Nervous Tissue PDF

Nervous tissue  Nervous tissue Complex communication system: rapid, precise and concrete communication, controlling and coordinating the different functions within the organism. Main cells: neurons - specialized receptors to receive stimuli and transform them into nerve impulses that are...

Nervous tissue  Nervous tissue Complex communication system: rapid, precise and concrete communication, controlling and coordinating the different functions within the organism. Main cells: neurons - specialized receptors to receive stimuli and transform them into nerve impulses that are transmitted to other cells, thus capturing information from the internal or external environment, or sending orders to different parts of the body. Other cells: glial cells or neuroglia - do not receive nor transmit impulses but give support to the neurons in different ways.  Neurons They are excitable cells that receive and transmit impulses. They consist of a cell body and prolongations coming out from it called dendrites and axon. The cell body, also called soma or perikaryon, is the central portion where the nucleus and most organelles are. They are usually polygonal or starry, although there are different shapes and sizes depending on their location. The nucleus is rounded, central and large, with scattered chromatin indicating high activity. The cytoplasm has abundant RER and ribosomes, which sometimes clump together and are observed as Nissl corpuscles. From the cell body come out the dendrites, which are extensions specialized in receiving stimuli. Most neurons have multiple dendrites. They tend to be very branched to facilitate the uptake of stimuli from multiple neurons and then transmit the stimulus to the soma. An axon emerges from each soma. They usually leave the cytoplasm on the opposite side of the dendrites. Axons conduct impulses from the soma to other cells. They can be branched and at their terminal end they have dilatations called terminal buttons or synaptic buttons, from where the impulse will be transmitted to the next cell. Axons are covered by myelin sheaths that makes conduction much faster. Between the sheaths, there are spaces without myelin called Ranvier nodules. Axonic transport In addition to conducting impulses, the axon must transport materials from the soma to the synaptic buttons and vice versa. The necessary materials for nervous conduction and for maintaining the structure of the axon are distributed moving along the axon. Organelles and vesicles, macromolecules and enzymes necessary for the synthesis of neurotransmitters are transported by filaments of the cytoskeleton, neurofilaments and neurotubules, that are important in axon structure and in axonal transport.  Morphological classification Depending on their shape and the arrangement of their extensions, the neurons are classified as: - Unipolar: they only have one extension that leaves the neuronal body. This prolongation works like an axon. They hardly occur in the human being. - Pseudounipolars: they only have a prolongation that leaves the neuronal body, but later it branches in two. They appear in some ganglia. - Bipolar: they have two extensions; one works as a dendrite and the other as an axon. They appear in the inner ear and in the olfactory epithelium of the nasal mucosa. - Multipolar: they are the most frequent type, appear throughout the nervous system. They have numerous dendrites and a single axon. Functional classification According to their function, neurons are classified as: - Motor: are those that originate in the CNS and drive impulses to muscles, glands or other neurons. - Sensitive: they receive sensory impulses from the internal or external environment and lead them to the CNS to be processed. - Interneurons: they are located in the CNS and act as a connection between sensory and motor neurons and other interneurons.  Action potential Nerve impulses are electrical signals that are generated as a result of a depolarization of the neuron's membrane. Neurons and other cells are polarized with a resting potential of about -70mV, due to different concentrations of ions on either side of the membrane. This polarity is maintained by the Na+-K+ pumps, which are responsible for maintaining high concentrations of Na+ in the exterior and high concentrations of K+ in the interior. When the rest potential is restored, the channels will be closed. This process of depolarization and repolarization is called action potential and follows an all-or-nothing law. Action Potential and Saltatory Conduction - Concept | Biology | JoVe The action potential that is generated in a specific point of the cell is transmitted to the adjacent channels and spread throughout the membrane of the cell until it reaches the axonic terminal.  