10.Nervous tissue_new pdf.pdf

Nervous tissue • It forms a complex communication system within the organism, controlling and coordinating the different functions. • Its main cells are neurons, which have specialized receptors to receive stimuli and transform them into nerve impulses that are transmitted to other cells, thus ca...

Nervous tissue • It forms a complex communication system within the organism, controlling and coordinating the different functions. • Its main cells are neurons, which have 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. It is therefore a system of rapid, precise and concrete communication. • The nervous tissue is formed by neurons and also by many other cells called glial cells or neuroglia, that do not receive nor transmit impulses, but give support to the neurons in different ways. Neurons They are cells that receive and transmit impulses, they are therefore excitable cells. 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, of variable diameter (greater diameter = greater speed) and length of up to one meter. 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, since it can propagate nervous impulses, although it can also receive stimuli. They hardly occur in the human being. - Pseudounipolars: they only have a prolongation that leaves the neuronal body, but later it branches in two. Both branches can propagate impulses and receive stimuli. 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. Refractory period When the neuron is stimulated, voltage-gated Na+ channels are opened and Na+ enters the cell. The potential is changing becoming increasingly positive until the resting potential is reversed, that is, the cell depolarizes. As a consequence, the Na+ channels are inactivated for a short period of time - refractory period. Then voltage-dependent K+ channels will open, allowing K+ to exit and restore the membrane potential, although a short time of hyperpolarization could be reached. 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-ornothing law. Usually, the action potential starts in the dendrites. In addition to being very branched, they have receptors that facilitate the capture of the stimuli. However, it could start anywhere on the membrane, even in the soma or in the axon. 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 axonic terminal, it will be transmitted to another cell. The place where it is transmitted is called synapse. The cell that transmits the impulse is the presynaptic cell, which is always a neuron; the cell that receives it, is the postsynaptic cell, which can be another neuron or a muscle cell or a glandular cell. 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. Synapse 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 into this space and will bind to specific receptors in the membrane of the postsynaptic cell. Neurotransmitters are molecules synthesized by neurons that are stored in the synaptic buttons and cause the opening of ion channels by binding to their membrane receptors. When an action potential reaches the synaptic button, Ca2+ channels open and this will cause the neurotransmitters to be released. If the stimulus of the neurotransmitter is capable of generating an action potential, it is an excitatory synapse; if it maintains the potential or increases it until it is hyperpolarized, it is an inhibitory synapse. 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 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 help eliminate neurotransmitters and remnants of neuronal metabolism, they also release glucose into the medium to contribute to the energy supply of neurons and form the blood-brain barrier controlling what reaches the CNS through the blood. 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 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. They are flattened cells with a flattened nucleus. 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. Each space between nodes will be occupied by a Schwann cell. 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. Unmyelinated fibers Some axons of the PNS are not surrounded by several layers of myelin. Around them there is a single layer of Schwann cell’s membrane and part of the cytoplasm of this cell. In this case, several unmyelinated axons may be wrapped by the same Schwann cell. Satellite cells They surround the neuronal bodies in the ganglia. They give physical support, protection and nutrition to these neurons. 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. A single neuron transmits impulses from the CNS to a skeletal muscle. ➢ Autonomous system: is responsible for all involuntary functions. The impulses that leave the CNS go to a ganglion through a neuron and then a second neuron will take them to a smooth muscle, a gland or the heart. Central nervous system Composed by the brain and the spinal cord. Here, the neurons are organized giving rise to two parts: - White matter: formed mostly by nerve fibers and neuroglial cells. Its white color is due to the abundance of myelin. - Gray matter: formed by aggregates of neuronal bodies, dendrites and unmyelinated portions of axons, as well as neuroglials cells. Spinal cord The gray matter is in the center of the spinal cord, with the shape of an H and around it is the white matter. In the center of the spinal cord is the ependymal channel, lined by ependymocytes. The posterior horns of the H receive prolongations of sensory neurons, whose bodies are in the ganglia of the posterior root. These extensions reach interneurons that are also found in these posterior horns. In the anterior horns there are bodies of motor neurons, whose axons will come out using the anterior roots. The disposition of the neurons in the spinal cord allow it to communicate with the brain and transmit the impulses received. The brain will elaborate a response that will also travel through the cord. However, the spinal cord can sometimes also develop simple and quick responses called reflex arcs. Reflex arc Brain In the brain, the gray matter is found in the cortex and the white matter in the interior. The majority of interneurons are found here, where they form communication networks for the integration of sensory and motor neurons. Cerebellum In the cerebellum there are the smallest neurons of the body and are distributed in two layers of gray matter: the outermost called molecular layer, with fewer neuronal bodies, and the innermost called granulosa, with a large number of neuronal bodies. Between these two layers, there is a row of large interneurons called Purkinje cells. Inside this gray matter we will find the white matter. Meninges They are three layers of connective tissue that surrounds the CNS to give it protection. The outermost, in contact with the bones of the skull and spine, is called dura mater. It has a periosteal layer formed by osteoprogenitor cells, fibroblasts and collagen fibers that bind to bone, and another meningeal layer formed by dense conjunctiva. The second meninge is the arachnoid, which is also formed by dense conjunctiva and has a flat layer adhered to the dura and another formed by a network of trabeculae that contact the pia mater. In the spaces between these trabeculae there is cerebrospinal fluid and blood vessels. The pia mater is the innermost of the meninges, formed by a thin layer of connective tissue and associated with nervous tissue. Peripheral nervous system • Nerves • Ganglia Nerves They are bundles of nerve fibers located outside the CNS surrounded by sheaths of connective tissue. Each fiber is surrounded by loose connective tissue called endoneuro. Several fibers are grouped together forming a fascicle, which is surrounded by dense connective tissue called perineuro; several fascicles form a nerve surrounded by more dense connective tissue called epineuro. There may be sensory nerves or motor nerves, but most are mixed. Autonomous nervous system It is a motor and involuntary system that helps to maintain the homeostasis of the organism since it controls the viscera (smooth muscle, cardiac muscle) and the secretion of the glands. Their neurons originate in the CNS and are directed to an autonomous ganglion, where they will synapse with a second neuron that will reach the effector organ. It is subdivided into two systems, sympathetic and parasympathetic. The sympathetic nervous system, generally prepares the body for action, acts in situations of stress. Increases breathing, blood pressure, heart rate, blood flow to skeletal muscles, dilates pupils and slows visceral functions. Their neurons originate in the dorsal and lumbar region of the cord and some synapses in a ganglion of the paravertebral chain and others cross this chain to enter the abdominal cavity and synapse in the collateral ganglia along the abdominal aorta. The parasympathetic nervous system tends to be sympathetic antagonist, produces the opposite reactions. It helps the maintenance of homeostasis. Its neurons originate in the brain or in the sacral region of the spinal cord and synapses in ganglia located in the vicinity of the effector organ or even in the organ itself (terminal ganglia). Nervous ganglia They are aggregates of neuronal bodies outside the CNS. They can be of two types: - Sensitive: they lodge bodies of sensitive neurons and are associated to some cranial pairs and to all the spinal nerves. They are surrounded by a dense connective capsule and inside of it there are abundant neurons all surrounded by satellite cells. - Autonomous: they lodge the neuronal bodies of the postsynaptic motor neurons of the ANS. They are the ganglia of the paravertebral sympathetic chain, the collateral ganglia and the terminal ganglia. They do not have their own capsule; they are usually inside the same nerve sheath and have few neurons and satellite cells.