Structure in the Brain - Article Example

 Structure in the Brain SUB TOPICS I. Brain Stem midbrain Structure in the Brain II. Cerebellum Structure in the Brain STRUCTURE IN THE BRAIN: The brain of human beings is a complex central nervous system organ located in the cranial cavity. BRAIN STEM The brainstem is made up of the I. Medulla oblongata, II. The pons III. The mesencephalon (midbrain) MEDULLA OBLONGATA Definition The medulla oblongata lies in the posterior cranial fossa and rests on the basilar part of the occipitalbone. Site It connects the pons superiorly with the spinal cord inferiorly, and contains most of the ascending and descending pathways represented in the cord. The junction of medulla and spinal cord is at the origin of the anterior and posterior roots of the first cervical spine nerve, which is at the level of the foramen magnum. This level is known as the cranio-cervical junction (CCJ). Function The medulla regulates vital functions of the body, such as respiratory, vasomotor and cardiac activity. It is also influences deglutition (swallowing), vomiting, sweating and gastric secretions. The medulla also contains the nuclei of origin of the ninth through to the twelfth cranial nerves. THE PONS Definition The pons contains the nuclei of origin of the fifth through to eighth cranial nerves. The word pons is Latin for 'bridge', and is so named because of the appearance presented on its anterior surface, that of a bridge connecting the two cerebellar hemispheres. Site The pons is situated in the posterior cranial fossa, and rests upon the dorsum sellae of the sphenoid bone, as well as the upper part of the occipital bone. Function The pons connects the medulla to the mesencephalon, and relays impulses from the cerebral cortex to the cerebellum. MESENCEPHALON Definition The mesencephalon is the shortest part of the brainstem. Anatomy Connects the pons and cerebellum with the prosencephalon. Its long axis inclines anteriorly, and consists of two cerebral peduncles, which converge upon each other caudally as they descend toward the pons. The notch between them is known as the interpeduncular fossa. Function A portion of the mesencephalon contains the reflex centres for visual, auditory and tactile impules. The mesencephalon contains the nuclei of origin of the third and fourth cranial nerves CEREBELLUM Definition The cerebellum consists of a central portion known as the vermis, and two lateral portions, the cerebellar hemispheres Site It is located in the posterior cranial fossa of the skull, and is separated from the cerebrum by a layer of dura mater known as the tentorium cerebelli. It is connected to the brain stem by three bands of fibres, the cerebellar peduncles. The superior peduncles connect with the mesencephalon, the middle peduncle with the pons and the inferior peduncle with the medulla oblongata. The symptom with pathology in the region may present with I. Disturbances of posture and gait, equilibrium, control of movements and of speech. II. Muscle tone will be decreased (hypotonia), III. action tremors may be present IV. Possibly also nystagmus (tremor-like movements of the eyes). Function The cerebellum maintains equilibrium and muscle tone, and controls and coordinates voluntary movements. The cerebrum is the prosencephalon, and is composed of the I. Diencephalon II. Telencephalon (the cerebral hemispheres). DIENCEPHALON The diencephalon consists of a central cavity, which is the third ventricle, and the structures that form its boundaries a. the thalamus b. hypothalamus c. epithalamus and d. subthalamus. THALAMUS Definition The thalamus forms the lateral walls of the third ventricle. Site It is one of the most important sensory centres of the brain Function It acts like an antechamber through which all sensory data must pass before it reaches the cerebral cortex. HYPOTHALAMUS Definition The hypothalamus is ventral to the thalamus, and forms the floor the third ventricle. Functions Maintaining the body water balance Maintaining the body temperature Reproductive functions and sexual development Emotional response Autonomic nervous system regulation Food intake control Circadian and cycle rhythm maintenance EPITHALAMUS Definition The epithalamus forms the thin roof of the third ventricle. It consists of the habenular nuclei and their connections, and the pineal gland. The habenular neucleus is believed to be a centre for the integration of olfactory, visceral and somatic afferent pathways. Anatomy At the point where the epithalamus becomes continuous with the midbrain, there is a thickening known as the posterior commissure. The symptom the patient suffering from lesion in the gland may present with clinical signs such as; I. precocious puberty The types of biogenic amines found in pineal gland include; I. serotonin II. norepinephrine III. melatonin Function The pineal gland is an endocrine gland, which can affect the activities of the pituitary gland, the islets of Langerhans, the parathyroids, the adrenals and the gonads. The actions of the pineal secretions are such that patients suffering from lesions in this gland may present with