Neuroglia, nerve fiber, action potential, synapse, neurotransmitters.ppt
- 1. • Nervous system Organization of nervous system, neuron, neuroglia, classification and properties of nerve fibre, electrophysiology, action potential, nerve impulse, receptors, synapse, neurotransmitters. Central nervous system: Meninges, ventricles of brain and cerebrospinal fluid.structure and functions of brain (cerebrum, brain stem, cerebellum), spinal cord (gross structure, functions of afferent and efferent nerve tracts, reflex activity)
- 2. Neuroglia • There are two types of nervous tissue, neurones and neuroglia • Neuroglia, any of several types of cell that function primarily to support neurons. • The term neuroglia means “nerve glue” • Neurones are supported by connective tissue, collectively known as neuroglia, which is formed from different types of glial cells. • Glia, also called glial cells (gliocytes) or neuroglia, are non- neuronal cells in the central nervous system (brain and spinal cord) and the peripheral nervous system that do not produce electrical impulses. • There are vast numbers of both cell types, 1 trillion glial cells and 10 times fewer neurones.
- 3. The neurones of the central nervous system are supported by non-excitable glial cells that greatly outnumber the neurones (Fig. 7.10). Unlike nerve cells, which cannot divide, glial cells continue to replicate throughout life. There are four types: astrocytes, oligodendrocytes, ependymal cells and microglia.
- 6. Astrocytes • These cells form the main supporting tissue of the central nervous system (Fig. 7.11). They are star shaped with fine branching processes and they lie in a mucopolysaccharide ground substance. • At the free ends of some of the processes are small swellings called foot processes. • Astrocytes are found in large numbers adjacent to blood vessels with their foot processes forming a sleeve round them. • This means that the blood is separated from the neurones by the capillary wall and a layer of astrocyte foot processes which together constitute the blood–brain barrier.
- 7. • The blood–brain barrier is a selective barrier that protects the brain from potentially toxic substances and chemical variations in the blood, e.g. after a meal. • Oxygen, carbon dioxide, glucose and other lipid-soluble substances, e.g. alcohol, quickly cross the barrier into the brain. • Some large molecules, many drugs, inorganic ions and amino acids pass more slowly, if at all, from the blood to the brain. • Oligodendrocytes These cells are smaller than astrocytes and are found in clusters round nerve cell bodies in grey matter, where they are thought to have a supportive function. • They are found adjacent to, and along the length of, myelinated nerve fibres. • Oligodendrocytes form and maintain myelin like Schwann cells in peripheral nerves.
- 8. Ependymal cells • These cells form the epithelial lining of the ventricles of the brain and the central canal of the spinal cord • Those cells that form the choroid plexuses of the ventricles secrete cerebrospinal fluid. Microglia • The smallest and least numerous glial cells, these cells may be derived from monocytes that migrate from the blood into the nervous system before birth. • They are found mainly in the area of blood vessels. • They enlarge and become phagocytic, removing microbes and damaged tissue, in areas of inflammation and cell destruction
- 9. Neuroglia Functions • It offers essential nutrients. It includes oxygen to neurons. • It also helps create the myelin sheath. The sheath is important in the functioning of the nervous system. It promotes and speeds up the electrical impulse conduction. It does so by wrapping around the axons. • Further, it also helps to maintain homeostasis within the neurons. Homeostasis is how a cell maintains a stable internal environment despite disturbances. • It destroys pathogens. It helps protect the neurons. • Finally, it also provides structural stability. It forms a support structure that the neurons can inhabit.
- 10. Neuroglial damage • Astrocytes. When these cells are damaged, their processes multiply forming a mesh or ‘scar’, which is thought to inhibit the regrowth of damaged CNS neurones. • Oligodendrocytes. These cells increase in number around degenerating neurones and are destroyed in demyelinating diseases such as multiple sclerosis. • Microglia. Where there is inflammation and cell destruction the microglia increase in size and become phagocytic.
- 11. Classification of Nerve Fibers A group of neurons form a nerve. Neurons are the structural and functional units of the nervous system. Nerve is an enclosed, cable-like bundle of axons and nerve fibres found in the peripheral nervous system. The function of the axon is to transmit information to different neurons, muscles, and glands. Nerve fibers are classified by their diameter, degree of myelination, and speed of conduction. There are three primary groups of nerve fibers: ■■ Group A fibers: These mostly serve the joints, skeletal muscles, and skin, and are primarily somatic sensory and motor fibers, with the largest diameter of all types of fibers and thick myelin sheaths. These fibers conduct impulses at speeds as high as 300 miles per hour. ■■ Group B fibers: of intermediate diameter, with light myelination, group B fibers conduct impulses at speeds averaging approximately 30 miles per hour. ■■ Group C fibers: These fibers are nonmyelinated with the smallest diameter and cannot create saltatory conduction; they conduct impulses at 2 miles per hour or less. Both B and C fibers include motor fibers of the ANS that serve the smaller somatic sensory fibers that transmit sensory impulses from the skin (including small touch and pain fibers), visceral sensory fibers, and those that serve the visceral organs.
- 13. Proprioception is the sense which allows perception of the location, movement, and action of parts of the body relative to each other.
