Action potential in the context of Efferent nerve fiber

In biology, the nervous system is the highly complex part of an animal that coordinates its actions and sensory information by transmitting signals to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the endocrine system to respond to such events. Nervous tissue first arose in wormlike organisms about 550 to 600 million years ago. In vertebrates, it consists of two main parts, the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The PNS consists mainly of nerves, which are enclosed bundles of the long fibers, or axons, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called motor nerves (efferent), while those nerves that transmit information from the body to the CNS are called sensory nerves (afferent). The PNS is divided into two separate subsystems, the somatic and autonomic nervous systems. The autonomic nervous system is further subdivided into the sympathetic, parasympathetic and enteric nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the gastrointestinal system. Nerves that exit from the brain are called cranial nerves while those exiting from the spinal cord are called spinal nerves.

The nervous system consists of nervous tissue which, at a cellular level, is defined by the presence of a special type of cell, called the neuron. Neurons have special structures that allow them to send signals rapidly and precisely to other cells. They send these signals in the form of electrochemical impulses traveling along thin fibers called axons, which can be directly transmitted to neighboring cells through electrical synapses or cause chemicals called neurotransmitters to be released at chemical synapses. A cell that receives a synaptic signal from a neuron may be excited, inhibited, or otherwise modulated. The connections between neurons can form neural pathways, neural circuits, and larger networks that generate an organism's perception of the world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called glial cells (or simply glia), which provide structural and metabolic support. Many of the cells and vasculature channels within the nervous system make up the neurovascular unit, which regulates cerebral blood flow in order to rapidly satisfy the high energy demands of activated neurons.

White matter refers to areas of the central nervous system that are mainly made up of myelinated axons, also called tracts. Long thought to be passive tissue, white matter affects learning and brain functions, modulating the distribution of action potentials, acting as a relay and coordinating communication between different brain regions.

White matter is named for its relatively light appearance resulting from the lipid content of myelin. Its white color in prepared specimens is due to its usual preservation in formaldehyde. It appears pinkish-white to the naked eye otherwise, because myelin is composed largely of lipid tissue veined with capillaries.

Sensory neurons, also known as afferent neurons, are in the nervous system which convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transduction. The cell bodies of the sensory neurons are located in the dorsal root ganglia of the spinal cord.

The sensory information travels on the afferent nerve fibers in a sensory nerve, to the brain via the spinal cord. Spinal nerves transmit external sensations via sensory nerves to the brain through the spinal cord. The stimulus can come from exteroreceptors outside the body, or interoreceptors inside the body.

A mechanoreceptor, also called mechanoceptor, is a sensory receptor that responds to mechanical pressure or distortion. Mechanoreceptors are located on sensory neurons that convert mechanical pressure into electrical signals that, in animals, are sent to the central nervous system.

A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance (endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemoreceptor is a neuron, or in the form of a neurotransmitter that can activate a nerve fiber if the chemoreceptor is a specialized cell, such as taste receptors, or an internal peripheral chemoreceptor, such as the carotid bodies. In physiology, a chemoreceptor detects changes in the normal environment, such as an increase in blood levels of carbon dioxide (hypercapnia) or a decrease in blood levels of oxygen (hypoxia), and transmits that information to the central nervous system which engages body responses to restore homeostasis.

In bacteria, chemoreceptors are essential in the mediation of chemotaxis.

Stimulation is the encouragement of development or the cause of activity in general. For example, "The press provides stimulation of political discourse." An interesting or fun activity can be described as "stimulating", regardless of its physical effects on senses. Stimulate means to act as a stimulus to; stimulus means something that rouses the recipient to activity; stimuli is the plural of stimulus.

A particular use of the term is physiological stimulation, which refers to sensory excitation, the action of various agents or forms of energy (stimuli) on receptors that generate impulses that travel through nerves to the brain (afferents). There are sensory receptors on or near the surface of the body, such as photoreceptors in the retina of the eye, hair cells in the cochlea of the ear, touch receptors in the skin and chemical receptors in the mouth and nasal cavity. There are also sensory receptors in the muscles, joints, digestive tract, and membranes around organs such as the brain, the abdominal cavity, the bladder and the prostate (providing one source of sexual stimulation). Stimulation to the external or internal senses may evoke involuntary activity or guide intentions in action. Such emotional or motivating stimulation typically is also experienced subjectively (enters awareness, is in consciousness). Perception can be regarded as conceptualised stimulation, used in reasoning and intending, for example. When bodily stimulation is perceived it is traditionally called a sensation, such as a kind of touch or a taste or smell, or a painful or pleasurable sensation. This can be thought of as psychological stimulation, which is a stimulus affecting a person's thinking or feeling processes.

