*Week 2: Neurons and Neuroglia

The nervous system is a complex network controlling body functions, consisting mainly of neurons and neuroglia.
Neurons: Primary functional units transmitting electrical and chemical signals.
Neuroglia (Glial Cells): Support neurons, outnumbering them and ensuring the nervous system's well-being.

Parts of a Neuron:
- Cell Body: Central part performing biosynthetic processes.
- Dendrites: Receive signals toward the cell body.
- Axon: Conducts signals away from the cell body; often wrapped in a myelin sheath to enhance conduction speed.
- Astrocytes: Type of neuroglia aiding in blood-brain barrier formation.

Conspicuous and metabolically active part of a neuron with:
- A single nucleus and prominent nucleolus.
- Abundant ribosomes and rough endoplasmic reticulum, called Nissl granules.
Amitotic; lacks centrioles required for cell division.

Short, branched extensions that resemble tree branches.
Receive electrical impulses from other neurons toward the cell body.
Contain organelles similar to the cell body, boosting surface area for reception.

A single long process conducting signals away from the soma.
Arises from the axon hillock and tapers to form thin axon, often insulated by myelin.
May have branches called axon collaterals and terminal ends known as telodendria.

Based on processes extending from soma:
1. Multipolar Neurons: One axon, multiple dendrites; most common type found in spinal cord and brain.
2. Bipolar Neurons: One axon, one dendrite; found in sensory organs (olfactory cells, retina).
3. Unipolar Neurons: One process that is a merged axon and dendrite, typically transmitting sensory information to the CNS.
4. Anaxonic Neurons: Lack distinct axon and dendrites; have a single thick, branched dendrite. Found in the brainstem and spinal cord.

Neurons classified into three functional groups:
1. Sensory Neurons (Afferent Neurons): Detect stimuli (heat, pressure) and send info to CNS; often pseudounipolar or bipolar.
2. Motor Neurons (Efferent Neurons): Carry messages from CNS to effectors (muscles, glands); primarily multipolar.
3. Interneurons (Association Neurons): Located in CNS, connect sensory and motor neurons; typically multipolar.

Role: Support, nourish, and protect neurons; non-conductive, retain ability to divide, fill gaps from dead neurons.
Types of Neuroglia in CNS:
- Astrocytes: Most abundant, maintain blood-brain barrier, regulate extracellular fluid composition, often rapidly divide during injury (risk of tumors).
- Oligodendrocytes: Form myelin sheath in CNS.
- Microglial Cells: Immune response, act as macrophages to eliminate pathogens and debris.
- Ependymal Cells: Line brain ventricles and spinal canal, produce cerebrospinal fluid (CSF).

Myelin Sheath: Insulating layer around nerve fibers, composed of 20% protein and 80% lipid.
Formation: Myelination starts in fetal development and completes in adolescence.
Schwann Cells: Produce myelin sheath in PNS, essential for regeneration of damaged axons if the cell body remains intact.
Satellite Cells: Regulate extracellular environment around neuronal cell bodies, similar to astrocytes in CNS.

Neurons and neuroglia collaborate to ensure proper nervous system functioning:
- Neurons: Transmit signals for perception and response.
- Neuroglial Cells: Provide crucial support, protection, and maintenance of the neural environment.
Key Types of Neurons:
- Multipolar Neurons: Most common types; involved in many functions.
- Regenerative Capacities: Neurons in PNS can regenerate if the cell body is intact, unlike those in CNS.
Other essential roles:
- Astrocytes: Form blood-brain barrier, potentially leading to tumors when rapidly dividing.
- Ciliated Ependymal Cells: Circulate CSF.