12. Nervous Tissue-vid

Introduction to Nervous Tissue

• Neural tissue consists of two main cell types: neurons and glia.

• Found primarily in the central nervous system (CNS) and also in peripheral nervous system (PNS).

• Functions as a fast communication system, transmitting messages in the form of electrical signals.

Neurons

• Responsible for transmitting messages via electrical signals.

• Most mature neurons do not divide, so damage to neurons is often irreversible.

• Neuron sizes vary from less than 1 mm to up to 1 meter in length.

• Transmission speeds vary from 0.5 to 130 meters per second depending on neuron structure and size.

Structure of a Neuron

• Comprised of three main regions:

  • Receptive Region: Receives signals from other neurons or stimuli.

  • Conducting Region: Transmits information along the neuron.

  • Secretory Region: Releases neurotransmitters or signals to other cells.

• Contains a cell body, which houses the nucleus and numerous mitochondria for energy.

Dendrites and Axons

• Dendrites: Structures that receive information and pass signals towards the axon.

• Axon: Long part of the neuron responsible for carrying the electrical signal; ends in axon terminals that communicate with other cells.

Classification of Neurons

• Sensory Neurons (Afferent Neurons): Transmit signals to the CNS from sensory receptors.

  • Afferent pathway leads to the CNS.

• Motor Neurons (Efferent Neurons): Transmit impulses away from the CNS to effector organs (muscles or glands).

  • Efferent pathway leads away from the CNS.

• Interneurons: Act as intermediaries between sensory and motor neurons.

Glial Cells (Neuroglia)

• Support cells in the nervous system, providing nutrition and insulation (myelin) for neurons.

• CNS Neuroglia Types:

  • Astrocytes: Largest, support neurons physically, and maintain the blood-brain barrier.

  • Oligodendrocytes: Form myelin sheaths around axons in the CNS.

  • Microglial Cells: Act as immune cells, removing debris and pathogens.

  • Ependymal Cells: Line cavities in the CNS and help circulate cerebrospinal fluid.

• PNS Neuroglia Types:

  • Schwann Cells: Insulate axons and speed up signal transmission.

  • Satellite Cells: Surround neuron cell bodies, providing support and regulating material passage.

Myelin and Signal Conduction

• Myelination increases the speed of electrical signal transmission:

  • Nodes of Ranvier: Gaps between Schwann cells that allow signals to jump, increasing speed.

  • Oligodendrocytes in the CNS can associate with multiple axons.

Synapses

• Site for neuron communication, transferring signals from one neuron to another or to muscle/gland cells.

• Electrical Synapse: Fast transmission via gap junctions.

• Chemical Synapse: Involves neurotransmitter release to transfer signals across the synapse.

Membrane Potentials

• Neurons generate electrical changes across their membranes to transmit signals.

• Chemical Gradient: Difference in ion concentrations inside and outside the neuron.

• Electrical Gradient: Charge differences due to ions (negatively charged proteins inside vs positively charged outside).

• Resting Membrane Potential: Typically around -70 mV, indicating the negative internal charge compared to the outside.

Ion Movement and Resting Membrane Potential

• Sodium-potassium pump maintains ion concentrations to establish resting membrane potential.

• Changes in ion permeability lead to variations in membrane potential (depolarization, repolarization, hyperpolarization).

  • Depolarization: Cell becomes less negative (approaches -40 mV).

  • Repolarization: Returns towards -70 mV.

  • Hyperpolarization: Becomes more negative (e.g., -80 mV).

Summary

• Neural tissue consists of neurons and glia; neurons communicate via electrical signals and have a resting membrane potential.

• Changes in ion permeability affect membrane potential and facilitate signal transmission.

• Neurons communicate at synapses through electrical or chemical means.

Introduction to Nervous Tissue

Neural tissue is a specialized tissue that forms the basis of the nervous system, which includes the brain, spinal cord, and peripheral nerves. It consists primarily of two main cell types: neurons and glial cells (neuroglia). This tissue is found predominantly in the central nervous system (CNS) and also in the peripheral nervous system (PNS). Neural tissue functions as a rapid communication system, efficiently transmitting messages and signals throughout the body in the form of electrical impulses.

Neurons

Neurons are the fundamental units of the brain and nervous system, responsible for transmitting messages via electrical signals. They come in various shapes and sizes, which allows them to carry out a diverse range of functions. Most mature neurons do not have the ability to divide (they are considered terminally differentiated). As a result, damage to neurons can often lead to irreversible loss of function, which is a significant concern in various neurological conditions. Neuron sizes can vary dramatically, ranging from less than 1 millimeter for certain types of interneurons to up to 1 meter in length for motor neurons extending from the spinal cord to the extremities. The transmission speeds of electrical signals in neurons can vary significantly, ranging from 0.5 meters per second in smaller unmyelinated fibers to as fast as 130 meters per second in larger myelinated fibers, depending on their structure and myelination.

Structure of a Neuron

A neuron comprises three main regions, each serving essential roles in signal transmission:

• Receptive Region: This area, primarily made up of dendrites, is where signals from other neurons or stimuli are received and processed.

