* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Nervous tissues (NS)
Signal transduction wikipedia , lookup
Nonsynaptic plasticity wikipedia , lookup
Multielectrode array wikipedia , lookup
Clinical neurochemistry wikipedia , lookup
Resting potential wikipedia , lookup
Subventricular zone wikipedia , lookup
Neurotransmitter wikipedia , lookup
Optogenetics wikipedia , lookup
Axon guidance wikipedia , lookup
Biological neuron model wikipedia , lookup
Synaptic gating wikipedia , lookup
Single-unit recording wikipedia , lookup
Molecular neuroscience wikipedia , lookup
Feature detection (nervous system) wikipedia , lookup
Development of the nervous system wikipedia , lookup
Circumventricular organs wikipedia , lookup
Node of Ranvier wikipedia , lookup
Chemical synapse wikipedia , lookup
Electrophysiology wikipedia , lookup
Neuroregeneration wikipedia , lookup
Neuropsychopharmacology wikipedia , lookup
Nervous system network models wikipedia , lookup
Synaptogenesis wikipedia , lookup
Channelrhodopsin wikipedia , lookup
Neuroanatomy wikipedia , lookup
Physiology Lect( 3 ) Asst.Lect/Shaimaa Hussein Nervous tissues (NS) The nervous system is the body s control center and communications network. In humans, the nervous system serves three broad functions: 1- Sensory functions: it senses changes within the body and in the outside environment. 2- Integrative function:it interprets the changes. 3- Motor function: it responds to the interpretation by initiating action in the form of muscular contractions or glandular secretions. Neurology: the branch of medical science deals with normal functioning and disorders of the NS. The general organization of the nervous system 1- Structural organization: 2- Functional organization: Cells of Nervous System Nervous tissues consists of two types of cells: 1- The neurons which conduct impulses and make up the impulse conducting portion of the brain, spinal cord and nerves. 2- The neuroglial cells which perform other functions. Neurons: consist of a cell body (soma), an axon, and usually several dendrites. In general, the axon conducts impulses away from the cell body, while the dendrites conduct impulse toward the cell body. The cell body (soma) contains most of the cytoplasm and many of the organells usually found in cells (mitochondria, colgi apparatus, nucleus and nucleolus). The cell body also contains Nissl granules(a complex of endoplasmic reticulum and ribosome that serves as the site of protein synthesis for the neuron). Neurofibrils are found in the cell body near the axon hillock (where the axon joins the cell body). The axon of a neuron is a long ,this process extending from the hillock. In most neurons it extends in only one direction from the cell body. Most axons are myelinated(that is they are surrounded by an insulating substance called myelin). بشكل يششك ي غالفيالسلك Dendrites are shorter processes than axon in most neurons. They connect directly with the cell body. Dendrites are not myelinated. Schwan cells: (sometimes considered a kind of neuroglial cells) are found wrapped around the axons of myelinated neurons of the PNS. Schwan cells are required to produce a myelin sheath on a singl axon. The myelin sheath has numerous small constrictions called node of Ranvier. These nodes represents minutes spaces between adjacent schwan cells. Classification of Neurons On the basis of their structure, neurons can also be classified into three main types: Unipolar Neurons. Sensory neurons have only a single process or fibre which divides close to the cell body into two main branches (axon and dendrite). Because of their structure they are often referred to as unipolar neurons. Multipolar Neurons. Motor neurons, which have numerous cell processes (an axon and many dendrites) are often referred to as multipolar neurons. Interneurons are also multipolar. Bipolar Neurons. Bipolar neurons are spindle-shaped, with a dendrite at one end and an axon at the other . An example can be found in the light-sensitive retina of the eye. Neuroglia: four types of neuroglia are found primarily in the CNS. 1-Ependymal cells: from a sheet that lines the ventricles of the brain (spaces that form and circulate cerebrospinal fluid) and the central canal of the spinal cord. 2- Astrocytes: star shaped cells with numerous processes. They are of two types: a- protoplasmic astrocytes: are found in the gray matter of the CNS. b- fibrous astrocytes: are found in the white matter of the CNS. It functions is a- create a supporting network for neurous and blood vessels. b- help to transport nutrients from blood to neurons. 