Nervous System: Nervous Tissue

The nervous system is a complex network of organs, tissues, and cells that coordinate and control the functions of the human body. It is responsible for receiving, processing, and responding to sensory input from the environment, as well as regulating and coordinating the body's internal functions. In this explanation, we will cover the major components and functions of the nervous system.

The nervous system is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS includes the brain and spinal cord, while the PNS consists of all the nerves that connect the CNS to the rest of the body.

The brain is the control center of the nervous system and is responsible for processing and integrating sensory input and generating appropriate motor responses. It is divided into three main regions: the forebrain, midbrain, and hindbrain. The forebrain includes the cerebral cortex, which is responsible for conscious thought, sensation, and voluntary movement, and the thalamus, which acts as a relay station for sensory information. The midbrain controls reflexes and eye movements, while the hindbrain includes the cerebellum, which coordinates movement and balance, and the brainstem, which controls vital functions such as breathing, heart rate, and blood pressure.

The spinal cord is a long, thin bundle of nerve fibers that runs from the base of the brain down through the center of the back. It serves as a conduit for sensory and motor information between the brain and the rest of the body. The spinal cord is also responsible for coordinating some reflexes.

The PNS is divided into two main branches: the somatic nervous system and the autonomic nervous system. The somatic nervous system controls voluntary movements and sensory input from the body's surface, while the autonomic nervous system regulates involuntary functions such as heart rate, digestion, and respiration.

The autonomic nervous system is further divided into two branches: the sympathetic and parasympathetic nervous systems. The sympathetic nervous system prepares the body for "fight or flight" responses in times of stress, while the parasympathetic nervous system promotes "rest and digest" activities during times of relaxation.

Nerves are the fundamental unit of the nervous system. They are long, thin fibers that transmit electrical impulses throughout the body. Sensory neurons carry information from the body's sensory receptors to the CNS, while motor neurons transmit information from the CNS to the muscles and glands.

The nervous system also contains a type of cell called a glial cell, or neuroglia. Glial cells support and protect neurons, and they also play a role in the formation and maintenance of the blood-brain barrier, which helps to protect the brain from toxins and pathogens.

Nervous Tissue

Nervous tissue is a specialized type of tissue that is found in the nervous system, which includes the brain, spinal cord, and nerves. It is composed of cells called neurons and supporting cells called neuroglia or glial cells. Nervous tissue is responsible for the coordination and control of all bodily functions, including sensation, movement, and thought.

Neurons are the primary cells of the nervous system, and they are responsible for transmitting and receiving electrical signals. They are composed of three main parts: the cell body (soma), dendrites, and axon. The cell body contains the nucleus and other organelles, and it is responsible for carrying out the metabolic functions of the neuron. Dendrites are short, branching projections that receive signals from other neurons or sensory cells. The axon is a long, slender projection that carries signals away from the cell body and transmits them to other neurons or muscles.  In the central nervous system, a bundle of neuron cell bodies is called a nucleus and a bundle of axons is called a tract.  In the peripheral nervous system, a bundle of neuron cell bodies is called a ganglion and a bundle of axons is called a nerve.

There are three types of neurons:

·         Sensory neurons are responsible for transmitting signals from sensory receptors, such as those found in the skin, to the spinal cord and brain.

·         Motor neurons transmit signals from the brain and spinal cord to muscles and glands, causing them to contract or secrete.

·         Interneurons connect sensory and motor neurons and are responsible for integrating information and coordinating responses.

Neurons are further classified by shape:

·         Multipolar neurons are the most common type of neurons and have multiple processes extending from their cell body. They have one axon and multiple dendrites. Multipolar neurons are typically found in the brain and spinal cord and are involved in processing and transmitting information.

·         Bipolar neurons have two processes extending from their cell body: one axon and one dendrite. They are found in specific sensory systems, such as the retina of the eye (for vision) and the olfactory epithelium (for smell). Bipolar neurons help relay sensory information from the environment to the brain.

·         Unipolar neurons, also known as pseudounipolar neurons, these neurons have a single process extending from the cell body that later branches into two: one axon and one dendrite. Unipolar neurons are primarily found in the peripheral nervous system and are involved in transmitting sensory information from the body to the spinal cord.

Neuroglia, or glial cells, are supporting cells that provide structural support and nutrients to neurons. There are several types of neuroglia, including astrocytes, oligodendrocytes, microglia, and ependymal cells.