• Conducting Region: The axon serves as the conducting region, transmitting electrical impulses away from the cell body to other neurons, muscles, or glands.

• Secretory Region: Located at the axon terminals, this region releases neurotransmitters or signaling molecules, which are essential for neuronal communication. The neuron also contains a cell body (soma), which houses the nucleus and organelles, including numerous mitochondria for energy production.

Dendrites and Axons

• Dendrites: These are branched structures that extend from the neuron’s cell body, resembling tree branches. They receive information from other neurons and convey these signals toward the axon.

• Axon: The axon is a long, slender projection that conducts electrical signals away from the neuron’s cell body. It terminates in axon terminals, which form synapses with other neurons or effector cells. The myelination of axons significantly increases the speed of signal transmission.

Classification of Neurons

Neurons can be classified based on their function:

• Sensory Neurons (Afferent Neurons): These neurons transmit sensory information from sensory receptors to the CNS. They are critical for relaying information about environmental stimuli, such as light, sound, and touch.

• Motor Neurons (Efferent Neurons): These neurons transmit impulses away from the CNS to effector organs (muscles or glands), directing actions based on CNS processing, such as movement or secretion.

• Interneurons: These neurons process information within the CNS and act as intermediaries between sensory and motor neurons. They play essential roles in reflexes and neural circuits.

Glial Cells (Neuroglia)

Glial cells, or neuroglia, are support cells in the nervous system that provide various functions critical to neuronal health and operation. They outnumber neurons in the CNS and are integral in maintaining homeostasis in the nervous system.

Types of CNS Neuroglia:

• Astrocytes: The largest type of glial cell, astrocytes support neurons physically, regulate ion concentrations in the extracellular environment, and maintain the blood-brain barrier, a selective permeability barrier that protects the brain from harmful substances in the bloodstream.

• Oligodendrocytes: These cells form myelin sheaths around axons in the CNS, which is vital for speeding up signal transmission.

• Microglial Cells: Acting as immune cells of the CNS, microglial cells are responsible for removing debris, dead cells, and pathogens through phagocytosis, thus playing an essential protective role.

• Ependymal Cells: These cells line the ventricles of the brain and spinal cord, playing a role in producing and circulating cerebrospinal fluid (CSF), which cushions the brain and spinal cord and facilitates nutrient exchange.

Types of PNS Neuroglia:

• Schwann Cells: These cells wrap around axons in the PNS to form myelin sheaths and enhance the speed of electrical signal conduction.

• Satellite Cells: Surrounding neuron cell bodies in the PNS, satellite cells provide structural support and regulate the exchange of materials between neurons and their environment.

Myelin and Signal Conduction

Myelination is a process that increases the speed and efficiency of electrical signal transmission in neurons. It involves the wrapping of axons in myelin sheaths, which are formed by oligodendrocytes in the CNS and Schwann cells in the PNS. When an action potential travels along a myelinated axon, it jumps between the nodes of Ranvier (gaps in the myelin sheath), thereby speeding up signal transmission significantly relative to non-myelinated axons.

Synapses

Synapses are the sites where neuron communication occurs. They can transfer signals from one neuron to another or from a neuron to a muscle or gland cell. There are two main types of synapses:

• Electrical Synapse: Characterized by fast transmission via gap junctions that allow for direct electrical communication between neurons.

• Chemical Synapse: These synapses involve the release of neurotransmitters from the presynaptic neuron, which diffuse across the synaptic cleft to bind to receptors on the postsynaptic neuron, allowing for a more versatile form of intercellular communication.

Membrane Potentials

Neurons generate electrical changes across their membranes to facilitate signal transmission. Two main factors contribute to membrane potential:

• Chemical Gradient: This refers to the difference in ion concentrations inside and outside the neuron, primarily involving sodium (Na+) and potassium (K+) ions.

• Electrical Gradient: This describes the charge differences due to ions, particularly the negatively charged proteins that reside inside the neuron compared to the positively charged ions outside.

The Resting Membrane Potential of a neuron is typically around -70 mV, indicating a negative internal charge relative to the outside. The maintenance of this potential is vital for neuronal function and is primarily controlled by the sodium-potassium pump, which actively transports Na+ out of and K+ into the cell.

Ion Movement and Resting Membrane Potential

Variations in ion permeability lead to shifts in membrane potential:

• Depolarization: When the cell membrane becomes less negative (approaching -40 mV), which can trigger action potentials if a certain threshold is met.

• Repolarization: Following depolarization, the membrane potential returns toward -70 mV, restoring the resting state.

• Hyperpolarization: This occurs when the membrane potential becomes more negative (e.g., -80 mV), making the neuron less likely to fire an action potential.

Summary

In conclusion, neural tissue plays a crucial role in the functioning of the nervous system, consisting mainly of neurons and supportive glial cells. Neurons facilitate communication via electrical signals and maintain a resting membrane potential that allows for the rapid transmission of impulses. Changes in ion permeability are essential for facilitating signal propagation and ultimately enable neurons to communicate effectively at synapses, whether through electrical or chemical means. Understanding the intricacies of neural tissue is foundational for comprehending both normal physiological functions and the pathophysiology of neurological disorders.