Nervous System Student Learning Objectives: Identify the primary parts of the neuron Identify the major structures of the central nervous system Identify the major structures of the peripheral nervous system Structures to be studied: Brain parts: Cerebrum Corpus callosum Cerebellum Thalamus Hypothalamus Pituitary gland Brainstem: Midbrain Pons Medulla oblongata Spinal cord: Spinal nerves Nerve plexuses: Cervical Phrenic nerve Brachial Radial nerve Median nerve Ulnar nerve Lumbar Sciatic nerve Sacral Femoral nerve Cranial nerves: Olfactory (I) Optic (II) Oculomotor (III) Trochlear (IV) Trigeminal (V) Abducens (VI) Facial (VII) Vestibulocochlear (VIII) Glossopharyngeal (IX) Vagus (X) Accessory (XI) Hypoglossal (XII) Introduction The nervous system is one of the two main controlling systems of the body (the other is the endocrine system). The neurons are cells in the nervous tissue that are capable of sending electrical impulses. These electrical impulses help to control the actions of muscle and glandular tissues in the other body organ systems. The nervous system is arranged into two main sections. The central nervous system includes the structures of the brain and spinal cord. The peripheral nervous system includes the nerves that conduct information between the central nervous system and the body structures. One portion of the peripheral nervous system, the special senses, will be explored during the next anatomy exercise.
Neuron The neuron is composed of a central cell body that contains a majority of the organelles of this cell. The Nissl bodies are clumps of rough endoplasmic reticulum. These structures manufacture the neurotransmitters used to communicate with other cells. Extending out from the cell body are various fibers. One of the fibers is always an axon. The axon transmits messages away from the cell body of the neuron to other cells. This fiber lacks organelles and can be distinguished from the other fibers that can be associated with the neuron using a special stain. The axons release neurotransmitters from synaptic bulbs found at the end of the axon. The other fibers there can be one or more of these are the dendrites. These fibers are attached to receptors (special detectors), and conduct messages towards the cell body. Organelles, such as the Nissl bodies, extend into these fibers. [Note: On this model, the axon has been sliced so the long fiber that you see at the top of the picture would normally be attached to the left of the axon that you see attached to the cell body.]
Central Nervous System The central nervous system is composed of the brain and spinal cord. The brain lies within the cranial vault of the skull, surrounded and protected by the skull bones. By the age of two, the skull bones have stopped growing and the brain becomes trapped in a relatively small box. The largest portion of the brain is the cerebrum. This structure has two halves, or hemispheres, divided by a large crevice at the center. The only communication between these hemispheres is through fibers of the corpus callosum. Voluntary muscle actions and much of the intellectual information processing are initiated here. From the side, most of the brain tissue that you can see is cerebrum. The cerebrum is divided up into different regions, or lobes. There are four lobes in each hemisphere. The lobes have names associated with the bones that lie close to the lobe in the skull. The frontal lobe is the most anterior of the lobes. This lobe lies directly behind the frontal bone near the forehead. The parietal lobe is at the center near the top of the brain. The temporal lobe is just below the parietal lobe, directly in from the ears. The occipital lobe is located at the back of the cerebrum, near the occipital bone. Also from the side, you can see a couple of additional structures. Inferior to the occipital and temporal lobes of the cerebrum is a structure called the cerebellum. This structure resembles a chunk of cauliflower! The cerebellum is the second largest part of the brain and is involved in muscle coordination and involuntary muscle actions.
Most of the rest of the structures that can be viewed from the side of the brain belong to the brainstem. Structures in the brainstem help control body functions that are necessary for your existence (i.e. to just keep you alive). Three structures the medulla oblongata, pons, and midbrain form the brainstem. The medulla oblongata, or medulla, is the largest part of the brainstem and attaches to the spinal cord. The junction between the medulla and the brainstem occurs at about the same place as the skull attaches to the first cervical vertebra not very good planning because damage to the medulla could result in instant death, and a blow to the base of the skull could kill you! Just in front of the medulla is the pons. This oval structure is about the size of a walnut. The final part of the brainstem, the midbrain cannot be seen from the side. We will look at this structure later. The final structure that can be viewed from the side of the brain is the pituitary gland. This structure is not part of the brainstem. The pituitary gland plays a major role in the endocrine system and will be investigated in more detail later in the semester with that system. If the brain was sliced open, from front to back, right through the center of the cerebrum where the two hemispheres meet, you would be able to see something resembling our next model. Notice from this view, that you can again see the cerebrum, cerebellum, medulla oblongata, pons, pituitary gland, and spinal cord. There are a number of additional structures, however, that you can also see from this central (i.e. mid-sagittal) view.
On the mid-sagittal view below, you can see some additional structures, including the corpus callosum, the band of fibers that helps connect the two cerebral hemispheres. The corpus callosum is the only way in which the two hemispheres of the cerebrum can communicate with one another. The midbrain, the final portion of the brainstem, can also be viewed on this picture. The midbrain lies just superior and posterior to the pons and medulla oblongata. In the center of the brain, you can view the thalamus which is involved in partially evaluating sensory information for the brain. Between the thalamus and the pituitary gland is the very important hypothalamus which helps control most of the body organs through its involvement with the endocrine and the autonomic nervous systems. The pineal body is a tiny structure that appears to be associated with biorhythms.
