Nervous System: Structure, Function, and Organization

• Astrocytes: Provide structural and metabolic support in the CNS.
• Microglia: Act as the resident macrophages (immune cells) within the brain's nervous tissue.
• Ependymal Cells: Line the cavities of the brain and spinal cord.
• Satellite Cells: Provide support within the PNS.

Exam Tip: If a question asks which cell acts as the "macrophage" of the brain, the answer is Microglia.

Myelination and Signal Speed

One of the most critical features of nervous tissue is the ability to transmit signals at high speeds. This is achieved through myelination, a process where a fatty insulating layer called a myelin sheath is wrapped around the axons of neurons.

The cells responsible for this insulation differ based on the location of the axon:

1. Oligodendrocytes: These cells form myelin sheaths around axons located within the Central Nervous System (CNS).
2. Schwann Cells: These cells perform the myelination function within the Peripheral Nervous System (PNS).

The importance of this insulation cannot be overstated. When myelin is damaged or lost, the efficiency of nerve impulse conduction drops significantly. A prime example of this is multiple sclerosis, a condition where the loss of myelin leads to impaired neurological function.

Communication at the Synapse

Neurons do not typically touch one another physically. Instead, they communicate across a specialized junction called a synapse.

In a chemical synapse, the process follows a specific sequence:

1. An electrical signal (nerve impulse) travels down the axon of the sending neuron.
2. This electrical signal triggers the release of chemical messengers (neurotransmitters) into the synaptic gap.
3. These chemicals cross the gap and bind to the next cell, triggering a new impulse or response.

A specific type of chemical synapse is the neuromuscular junction, where a neuron communicates with a muscle cell to initiate movement.

Structural Organization: Nerves and Connective Tissue

The nervous system is organized into distinct structures based on whether they reside in the CNS or the PNS. It is vital to distinguish between these terms for anatomical accuracy:

Finally, these bundles of axons (nerves) are not loose; they are highly organized and protected by various layers of connective tissue. This packaging ensures that the delicate axons are shielded from mechanical stress and maintain their structural integrity as they travel throughout the body.