Physiology of the Endocrine System
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The brain is generally larger in those insects that have more complex social lives. Although much smaller than a human brain, containing only one thousandth as many cells, it is still immensely complex. There is also less replication of function - fewer neurons perform each function.
The supraoesophageal ganglion consists of several fused ganglia or lobes. The paired ganglia of the first frontmost head segment form the protocerebrum, concerned with vision, time-keeping, higher functions, memory and combining information from different sensory modalities.
Those of the segment segment form the deutocerebrum, which is concerned with processing sensory inputs from the antennae, and also the labial palps and parts of the tegument body wall.
The optic lobe connects directly to the sensory cells retinula cells in the retina of the compound eye. It contains three distinct regions neuropils: The protocerebrum also receives inputs via the ocelli, when present, via the ocellar nerves. These are thought to function as higher centres responsible for the most sophisticated computations occurring in the insect brain.
Each consists of a topmost cap and a stalk or peduncle which branches into at least two lobes. The cap consists of a pair of cup-like structures, the medial calyx and the lateral calyx plural of calyx is calyces. The mushroom bodies receive sensory inputs from the lobula of the optic lobe and from the antennal lobes of the deutocerebrum. Most sensory inputs enter the MB through the calyx. There are about to specialised neurons, called Kenyon cells , in each mushroom body.
Dragonfly mushroom bodies have no calyces and no Kenyon cells. The mushroom bodies are also involved in learning, and in the honeybee have been shown to process memories, transferring data from short-term memory STM into long-term memory LTM. The central body receives inputs from the mushroom bodies and integrates sensory inputs from different sensory modalities such as small and vision - so-called multimodal sensory perception.
It functions as an activating centre , switching on appropriate locomotor activity patterns which are central programs located in the thoracic ganglia.
That is it instructs the thoracic ganglia which programs to run - programs that control the legs and wings. These hard-wired programs are sometimes called central pattern generators and require no sensory input for their execution, though sensory inputs may start and stop these programs or modify them slightly.
The pars intercerebralis is a mass of cell bodies, including neurosecretory cells which send their axons to the pair of corpora cardiaca see the neuroendocrine system in insect development. The corpora cardiac are sometimes fused into a single medial ganglion.
They send out nerves to innervate the dorsal blood vessel, forming a cardio-aortic system , which controls the rate of heart beat, as well as having a secretory hormona lfunction.
The insect brain contains about to neurons, compared to one billion one thousand million found in the human brain. Biological Clocks Another function associated with the protocerebrum is time-keeping. This resetting by use of external signals enables the insect to adjust to different local conditions depending, for example, on latitude.
Many body parts and organs have their own circadian clocks, indeed each cell appears capable of keeping time, but these appear to be set and synchronised by a central master clock , which resides in the protocerebrum and is both neural and hormonal. In some insects, a master clock is found in each optic lobe, which makes sense as these process light signals. There is also a daily movement of screening pigments in the ommatidia of the compound eye, as the insect adjusts to night-time darkness by increasing the sensitivity of its retina it will continue to do this at the correct time for days when kept in constant light or dark for several days, so the response is coordinated, in part, by a central clock.
Severing of the optic lobes prevents these clocks from synchronising bodily activities. In other species, however, the clock is only abolished if the brain is cut in two, which suggests that it may reside in the central body. Deutocerebrum This consists of two nerve centres - the main antennal lobe AL and the smaller antennal mechanosensory and motor centre AMMC or dorsal lobe. The AL receives inputs from the third terminal antennal segment the flagellum, which is made-up of sub-segments called flagellomeres via the antennal nerves.
It contains from less than 10 to more than sub-centres called glomeruli singular glomerulus. Inputs to the AL appear to be mainly or exclusively from chemoreceptors i. Each antenna sends signals to the AL on the same side of the head ipsilateral pathways although some may also send signals to the AL on the opposite side contralateral pathways. Each glomerulus is a region of neuropil nerve cell processes and synapses where computations occur. It is thought that each glomerulus may, in some species at least, receive inputs from a specific class of receptor sensor on the antenna.
For example, in the males of some species there is a specially large glomerulus, called the macroglomerular complex MGC which receives inputs from pheromone olfactory sensors on the antenna. The AL does not receive one input line from each chemoreceptor, as sensors of the same type converge - their axons fuse into a smaller number of axons in the antennal nerve typically inputs from 15 sensors are combined, a These sensory input axons, and also input axons from the CB of the protocerebrum, synapse with local interneurones within the AL amacrine cells.
