Nervous System: Structure, Function, and Physiology
The Spinal Cord
Location: Within the vertebral foramen (spinal canal) at the back middle of the body. It extends from the foramen magnum, connecting with the brainstem, to the lumbar region. It measures approximately 40-45 cm in length.
Anatomy: 31 pairs of spinal nerves emerge from the spinal cord: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. It’s composed of gray matter and white matter. The central canal, lined with ependymal cells, runs through the spinal cord.
Gray Matter: Forms an H-shape in cross-section, consisting of two symmetrical, crescent-shaped halves connected by a central region. It contains neuronal cell bodies and is organized into horns: two dorsal, two ventral, and two intermediate.
- Dorsal Horns: Contain neurons involved in processing sensory information.
- Ventral Horns: Contain motor neurons that control skeletal muscles.
- Intermediate Horns: Contain neurons of the autonomic nervous system.
White Matter: Surrounds the gray matter and consists of myelinated nerve fibers, forming ascending and descending tracts.
- Ascending Tracts: Carry sensory impulses from the skin, muscles, and joints to the brain.
- Descending Tracts: Carry motor impulses from the brain to muscles and glands.
The Brain
The brain is composed of the cerebrum, cerebellum, and brainstem (medulla oblongata, pons, and midbrain).
Protective Structures
- Skull
- Meninges: Three protective membranes: dura mater, arachnoid mater, and pia mater.
- Dura Mater Extensions:
- Falx cerebri: Separates the two cerebral hemispheres.
- Falx cerebelli: Separates the two cerebellar hemispheres.
- Tentorium cerebelli: Separates the cerebrum from the cerebellum.
- Cerebrospinal Fluid
Brainstem
The brainstem consists of the medulla oblongata, pons, and midbrain. It contains the reticular formation, a network of white and gray matter.
Medulla Oblongata
The medulla is continuous with the spinal cord and forms the lower part of the brainstem. It extends from the foramen magnum to the lower edge of the pons (about 3 cm). It contains ascending sensory and descending motor tracts, as well as nuclei that regulate vital functions like the cardiovascular and respiratory centers. It also houses nuclei for cranial nerves VIII, IX, X, XI, and XII.
Pons
Located above the medulla and in front of the cerebellum, the pons measures about 2.5 cm in length. It connects the medulla, cerebrum, cerebellum, and the two cerebellar hemispheres. It contains ascending and descending tracts and nuclei for cranial nerves V, VI, VII, and VIII.
Midbrain (Mesencephalon)
Extending from the pons to the diencephalon, the midbrain measures about 2.5 cm. The cerebral aqueduct connects the third and fourth ventricles. It contains superior and inferior colliculi, substantia nigra, red nuclei, and nuclei for cranial nerves III and IV.
Cerebellum
Location: Occupies the back and bottom of the cranial cavity, behind the medulla and pons.
Structure: Butterfly-shaped with a central vermis and two lateral cerebellar hemispheres. The cerebellar hemispheres are divided into lobes by deep fissures. It has three pairs of cerebellar peduncles that connect it to the brainstem.
- Cerebellar Cortex: Composed of thin, parallel folds of gray matter (folia).
- White Matter: Forms the “tree of life” within the cerebellar cortex.
- Cerebellar Nuclei: Gray matter regions from which nerve fibers carry impulses from the cerebellum to other brain regions.
Functions: Coordinates movement, maintains balance, and may also play a role in cognitive functions and language processing.
Nervous Tissue
Nervous tissue contains two main cell types:
- Neurons: The functional units of the nervous system.
- Glial Cells: Provide support, protection, and nutrition to neurons.
Neuron Structure: Consists of a cell body and extensions (axon and dendrites).
Myelination: Myelin sheath is a lipid and protein covering around many axons, interrupted by Nodes of Ranvier. It increases the speed of nerve impulse conduction.
Synapses: Junctions between neurons, enabling communication. Can be electrical or chemical.
Physiological Properties of Neurons: Excitability and conductivity.
Action Potential: A rapid change in membrane potential that propagates along the axon.
Resting Membrane Potential: The electrical potential difference across the neuron’s membrane at rest.