12.2 Nervous Tissue

Neurons and neuroglia are the building blocks of the nervous system. Neurons transmit electrical signals, while glial cells support and protect them. Together, they form a complex network that enables our bodies to sense, think, and respond to stimuli.

Understanding the structure and function of neurons and neuroglia is crucial for grasping how the nervous system works. From the specialized parts of neurons to the diverse roles of glial cells, these components work in harmony to maintain brain health and function.

Neurons and Neuroglia

Structure and components of neurons

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• Cell body (soma) contains the nucleus and organelles such as Nissl bodies (rough endoplasmic reticulum) that synthesize proteins needed for neuron function
• Dendrites are branching extensions covered in dendritic spines that receive signals from other neurons and transmit them towards the cell body
• Axon is a long, thin projection that transmits electrical signals away from the cell body to other neurons
  • Axon hillock is the initial segment where action potentials are generated (trigger zone)
  • Axon terminals are the end of the axon that forms synapses with other neurons to release neurotransmitters
• Myelin sheath is an insulating layer around the axon formed by Schwann cells (PNS) or oligodendrocytes (CNS) that increases the speed of signal transmission
  • Nodes of Ranvier are gaps in the myelin sheath that allow for saltatory conduction, enabling faster signal propagation along the axon

Types of neurons by polarity and function

• Unipolar neurons have a single process extending from the cell body and are primarily sensory neurons in the peripheral nervous system (dorsal root ganglia)
• Bipolar neurons have two processes extending from the cell body and are specialized sensory neurons (retinal cells, olfactory neurons)
• Multipolar neurons are the most common type in the nervous system with multiple dendrites and a single axon, including motor neurons and interneurons
• Sensory (afferent) neurons transmit sensory information from receptors (touch, pain) to the central nervous system for processing
• Motor (efferent) neurons transmit signals from the central nervous system to effectors (muscles, glands) to initiate a response
• Interneurons connect sensory and motor neurons within the central nervous system to process, integrate, and modulate information

Roles of glial cells in nervous systems

• Astrocytes (CNS) provide structural support, maintain the blood-brain barrier, and regulate neurotransmitter levels and ion concentrations in the synaptic cleft
• Oligodendrocytes (CNS) form the myelin sheath around axons in the central nervous system to facilitate rapid signal transmission
• Microglia (CNS) are the immune cells of the central nervous system that phagocytose debris and pathogens to protect neurons
• Ependymal cells (CNS) line the ventricles and central canal of the spinal cord and produce and circulate cerebrospinal fluid
• Schwann cells (PNS) form the myelin sheath around axons in the peripheral nervous system to facilitate rapid signal transmission
• Satellite cells (PNS) surround and support neuron cell bodies in ganglia to maintain the local environment
• Glial cells play a crucial role in supporting neuroplasticity by regulating the extracellular environment and promoting synaptic remodeling

Neuroglia in CNS vs PNS

• Central Nervous System (CNS)
  1. Astrocytes provide structural support, maintain the blood-brain barrier, and regulate neurotransmitters and ions
  2. Oligodendrocytes form the myelin sheath around axons
  3. Microglia are the immune cells that phagocytose debris and pathogens
  4. Ependymal cells line the ventricles and central canal and produce cerebrospinal fluid
• Peripheral Nervous System (PNS)
  1. Schwann cells form the myelin sheath around axons
  2. Satellite cells surround and support neuron cell bodies in ganglia
• Similarities between CNS and PNS neuroglia include cells that form myelin sheath (oligodendrocytes, Schwann cells) and cells that provide support and protection to neurons (astrocytes, satellite cells)
• Differences include the presence of microglia and ependymal cells in the CNS, which are not found in the PNS, and satellite cells in the PNS, which are not present in the CNS

Neuronal Communication

• Neurons communicate through chemical and electrical synapses, with chemical synapses being more common
• Synapses are specialized junctions where neurons transmit signals to target cells (neurons, muscle cells, or gland cells)
• Neurotransmitters are chemical messengers released from presynaptic neurons that bind to receptors on postsynaptic cells
• The membrane potential of a neuron is the difference in electrical charge between the inside and outside of the cell, which is crucial for signal transmission
• Neurogenesis, the formation of new neurons, occurs in specific regions of the adult brain and contributes to learning, memory, and brain repair