Outputs from the AL are carried along the axons of output neurons to the MB of the protocerebrum. The AMMC receives mechanosensory inputs from mechanosensors mechanoreceptors on the first two antennal segments scape and pedicel via the antennal nerves. It also sends motor outputs to the muscles of the scape. It also receives inputs from mechanosensors on the labial palps, some tegument body wall mechanosensors, and some inputs from the flagellum possibly from the mechanosensors found on the flagellum.
The antennal nerve is therefore a mixed nerve - containing both sensory and motor axons. Some of the antennal mechanoreceptors also send outputs to the SOG, the protocerebrum and the thoracic ganglia. A single medial recurrent nerve runs back up to a ganglion situated beneath and behind the supraoesophageal ganglion. This ganglion may be called the stomachic ganglion or the hypocerebral ganglion HG.
In the locust, the HG sends out one pair of outer oesophageal nerves and one pair of inner oesophageal nerves ventricular nerves. Each of the latter terminates in a ventricular ganglion ingluvial ganglion on the crop of the foregut see insect nutrition.
These then control crop movements. In Dytiscus , it has been shown that the FG also controls swallowing. Thus, the tritocerebrum and frontal ganglion control the foregut, forming the stomatogastric system. The tritocerebrum also innervates the labrum.
Suboesophageal Ganglion The suboesophageal ganglion SOG and the segmental ganglia of the double ventral nerve-cord each send out pairs of nerves, one of which innervates the pair of spiracles on that segment and so help regulate breathing.
As an aside, there is actually another mushroom that may prove helpful with demyelination— Phellinus igniarius , otherwise known as willow bracket. One study found its extract suppressed demyelination as well as suppressing many of the immune cells active or overactive in multiple sclerosis.
Beta-amyloid plaques are proteins that form in the fatty membranes that surround nerve cells, interfering with neurotransmission. These plaques are thought to play a role in neurodegenerative diseases such as Alzheimer's and Parkinson's. In addition to regaining their former cognitive skills, they gained NEW cognitive skills—something akin to curiosity, as measured by greater time spent exploring novel objects compared to familiar ones. The reduction of beta amyloid plaques in the mushroom-fed mice was remarkable.
The exact molecule underlying this effect is not currently known, but it is thought to be a bioactive peptide. Increases the expression of several genes involved in fat metabolism. Topical application of the extract was found to accelerate wound healing. Does it taste like chicken? No, in this case lobster According to Mushroom Forager: Some serious allergic reactions have been reported, so please take ample precautions. The brains of many molluscs and insects also contain substantial numbers of identified neurons.
Each Mauthner cell has an axon that crosses over, innervating neurons at the same brain level and then travelling down through the spinal cord, making numerous connections as it goes. The synapses generated by a Mauthner cell are so powerful that a single action potential gives rise to a major behavioral response: Functionally this is a fast escape response, triggered most easily by a strong sound wave or pressure wave impinging on the lateral line organ of the fish.
Mauthner cells are not the only identified neurons in fish—there are about 20 more types, including pairs of "Mauthner cell analogs" in each spinal segmental nucleus. Although a Mauthner cell is capable of bringing about an escape response individually, in the context of ordinary behavior other types of cells usually contribute to shaping the amplitude and direction of the response.
Mauthner cells have been described as command neurons. A command neuron is a special type of identified neuron, defined as a neuron that is capable of driving a specific behavior individually.
The concept of a command neuron has, however, become controversial, because of studies showing that some neurons that initially appeared to fit the description were really only capable of evoking a response in a limited set of circumstances. At the most basic level, the function of the nervous system is to send signals from one cell to others, or from one part of the body to others. There are multiple ways that a cell can send signals to other cells.
One is by releasing chemicals called hormones into the internal circulation, so that they can diffuse to distant sites. In contrast to this "broadcast" mode of signaling, the nervous system provides "point-to-point" signals—neurons project their axons to specific target areas and make synaptic connections with specific target cells. It is also much faster: At a more integrative level, the primary function of the nervous system is to control the body.
The evolution of a complex nervous system has made it possible for various animal species to have advanced perception abilities such as vision, complex social interactions, rapid coordination of organ systems, and integrated processing of concurrent signals. In humans, the sophistication of the nervous system makes it possible to have language, abstract representation of concepts, transmission of culture, and many other features of human society that would not exist without the human brain.
Most neurons send signals via their axons , although some types are capable of dendrite-to-dendrite communication.