·         Central Nervous Glial Cells

o    Astrocytes are star-shaped cells that provide structural support and regulate the chemical environment around neurons.

o    Oligodendrocytes produce myelin sheaths, a fatty substance that insulates the axons of neurons and increases the speed of electrical signals.

o    Microglia are immune cells that protect the nervous system from infection and injury.

o    Ependymal cells line the ventricles of the brain and spinal cord and produce cerebrospinal fluid, which provides nutrients and cushioning for the nervous system.

·         Peripheral Nervous Support Cells

o    Schwann cells are myelin producing cells that wrap axons of neurons in little packets of fatty tissue that have the appearance of jelly rolls.

o    Satellite cells cover the neuron cell body, or soma, supporting and protecting the cell.

The functions of nervous tissue are diverse and complex. The nervous system is responsible for sensation, perception, movement, thought, and emotion. It receives information from sensory receptors and processes it to produce appropriate responses. It also coordinates the functions of all the other systems of the body, including the cardiovascular, respiratory, digestive, and endocrine systems.

Action Potentials

Nervous tissue is an essential component of the human body that plays a crucial role in transmitting electrical signals throughout the body. These electrical signals, known as action potentials, are generated by specialized cells called neurons, which are responsible for transmitting information from one part of the body to another.

Action potentials are a form of electrochemical signaling that occur when there is a change in the electrical potential across the membrane of a neuron. When a neuron is at rest, the resting membrane potential is polarized, meaning there is a difference in electrical charge between the negative inside and the positive outside of the cell. This is maintained by the sodium-potassium pump, which pumps out three sodium (Na+) ions for every two potassium ions that are pumped in. As a result, the inside of the cell is negatively charged relative to the outside.

When a neuron is stimulated, either by a chemical signal or an electrical signal from another neuron, Na+ channels on the membrane open, allowing positively charged Na+ ions to enter the cell. This depolarizes the membrane, making the inside of the cell more positively charged relative to the outside. If the depolarization reaches a certain threshold, voltage-gated Na+ channels open, causing a rapid influx of Na+ ions into the cell, which further depolarizes the membrane. This rapid depolarization triggers an action potential, which is a brief, all-or-nothing electrical pulse that travels down the length of the neuron. The action potential is generated by the opening of voltage-gated ion channels, which allow ions to flow across the membrane in response to changes in electrical charge. Specifically, the opening of Na+ channels causes the rapid depolarization, while the opening of potassium channels allows positively charged potassium ions to leave the cell, which repolarizes the membrane.

Once an action potential is initiated or generated, it propagates or travels down the length of the neuron, from the dendrites (where it receives signals from other neurons) to the axon terminal (where it communicates with other neurons or effector cells such as muscles). During the signaling the cell enters a refractory phase. Refractory times are periods during which a neuron is less likely or unable to generate another action potential. Absolute refractory period is the time during which a neuron cannot generate another action potential, no matter how strong the stimulus is. This period occurs because the voltage-gated sodium channels, which are responsible for the depolarization phase of the action potential, are inactivated. The absolute refractory period ensures that action potentials only travel in one direction along the axon and prevents them from overlapping, which would make it difficult for the nervous system to interpret the signal. Relative refractory period is the time following the absolute refractory period when a neuron can generate another action potential, but only if the stimulus is stronger than usual. During the relative refractory period, the voltage-gated potassium channels are still open, causing the membrane potential to be more negative (hyperpolarized) than the resting potential. A stronger stimulus is required to overcome this hyperpolarization and reach the threshold for generating an action potential. The speed of the action potential is determined by the thickness of the axon and the presence of myelin, a fatty substance that insulates the axon and speeds up the transmission of the signal. Myelin plays a role in the propagation of action potentials by speeding the transmission of the signal up to 30 times faster.

Neurotransmitters

Neurotransmitters are chemical messengers that transmit signals across synapses in the nervous system. They play a critical role in controlling a wide range of physiological functions, including movement, sensation, emotion, and cognition. This article will provide an overview of neurotransmitter function and types. Neurotransmitters are released by neurons into the synaptic cleft, which is the small gap between two neurons. The neurotransmitter molecules then bind to receptors on the postsynaptic neuron, causing a change in the membrane potential of the cell. This change in membrane potential can result in an action potential, which is the electrical signal that allows neurons to communicate with each other. Neurotransmitters can have either excitatory or inhibitory effects on the postsynaptic neuron. Excitatory neurotransmitters cause depolarization of the membrane potential, making it more likely that the postsynaptic neuron will fire an action potential. Inhibitory neurotransmitters cause hyperpolarization of the membrane potential, making it less likely that the postsynaptic neuron will fire an action potential.

