Nerve function synapse

The cyclase produces cAMP which results in opening of a Na" ion channel in the membrane of the sensory cell. If a sufficient number of Na " ions enter, this depolarises the membrane and initiates an action potential along the axon to the olfactory nerve. Further effects depend upon interaction between the nerves and synapses within the olfactory centre in the brain. This can result in physiological effects in other parts of the body which define the function of the pheromone. The effects of pheromones on the sexual responses of men and women are discussed in Chapter 19 (see Figure 19.17). 

Organophosphates are characterized by their similar mechanism of toxic action in insects and mammals, resulting in the irreversible inhibition of the enzyme cholinesterase, and the accumulation of acetylcholine at nerve endings (synapses). The primary mechanism is phosphorylation of the enzyme critical to normal transmission of nerve impulses from fibers to innervated tissues. A critical fraction of tissue enzyme must be inactivated before the symptoms of toxicity appear. At sufficient dose, the loss of enzyme function results in... 

Nicotinic acetylcholine (ACh) receptors are responsible for transmission of nerve impulses from motor nerves to muscle fibers (muscle types) and for synaptic transmission in autonomic ganglia (neuronal types). They are also present in the brain, where they are presumed to be responsible for nicotine addiction, although little is known about their normal physiological function there. Nicotinic receptors form cation-selective ion channels. When a pulse of ACh is released at the nerve-muscle synapse, the channels in the postsynaptic membrane of the muscle cell open, and the initial electrochemical driving force is mainly for sodium ions to pass from the extracellular space into the interior of the cell. However, as the membrane depolarizes, the driving force increases for potassium ions to go in the opposite direction. Nicotinic channels (particularly some of the neuronal type) are also permeable to divalent cations, such as calcium. 

Choline functions in fat metaboHsm and transmethylation reactions. Acetylcholine functions as a neurotransmitter in certain portions of the nervous system. Acetylcholine is released by a stimulated nerve cell into the synapse and binds to the receptor site on the next nerve cell, causing propagation of the nerve impulse. 

Tetanus occurs when Cl. tetani, ubiquitous in the soil and faeces, contaminates wounds, especially deep puncture-type lesions. These might be minor traumas such as a splinter, or major ones such as battle injury. At these sites, tissue necrosis and possibly microbial growth reduce the oxygen tension to allow this anaerobe to multiply. Its growth is accompanied by the production of a highly potent toxin which passes up peripheral nerves and diSuses locally within the central nervous system. It acts like strychnine by affecting normal function at the synapses. Since the motor nerves of the brain stem are the shortest, the cranial nerves are the first affected, with twitches of the eyes and spasms of the jaw (lockjaw). 

Interneurons are found in all areas of the spinal cord gray matter. These neurons are quite numerous, small, and highly excitable they have many interconnections. They receive input from higher levels of the CNS as well as from sensory neurons entering the CNS through the spinal nerves. Many intemeurons in the spinal cord synapse with motor neurons in the ventral hom. These interconnections are responsible for the integrative functions of the spinal cord including reflexes. 

Tyrosine phosphorylation plays an important role in synaptic transmission and plasticity. Evidence for this role is that modulators of PTKs and PTPs have been shown to be intimately involved in these synaptic functions. Among the various modulators of PTKs, neuro-trophins have been extensively studied in this regard and will be our focus in the following discussion (for details of growth factors, see Ch. 27). BDNF and NT-3 have been shown to potentiate both the spontaneous miniature synaptic response and evoked synaptic transmission in Xenopus nerve-muscle cocultures. Neurotrophins have also been reported to augment excitatory synaptic transmission in central synapses. These effects of neurotrophins in the neuromuscular and central synapses are dependent on tyrosine kinase activities since they are inhibited by a tyrosine kinase inhibitor, K-252a. Many effects of neurotrophins on synaptic functions have been attributed to the enhancement of neurotransmitter release BDNF-induced increase in neurotransmitter release is a result of induced elevation in presynaptic cytosolic calcium. Accordingly, a presynaptic calcium-depen-dent phenomenon - paired pulse facilitation - is impaired in mice deficient in BDNF. 

The taste bud is a polarized structure with a narrow apical opening, termed the taste pore, and basolateral synapses with afferent nerve fibers. Solutes in the oral cavity make contact with the apical membranes of the TRCs via the taste pore. There is a significant amount of lateral connectedness between taste cells within a bud both electrical synapses between TRCs and chemical synapses between TRCs and Merkel-like basal cells have been demonstrated to occur [39]. Furthermore, there are symmetrical synapses between TRCs and Merkel-like basal cells [39]. In addition, these basal cells synapse with the afferent nerve fiber, suggesting that they may function in effect as interneurons [39]. The extensive lateral interconnections... 