In fact, the types of neurons called amacrine cells have no axons, and communicate only via their dendrites. Neural signals propagate along an axon in the form of electrochemical waves called action potentials , which produce cell-to-cell signals at points where axon terminals make synaptic contact with other cells. Synapses may be electrical or chemical. Electrical synapses make direct electrical connections between neurons,  but chemical synapses are much more common, and much more diverse in function.
Both the presynaptic and postsynaptic areas are full of molecular machinery that carries out the signalling process. The presynaptic area contains large numbers of tiny spherical vessels called synaptic vesicles , packed with neurotransmitter chemicals. The neurotransmitter then binds to receptors embedded in the postsynaptic membrane, causing them to enter an activated state.
For example, release of the neurotransmitter acetylcholine at a synaptic contact between a motor neuron and a muscle cell induces rapid contraction of the muscle cell. There are literally hundreds of different types of synapses. In fact, there are over a hundred known neurotransmitters, and many of them have multiple types of receptors. Molecular neuroscientists generally divide receptors into two broad groups: When a chemically gated ion channel is activated, it forms a passage that allows specific types of ions to flow across the membrane.
Depending on the type of ion, the effect on the target cell may be excitatory or inhibitory. When a second messenger system is activated, it starts a cascade of molecular interactions inside the target cell, which may ultimately produce a wide variety of complex effects, such as increasing or decreasing the sensitivity of the cell to stimuli, or even altering gene transcription. According to a rule called Dale's principle , which has only a few known exceptions, a neuron releases the same neurotransmitters at all of its synapses.
Nevertheless, it happens that the two most widely used neurotransmitters, glutamate and GABA , each have largely consistent effects. Glutamate has several widely occurring types of receptors, but all of them are excitatory or modulatory. Similarly, GABA has several widely occurring receptor types, but all of them are inhibitory. Strictly speaking, this is an abuse of terminology—it is the receptors that are excitatory and inhibitory, not the neurons—but it is commonly seen even in scholarly publications.
One very important subset of synapses are capable of forming memory traces by means of long-lasting activity-dependent changes in synaptic strength. This change in strength can last for weeks or longer. Since the discovery of LTP in , many other types of synaptic memory traces have been found, involving increases or decreases in synaptic strength that are induced by varying conditions, and last for variable periods of time.
The basic neuronal function of sending signals to other cells includes a capability for neurons to exchange signals with each other. Networks formed by interconnected groups of neurons are capable of a wide variety of functions, including feature detection, pattern generation and timing,  and there are seen to be countless types of information processing possible. Warren McCulloch and Walter Pitts showed in that even artificial neural networks formed from a greatly simplified mathematical abstraction of a neuron are capable of universal computation.
Historically, for many years the predominant view of the function of the nervous system was as a stimulus-response associator. Descartes believed that all of the behaviors of animals, and most of the behaviors of humans, could be explained in terms of stimulus-response circuits, although he also believed that higher cognitive functions such as language were not capable of being explained mechanistically. However, experimental studies of electrophysiology , beginning in the early 20th century and reaching high productivity by the s, showed that the nervous system contains many mechanisms for generating patterns of activity intrinsically, without requiring an external stimulus.
The simplest type of neural circuit is a reflex arc , which begins with a sensory input and ends with a motor output, passing through a sequence of neurons connected in series. The circuit begins with sensory receptors in the skin that are activated by harmful levels of heat: If the change in electrical potential is large enough to pass the given threshold, it evokes an action potential, which is transmitted along the axon of the receptor cell, into the spinal cord.
There the axon makes excitatory synaptic contacts with other cells, some of which project send axonal output to the same region of the spinal cord, others projecting into the brain. One target is a set of spinal interneurons that project to motor neurons controlling the arm muscles. The interneurons excite the motor neurons, and if the excitation is strong enough, some of the motor neurons generate action potentials, which travel down their axons to the point where they make excitatory synaptic contacts with muscle cells.
The excitatory signals induce contraction of the muscle cells, which causes the joint angles in the arm to change, pulling the arm away. In reality, this straightforward schema is subject to numerous complications. Furthermore, there are projections from the brain to the spinal cord that are capable of enhancing or inhibiting the reflex. Although the simplest reflexes may be mediated by circuits lying entirely within the spinal cord, more complex responses rely on signal processing in the brain.
The initial sensory response, in the retina of the eye, and the final motor response, in the oculomotor nuclei of the brain stem, are not all that different from those in a simple reflex, but the intermediate stages are completely different. Instead of a one or two step chain of processing, the visual signals pass through perhaps a dozen stages of integration, involving the thalamus, cerebral cortex, basal ganglia, superior colliculus, cerebellum, and several brainstem nuclei.
