13 The Nervous System Neural Tissue

Human Anatomy, , Martini/Timmons/Tallitsch The Nervous System – Neural Tissue Cellular Organization in Neural Tissue Neural tissue contains two distinct cell types: (see lines below and fill in) *____Neurons_____ are responsible for the transfer and processing of information in the nervous system. Structure of Neurons The cell body usually has several branching dendrites which are like a receiver on a football team In the CNS, a neuron receives information from other neurons primarily at the dendritic spines, which represent 80-90 percent of the neuron’s total surface area. The cell body of a representative neuron contains a relatively large, round nucleus with a prominent nucleolus. The cell body is attached to an elongated axon that ends at one or more synaptic terminals. An axon, or nerve fiber, is a long cytoplasmic process capable of propagating an action potential. The larger the diameter, the more rapidly the impulse will be conducted. The largest myelinated axons, with diameters ranging from 4 to 20 ?m, conduct nerve impulses at speeds of up to 140 m/s (300 mph). Each synaptic terminal is part of a synapse, a specialized site where the neuron communicates with another cell. Synaptic communication most often involves the release of specific chemicals called neurotransmitters Structural Classification of Neurons The structural classification is based on the number of processes that project from the cell body. 4 of em Anaxonic neurons _ are small, and there are no anatomical clues to distinguish dendrites from axons. Are found only in the CNS and in special sense organs, and their functions are poorly understood . __Bipolar________ neurons have a number of fine dendrites that fuse to form a single dendrite. Are rare but play an important role in relaying sensory information concerning sight, smell, and hearing. Their axons are non-myelinated Psuedo-unipolar neurons have continuous dendritic and axonal processes, and the cell body lies off to one side. Sensory neurons of the peripheral nervous system are usually Multipolar neurons have several dendrites and a single axon that may have one or more branches. Multipolar neurons are the most common type of neuron in the CNS. * Neuroglia Cells They support the neurons – helper cells “ if you will.” Four types of glial cells are found in the central nervous system: Astrocytes – also called _Satellite Cells______in the___ (PNS) Peripheral nervous system. The largest and most numerous glial cells are the astrocytes. Astrocytes act both to shield neurons from direct contact with other neurons and limit their exposure to the surrounding interstitial fluid. Maintaining the blood–brain barrier The endothelial cells lining CNS capillaries have very restricted permeability characteristics that control the chemical exchange between blood and interstitial fluid. Chemicals secreted by astrocytes are essential for the maintenance of the blood–brain barrier. Creating a three-dimensional framework for the CNS Performing repairs in damaged neural tissue After damage to the CNS, astrocytes make structural repairs that stabilize the tissue and prevent further injury by producing scar tissue at the injury site. Guiding neuron development Oligodendrocytes also called _ _Schwann Cells_______________________________________(PNS) Improve the functional performance of neurons by wrapping axons in myelin, a material with insulating properties. Many axons in the CNS are completely sheathed by the processes of oligodendrocytes. Myelin improves the speed at which an action potential, or nerve impulse is conducted along an axon. A myelinated axon conducts impulses five to seven times faster than an unmyelinated axon. Microglia Acting as a roving security force. Microglia engulf cellular debris, waste products, and pathogens. Ependymal Cells The ventricles of the brain and central canal of the spinal cord are lined by a cellular layer called the ependyma. Ependymal cells participate in the secretion of cerebrospinal fluid. Functional Classification of Neurons Neurons can be categorized into three functional groups: Sensory neurons Motor neurons Interneurons, or association neurons Sensory neurons are_Psuedo-unipolor_ neurons with their cell bodies located outside the CNS in peripheral sensory ganglia. Their function is to deliver information to the CNS. The axons of sensory neurons, called afferent fibers, extend between a sensory receptor and the spinal cord or brain. Somatic sensory neurons transmit information about the outside world and our position within it. Visceral sensory neurons transmit information about internal conditions and the status of other organ systems. Types of receptors: Exteroreceptors_ provide information about the external environment in the form of touch, temperature, and pressure sensations and the more complex special senses of sight, smell, and hearing. Proprioreceptors_AKA Kinesthestic receptors monitor the position and movement of skeletal muscles and joints. (Proprioreception). Muscle Spindles: are located in the belly of a muscle. They detect changes in the length of this muscle The Stretch Reflex aka myotatic reflex is a muscle contraction in response to stretching within the muscle itself. Jendrassik maneurver - is a maneuver wherein the patient clenches the teeth, flexes both sets of fingers into a hook-like form and interlocks those sets of fingers together. Used to distract a patients when doing a patellar reflex test Golgi Tendon Organ_or GTO is a tendon reflex – detects changes in force or tension in a tendon. PNF stretching, or proprioceptive neuromuscular facilitation stretching : involves a shortening contraction of the opposing muscle to place the target muscle on stretch. This is followed by an isometric contraction of the target muscle. Uses Reciprocal Inhibition which: Describes a process where muscles on one side of a joint relax to accommodate contraction on the other side of that joint. The body handles this pretty well during physical activities like running, where muscles that oppose each other are engaged and disengaged sequentially to produce coordinated movement. Interoceptors: monitor the digestive, respiratory, cardiovascular, urinary, and reproductive systems and provide sensations of deep pressure and pain as well as taste, another special sense. Motor neurons - (efferent) are Multipolar neurons that form the efferent fibers…this going out from the CNS to the Periphery The somatic nervous system (SNS) includes all of the somatic motor neurons that innervate skeletal muscles. The cell bodies of these motor neurons lie inside the CNS, and their axons extend to the neuromuscular junctions that control skeletal muscles. Types of Motor Neurons Somatic ones are consciously controlled. The autonomic nervous system includes all the visceral motor neurons that innervate peripheral effectors other than skeletal muscles. Interneurons (association neurons) may be situated between sensory and motor neurons. Interneurons are located entirely within the brain and spinal cord. They outnumber all other neurons combined both in total number and in types. Interneurons can be classified as excitatory or inhibitory on the basis of their effects on the postsynaptic membranes of other neurons. The Nerve Impulse Excitability is the ability of a cell membrane to conduct electrical impulses. An electrical impulse, or action potential, develops after the membrane is stimulated to a level known as the threshold. After the threshold level has been reached, the membrane permeability to sodium and potassium ions changes. The ion movements that result produce a sudden change in the transmembrane potential, and this change constitutes an action potential. In the nervous system, action potentials traveling along axons are known as nerve impulses. Synaptic Communication A synapse permits communication between a neuron and another cell type; such synapses are called neuroeffector junctions. The neuromuscular junction described in Chapter 9 was an example of a neuroeffector junction. Or a The motor-end plate where it attaches to the muscle cell Chemical Synapses Chemical synapses are by far the most abundant; there are several different types. All somatic neuromuscular junctions utilize Ach (acetylcholine); Arrival of the action potential at the synaptic knob triggers release of neurotransmitter. The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic membrane. 2. Electrical Synapses - are found between neurons in both the CNS and PNS, but they are relatively rare. See in _retina_ and _cerebral_cortex_. Quick and are Bi-directional. 1of 6