Neurons in the Nervous System

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This reflex does not require the CNS as it occurs even if the nerve cord is removed. Each segment, however, has its own nerve plexi and the plexus of one segment is not connected directly to the nerve plexus of adjacent segments - the nerve cord is required to connect the nervous systems of the segments together. Scientists have ascertained many of these features by the following experiment: They removed segments 40 to 60 from the worm, removed the gut and nerve cord from this portion and then turned the body wall inside out and placed it in saline attached to levers that could record the pull of the muscles in the portion of body wall.

Electrical activity in the muscles and nerves could also be measured and the nervous system could be stimulated by a physical prod and by electrodes. These pieces of body wall even remained alive if kept in saline in the fridge overnight! When one segment of such a preparation is stimulated, the longitudinal muscles in that segment contracted and a wave of contraction spread to the other segments, one after the other, but only when the segments were free to move, which showed that it was simply the mechanical pull of the stimulated segment that stimulated the neighbouring segments to contract in turn.

Immobilising some of the segments stopped the spread of contraction, showing that the nerve plexus of one segment was not connected to that of other segments. If the nerve cord was intact, however, then a nervous electrical signal passed along it to the adjacent segments, even if some of the segments were immobilised. Thus, only the nerves in the nerve cord connect the nervous systems of adjacent segments together.

The cord is enclosed in a three-layered trilaminar sheath. The outermost layer is a single layer of flat epithelial cells that are part of the peritoneum the lining of the coelom. Beneath this is a layer of longitudinal muscle cells, and beneath this is the innermost fibrous sheath that invests the nerve cells.

In animal nervous systems there are two principle types of nerve cell: The main wires in the system are long slender processes from neurones, and these processes are called axons. There are many axons in the nerve cord, but most are small, however, five are especially large and easily seen under the microscope - these are the giant axons.

These are axons with very large diameters. The dorsal giant axons is the widest, with a diameter of about 50 micrometres. An axon is rather like an electrical wire, and just as electricity flows more easily and faster along a wide wire than along a narrow wire which provides more resistance to the flow of electricity so the nervous signal flows faster along giant axons.

The nervous signal is positive electricity, consisting of the flow of positively charged ions, whereas electricity in electrical wires is negative electricity, consisting of the flow of negatively charged electrons. These giant axons are used to carry the fastest signals along the nerve cord, and these are emergency signals that initiate escape reflexes.

The larger dorsal giant axon conducts signals the fastest and in the direction from aft to fore only. This sends signals from the rear of the worm forwards, such that if something unexpected touches the rear of the worm, a rapid signal is sent forwards along the nerve cord, causing the longitudinal muscle sin each segment to rapidly contract, shortening the worm rapidly in an attempt to escape from a would-be predator or other threat.

The two dorso-lateral giant axons connect with each other and send signals from the front end to the rear, in case the worm has to make a rapid retreat perhaps contracting into its burrow to escape a curious bird.

The giant axons are made up of a number of cylindrical cells all connected end-to-end. The junctions between neighbouring cells contain fast electrical synapses nexuses that allow the electrical signal to rapidly travel from one cell to another on its way down the axon. Sensory systems The CNS is a series of tiny but powerful! This sends out nerves, such as the segmental nerves, to the muscles and sensors of the worm.

The job of the CNS is to analyse the incoming sensory information from the various sensors and determine a course of action and then send out the appropriate instructions to the muscles. One type of important sensor are photoreceptors light sensors. Earthworms have no eyes although many other worms do, eyes are not so useful when you spend most of your life buried in soil or crawling about at night.

However, worms do have light sensors in the form of specialised nerve endings called light cells of Hess. These sensors occur in most parts of the worm's skin, but they are concentrated in certain areas.

They are more concentrated on the back and sides of the worm, though a few do occur on the ventral surface at the front end 1st segment of the worm. They are also much more concentrated toward the front end of the worm, being most numerous in the prostomium the front-most lobe which is not a true segment and dwindling in density in the first three segments and are very few in number past the third segment.