A nerve is an enclosed, cable-like bundle of nerve fibers (called axons). Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called action potentials that are transmitted along each of the axons to peripheral organs or, in the case of sensory nerves, from the periphery back to the central nervous system. Each axon is an extension of an individual neuron, along with other supportive cells such as some Schwann cells that coat the axons in myelin.

Each axon is surrounded by a layer of connective tissue called the endoneurium. The axons are bundled together into groups called fascicles, and each fascicle is wrapped in a layer of connective tissue called the perineurium. The entire nerve is wrapped in a layer of connective tissue called the epineurium. Nerve cells (often called neurons) are further classified as either sensory or motor.

Multiple sclerosis (MS) is an autoimmune disease resulting in damage to myelin which is the insulating covers of nerve cells in the brain and spinal cord. As a demyelinating disease, MS disrupts the nervous system's ability to transmit signals, resulting in a range of signs and symptoms, including physical, mental, and sometimes psychiatric problems. Symptoms include double vision, vision loss, eye pain, muscle weakness, and loss of sensation or coordination.

Alcohol, sometimes referred to by the chemical name ethanol, is the active ingredient in alcoholic drinks such as beer, wine, and distilled spirits (hard liquor). Alcohol is a central nervous system (CNS) depressant, decreasing electrical activity of neurons in the brain, which causes the characteristic effects of alcohol intoxication ("drunkenness"). Among other effects, alcohol produces euphoria, decreased anxiety, increased sociability, sedation, and impairment of cognitive, memory, motor, and sensory function.

Alcohol has a variety of adverse effects. Short-term adverse effects include generalized impairment of neurocognitive function, dizziness, nausea, vomiting, and symptoms of hangover. Alcohol is addictive and can result in alcohol use disorder, dependence, and withdrawal upon cessation. The long-term effects of alcohol are considered to be a major global public health issue and include liver disease, hepatitis, cardiovascular disease (e.g., cardiomyopathy), polyneuropathy, alcoholic hallucinosis, long-term impact on the brain (e.g., brain damage, dementia, and Marchiafava–Bignami disease), and cancers; alcohol and some of its metabolites (such as acetaldehyde) are IARC group 1 carcinogens. The adverse effects of alcohol on health are most significant when it is used in excessive quantities or with heavy frequency. However, in 2023, the World Health Organization published a statement in The Lancet Public Health that concluded, "no safe amount of alcohol consumption for cancers and health can be established." In high amounts, alcohol may cause loss of consciousness or, in severe cases, death. Many governmental agencies and organizations issue alcohol consumption recommendations.

An axon (from Greek ἄξων áxōn, axis) or nerve fiber (or nerve fibre: see spelling differences) is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action potentials away from the nerve cell body. The function of the axon is to transmit information to different neurons, muscles, and glands. In certain sensory neurons (pseudounipolar neurons), such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body and from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction can be the cause of many inherited and many acquired neurological disorders that affect both the peripheral and central neurons. Nerve fibers are classed into three types – group A nerve fibers, group B nerve fibers, and group C nerve fibers. Groups A and B are myelinated, and group C are unmyelinated. These groups include both sensory fibers and motor fibers. Another classification groups only the sensory fibers as Type I, Type II, Type III, and Type IV.

An axon is one of two types of cytoplasmic protrusions from the cell body of a neuron; the other type is a dendrite. Axons are distinguished from dendrites by several features, including shape (dendrites often taper while axons usually maintain a constant radius), length (dendrites are restricted to a small region around the cell body while axons can be much longer), and function (dendrites receive signals whereas axons transmit them). Some types of neurons have no axon and transmit signals from their dendrites. In some species, axons can emanate from dendrites known as axon-carrying dendrites. No neuron ever has more than one axon; however in invertebrates such as insects or leeches the axon sometimes consists of several regions that function more or less independently of each other.

