Nervous System
- Nerves: Whitish cords connecting the CNS to peripheral organs. These are classified as cranial nerves (connected to the brain) and spinal nerves (connected to the spinal cord).
- Ganglia: Dilations containing nerve cell bodies. These can be sensory or visceral motor ganglia.
- Nerve Endings: Located at the end of nerve fibers, these can be sensory (afferent) or motor (efferent).
Functional Division
- Somatic Nervous System: Connects the organism to its environment, controlling both conscious and unconscious afferent information and the motor control of skeletal muscle.
- Autonomic Nervous System: Manages inputs from the viscera and controls smooth and cardiac muscle, as well as glands.
Somatic Nervous System
- Afferent Component: Carries impulses from peripheral receptors to the CNS, providing information about the environment.
- Efferent Component: Carries information from nerve centers to skeletal muscles, enabling voluntary movements.
Visceral Nervous System
- Functions: Integrates organ function and maintains the internal environment.
- Components:
- Afferent Component: Conducts impulses from receptors in internal organs (visceroceptors) to specific areas of the nervous system.
- Efferent Component (Autonomic Nervous System): Carries impulses from nerve centers to the viscera. It is further divided into the sympathetic and parasympathetic systems.
Segmental Division
- Segmental Nervous System: Includes the PNS and parts of the CNS related to spinal nerves, such as the spinal cord and brainstem (excluding the cortex). Gray matter is located inside the white matter.
- Suprasegmental Nervous System: Includes the cerebrum and cerebellum. Gray matter is located on the outside, surrounding the white matter.
Morphofunctional Organization
The neuraxis, the longitudinal axis of the brain and spinal cord, has a T-shape. The vertical portion forms the spinal cord and brainstem, while the horizontal portion extends from the occipital pole to the front of the brain.
The CNS has two main types of tissue:
- Gray Matter: Contains neuronal cell bodies, dendrites, axon terminals, synapses, and glial cells. It is richly vascularized.
- White Matter: Primarily composed of myelinated axons (giving it a whitish color) and oligodendrocytes. It contains no neuronal cell bodies and has less vascularization.
Gray matter can form a cortex (e.g., cerebral cortex, cerebellar cortex) or other structures like nuclei, bodies, plates, horns, or formations.
White matter forms tracts, bundles, arms, lemnisci, peduncles, angles, loops, or capsules. A commissure is a set of nerve fibers crossing the midline perpendicular to the neuraxis.
Neuronal Organization
- Sensory Neurons: Located in sensory ganglia, these neurons carry impulses from receptors (skin and viscera) to the spinal cord and brainstem. They connect to motor neurons via central extensions (simple reflex arc) or association neurons.
- Motor Neurons: Somatic and visceral motor neurons carry impulses to muscles or glands, forming monosynaptic or polysynaptic reflex arcs.
Pathways
- Ascending (Sensory) Pathways: Carry information from receptors to higher centers for conscious or unconscious processing.
- Descending (Motor) Pathways: Carry motor information to effector organs (muscles, glands).