The brain and spinal cord the CNS function as the control center. They receive data and feedback from the sensory organs and from nerves throughout the body, process the information, and send commands back out. Nerve pathways of the PNS carry the incoming and outgoing signals. Twelve pairs of cranial nerves connect the brain to eyes, ears, and other sensory organs and to head and neck muscles. Thirty-one pairs of spinal nerves branch out from the spinal cord to tissues of the thorax, abdomen, and limbs.
Each nerve is responsible for relaying sensory information, sending motor commands, or both. All nervous tissue, from the brain to the spinal cord to the furthest nerve branch, includes cells called neurons.
Neurons are charged cells: they conduct electrical signals to pass information through the body. A typical neuron consists of a cell body, dendrites, and an axon with an axon terminal. The dendrites receive signals from body tissues or other neurons and pass them into the cell body. If an outgoing signal is produced, it zips down the axon to the axon terminal and passes to the next neuron or target cell. This conductive capability sends information up and down nerve pathways and through the central nervous system at incredible speed.
In the PNS, sensory receptor neurons respond to physical stimuli in our environment, like touch or temperature, and send signals that inform the CNS of the state of the body and the external environment.
This sensory information is then processed by the CNS, predominantly by the brain. After information is processed, motor neurons return signals to the muscles and glands of the PNS, which responds with motor output. Central neurons, which in humans greatly outnumber the sensory and motor neurons, make all of their input and output connections with other neurons. The connections of these neurons form neural circuits that are responsible for our perceptions of the world and determine our behavior.
Along with neurons, the nervous system relies on the function of other specialized cells called glial cells, or glia, that provide structural and metabolic support to the nervous system. Learning Objectives Describe the organization of the nervous system. It should be noted that some functions can be contained entirely within the CNS; for example, dreaming, thinking, or even information storage.
Neurons of the efferent division of the PNS can be further subdivided into the somatic nervous system, which controls the voluntary movement of skeletal muscle and the autonomic nervous system which regulates involuntary functions of organs and tissues. Autonomic neurons are further subdivided into sympathetic and parasympathetic systems see first figure.
The autonomic nervous system will be addressed in a separate module. A third division of the PNS is a semi-independent nervous system called the enteric nervous system which controls the gastrointestinal tract see first figure. This system is considered semi-independent because it can run independently, or through modulation by the autonomic nervous system. It is also interesting to note that the enteric nervous system contains more neurons than the entire spinal cord.
Introduction to the Nervous System Imagine that you suddenly lost the ability to stand unless you looked down at your feet. The same axons extend from the eye to the brain through these two bundles of fibers, but the chiasm represents the border between peripheral and central. A similar situation outside of science can be described for some roads. Table 1 helps to clarify which of these terms apply to the central or peripheral nervous systems.
This is a tool to see the structures of the body not just the nervous system that depends on magnetic fields associated with certain atomic nuclei. The utility of this technique in the nervous system is that fat tissue and water appear as different shades between black and white. Because white matter is fatty from myelin and gray matter is not, they can be easily distinguished in MRI images. The nervous system can also be divided on the basis of its functions, but anatomical divisions and functional divisions are different.
The CNS and the PNS both contribute to the same functions, but those functions can be attributed to different regions of the brain such as the cerebral cortex or the hypothalamus or to different ganglia in the periphery. The problem with trying to fit functional differences into anatomical divisions is that sometimes the same structure can be part of several functions.
For example, the optic nerve carries signals from the retina that are either used for the conscious perception of visual stimuli, which takes place in the cerebral cortex, or for the reflexive responses of smooth muscle tissue that are processed through the hypothalamus.
There are two ways to consider how the nervous system is divided functionally. First, the basic functions of the nervous system are sensation, integration, and response.
Secondly, control of the body can be somatic or autonomic—divisions that are largely defined by the structures that are involved in the response. There is also a region of the peripheral nervous system that is called the enteric nervous system that is responsible for a specific set of the functions within the realm of autonomic control related to gastrointestinal functions. The nervous system is involved in receiving information about the environment around us sensation and generating responses to that information motor responses.
The nervous system can be divided into regions that are responsible for sensation sensory functions and for the response motor functions. But there is a third function that needs to be included. Sensory input needs to be integrated with other sensations, as well as with memories, emotional state, or learning cognition. Some regions of the nervous system are termed integration or association areas.
The process of integration combines sensory perceptions and higher cognitive functions such as memories, learning, and emotion to produce a response. The first major function of the nervous system is sensation—receiving information about the environment to gain input about what is happening outside the body or, sometimes, within the body.
The sensory functions of the nervous system register the presence of a change from homeostasis or a particular event in the environment, known as a stimulus.
The stimuli for taste and smell are both chemical substances molecules, compounds, ions, etc. There are actually more senses than just those, but that list represents the major senses.
Those five are all senses that receive stimuli from the outside world, and of which there is conscious perception. Additional sensory stimuli might be from the internal environment inside the body , such as the stretch of an organ wall or the concentration of certain ions in the blood.
The nervous system produces a response on the basis of the stimuli perceived by sensory structures. An obvious response would be the movement of muscles, such as withdrawing a hand from a hot stove, but there are broader uses of the term. The nervous system can cause the contraction of all three types of muscle tissue. For example, skeletal muscle contracts to move the skeleton, cardiac muscle is influenced as heart rate increases during exercise, and smooth muscle contracts as the digestive system moves food along the digestive tract.
Responses also include the neural control of glands in the body as well, such as the production and secretion of sweat by the eccrine and merocrine sweat glands found in the skin to lower body temperature. Responses can be divided into those that are voluntary or conscious contraction of skeletal muscle and those that are involuntary contraction of smooth muscles, regulation of cardiac muscle, activation of glands. Voluntary responses are governed by the somatic nervous system and involuntary responses are governed by the autonomic nervous system, which are discussed in the next section.
Stimuli that are received by sensory structures are communicated to the nervous system where that information is processed. This is called integration.
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