- 14. Classification of Nerve Fibers
- 15. Classification of Nerve Fibers
- 16. Classification of Nerve Fibers
- 17. The sodium–potassium pump (K-in; Na-Out) • All cells possess this pump, which indirectly supports other transport mechanisms such as glucose uptake, and is essential in maintaining the electrical gradient needed to generate action potentials in nerve and muscle cells. • This active transport mechanism maintains the unequal concentrations of sodium (Na+ ) and potassium (K+ ) ions on either side of the plasma membrane. • It may use up to 30% of cellular ATP (energy) requirements. • Potassium levels are much higher inside the cell than outside – it is the principal intracellular cation. • Sodium levels are much higher outside the cell than inside – it is the principal extracellular cation. • These ions tend to diffuse down their concentration gradients, K+ outwards and Na+ into the cell. • In order to maintain their concentration gradients, excess Na+ is constantly pumped out across the cell membrane in exchange for K+ .
- 20. Action Potential • Neurones generate and transmit electrical impulses called action potentials. • The initial strength of the impulse is maintained throughout the length of the neurone. • Some neurones initiate nerve impulses while others act as ‘relay stations’ where impulses are passed on and sometimes redirected. • Nerve impulses can be initiated in response to stimuli from: • outside the body, e.g. touch, light waves • inside the body, e.g. a change in the concentration of carbon dioxide in the blood alters respiration; a thought may result in voluntary movement. • Transmission of nerve signals is both electrical and chemical. • The action potential travelling down the nerve axon is an electrical signal, but because nerves do not come into direct contact with each other, the signal between a nerve cell and the next cell in the chain is nearly always chemical.
- 21. The nerve impulse (action potential) • An impulse is initiated by stimulation of sensory nerve endings or by the passage of an impulse from another nerve. • Transmission of the impulse, or action potential, is due to movement of ions across the nerve cell membrane (from dendrite to axon terminal). • In the resting state the nerve cell membrane is polarised due to differences in the concentrations of ions across the plasma membrane. • This means that there is a different electrical charge on each side of the membrane, which is called the resting membrane potential. • At rest the charge on the outside is positive and inside it is negative. • The principal ions involved are: • sodium (Na+ ), the main extracellular cation • potassium (K+ ), the main intracellular cation.
- 27. • An action potential begins when a depolarization increases the membrane voltage so that it crosses a threshold value (usually around ). • At this threshold, voltage-gated channels in the membrane open, allowing many sodium ions to rush into the cell. This influx of sodium ions makes the membrane potential increase very rapidly, going all the way up to about . • After a short time, the sodium channels self-inactivate (close and become unresponsive to voltage), stopping the influx of sodium. • A set of voltage-gated potassium channels open, allowing potassium to rush out of the cell down its electrochemical gradient. • These events rapidly decrease the membrane potential, bringing it back towards its normal resting state. • The voltage-gated potassium channels stay open a little longer than needed to bring the membrane back to its resting potential. This results in a phenomenon called “undershoot,” in which the membrane potential briefly dips lower (more negative) than its resting potential. • Eventually, the voltage-gated potassium channels close and the membrane potential stabilizes at resting potential. • The sodium channels return to their normal state (remaining closed, but once more
- 28. Graded potentials • A hyperpolarization or depolarization event may simply produce a graded potential, a smallish change in the membrane potential that is proportional to the size of the stimulus. • As its name suggests, a graded potential doesn’t come in just one size – instead, it comes in a wide range of slightly different sizes, or gradations. • Thus, if just one or two channels open (due to a small stimulus, such as binding of a few molecules of neurotransmitter), the graded potential may be small, while if more channels open (due to a larger stimulus), it may be larger. • Graded potentials don’t travel long distances along the neuron’s membrane, but rather, travel just a short distance and diminish as they spread, eventually disappearing.
- 32. Synapses • There is always more than one neurone involved in the transmission of a nerve impulse from its origin to its destination, whether it is sensory or motor. • There is no physical contact between two neurones. • The point at which the nerve impulse passes from the presynaptic neurone to the postsynaptic neurone is the synapse • Synapses most often form between axons and dendrites, and consist of a presynaptic neuron, synaptic cleft, and a postsynaptic neuron. • At its free end, the axon of the presynaptic neurone breaks up into minute branches that terminate in small swellings called synaptic knobs, or terminal boutons. • Synaptic knobs contain spherical membrane bound synaptic vesicles, which store a chemical, the neurotransmitter that is released into the synaptic cleft.
- 43. Neurotransmitters Neurotransmitters are synthesised by nerve cell bodies, actively transported along the axons and stored in the synaptic vesicles. They are released by exocytosis in response to the action potential and diffuse across the synaptic cleft. They act on specific receptor sites on the postsynaptic membrane. Their action is short lived, because immediately they have acted on the postsynaptic cell such as a muscle fibre, they are either inactivated by enzymes or taken back into the synaptic knob (Reuptake). Some important drugs mimic, neutralise (antagonise) or prolong neurotransmitter activity. Neurotransmitters usually have an excitatory effect on postsynaptic receptors but they are sometimes inhibitory
- 44. • There are more than 50 neurotransmitters in the brain and spinal cord including • noradrenaline (norepinephrine), • adrenaline (epinephrine), • dopamine, • histamine, • serotonin, • gamma aminobutyric acid (GABA) and • acetylcholine. • Other substances, such as enkephalins, endor phins and substance P, have specialised roles in, for example, transmission of pain signals.