3.4 Nervous tissue

Nervous tissue is the star of the nervous system, allowing our bodies to react and communicate at lightning speed. It's made up of neurons that send signals and glial cells that support them, working together to keep us functioning.

This tissue is crucial for processing information from our senses and controlling our movements. It's found in our brain, spinal cord, and nerves, forming a complex network that helps us think, feel, and respond to the world around us.

Nervous Tissue Characteristics and Functions

General Characteristics of Nervous Tissue

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• Nervous tissue is one of the four main tissue types in the body specializes in the conduction of electrical impulses and communication between different parts of the body
• Nervous tissue is primarily composed of two cell types: neurons transmit electrical and chemical signals, and neuroglia provide support and protection for the neurons
• Nervous tissue is found in the brain, spinal cord, and peripheral nerves, forming a complex network allows for rapid communication and coordination throughout the body

Functions of Nervous Tissue

• The main functions of nervous tissue include receiving sensory input, integrating and processing information, and generating appropriate motor output to effectors (muscles and glands)
• Nervous tissue facilitates rapid communication and coordination between different parts of the body, enabling complex behaviors and responses to stimuli
• Nervous tissue plays a crucial role in maintaining homeostasis by regulating various physiological processes (heart rate, breathing, and digestion)

Nervous Tissue Cell Types and Roles

Neurons: The Functional Units

• Neurons are the primary functional units of the nervous system responsible for receiving, processing, and transmitting electrical and chemical signals
• Neurons communicate with each other and with effectors through specialized junctions called synapses, which can be electrical or chemical (chemical synapses being more common)
• Neurons can be classified based on their structure (unipolar, bipolar, or multipolar) and function (sensory, motor, or interneurons)

Neuroglia: The Supportive Cells

• Neuroglia, also known as glial cells, are non-neuronal cells provide support, protection, and maintenance for neurons in the nervous system
• The main types of neuroglia in the central nervous system include astrocytes, oligodendrocytes, microglia, and ependymal cells
• In the peripheral nervous system, the main types of neuroglia are Schwann cells and satellite cells
• Astrocytes are star-shaped cells provide structural support, regulate the extracellular environment, and assist in the formation of the blood-brain barrier
• Oligodendrocytes and Schwann cells produce myelin, an insulating substance wraps around axons to facilitate rapid signal transmission and protect the axons

Neuron Structure and Function

Neuron Components

• Neurons are highly specialized cells consisting of a cell body (soma), dendrites, and an axon
  • The soma contains the nucleus and other organelles necessary for cellular function and protein synthesis
  • Dendrites are short, branched extensions receive incoming signals from other neurons
  • The axon is a long, thin extension conducts electrical signals away from the soma to other neurons or effectors
• The axon hillock is the junction between the soma and the axon, where the electrical signal, called an action potential, is generated

Signal Transmission in Neurons

• Axons are often covered by a myelin sheath, which is produced by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system
• Myelin insulates the axon and allows for faster signal transmission through saltatory conduction, where the action potential jumps from one node of Ranvier to the next
• In a chemical synapse, the presynaptic neuron releases neurotransmitters (glutamate, GABA, dopamine) into the synaptic cleft, which then bind to receptors on the postsynaptic cell, triggering a response

Neuroglia in Nervous Tissue Support

Astrocytes and the Blood-Brain Barrier

• Astrocytes are the most abundant type of neuroglia in the central nervous system provide structural support by forming a scaffolding for neurons and blood vessels
• Astrocytes regulate the extracellular environment by taking up excess neurotransmitters and ions (potassium)
• Astrocytes play a crucial role in the formation and maintenance of the blood-brain barrier selectively permits the passage of substances between the blood and the brain

Myelination by Oligodendrocytes and Schwann Cells

• Oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system produce myelin wraps around axons in segments called internodes
• Myelin acts as an insulator, allowing for rapid and efficient signal transmission through saltatory conduction
• Myelination increases the speed of action potential propagation (up to 100 m/s) compared to unmyelinated axons (0.5-2 m/s)

Microglia: The Immune Cells of the CNS

• Microglia are the immune cells of the central nervous system constantly survey their environment for signs of damage, infection, or cellular debris
• Upon detection, microglia become activated and phagocytose the offending substances (bacteria, damaged cells), protecting the nervous tissue from harm
• Microglia also release cytokines and other signaling molecules to modulate the immune response and promote tissue repair

Ependymal Cells and Cerebrospinal Fluid

• Ependymal cells line the ventricles of the brain and the central canal of the spinal cord have cilia help circulate cerebrospinal fluid
• Cerebrospinal fluid provides mechanical protection and chemical support for the brain and spinal cord
• Ependymal cells play a role in regulating the composition of cerebrospinal fluid and facilitating communication between the cerebrospinal fluid and the interstitial fluid of the brain