These areas perform signal-processing functions that include feature detection , perceptual analysis, memory recall , decision-making , and motor planning.
Feature detection is the ability to extract biologically relevant information from combinations of sensory signals. At each stage, important information is extracted from the signal ensemble and unimportant information is discarded.
By the end of the process, input signals representing "points of light" have been transformed into a neural representation of objects in the surrounding world and their properties.
The most sophisticated sensory processing occurs inside the brain, but complex feature extraction also takes place in the spinal cord and in peripheral sensory organs such as the retina.
Although stimulus-response mechanisms are the easiest to understand, the nervous system is also capable of controlling the body in ways that do not require an external stimulus, by means of internally generated rhythms of activity.
Because of the variety of voltage-sensitive ion channels that can be embedded in the membrane of a neuron, many types of neurons are capable, even in isolation, of generating rhythmic sequences of action potentials, or rhythmic alternations between high-rate bursting and quiescence. When neurons that are intrinsically rhythmic are connected to each other by excitatory or inhibitory synapses, the resulting networks are capable of a wide variety of dynamical behaviors, including attractor dynamics, periodicity, and even chaos.
A network of neurons that uses its internal structure to generate temporally structured output, without requiring a corresponding temporally structured stimulus, is called a central pattern generator.
Internal pattern generation operates on a wide range of time scales, from milliseconds to hours or longer. One of the most important types of temporal pattern is circadian rhythmicity —that is, rhythmicity with a period of approximately 24 hours. All animals that have been studied show circadian fluctuations in neural activity, which control circadian alternations in behavior such as the sleep-wake cycle. Experimental studies dating from the s have shown that circadian rhythms are generated by a "genetic clock" consisting of a special set of genes whose expression level rises and falls over the course of the day.
Animals as diverse as insects and vertebrates share a similar genetic clock system. The circadian clock is influenced by light but continues to operate even when light levels are held constant and no other external time-of-day cues are available.
The clock genes are expressed in many parts of the nervous system as well as many peripheral organs, but in mammals, all of these "tissue clocks" are kept in synchrony by signals that emanate from a master timekeeper in a tiny part of the brain called the suprachiasmatic nucleus.
A mirror neuron is a neuron that fires both when an animal acts and when the animal observes the same action performed by another. Such neurons have been directly observed in primate species. Some researchers also speculate that mirror systems may simulate observed actions, and thus contribute to theory of mind skills,   while others relate mirror neurons to language abilities.
In vertebrates, landmarks of embryonic neural development include the birth and differentiation of neurons from stem cell precursors, the migration of immature neurons from their birthplaces in the embryo to their final positions, outgrowth of axons from neurons and guidance of the motile growth cone through the embryo towards postsynaptic partners, the generation of synapses between these axons and their postsynaptic partners, and finally the lifelong changes in synapses which are thought to underlie learning and memory.
All bilaterian animals at an early stage of development form a gastrula , which is polarized, with one end called the animal pole and the other the vegetal pole. The gastrula has the shape of a disk with three layers of cells, an inner layer called the endoderm , which gives rise to the lining of most internal organs, a middle layer called the mesoderm , which gives rise to the bones and muscles, and an outer layer called the ectoderm , which gives rise to the skin and nervous system.
In vertebrates, the first sign of the nervous system is the appearance of a thin strip of cells along the center of the back, called the neural plate. The inner portion of the neural plate along the midline is destined to become the central nervous system CNS , the outer portion the peripheral nervous system PNS.
As development proceeds, a fold called the neural groove appears along the midline. This fold deepens, and then closes up at the top. At this point the future CNS appears as a cylindrical structure called the neural tube , whereas the future PNS appears as two strips of tissue called the neural crest , running lengthwise above the neural tube.
The sequence of stages from neural plate to neural tube and neural crest is known as neurulation. In the early 20th century, a set of famous experiments by Hans Spemann and Hilde Mangold showed that the formation of nervous tissue is "induced" by signals from a group of mesodermal cells called the organizer region.
Induction of neural tissue requires inhibition of the gene for a so-called bone morphogenetic protein , or BMP. Specifically the protein BMP4 appears to be involved. Two proteins called Noggin and Chordin , both secreted by the mesoderm, are capable of inhibiting BMP4 and thereby inducing ectoderm to turn into neural tissue. It appears that a similar molecular mechanism is involved for widely disparate types of animals, including arthropods as well as vertebrates. In some animals, however, another type of molecule called Fibroblast Growth Factor or FGF may also play an important role in induction.
Induction of neural tissues causes formation of neural precursor cells, called neuroblasts.