Functional anatomy of the peripheral nervous system
The concept of the peripheral nervous system
After studying the material of the chapter, the student must:
• classification of peripheral nervous system structures by topographic and functional principles;
be able to
• distinguish the main components of the cranial and spinal parts of the peripheral nervous system;
• skills in determining the composition of peripheral nerves but the composition of the fibers.
Peripheral Nervous System is a collection of nerve structures located outside the spinal cord and brain (Figure 6.1). The structures of the peripheral nervous system provide for the perception of stimuli from the external or internal media, transform the energy of stimulation into a nerve impulse, conduct it to the spinal cord or brain and from them to the executive organs (for example, to the muscles and glands).
According to the topographic principle, cranial (cranial) and spinal (spinal) parts of the peripheral nervous system are distinguished. The functional principle distinguishes between somatic and vegetative departments. The somatic department ensures the innervation of soma - the body, and the vegetative - of the internal organs.
The cranial section is represented by nervous structures closing on the brain stem (cranial nerves, sensitive nodes of cranial nerves, nerve plexuses, organ nerves and nerve endings). The spinal column is represented by nervous structures that lock on the spinal cord (spinal nerves, sensory nodes of the spinal nerves (spinal nodes), branches of the spinal nerves, plexus and organ nerves, nerve endings.)
Fig. 6.1. Peripheral Nervous System:
I - cervical plexus; II - brachial plexus; III - intercostal nerves; IV - lumbar plexus; V - sacral plexus; 1 - the orbital nerve; 2 - maxillary nerve; 3 - the mandibular nerve; 4 - facial nerve; 5 - the vagus nerve; 6 - intercostal nerve; 7 - musculocutaneous nerve; 8 - radial nerve; 9 - median nerve; 10 - ilio-inguinal nerve; 11 - ilio-hypogastric nerve; 12 - ulnar nerve; 13 - femoral nerve; 14 - sciatic nerve; 15 - the occlusal nerve; 16 - common peroneal nerve; 17 - superficial peroneal nerve; 18 - deep peroneal nerve; 19 - tibial nerve; 20 - lateral cutaneous nerve of the thigh; 21 - sympathetic trunk; 22 - celiac plexus
Nerves are formed by processes of nerve cells that combine into bundles of nerve fibers. The latter are covered with loose connective tissue - perineurium. Perineuria processes penetrate between separate nerve fibers, forming an inner connective tissue - endoneurium. The nerve, including several bundles, is also surrounded from the outside by a connective tissue called an epineurium. In the epineurium, the blood and lymph vessels of nerves and nerves that supply and innervate the nerve pass.>
The impulses are carried along the nerves. As a rule, nerves are mixed in composition of fibers, i. E. contain in various ratios sensitive, motor and vegetative conductors. Therefore, the composition of fibers distinguish between motor, sensory, mixed and autonomic nerves.
The motor nerve consists of nerve fibers formed by axons of nerve cells located in the motor nuclei of the anterior horns of the spinal cord or in the motor nuclei of the cranial nerves. In addition, they contain a small number of proprioceptive and sympathetic fibers.
The sensory nerve consists of afferent nerve fibers, which are peripheral processes of pseudo-unipolar or bipolar cells that are part of the sensory nodes of the spinal or cranial nerves. In addition, these nerves contain a small number of sympathetic nerve fibers.
Mixed nerve can include afferent, efferent, sympathetic or parasympathetic fibers in various combinations and percentages.
Vegetative nerves are formed by preganglionic or postganglionic fibers. Preganglionic fibers go from the cells of the autonomic nuclei of the central nervous system to the vegetative node. Postganglionic fibers follow from the cells of the vegetative nodes to the innervated organs and tissues. An important aspect of the structure of these structures is that the preganglionic fibers have a myelin sheath and the postganglionic fibers are devoid of it.
Pulsing through nerve fibers is a complex physiological process. In the center of the myelinic nerve fiber passes the process of the nerve cell (axial cylinder). Around him several layers are wrapped glial membrane, between the layers of which is myelin - a protein-lipid compound possessing the properties of a dielectric (insulator). Myelin sheath covers the axial cylinder not all over, but with interruptions. These gaps are called Ranvier intercepts. In these areas, the fiber is covered with myelin sheath.
At rest between the outer and inner sides of the membrane of the nerve cell a certain difference of charges (potentials) is maintained. This is due to the different content of ions outside and inside the axon. When summing up the charges of all ions from the outside and from the inside of the cytolemma, it turns out that the inner side of the membrane is negatively charged with respect to the outer side of the membrane. This state is called the membrane resting potential.
Special protein channels are built into the axon cytomemma, which pass ions in the direction of their lower concentration. However, these channels do not function at rest. If the cell gets irritated, these channels open and ions move to the opposite side of the membrane. A condition arises when the inner membrane becomes positively charged with respect to the outer membrane. This change is called the membrane action potential.
The electric field arising from the change in the charge difference propagates along the nerve fiber. It activates the ion channels of neighboring regions, and the excitement spreads further. In myelinated nerve fibers, action potentials only occur in the Ranvier intercepts, where neuronal processes contact the intercellular substance. The transition of a pulse from one interception to another is achieved by the arising electric field. The process of occurrence of the action potential takes a fraction of a second. The speed of the impulse along the myelin fibers ranges from 10 to 120 m/s. After the passage of the pulse, the channels are closed and the special protein-pumps equalize the ion concentration to the characteristic state of rest. This process requires the expenditure of ATP synthase energy.
Bessmielin fibers carry a nerve impulse at a much slower rate (about 1-2 m/s), which is due to "scattering" pulse in the surrounding tissue.
Thus, the transmission of a nerve impulse is not a purely electrical phenomenon, but a set of complex physiological processes of redistribution of ions relative to the membrane of a nerve cell. As such, the electrical currents in the nerves are not observed.
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