3- Oligodendrocytes: resemble astrocytes in some ways, but processes are fewer and shorter. It have supporting function by : a- forming semirigid connective tissue rows between neurous in brain and spinal cord. b- produce a phospholipid myelin sheath around axons of neurous of CNS. 4- Microglia: small cells with few processes derived from monocytes (also called brain macrophage). They are thought to protect nerve cells against infection. These cells are phagocytic cells migrate to the site of an injury in the CNS and destroy microorganisms and cellular debris. Nerves: are bundles of nerve fibers in the PNS Tracts: are bundles of nerve fibers in the CNS Nerve: contain mostly myelinated and few unmylinated axons, surrounded by several C.T. sheath most nerves excepts for some cranial nerves (nerve that branch directly from brain), contain both sensory and motor fibers. A connective tissue sheath (the endoneurium), surrounds each fibers. Another C.T.sheath(the perineurium) binds groups of fibers together into fascicles. Yet another C.T. sheath (the epineurium) covers the whole nerve (including its several fascicles. Blood and lymph vessels are often located within the C.T. between fascicles of large nerves. Nerve fibers in tracts of the CNS are mostly myelinated (they acquired their myelin from the activities of oligodendrocytes rather than from Sch. cells). The myelin that surrounds the fibers of tracts gives the tracts a whitish color (they are recognized as white matter of the CNS). Cell bodies and dendrites lack myelin and these areas are recognized as gray matter. Ganglia and Nuclei : The cell bodies of the fibers of both nerves and tracts are usually aggregated in large groups to the side of the main pathway of the fibers. In the peripheral nervous system these aggregations are called ganglia. Within the CNS they are called nuclei (but they are not to be confused cellular nuclei). Each nucleus in the brain of many cell bodies, each having its own cellular nucleus. The Synapse: After a signal has traveled the length of a neuron transmission of the signal to the next neuron in the neural pathway occurs, Such transmission takes place across, asynapse aspecilized junction between the axon terminal of one neroun and the dendrite(or cell body or axon) of the next neroun. Transmission across asynapse is accomplished by a chemical substance called a neurotransmitter. The neuron whose axon release the neurotransmitter is the presynaptic neuron, the neuron that receives the neurotransmitter is the postsynaptic neuron. Atypical synapse consist of the presynaptic knob of an axon separated from the postsynaptic region of adendrite or nerve cell body by the synaptic cleft. Excitability: the ability to respond to a stimulus and initiate and conduct an electrical impulse. In living cells potential differences are created by charged particles (ions) in and out the cells. Each cell contains intracellular fluid and is surrounded by extracellular fluid. Both of these fluids contain ions, but the concentration of the various ions in each of the fluids differ. The concentration of (Na+) in extracellular fluids is abound (20) times greater than that in intracellular. The concentration of (K+) in intracellular fluids is about (25) times greater than that in extracellular fluids. Inracellular fluids also contain large numbers of negatively charged protein molecules. Differences in the concentrations of charged particles create a potential difference across the cell membrane, and the cell is said to be polarized. In resting, polarized neurons, the potential difference about (-70mv) is called the resting potential. Thus, the inside of the membrane is more negative than the outside by 70mv. In resting, cell potassium ions tend to diffuse out of the cell more easily than sodium ions diffuse in. Because large protein molecules and negatively chargedions cannot easily diffuse across the cell membrane, the inside of a cell tends to become more negative than the outside. These events create the negative potential difference . Furthermore, as Na+ leak slowly into the cell, they are actively transported out again, and as K+ leak out of the cell, they are actively transported back into the cell by the action of Na-K pump. This combination of active transport and passive diffusion of ions maintains the potential difference across the membrane. Initiation and Conduction of Nerve impulse: The property of excitability: 1- means the ability of neurons to respond to stimulus and to conduct an impulse. 2- make it possible for the NS to receive information and use it to regulate function. The processes of initiation and conduction of an impulse only a neuron occure as follows: 1-When a neroun is stimulated, the movment of ions across its membrane cerates a change in the potential differences. 2-When a resting neroun is stimulated a small number of Na + move into the cell creating an action potential, which move along the membrane as an impulse. 3-Following a wave of depolarization is a wave of repalarization,which prepares the neroun to receive another stimulus. *Factors that affect the generation and conduction of an impulse include: 1-strength of stimulus. 2-summation. 3-the all or non principle. 4-refractoriness. 5-saltatory conduction. 6-myelination. 7-axon diameter. *Multiple synapses on the same neroun may: 1-create excilatory postsynaptic potential (EPSPs) and facilitate transmission. 2-create inhibitory postsynaptic potentials(IPSPs) and inhibit transmission. Spinal Cord: -The spinal cord extends from the base of the brain to the second lumbar vertebra and give rise to thirty-one paris of spinal nerves. -In cross section the spinal cord displays: 1-a butterfly-shaped area of gray matter which contains large numbers of neroun cell bodies and synapses. 2-an outer area of white matter which contains the myelinated spinal tracts. - The properties of the spinal tracts are summarized in a special table.