Types of Neurotransmitters

There are several different types of neurotransmitters, including, but not limited to:

·         Acetylcholine (ACh) is the primary neurotransmitter in the parasympathetic nervous system and is also involved in muscle contraction and memory formation. ACh was the first neurotransmitter discovered and is the most understood. Nicotinic and muscarinic receptors are two types of ACh receptors in the nervous system that bind to different neurotransmitters and play important roles in various physiological processes.

o Nicotinic receptors allow the influx of Na+ and calcium Ca2+ ions into the cell, leading to depolarization. They are named after nicotine because this compound specifically activates them. Nicotinic receptors are found in both CNS and PNS. Nicotinic receptors are involved in muscle contraction, synaptic transmission, and cognitive processes.

o Muscarinic receptors trigger intracellular signaling cascades involving G-proteins. They are named after muscarine, a compound derived from certain mushrooms that selectively activates these receptors. Muscarinic receptors are primarily found in the CNS, smooth muscles, and some glands. They are involved in regulating heart rate, smooth muscle contraction, glandular secretion, and neuronal signaling.

·         Dopamine is involved in the reward system and is associated with feelings of pleasure and motivation. It is also involved in movement and coordination.

·         Serotonin is involved in mood regulation and is associated with feelings of well-being and happiness. It is also involved in appetite, sleep, and sexual behavior.

·         Norepinephrine is involved in the sympathetic nervous system and is associated with the "fight or flight" response. It is also involved in attention and alertness.

·         GABA (Gamma-aminobutyric acid) is the primary inhibitory neurotransmitter in the brain and is involved in reducing anxiety and promoting relaxation.

·         Glutamate is the primary excitatory neurotransmitter in the brain and is involved in learning and memory.

·         Endorphins are neurotransmitters that are involved in pain relief and are released during exercise and other pleasurable activities.

Neurotransmitters play a crucial role in controlling many physiological functions in the body. There are several different types of neurotransmitters, each with their own specific functions and effects on the body.

Overview

The nervous system is a complex network of organs, tissues, and cells that coordinate and control the functions of the human body. It is divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord, while the PNS includes all the nerves that connect the CNS to the rest of the body. The brain is the control center of the nervous system and is responsible for processing sensory input and generating motor responses. The spinal cord serves as a conduit for sensory and motor information between the brain and the body.

The nervous system also includes nervous tissue, which is found in the brain, spinal cord, and nerves. Nervous tissue is composed of neurons and neuroglia (or glial cells). Neurons are responsible for transmitting and receiving electrical signals, and they consist of a cell body, dendrites, and an axon. There are three types of neurons: sensory neurons, motor neurons, and interneurons. Sensory neurons transmit signals from sensory receptors to the CNS, while motor neurons transmit signals from the CNS to muscles and glands. Interneurons connect sensory and motor neurons and integrate information.

Neuroglia, or glial cells, provide support and nutrients to neurons. There are different types of glial cells, including astrocytes, oligodendrocytes, microglia, and ependymal cells. Glial cells have various functions, such as providing structural support, regulating the chemical environment around neurons, producing myelin (which insulates axons and speeds up signal transmission), protecting the nervous system from infection and injury, and producing cerebrospinal fluid.

Action potentials are electrical signals generated by neurons. They occur when there is a change in the electrical potential across the neuron's membrane. Action potentials are initiated by depolarization, which is caused by the opening of Na+ channels and the influx of Na+ ions into the cell. This depolarization triggers a rapid influx of Na+ ions, resulting in an action potential. The action potential then travels along the neuron's axon to transmit the signal to other neurons or effector cells. The speed of the action potential is influenced by the axon's thickness and the presence of myelin.

Neurotransmitters are chemical messengers that transmit signals across synapses in the nervous system. They are released by neurons into the synaptic cleft and bind to receptors on the postsynaptic neuron, leading to changes in the membrane potential and the generation of action potentials. Neurotransmitters can have either excitatory or inhibitory effects on the postsynaptic neuron, influencing its likelihood of firing an action potential. There are several types of neurotransmitters, including acetylcholine, dopamine, serotonin, norepinephrine, GABA, glutamate, and endorphins, each with their own specific functions and effects on the body.

The nervous system, composed of the CNS, PNS, neurons, glial cells, action potentials, and neurotransmitters, plays a vital role in coordinating and controlling the functions of the human body, including sensation, movement, thought, and emotion. Understanding the structure and function of the nervous system is essential for understanding human anatomy and physiology.