|
The Nervous System - Advanced Version / The Brain
Page: 5
 Below the diencephalon are the midbrain (or mesencephalon) and the hindbrain (or rhombencephalon). The hindbrain consists of the pons (or metencephalon) and the medulla oblongata (or myelencephalon). These parts of the brain stem are characterized by three main features: (1) a roof plate superior to the cerebral aqueduct and fourth ventricle; (2) a central core of gray matter, known as the reticular formation; and (3) a massive basal collection of fibers descending from the cerebral cortex to the brain stem and spinal cord.
The roof plate of the midbrain is formed by two paired rounded eminences, the superior and inferior colliculi. The superior colliculus receives input from the retina and the visual cortex and participates in a variety of visual reflexes, particularly the tracking of objects in the contralateral visual field. The inferior colliculus receives both crossed and uncrossed auditory fibers and projects upon the medial geniculate body, the auditory relay nucleus of the thalamus (see above Diencephalon). In the hindbrain the roof plate is formed by the cerebellum and a membrane containing the choroid plexus of the fourth ventricle.
The reticular formation contains a core collection of cells of various sizes that project to the thalamus, the cerebellum, and the spinal cord. Surrounding this core are long ascending and descending tracts in which various cranial nerve nuclei are embedded.
Fibers derived from the cerebral cortex lie on or near the ventral surface of the midbrain, pons, and medulla. At the midbrain they gather into two bundles called the crura cerebri; from there they descend into the pons, where most terminate upon cell nuclei that project into the cerebellum. These constitute the corticopontine tract. The other major tract, called the corticospinal tract, forms the medullary pyramids before descending to the spinal cord.
The midbrain contains the nuclear complex of the oculomotor nerve as well as the trochlear nucleus; these cranial nerves innervate muscles that move the eye and control the shape of the lens and the diameter of the pupil. In addition, between the midbrain reticular formation (known here as the tegmentum) and the crus cerebri is a large, pigmented structure called the substantia nigra. This nucleus consists of two parts, the pars reticulata and the pars compacta. Cells of the pars compacta contain the black pigment melanin; these synthesize dopamine and project to cells of either the caudate nucleus or the putamen but not to both. By exercising an inhibitory action on large aspiny cholinergic neurons in the neostriatum, the dopaminergic cells of the pars compacta influence the output of the neurotransmitter GABA from spiny striatal neurons. These spiny neurons in turn project to cells of the pars reticulata, which, by projecting fibers to the thalamus, are in effect part of the output system of the corpus striatum.
At the caudal midbrain, crossed fibers of the superior cerebellar peduncle (the major output system of the cerebellum) surround and partially terminate in a large, centrally located structure known as the red nucleus. Most crossed ascending fibers of this bundle project to thalamic nuclei, which have access to the primary motor cortex. A smaller number of fibers synapse on large cells in caudal regions of the red nucleus; these give rise to the crossed fibers of the rubrospinal tract (see below The spinal cord: Descending spinal tracts).
 The pons consists of two parts: the tegmentum, a phylogenetically older part that contains the reticular formation; and the pontine nuclei, a larger part composed of masses of neurons that lie among large bundles of longitudinal and transverse fibers.
Fibers originating from neurons in all major lobes of the cerebral cortex terminate upon the pontine nuclei, which in turn project to the opposite cerebellar hemisphere. These massive crossed fibers form the middle cerebellar peduncle—in effect serving as the bridge that connects each cerebral hemisphere with the opposite half of the cerebellum.
The reticular formation in the pontine tegmentum contains multiple cell groups that exert facilitating influences upon motor function. It also contains the nuclei of several cranial nerves. The facial nerve and the two components of the vestibulocochlear nerve, for example, emerge from and enter the brain stem at the junction of the pons, medulla, and cerebellum. Motor nuclei for the trigeminal nerve lie in the upper pons. Located on the periphery of the pons are long ascending and descending tracts that connect the brain to the spinal cord.
 The medulla oblongata is closest to the spinal cord, and is involved with the regulation of heartbeat, breathing, vasoconstriction (blood pressure), and reflex centers for vomiting, coughing, sneezing, swallowing, and hiccuping.
