The peripheral nervous system sends back the status report to the brain by relaying information via sensory nerves see above image. As with the central nervous system, the basic cell units of the peripheral central nervous system are neurons. Each neuron has a long process, known as the axon, which transmits the electrochemical signals through which neurons communicate.
Axons of the peripheral nervous system run together in bundles called fibres , and multiple fibres form the nerve , the cable of the electric circuit. The nerves, which also contain connective tissue and blood vessels, reach out to the muscles, glands and organs in the entire body. Nerves of the peripheral nervous system are classified based on the types of neurons they contain - sensory, motor or mixed nerves if they contain both sensory and motor neurons , as well as the direction of information flow — towards or away from the brain.
The afferent nerves, from the Latin "afferre" that means "to bring towards", contain neurons that bring information to the central nervous system. In this case, the afferent are sensory neurons, which have the role of receiving a sensory input — hearing, vision, smell, taste and touch - and pass the signal to the CNS to encode the appropriate sensation. The afferent neurons have also another important subconscious function. In this case, the peripheral nervous system brings information to the central nervous system about the inner state of the organs homeostasis , providing feedback on their conditions, without the need for us to be consciously aware.
For example, afferent nerves communicate to the brain the level of energy intake of various organs. The efferent nerves, from the Latin "efferre" that means "to bring away from", contain efferent neurons that transmit the signals originating in the central nervous system to the organs and muscles, and put into action the orders from the brain.
For example, motor neurons efferent neurons contact the skeletal muscles to execute the voluntary movement of raising your arm and wiggling your hand about.
Reflexes are triggered when sensory information is powerful enough to reach a given threshold and the interneurons in the spinal cord act to send a message back through the motor neurons without relaying the information to the brain see Figure 4. When you touch a hot stove and immediately pull your hand back, or when you fumble your cell phone and instinctively reach to catch it before it falls, reflexes in your spinal cord order the appropriate responses before your brain even knows what is happening.
If the central nervous system is the command centre of the body, the peripheral nervous system PNS represents the front line. As you can see in Figure 4. The autonomic nervous system ANS is the division of the PNS that governs the internal activities of the human body, including heart rate, breathing, digestion, salivation, perspiration, urination, and sexual arousal.
Many of the actions of the ANS, such as heart rate and digestion, are automatic and out of our conscious control, but others, such as breathing and sexual activity, can be controlled and influenced by conscious processes. The somatic nervous system SNS is the division of the PNS that controls the external aspects of the body , including the skeletal muscles, skin, and sense organs. The somatic nervous system consists primarily of motor nerves responsible for sending brain signals for muscle contraction.
The autonomic nervous system itself can be further subdivided into the sympathetic and parasympathetic systems. The sympathetic division of the ANS is involved in preparing the body for behaviour, particularly in response to stress, by activating the organs and the glands in the endocrine system.
The parasympathetic division of the ANS tends to calm the body by slowing the heart and breathing and by allowing the body to recover from the activities that the sympathetic system causes. The sympathetic and the parasympathetic divisions normally function in opposition to each other, with the sympathetic division acting a bit like the accelerator pedal on a car and the parasympathetic division acting like the brake.
Our everyday activities are controlled by the interaction between the sympathetic and parasympathetic nervous systems. For example, when we get out of bed in the morning, we would experience a sharp drop in blood pressure if it were not for the action of the sympathetic system, which automatically increases blood flow through the body.
Similarly, after we eat a big meal, the parasympathetic system automatically sends more blood to the stomach and intestines, allowing us to efficiently digest the food. And perhaps you have had the experience of not being at all hungry before a stressful event, such as a sports game or an exam when the sympathetic division was primarily in action , but suddenly finding yourself feeling starved afterward, as the parasympathetic takes over.
The nervous system is designed to protect us from danger through its interpretation of and reactions to stimuli. But a primary function of the sympathetic and parasympathetic nervous systems is to interact with the endocrine system to elicit chemicals that provide another system for influencing our feelings and behaviours.
A gland in the endocrine system is made up of groups of cells that function to secrete hormones. A hormone is a chemical that moves throughout the body to help regulate emotions and behaviours. When the hormones released by one gland arrive at receptor tissues or other glands, these receiving receptors may trigger the release of other hormones, resulting in a series of complex chemical chain reactions.
The endocrine system works together with the nervous system to influence many aspects of human behaviour, including growth, reproduction, and metabolism.
And the endocrine system plays a vital role in emotions. Because the glands in men and women differ, hormones also help explain some of the observed behavioural differences between men and women.
The major glands in the endocrine system are shown in Figure 4. The pituitary secretes hormones that influence our responses to pain as well as hormones that signal the ovaries and testes to make sex hormones. The pituitary gland also controls ovulation and the menstrual cycle in women.
Other glands in the endocrine system include the pancreas , which secretes hormones designed to keep the body supplied with fuel to produce and maintain stores of energy ; the pineal gland , located in the middle of the brain, which secretes melatonin , a hormone that helps regulate the wake-sleep cycle; and the thyroid and parathyroid glands , which are responsible for determining how quickly the body uses energy and hormones, and controlling the amount of calcium in the blood and bones.
The body has two triangular adrenal glands , one atop each kidney. The adrenal glands produce hormones that regulate salt and water balance in the body, and they are involved in metabolism, the immune system, and sexual development and function.
The most important function of the adrenal glands is to secrete the hormones epinephrine also known as adrenaline and norepinephrine also known as noradrenaline when we are excited, threatened, or stressed. Epinephrine and norepinephrine stimulate the sympathetic division of the ANS, causing increased heart and lung activity, dilation of the pupils, and increases in blood sugar, which give the body a surge of energy to respond to a threat. The activity and role of the adrenal glands in response to stress provide an excellent example of the close relationship and interdependency of the nervous and endocrine systems.
A quick-acting nervous system is essential for immediate activation of the adrenal glands, while the endocrine system mobilizes the body for action.
The male sex glands , known as the testes , secrete a number of hormones, the most important of which is testosterone , the male sex hormone. Testosterone regulates body changes associated with sexual development, including enlargement of the penis, deepening of the voice, growth of facial and pubic hair, and the increase in muscle growth and strength.
The ovaries , the female sex glands , are located in the pelvis. A nerve may also have blood vessels enclosed in its connective tissue wrappings. Twelve pairs of cranial nerves emerge from the inferior surface of the brain. All of these nerves, except the vagus nerve , pass through foramina of the skull to innervate structures in the head , neck , and facial region. The cranial nerves are designated both by name and by Roman numerals, according to the order in which they appear on the inferior surface of the brain.
Most of the nerves have both sensory and motor components. Three of the nerves are associated with the special senses of smell , vision, hearing , and equilibrium and have only sensory fibers. Five other nerves are primarily motor in function but do have some sensory fibers for proprioception.
The remaining four nerves consist of significant amounts of both sensory and motor fibers. Acoustic neuromas are benign fibrous growths that arise from the balance nerve, also called the eighth cranial nerve or vestibulocochlear nerve. These tumors are non- malignant , meaning that they do not spread or metastasize to other parts of the body.
The location of these tumors is deep inside the skull, adjacent to vital brain centers in the brain stem. As the tumors enlarge, they involve surrounding structures which have to do with vital functions. In the majority of cases, these tumors grow slowly over a period of years.
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