When viewing the brain from the bottom (or ventral surface), you will see some familiar structures: cerebrum, cerebellum, medulla oblongata, pons, and pituitary gland. There are, however, a couple of additional structures that you can also see. Near the anterior/front of the brain, you will see some bands of tissue that are part of the olfactory bulbs and tracts. These structures help conduct nerve messages from the nose to the brain for smell. Near the center of the brain, just anterior to the pituitary gland are additional bands of tissue that form an X and are known as the optic chiasm. These structures help conduct nerve messages associated with vision from the eyes to the brain. The spinal cord attaches to the brain at the medulla and then travels out of the foramen magnum of the occipital bone and through the vertebral canal of the vertebra. This tunnel, bordered by the body of the vertebra and the dorsal and transverse processes, extends all the way down through the sacrum. Every place where a pair of vertebra meet, there will be a pair of spinal nerves that emerges from the spinal cord and exits from the spinal canal. These nerves are named for the section of the spinal column from which they emerge: cervical, thoracic, lumbar, sacral, and coccygeal. The discussion of the spinal cord will continue with the Peripheral Nervous System in the next section.
Peripheral Nervous System The peripheral nervous system relates to the nervous tissue structures that connect the central nervous system to the rest of the body organs. This will involve not only the spinal nerves that are associated with the spinal cord, but also cranial nerves that originate from the brain. Most of the spinal nerves emerge from the spinal cord and after a short distance join with other spinal nerves to form nerve plexuses. The fibers of the different spinal nerves in a nerve plexus mix together and then redistribute to form new nerves that go out to specific body regions. The most superior of the nerve plexuses is the cervical plexus which forms from several of the cervical spinal nerves. Leaving the cervical plexus are new nerves which serve the head and neck area. One of the important nerves associated with the cervical plexus is the phrenic nerve which is the only cervical plexus nerve not associated with structures of the head and neck. This nerve innervates the diaphragm and is involved in breathing. It is not easy to see the phrenic nerve on the model. The brachial plexus is formed by the lower cervical spinal nerves and the first thoracic nerve. The nerves leaving the brachial plexus go to the structures of the arm. Three important nerves emerge from this plexus. The radial nerve runs down the lateral portion of the arm towards the thumb, innervating muscles along that side of the arm. The ulnar nerve runs down the medial side of the arm to the little finger, providing innervation for medial structures of the arm. The median nerve runs between the other two nerves and innervates the remaining structures of the arm. The thoracic spinal nerves do not form a plexus like the rest of the spinal nerves. These nerves are referred to as the intercostals nerves and are associated with the rib region. They go directly to the body area without forming a plexus.
The lumbar plexus involves nerves of the lower back (i.e. the lumbar nerves). The nerves emerging from this plexus provide nerve supply to the pelvic region and the anterior/front of the leg. The most important of the nerves associated with this plexus is the femoral nerve which innervates the quadriceps muscles of the thigh. The sacral plexus combines the sacral spinal nerves and supplies nerves to the pelvic region and the posterior/back of the leg. The sciatic nerve is a very large nerve that leaves this plexus, runs down the back of the leg, and helps control actions of the hamstrings and muscles of the lower leg. When giving intramuscular injections in the buttocks, one needs to be careful not to inject materials into or near this nerve. This is why the gluteus medius muscle is used, rather than the gluteus maximus, for such injections. The gluteus maximus lies directly over the sciatic nerve. The gluteus medius is off to the side of the sciatic nerve.
The cranial nerves originate from the brain tissue and are numbered using Roman numerals. You will not be asked to identify these nerves on the anatomy quiz, but the information provided here is to assist you with learning lecture material. There are twelve pairs of cranial nerves: You have already looked at the olfactory nerve (CN I) when you studied the olfactory bulbs and tracts of the brain. This nerve is associated with smell. You also looked at the optic nerve (CN II) when you studied the optic chiasm of the brain tissue. This nerve is associated with vision. The occulomotor nerve (CN III) helps control eye movement and the size/shape of some of the internal eye structures used for focusing and light control. The trochlear nerve (CN IV) is also involved in eye movement. The trigeminal nerve (CN V) is one of the major nerves associated with the face. Most of the sensory information from the face travels through this nerve. This nerve also controls the muscles associated with mastication (chewing). The abducens nerve (CN VI) is a third nerve involved in the movement of the eye. The facial nerve (CN VII) is the other major nerve of the face region. This nerve is involved in controlling the muscles of facial expression. The vestibulocochlear nerve (CN VIII) is associated with hearing and equilibrium. The glosspharyngeal nerve (CN IX) helps control movements of the throat and tongue, as well as detect changes in blood pressure. The vagus nerve (CN X) is the major nerve associated with the organs of the thoracic, abdominal, and pelvic cavities and conducts a majority of the parasympathetic nervous system impulses for the autonomic nervous system. The spinal accessory nerve (CN XI) innervates the muscles of the throat and neck region to control their motor activities. The hypoglossal nerve (CN XII) is associated with movements of the tongue.