What is a synapse In the brain, the nerve cells or neurons are connected at special functional junctions called synapses, which depend on many proteins, including large complexes. They participate in basic functions with important roles in coordinating every characteristic of the nervous system, including physiology, emotions, learning, sleep, memory, and pain signal transmission. 

The nervous system consists of two main units the central nervous system (CNS), which includes the brain and the spinal cord and the peripheral nervous system (PNS), which includes the body s system of nerves that control the muscles (motor function), the senses (the sensory nerves), and which are involved in other critical control functions. The individual units of the nervous system are the nerve cells, called neurons. Nenrons are a nniqne type of cell becanse they have the capacity to transmit electrical messages aronnd the body. Messages pass from one nenron to the next in a strnctnre called a synapse. Electric impnlses moving along a branch of the nenron called the axon reach the synapse (a space between nenrons) and canse the release of certain chemicals called neurotransmitters, one of which, acetylcholine, we described earlier in the chapter. These chemicals migrate to a nnit of the next nenron called the dendrites, where their presence canses the bnild-np of an electrical impnlse in the second nenron. 

Although pheromones can be considered as a special form of odorants (scents), their actions, effects and functions have similarities to those of hormones. They bind to a specific receptor which then activates an effector system, which initiates an action potential. They bind to specific sensory cells, the neurones, in the olfactory epithelium, which is located on the roof of the nasal cavities. The epithelium consists of three types of cells, basal, supporting and sensory cells (neurones). The neurones are bipolar, that is they possess a single dendrite, which extends from the cell body to the surface of the olfactory epithelium, and an axon that forms a synapse with a nerve that transfers information to the olfactory centre in the brain. The epithelium is covered with a thick layer of mucus, in which the pheromones dissolve. The mucus contains proteins that bind the pheromone(s) for delivery to the olfactory receptors and then to remove them once they have been detected. 

The concept of chemical transmission in the nervous system arose in the early years of the century when it was discovered that the functioning of the autonomic nervous system was largely dependent on the secretion of acetylcholine and noradrenaline from the parasympathetic and sympathetic nerves respectively. The physiologist Sherrington proposed that nerve cells communicated with one another, and with any other type of adjacent cell, by liberating the neurotransmitter into the space, or synapse, in the immediate vicinity of the nerve ending. He believed that transmission across the synaptic cleft was unidirectional and, unlike conduction down the nerve fibre, was delayed by some milliseconds because of the time it took the transmitter to diffuse across the synapse and activate a specific neurotransmitter receptor on the cell membrane. 

The neurochemical basis for these effects has also heen studied in some detail. Researchers have learned that MDMA (and its phenyl-ethylamine cousins) interferes with the normal function of at least two neurotransmitters in the brain, serotonin and dopamine. Under normal circumstances, nerve messages are transmitted through the CNS when an axon on one neuron releases a neurotransmitter, such as serotonin or dopamine, which travels across the synapse between two neurons and is taken up at a receptor site in the second neuron. 

The transfer of stimuli occurs at the synapses, which link the individual neurons to each other as well as linking neurons functionally to muscle fibers. Neurotransmitters (see p. 352) are stored in the axonal nerve endings. These signaling substances are released in response to electrical signals in order to excite neighboring neurons (or muscle cells). It is estimated that each neuron in the brain is in contact via synapses with approximately 10 000 other neurons. 

The sympathetic nervous system plays an important role in the involuntary regulation of cardiac activity, vascular tonicity, functional activity of smooth muscle, and glands by releasing endogenic adrenergic substances, cateeholines, from peripheral nerve endings into the synapses of the central nervous system (CNS). 

The aminopyridines (4-aminopyridine 3,4-diaminopyri-dine) accelerate spontaneous exocytosis at central and peripheral synapses. There is also an increase in the number of transmitter quanta released by a nerve action potential. This is probably the result of increased Ca++ inflow at the terminals due to a reduction of K+ conductance and prolongation of the nerve action potential. Muscle strength is increased in patients with the Lambert-Eaton myasthenic syndrome and in others poisoned with botuUnum E toxin (discussed later). Improvement in uncontrolled spasms, muscle tone, and pulmonary function is noted in patients with multiple sclerosis or long-standing spinal cord damage. Side effects that limit clinical utility include convulsions, restlessness, insomnia, and elevated blood pressure. Of the two agents, 3,4-diaminopyridine is the more potent and crosses the blood-brain barrier less readily. 

Acetylcholine (ACh) has been known as a neurotransmitter since the mid-1920s. In fact, the demonstration that acetylcholine is the Vagusstoff ( vagus-substance ) released from the vagus nerve to modulate heart function was the first proof for the chemical mediation of nerve impulses (Loewi and Navratil, 1926). In the peripheral nervous system, ACh is found as the neurotransmitter in the autonomic ganglia, the parasympathetic postganglionic synapse, and the neuromuscular endplate. 

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