There are also quite a few on the rear-most segment of the worm, including its ventral surface. Note that the front and back ends of the worm are often lifted up above the ground and so having some light sensors underneath the front and back ends, as well as one the sides and back, makes sense.

So, the prostomium is the main light-sensing region of the worm. The prostomium is innervated by a pair of branching nerves from the cerebral ganglia. These branches from the subepidermal and intermuscular plexuses. The brain, the seat of higher mental functions such as consciousness, memory, planning, and voluntary actions, also controls lower body functions such as the maintenance of respiration, heart rate, blood pressure, and digestion.

The spinal cord is a long, thin mass of bundled neurons that carries information through the vertebral cavity of the spine beginning at the medulla oblongata of the brain on its superior end and continuing inferiorly to the lumbar region of the spine. The white matter of the spinal cord functions as the main conduit of nerve signals to the body from the brain.

The grey matter of the spinal cord integrates reflexes to stimuli. Nerves are bundles of axons in the peripheral nervous system PNS that act as information highways to carry signals between the brain and spinal cord and the rest of the body. Each axon is wrapped in a connective tissue sheath called the endoneurium. Individual axons of the nerve are bundled into groups of axons called fascicles, wrapped in a sheath of connective tissue called the perineurium.

Finally, many fascicles are wrapped together in another layer of connective tissue called the epineurium to form a whole nerve. The wrapping of nerves with connective tissue helps to protect the axons and to increase the speed of their communication within the body. The meninges are the protective coverings of the central nervous system CNS. They consist of three layers: CSF is formed from blood plasma by special structures called choroid plexuses. The choroid plexuses contain many capillaries lined with epithelial tissue that filters blood plasma and allows the filtered fluid to enter the space around the brain.

Newly created CSF flows through the inside of the brain in hollow spaces called ventricles and through a small cavity in the middle of the spinal cord called the central canal. CSF also flows through the subarachnoid space around the outside of the brain and spinal cord. CSF is constantly produced at the choroid plexuses and is reabsorbed into the bloodstream at structures called arachnoid villi. What are known as the special senses—vision, taste, smell, hearing, and balance—are all detected by specialized organs such as the eyes , taste buds , and olfactory epithelium.

Sensory receptors for the general senses like touch, temperature, and pain are found throughout most of the body. All of the sensory receptors of the body are connected to afferent neurons that carry their sensory information to the CNS to be processed and integrated. Did you know that DNA testing can help you discover your genetic risk of acquiring certain health conditions that affect the organs of our nervous system? The brain and spinal cord together form the central nervous system, or CNS.

The CNS acts as the control center of the body by providing its processing, memory, and regulation systems. The CNS is also responsible for the higher functions of the nervous system such as language, creativity, expression, emotions, and personality. The brain is the seat of consciousness and determines who we are as individuals. The peripheral nervous system PNS includes all of the parts of the nervous system outside of the brain and spinal cord. These parts include all of the cranial and spinal nerves, ganglia, and sensory receptors.

The SNS is the only consciously controlled part of the PNS and is responsible for stimulating skeletal muscles in the body. The ANS controls subconscious effectors such as visceral muscle tissue, cardiac muscle tissue, and glandular tissue. There are 2 divisions of the autonomic nervous system in the body: The enteric nervous system ENS is the division of the ANS that is responsible for regulating digestion and the function of the digestive organs.

The ENS receives signals from the central nervous system through both the sympathetic and parasympathetic divisions of the autonomic nervous system to help regulate its functions.

Neurons function through the generation and propagation of electrochemical signals known as action potentials APs. An AP is created by the movement of sodium and potassium ions through the membrane of neurons.

See Water and Electrolytes. A synapse is the junction between a neuron and another cell. Synapses may form between 2 neurons or between a neuron and an effector cell. There are two types of synapses found in the body: The axons of many neurons are covered by a coating of insulation known as myelin to increase the speed of nerve conduction throughout the body. Myelin is formed by 2 types of glial cells:

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