![]() These nerves relay impulses to and from the rest of the body. Thirty-one pairs of spinal nerves branch off the spinal cord, exiting between each level of vertebrae. These nerves relay impulses from the sensory organs, such as the eyes or ears. There are 12 pairs of cranial nerves branching off the brain. The brain constantly receives information and sends out instructions to the body through the spinal cord and the body’s vast network of nerves. The brain is the control center of the nervous system. Nerves consist of bundles of both sensory and motor neurons. Association neurons make up 90 percent of all neurons and are found only in the brain and spinal cord. A nerve impulse to a muscle, for example, may cause it to contract. Motor neurons transmit nerve impulses from the brain and spinal cord to a specific area of the body. They also carry nerve impulses to the brain and spinal cord. Sensory neurons transmit nerve impulses from sense organs (eyes, ears, nose, tongue, and touch) to the brain. There are three types of neurons: sensory, motor, and association. When an impulse arrives at the end of an axon, it releases chemicals that generate an impulse in the dendrites of the neighboring neuron. Neurons do not touch, but are separated by a tiny gap called a synapse. An axon, or nerve fiber, carries nerve impulses to other neurons or to muscle. Neurons have many branched endings, called dendrites, which receive impulses from other neurons. Neurons are long, thin cells that transmit electrical impulses. The nervous system is made up of cells called neurons. The PNS consists of nerves bundles made up of sensory and motor neurons. The nerves branching off the central nervous system make up the peripheral nervous system, or PNS. The brain and spinal cord make up the central nervous system, or CNS. The nervous system has two main parts: the central and the peripheral nervous system. Others, we consciously control, such as movement, talking, eating, and thinking. People are not generally conscious of some of these activities as they occur, such as regulating body temperature, breathing, or heart rate. The nervous system is a communication network that controls and coordinates most body actions. The occipital condyles on either side of the foramen magnum articulate with the first vertebra (C1) of the spine to permit up-and-down movement of the head. The spinal cord passes through the largest hole, called the foramen magnum, in the base of the cranium to join the brain. Small holes in the skull bones, called foraminae, and canals enable blood vessels, such as the carotid arteries and nerves, to enter and leave the skull. The maxillae and mandible secure the teeth. The vomer is a single bone that makes up part of the nasal septum, which divides the nostrils, and the mandible forms the lower jaw. Each of the following facial bones are paired: the maxillae form the upper jaw and front of the hard palate the zygomatic bones form the cheeks the nasal bones form the bridge of the nose the lacrimal bones form part of the orbit, or eye socket the palatine bones form the rear of the hard palate and the inferior nasal conchae divide the nasal cavity. The 14 facial bones provide the structure of the face and form the openings through which food, water, and air enter the body. Shaped like a butterfly, the sphenoid bone forms the middle part of the cranial floor. The ethmoid bone forms part of the nasal cavity. The occipital bone forms the cranial rear and floor. Two temporal bones form the lower cranial sides. Two parietal bones form the sides of the cranial roof. By one-and-a-half years of age, the skull sutures have formed, and the fontanels have disappeared. Fontanels allow the skull to be compressed slightly during birth and accommodate growth of the brain during early infancy. In newborns, the skull bones are not completely fused they are linked by fibrous membranes called fontanels. The mandible, or lower jaw, the only moveable skull bone, allows the mouth to open and close. The sutures lock the edges of the skull bones together, like pieces in a puzzle, to form a structure that is both rigid and strong. In adults, all but one of the skull bones are fused together by immovable joints called sutures. The skull is made up of 22 bones: the cranium includes eight bones that surround and protect the brain and 14 bones that form the face. ![]()
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