Chapter 11 The Nervous System The Soma4c Nervous System Anatomy of the Autonomic Nervous System The primary func6on of the ANS is to regulate the func6on of e ector organs so as to maintain homeostasis Two branches both innervate most organs and act in opposite manners and are typically most ac6ve under di erent condi6ons Parasympathe6c NS is most ac6ve during res6ng condi6ons enhancing the diges6ons and absorp6on of nutrients Sympathe6c NS is most ac6ve during periods of excita6on or physiological ac6vity to coordinate ght or ight response The Soma6c Nervous System Unlike the autonomic nervous system which controls the func6ons of many di erent types of e ector organs the soma6c nervous system controls skeletal muscle In addi6on the soma6c nervous system has only a single type of e erent neuron that innervate skeletal muscle motor neurons Most skeletal muscle is under voluntary control you can consciously decide to contract a muscle Comple6on of Voluntary Motor Tasks There are several cor6cal motor areas Cor6cal Control of voluntary Movement Upper motor neurons Descending input from primary motor cortex through direct pathways called the cor4cospinal tracts pyramidal tracts provide input onto motor neurons in the spinal cord The axons of these tracts terminate in the ventral horn of the spinal cord Lower motor neurons Lower Motor Neurons Lower motor neurons in the ventral horn of the spinal cord receive input from motor cortex and also from spinal re exes brainstem and the extrapyramidal tract A single motor neuron innervates many muscle cells muscle bers Lower motor neuron Structure of Skeletal Muscle Skeletal muscle contains many bundles called fascicles of individual muscles cells Each fascicle contains hundred to thousands of muscle cells which are called muscle bers due to their elongated shape Motor Unit A motor neuron plus all of the muscle bers it innervates is called motor unit When a motor unit is ac6vated it s6mulates all the muscle bers in its unit to contract moving the body Neuromuscular Junc6on Each branch of a motor neuron synapses with a skeletal muscle ber at a highly specialized central region of the ber called the neuromuscular junc6on The axon terminals of the motor neuron are called terminal boutons which store and release acetylcholine Neuromuscular Junc6on This junc6on is a chemical synapse between the axon of the motor neuron and the motor end plate of the skeletal muscle cell Neuromuscular Junc6on The signal transmission mechanism at the NMJ is similar to that of excitatory neuron to neuron synapses Ac6on poten6als from motor neurons cause acetylcholine to be released which binds to nico6nic receptors at the motor end plate causing Na to ow into the muscle ber to produce a depolariza6on called an end plate poten6al EPP Neuromuscular Junc6on Curare Curare is an extract of a plant Chondrodendron tomentosum found in South America Indigenous people of that region crush and cook the roots and stems of the plant to produce a poison for the 6ps of arrows and darts Curare Curare was and s6ll is used for hun6ng game by paralyzing game which would eventually die of respiratory failure The e ec6ve compound is called tubocuranine which binds to nico6nic cholinergic receptors thereby preven6ng acetylcholine from binding so that skeletal muscle are unable to contract even when motor neurons are ring ac6on poten6als Myasthenia Gravis A disease a ec6ng mostly women resul6ng in unusually rapid onset and severity following the use of certain muscle groups The most frequently a ected muscles are those of the head making di cul6es in speaking dysarthria swallowing dysphagia and drooping of the face ptosis common symptoms In some people muscles used in breathing are a ected some6mes necessita6ng the use of a mechanical ven6lator Myasthenia Gravis Wasn t un6l the mis 1900 s that it was discovered that the symptoms of MG were due to a failure of motor neurons to excite muscle cells enough to cause contrac6on by measuring EPP s We now know that MG is an autoimmune disease in which the body s own an6bodies bind to nico6nic cholinergic receptors triggering their removal from the plasma membrane and degrada6on This results in a decreased number of func6onal receptors The Thymus and the Immune System The thymus is a lymphoid organ that plays an important role in the development of the bodies immune system which protects the body against disease causing pathogens The Thymus and Myasthenia Gravis The thymus contains immature T cells which migrated from the bone marrow and develop and mature in the thymus In adults with myasthenia gravis the thymus is o en abnormal which has lead to the hypothesis that the thymus in these pa6ents gives incorrect instruc6ons to developing immune cells resul6ng in autoimmunity and the produc6on of an6bodies against acetylcholine receptors
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