Nervous System Structure: Neurons, Glia, and Brain Anatomy
Types of Neurons
Depending on the number of extensions, neurons are classified into:
- Monopolar: Possess only one axon.
- Bipolar: Possess one axon and dendrites.
- Multipolar: Possess more than one axon and dendrite.
Glia Cells
Glia cells are the most abundant cells in the nervous system. Their primary functions are to support, protect, and nourish neurons. There are five main types:
Central Nervous System (CNS) Glia
- Astrocytes: Provide structural support and nutrients to neurons.
- Microglia: Act as macrophages to remove dead cell debris.
- Oligodendrocytes: Produce myelin sheaths covering CNS axons.
- Ependymal cells: Line the brain ventricles and spinal canal; they produce cerebrospinal fluid.
Peripheral Nervous System (PNS) Glia
- Schwann cells: Produce the myelin sheaths for axons in the peripheral nervous system.
Organization of the Nervous System
The nervous system consists of two primary divisions: the central nervous system (CNS) and the peripheral nervous system (PNS).
The Central Nervous System
The CNS consists of the brain and spinal cord, protected by the skull and spine, respectively. Both are surrounded by three membranes known as the meninges (from outside to inside: dura mater, arachnoid, and pia mater).
The brain and spinal cord are cushioned by cerebrospinal fluid, which circulates between the arachnoid and pia mater. This fluid protects, nourishes, and removes waste from neural structures.
Brain Anatomy
The brain is composed of three main structures: the cerebrum, cerebellum, and brainstem.
The Cerebrum
The cerebrum is the most voluminous part of the brain. It consists of two hemispheres separated by a central fissure. The outer layer, the cortex, is gray due to a dense concentration of cell bodies. The inner region is white matter, consisting primarily of myelinated axons.