clinical signs such as precocious puberty. SUBTHALAMUS Site The subthalamus lies inferior to the thalamus. Function Lesions in this region result in sudden, forceful involuntary movements in a contralateral extremity. TELENCEPHALON Site The right and left cerebral hemispheres are separated by the longitudinal cerebral fissure. This fissure contains a fold of dura mater known as the falx cerebri. Deep in this fissure is the corpus callosum, connecting the hemispheres across the midline. The hemispheres are also separated from the cerebellum by the transverse fissure. SULCI There are four main sulci, and these are used to delineate the lobar anatomy of the cerebral hemispheres a. Sparieto-occipital; sulcus begins on the superior medial margin of each hemisphere, anterior to the occipital pole. It passes downward and anteriorly on the medial surface, where it joins the calcarine b. Central sulcus; starts at the midpoint of the longitudinal fissure, and runs down the lateral surface of the cerebral hemisphere .It is an important anatomical landmark, as it separates the motor strip or pre-central gyrus, which contains the motor cells that initiate movement in the opposite side of the body, from the sensory strip or post-central gyrus, which contains the general sensory cortex that receives sensory information from the opposite side of the body. c. Lateral sulcus; This fissure begins just above the temporal pole and runs obliquely upward on the lateral surface of the cerebrum .Centres for speech and hearing lie close to this sulcus, and it separates the temporal lobe from the rest of the cerebrum d. Calcarine sulcus; is on the medial surface of the posterior part of the hemisphere. It arches upward and backward to reach the occipital pole. The visual area occupies the walls of the posterior part of the calcarine fissure LOBAR ANATOMY There are four main lobes of the cerebral hemisphere, namely I. the frontal, II. parietal, temporal III. occipital lobes; I. The frontal lobe is separated from the parietal lobe by the central sulcus. II. The temporal lobe is separated from the frontal and parietal lobes by the lateral sulcus. III. The occipital lobe is separated from the parietal lobe on the medial surface of the hemisphere by the parieto-occipital sulcus. VENTRICULAR SYSTEM Defination The ventricular system of the brain consists of two lateral ventricles, a third and a fourth ventricle. A lateral ventricle is present in each hemisphere. They are both roughly C-shaped, and consist of a body, with anterior, posterior and inferior horns. Anatomy Each ventricle has a choroid plexus on its medial aspect, which is a vascular fringe composed of pia mater covered with ependymal lining. The two lateral ventricles communicate with the third ventricle through the interventricular foramen, also known as the foramen of Monro. The third ventricle is a slitlike cleft between the two thalami, with the choroid plexus in the roof. This third ventricle communicates with the fourth ventricle via the cerebral aqueduct, also known as the Aqueduct of Sylvius. The fourth ventricle is a cavity anterior to the cerebellum and posterior to the pons and superior half of the medulla oblongata. It is continuous inferiorly with the central canal of the spinal cord. MENINGES OF THE BRAIN The brain and spinal cord are enclosed in three membranes, known as the meninges. Named from outer to inner they are the I. dura mater, II. arachnoid mater III. pia mater DURA MATER Definition The outer layer of the dura is closely related to the periosteum of the inner surface of the cranial bones, and is thus known as the periosteal dura. Function The inner meningeal dura folds upon itself, and pushes between parts of the brain to aid in support and protection. It ultimately forms the a. falx cerebri; situated in the midline and separates the two hemispheres above the corpus callosum b. the tentorium cerebella; separates the cerebellum from the occipital lobe. c. falx cerebella; a small fold which extends forwards from the internal occipital crest into the posterior cerebellar notch. d. the diaphragma sella; a roof over the pituitary fossa ARACHNOID MATER Definition The arachnoid mater is a delicate, nonvascular membrane, which passes over the sulci, but does not dip into them. Site Between the dura and arachnoid mater is a potential space, known as the subdural space. PIA MATER Definition The pia mater is a vascular membrane, which closely invests the brain. Function It covers the gyri and descends into the sulci. Between the arachnoid and pia mater is the subarachnoid space, which is filled with cerebral spinal fluid and contains the large blood vessels of the brain. In certain locations, this space is quite large, and is known as a cistern. Examples of cisterns include the cisterna magna (between the inferior surface of the cerebellum and the medulla oblongata), the pontine cistern (ventral to the pons) and the lumbar cistern (beyond the caudal end of the spinal cord). CEREBROSPINAL FLUID Cerebrospinal fluid (CSF) is produced by the choroid plexuses in the lateral, third and fourth ventricles. It has many functions