Myelin (/ˈmaɪ.əlɪn/ MY-ə-lin) is a lipid-rich material that in most vertebrates surrounds the axons of neurons to insulate them and increase the rate at which electrical impulses (called action potentials) pass along the axon. The myelinated axon can be likened to an electrical wire (the axon) with insulating material (myelin) around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Myelin ensheaths part of an axon known as an internodal segment, in multiple myelin layers of a tightly regulated internodal length.

The ensheathed segments are separated at regular short unmyelinated intervals, called nodes of Ranvier. Each node of Ranvier is around one micrometre long. Nodes of Ranvier enable a much faster rate of conduction known as saltatory conduction where the action potential recharges at each node to jump over to the next node, and so on until it reaches the axon terminal. At the terminal the action potential provokes the release of neurotransmitters across the synapse, which bind to receptors on the post-synaptic cell such as another neuron, myocyte or secretory cell.

Nervous tissue, also called neural tissue, is the main tissue component of the nervous system. The nervous system regulates and controls body functions and activity. It consists of two parts: the central nervous system (CNS) comprising the brain and spinal cord, and the peripheral nervous system (PNS) comprising the branching peripheral nerves. It is composed of neurons, also known as nerve cells, which receive and transmit impulses to and from it, and neuroglia, also known as glial cells or glia, which assist the propagation of the nerve impulse as well as provide nutrients to the neurons.

Nervous tissue is made up of different types of neurons, all of which have an axon. An axon is the long stem-like part of the cell that sends action potentials to the next cell. Bundles of axons make up the nerves in the PNS and tracts in the CNS.

Glia, also called glial cells (gliocytes) or neuroglia, are non-neuronal cells in the central nervous system (the brain and the spinal cord) and in the peripheral nervous system that do not produce electrical impulses. The neuroglia make up more than one half the volume of neural tissue in the human body. They contribute to the maintenance of homeostasis, help form myelin, and provide support and protection for neurons. In the central nervous system, glial cells include oligodendrocytes (that produce myelin), astrocytes, ependymal cells and microglia, and in the peripheral nervous system they include Schwann cells (that produce myelin), and satellite cells.

Chemical synapses are biological junctions through which neurons' signals can be sent to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous system. They are crucial to the biological computations that underlie perception and thought. They allow the nervous system to connect to and control other systems of the body.

At a chemical synapse, one neuron releases neurotransmitter molecules into a small space (the synaptic cleft) that is adjacent to the postsynaptic cell (e.g., another neuron). The neurotransmitter molecules are contained within small sacs called synaptic vesicles, and are released into the synaptic cleft by exocytosis. These molecules then bind to neurotransmitter receptors on the postsynaptic cell. Finally, to terminate its action, the neurotransmitter is cleared from the cleft through one of several mechanisms, including enzymatic degradation or re-uptake, by specific transporters, either into the presynaptic cell or to neuroglia.

In biology, depolarization or hypopolarization is a change within a cell, during which the cell undergoes a shift in electric charge distribution, resulting in less negative charge inside the cell compared to the outside. Depolarization is essential to the function of many cells, communication between cells, and the overall physiology of an organism.

Most cells in higher organisms maintain an internal environment that is negatively charged relative to the cell's exterior. This difference in charge is called the cell's membrane potential. In the process of depolarization, the negative internal charge of the cell temporarily becomes more positive (less negative). This shift from a negative to a more positive membrane potential occurs during several processes, including an action potential. During an action potential, the depolarization is so large that the potential difference across the cell membrane briefly reverses polarity, with the inside of the cell becoming positively charged.

Hyperpolarization is a change in a cell's membrane potential that makes it more negative. Cells typically have a negative resting potential, with neuronal action potentials depolarizing the membrane. When the resting membrane potential is made more negative, it increases the minimum stimulus needed to surpass the needed threshold. Neurons naturally become hyperpolarized at the end of an action potential, which is often referred to as the relative refractory period. Relative refractory periods typically last 2 milliseconds, during which a stronger stimulus is needed to trigger another action potential. Cells can also become hyperpolarized depending on channels and receptors present on the membrane, which can have an inhibitory effect.

Hyperpolarization is often caused by efflux of K (a cation) through K channels, or influx of Cl (an anion) through Cl channels. On the other hand, influx of cations, e.g. Na through Na channels or Ca through Ca channels, inhibits hyperpolarization. If a cell has Na or Ca currents at rest, then inhibition of those currents will also result in hyperpolarization. This voltage-gated ion channel response is how the hyperpolarization state is achieved.