Myelinated axon Non-myelinated axon In areas covered by myelin, there are very few or none Na+ channels, so action potentials are not produced in these areas. The action potential will have to spread from one nodule of Ranvier to the next - saltation conduction. This conduction allows the myelinated fibers to transmit the impulse much faster than an unmyelinated fiber.  Synapse Presynaptic neuron Postsynaptic neuron When the action potential reaches the axon terminal, it will be transmitted to another cell. The place where it is transmitted is what we call synapse. Presynaptic cell. The cell that transmits the impulse. It is always a neuron. Postsynaptic cell. The cell that receives the impulse. It can be another neuron or a muscle cell or a glandular cell. Synapse There are electrical synapses, in which the presynaptic and postsynaptic cell are joined by nexus/GAP junctions, which allow the flow of ions from one cell to the other and so the impulse is transmitted directly. Therefore, it is a very rapid transmission, but infrequent in mammals. They only occur in the trunk of the brain, the retina and the cerebral cortex. The most common type of synapse is the chemical synapse, in which the presynaptic and postsynaptic cells are not in direct contact but leave a space between them called synaptic cleft. The presynaptic neuron releases neurotransmitters (synaptic buttons) into this space and will bind to specific receptors in the membrane of the postsynaptic cell. Excitatory synapse. If the neurotransmitter stimulus depolarizes the postsynaptic cell and can generate an action potential. Inhibitory synapse. If it maintains the potential or hyperpolarizes the postsynaptic cell. The Synapse and Neurotransmitters - Concept | Biology | JoVe Neuroglia Glia of the CNS They are a group of cells that provide metabolic and mechanical support and protection to neurons, in addition to helping the propagation of impulses. Types of neuroglial cells: astrocytes, oligodendrocytes, microcytes and ependymocytes in the CNS; Schwann cells and satellite cells in the PNS.  Astrocytes They are the largest glial cells and can be of two types: The protoplasmic astrocytes appear in the gray matter of the CNS, have a starry shape and abundant short and branched extensions. Fibrous astrocytes appear in the white matter of the CNS and also have prolongations, but long and usually unbranched. The ends of some extensions in the Vessel Pedicel astrocytes, end up as pedicels (vascular feet) that come in contact with blood vessels and with the pia mater, separating these tissues from the neurons. They form the blood-brain barrier controlling what reaches the CNS through the blood. Protoplasmic Fibrous astrocyte astrocyte  Oligodendrocytes They are similar to astrocytes, but smaller and with fewer extensions. They make and maintain myelin in the CNS. Each branch of an oligodendrocyte will form a myelin sheath, so a single oligodendrocyte can coat fragments of several axons.  Microcytes The microglial cells are found throughout the CNS, are small and have short and irregular branches. They are phagocytes that eliminate cellular debris and damaged structures, as well as protect against infections. Ependymocytes They are low cylindrical epithelial cells that line the ventricles of the brain and the central channel of the spinal cord. In some regions, they present cilia to facilitate the movement of cerebrospinal fluid. Glia of the PNS Schwann cells Satellite cells Schwann cells They are flattened cells with a flattened nucleus. They surround the axons in the PNS to form myelinated or unmyelinated sheaths. The axon together with its enveloping sheath (myelinated or not) is known as nerve fiber. Myelin corresponds to the membrane of the Schwann cell that is wrapped several times around the axon. The myelin sheath covering the axon is interrupted several times in unmyelinated spots called Ranvier nodules. Myelinated fibers. In this type of fibers, the Schwann cell wraps its membrane around a single axon, making several turns around it. They are fast fibers. Satellite cells They surround the neuronal bodies in the ganglia. They give physical support, protection and nutrition to these neurons. Glial Cells: | Biology | JoVe  Nervous system To carry out its functions, the nervous system is organized anatomically in: Central Nervous System (CNS): formed by the encephalon and spinal cord. Peripheral Nervous System (PNS): located outside the CNS and formed by nerves and ganglia. The PNS can functionally be divided into: o Sensitive (afferent): receives signals from the internal or external environment and sends them to the CNS. o Motor (efferent): originating in the CNS and sending impulses to effector organs. This motor component can also be divided into: Somatic system: is responsible for all voluntary functions. Autonomous system: is responsible for all involuntary functions.