Chapter 2. General Description of the Central Nervous System
The two hemispheres of the brain are covered by a layer of cells called the cortex. (Cortex means "bark" in Latin.) The surface of the cortex is ridged, as it is made up of gyri and sulci.
A gyrus is a raised fold of tissue.
A sulcus is a groove between two gyri. A particularly deep sulcus is called a fissure as in the fissure of Rolando or the central sulcus which separates the frontal and parietal lobes. So sometimes the labels can be interchangeable.
A convolution includes both a gyrus and sulcus.
Because it is convoluted, a large amount of cortical tissue fits into a relatively small area.
The basal ganglia or basal nuclei is made up of two structures, the caudate nucleus and the lenticular nucleus.
The caudate nucleus is bounded on one side by the lateral ventricle and is divided into a head, body and tail.
The lenticular nucleus is a lens-shaped structures that has two components: the globus pallidus and the putamen. The putamen is the more lateral of the two. The basal ganglia is a very important part of the extrapyramidal tract. It is the basal ganglia that is responsible for a natural and spontaneous smile, which is quite different from the phony smiles we see in photographs, which originate in the cortex and are transmitted on the pyramidal tract which is responsible for volitional movement.
The internal capsule lies between the lenticular and caudate nuclei. It is a group of myelinated (therefore white) afferent and descending fiber tracts including the pyramidal tract that connects the cortex to other parts of the central nervous system. The pyramidal tract begins as a corona radiata (radiating crown) from motor cells in the premotor, primary motor (where most of the motor cells originate), and primary sensory areas of the cortex and converges into the internal capsule.The capsule itself ends within the cerebrum, but the axons that pass through it continue down to the brain stem, and spinal cord.
Because so many axons join together to pass through this area, the internal capsule is sometimes referred to as a bottleneck of fibers.
Despite its close proximity to the caudate nucleus and lenticular nucleus, the internal capsule is not part of the basal ganglia.
The internal capsule and the basal ganglia are collectively referred to as the corpus striatum.
This is the most ancient and primitive part of the brain. It is sometimes called the rhinencephalon as the term "rhino" means nose in Latin and some of this area is dedicated to the processing of olfactory stimuli.
The limbic system is involved, among other things, with emotion and memory, particularly new memories.
This subcortical structure sits within the brain at the level of the temporal lobe. It is well protected in this location.
The thalamus is made up of three parts, including two thalamic bodies and the tissue that connects them which is called the massa intermedia, or the interthalamic adhesion. The thalamic bodies are separated by the third ventricle, one of the spaces in the brain that is filled with cerebral spinal fluid. The massa intermedia lies within the third ventricle.
The thalamus receives and organizes sensory information from the periphery. Messages from all sensory modalities with the exception of smell pass through the thalamus on their way to cortical centers and other structures for further processing. (Information about smell travels directly to the temporal lobe.)
Sensory information, including touch and kinesthesia passes from the thalamus to the parietal lobe.
Auditory information comes into the thalamus from the inferior colliculi of the midbrain. It is processed by the medial geniculate bodies of the thalamus before being sent to the temporal lobe. In the temporal lobe, it first arrives at Heschl's gyrus, which is the primary auditory area. From there it is sent to association areas for further processing.
Visual information comes into the thalamus from the superior colliculi of the midbrain. It is processed by the lateral geniculate bodies of the thalamus and sent to the primary visual area in the occipital lobe. The visual association areas, also in the occipital lobe, further analyze the information. There are two very important motor relay nuclei in the thalamus: the ventral lateral and ventral anterior. These nuclei receive signals from the basal ganglia and in turn send signals to the motor cortex. They are part of the extrapyramidal tract..
Lesions in the thalamus can cause a type of aphasia.
The hypothalamus is located immediately below the thalamus. Part of it is also slightly anterior to the thalamus. The hypothalamus regulates the functioning of the pituitary gland, so it controls basic biological functions like appetite, body temperature, sex drive, etc. The hypothalamus is the part of the brain that makes you shiver when you are cold and sweat when you are hot. A part of the hypothalamus monitors the level of glucose in the blood and, when it notices a significant decrease, it sends messages to the stomach producing sensations of hunger.
Diabetes Insipidus, the most serious type of diabetes, is caused by lesions in the hypothalamus, or between the hypothalamus and the pituitary gland.
While the thalamus is mainly an input structure, sending messages to higher brain areas, the hypothalamus is an output structure, sending messages to glands and other parts of the body.
The Midbrain (Mesencephalon)
This is the most superior part of the brain stem. The corpora quadrigemina, the red nucleus, the substantia nigra, the cerebral peduncles, and the cell bodies of two cranial nerves are located in the midbrain.
The corpora quadrigemina consists of the tectum which is the roof of the brain stem, and of four protrusions located on the tectum which are called colliculi.
The two superior colliculi are involved in vision. They relay information to the lateral geniculate bodies of the thalamus.
The two inferior colliculi are involved in hearing. They relay information to the medial geniculate bodies of the thalamus.
The red nucleus is part of the extrapyramidal tract and connects the cerebellum to the thalamus and spinal cord.
The substantia nigra is a group of dark colored cell bodies which produce dopamine. It is also part of the extrapyramidal tract.
The cerebral peduncles connect the pons to the cerebrum.
The nuclei of cranial nerve III, the oculomotor cranial nerve, and of cranial nerve IV, the trochlear cranial nerve which both provide innervation for eye movement are also located in the midbrain.
The word "pons" is Latin for "bridge." Fibers found there connect the brain stem to the cerebellum.
The cell bodies for cranial nerves V and VI, the trigeminal and abducens, as well as nuclei of cranial nerve VII, the facial nerve, are located there.
This structure, which is the most inferior part of the brain stem, sits on top of the superior end of the spinal cord. Because it has a rounded shape, it was once called "the bulb." (The term "bulbar" refers to the brain stem.) It is involved in circulation and respiration and has several important landmarks.
The pyramids, which mark the decussation of the pyramidal tract, lie on either side of the median fissure.
The olivary nuclei are posterior to the pyramids. They are involved in the processing and relay of auditory information.
The cell bodies of cranial nerves VIII-XII are located here. Some of the nuclei of CN VII are also found in the medulla.
The word "cerebellum" means "little brain" in Latin. This structure has two hemispheres, each of which is divided into lobes and is covered by the cortex. It is one of the newer parts of the brain and is very important for the production of speech. It organizes muscle activity and plays a role in the coordination of fine motor movements and also in balance.
The cerebellum receives both motor and sensory input, and so is the center of a feedback loop. All motor messages that leave the brain also go to the cerebellum, including information about the strength of the impulses. The cerebellum integrates motor output so that movements are smooth and coordinated. Muscle spindles, joints and tendons send information about movement back to this area. The cerebellum then relays these messages to the cortex, completing the feedback loop.
Lesions here will cause cerebellar or ataxic dysarthria which involves jerky, uncoordinated movements of the speech musculature.
The cerebellum is connected to the brain stem by three pairs of tracts called the cerebellar peduncles.
The spinal cord contains the cell bodies of the spinal nerves as well as their afferent and efferent fibers. It begins below a large opening in the base of the skull called the magnum foramen and extends downward, surrounded and protected by the vertebral column. It does not continue through the whole length of the column, terminating instead slightly above the level of the waist. The part of the vertebral column that lies below the spinal chord is called the cauda equina, which is Latin for horse's tail.
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