It is the most caudal segment of the brain stem, and appears as a conical expansion of the spinal cord. Both the pons and the medulla are separated from the overlying cerebellum by the fourth ventricle, and cerebrospinal fluid entering the fourth ventricle from the cerebral aqueduct passes into the cisterna magna, a subarachnoid space surrounding the medulla and the cerebellum, via foramina in the lateral recesses and in the midline of the ventricle.
At the transition from the medulla to the spinal cord, there are two major decussations, or crossings, of nerve fibers. The corticospinal decussation is the site at which 90 percent of the fibers of the medullary pyramid cross and enter the dorsolateral funiculus of the spinal cord. Signals conveyed by this tract provide the basis for voluntary motor function on the opposite side of the body (see below The spinal cord: Descending spinal tracts). In the other decussation, sensory fibers ascending in the fasciculus gracilis and fasciculus cuneatus of the spinal cord terminate upon large nuclear masses on the dorsal surface of the medulla. Known as the nuclei gracilis and cuneatus, these masses give rise to fibers that decussate above the corticospinal tract and form a major ascending sensory pathway known as the medial lemniscus. Present at all brain-stem levels, the medial lemniscus projects upon the somesthetic relay nuclei of the thalamus.
The medulla contains nuclei associated with the hypoglossal, accessory, vagus, and glossopharyngeal cranial nerves. In addition, it contains portions of the vestibular nuclear complex, parts of the trigeminal nuclear complex concerned with pain and thermal sense, and solitary nuclei related to the vagus, glossopharyngeal, and facial nerves that subserve the sense of taste.
Of several medullary relay nuclei that project to the cerebellum via the inferior cerebellar peduncle, the largest is the inferior olive.
 The cerebellum is the second largest part of the brain, after the cerebrum. It functions for muscle coordination and maintains normal muscle tone and posture. The cerebellum coordinates balance.
It overlies the posterior aspect of the pons and medulla and fills the greater part of the back portion of the skull. It consists of two paired lobes, or hemispheres, and a midline portion known as the vermis. Cerebellar cortex appears very different from cerebral cortex in that it consists of small, leaflike laminae, referred to as folia. Structurally the cerebellum consists of a three-layered, gray cellular mantle called the cerebellar cortex and a core of white matter containing four paired intrinsic (i.e., deep) nuclei, the dentate, globose, emboliform, and fastigial. Three paired fiber bundles—the superior, middle, and inferior peduncles—connect the cerebellum with the midbrain, pons, and medulla, respectively.
On an embryological basis the cerebellum can be divided into three parts: (1) the archicerebellum, related primarily to the vestibular system; (2) the paleocerebellum, or anterior lobe, concerned with control of muscle tone; and (3) the neocerebellum, known as the posterior lobe. Receiving input from the cerebral hemispheres via the middle cerebellar peduncle, the neocerebellum is the part most concerned with coordination of voluntary motor function.
The three layers of the cerebellar cortex are an outer synaptic layer (also called the molecular layer), an intermediate discharge layer (the Purkinje layer), and an inner receptive layer (the granular layer). Sensory input from all sorts of receptors are conveyed to specific regions of the receptive layer, which consists of enormous numbers of small nerve cells (hence the name granular) that project axons into the synaptic layer. There they excite the dendrites of the Purkinje cells, which in turn project axons to portions of the four intrinsic nuclei and upon dorsal portions of the lateral vestibular nucleus. Because most Purkinje cells are GABAergic and therefore exert strong inhibitory influences upon the cells that receive their terminals, all sensory input into the cerebellum results in inhibitory impulses being exerted upon the deep cerebellar nuclei and parts of the vestibular nucleus. Cells of all deep cerebellar nuclei, on the other hand, are excitatory (secreting the neurotransmitter glutamate) and project upon parts of the thalamus, red nucleus, vestibular nuclei, and reticular formation.
The cerebellum thus functions as a kind of computer, providing a quick and clear response to any set of sensory signals. It plays no role in sensory perception, but it exerts profound influences upon equilibrium, muscle tone, and the coordination of voluntary motor function. Lesions of the cerebellum produce a constellation of disturbances, including intention tremor, ataxia, hypotonus, easy fatigability, and disturbances of speech.
Take this preliminary to see if your condition could respond to treatment.
Not sure on your treatment options? For a limited time you can schedule a
to talk with a licensed doctor or clinician regarding your condition. Use our ChiroLocator to find a doctor in your area.
|