including: • the support of the brain, brainstem and spinal cord • preventing concussion of the brain due to acceleration/deceleration by its buoyancy • removing waste products of nerve cell metabolism • removing drugs and other chemicals that have diffused across the blood-brain barrier. From the fourth ventricle, the majority of CSF enters the subarachnoid space via three openings; namely, the two lateral openings of the ventricle known as the foramina of Luschka the medial opening, the foramen of Magendie. Some CSF also proceeds down the central canal of the spinal cord. Upon entering the subarachnoid space, CSF slowly spreads over the surface of the cerebral hemispheres, and is ultimately absorbed into the blood stream predominantly via the superior sagittal (venous) sinus. Movement of CSF in the subarachnoid space is believed to be due to pulsations of the cerebral and spinal arteries, as well as movement of the vertebral column GREY AND WHITE MATTER Defination Grey matter consists of nerve cells, and the proximal portions of their processes are embedded in neuroglia. Neuroglia is specialised supporting tissue found within the central nervous system. Anatomy The cerebral cortex forms a complete covering of gray matter over each cerebral hemisphere, and can reach up to 5 mm in thickness. White matter consists of nerve fibres (axons) embedded in the neuroglia, and is white in colour due to the myelin around them. The basal ganglia (nuclei) constitute the central gray matter, and lie between the white matter and the thalamus. These ganglia are the caudate nucleus putamen, claustrum globus pallidus. The globus pallidus and the putamen together are known as the lentiform nucleus. Diseases of the basal ganglia such as Parkinson's Disease and Huntington's chorea cause a variety of disorders involving; i. Voluntary muscular symptoms ii. Loss of control and coordination of movements. INTERNAL AND EXTERNAL CAPSULE Definition Fibre tracts of white matter connect the cerebral cortex to the brainstem and spinal cord. As the tracts approach the brainstem, they divide and pass around the gray matter of the basal ganglia. One part of this divided tract lies between the caudate nucleus (which is medial) and the lentiform nucleus (which is lateral), and forms a broad mass of fibres known as the internal capsule. Site Because of the wedge shaped form of the lentiform nucleus, the internal capsule is bent, forming anterior and posterior limbs, connected by the genu. The remaining fibres pass to the outside of the lentiform nucleus, and are known as the external capsule. PITUITARY GLAND Definition The pituitary gland is part of the endocrine system, and is a pea-sized organ located in the sella turcica of the sphenoid bone. Anatomy Superiorly, it is attached to the hypothalamus via a stalk, the infundibulum. The gland has a dual origin, and is composed of an anterior, intermediate and posterior lobe. The anterior and posterior lobes are physiologically and anatomically distinct. The larger anterior lobe (adenohypophysis) produces a number of hormones including; I. follicle stimulating hormone(FSH). II. luteinising hormone. III. adrenocorticotrophin (ACTH). The posterior lobe (neurohypophysis) does not produce any hormones itself, but does store; I. vasopressin (ADH) II. Oxytocin which are produced by the hypothalamus CRANIAL NERVES There are 12 pairs of cranial nerves which leave the brain and pass through the various foramina and fissures of the skull. They include; I. Olfactory II. optic III. oculometer IV. trochlear V. trigeminal VI. abducens VII. facial VIII. vestibulocochlear IX. glossopharyngeal X. vagus XI. accessory XII. hypoglossal. Whilst MRI can be used to evaluate any of these nerves, the VIII vestibulochchlear nerve is one of the most commonly imaged nerves. The internal ear is the organ of hearing and equilibrium, consisting of both an osseous and membranous labyrinth. The osseous component is within a hollowed out section of the petrous portion of temporal bone, and consists of three components: • vestibule: a chamber medial to the middle ear • semicircular canals: three in number, one in each plane of space • cochlea: shaped like a shell of a snail, and consists of two and three quarter spiral coils. The membranous labyrinth lies within the osseous labyrinth. The vestibulocochlear nerve has two distinct parts namely; I. the vestibular nerve II. the cochlear Nerve. a. the cochlear Nerve; The cochlear nerve is the nerve used for hearing arises in the cochlea It passes through the internalauditory meatus and terminates at the lower border of the pons. b. the vestibular nerve; The vestibular nerve provides information concerning the position and movements of the head. It passes through the internal auditory meatus and enters the upper part of the medulla oblongata. Some fibres also pass directly to the cerebellum via the inferior cerebellar peduncle. Other fibresdescend to the spinal cord and form the vestibulospinal tract, and still others pass to the nuclei of the oculomotor, trochlear and abducent cranial nerves. These connections enable the movements of the head and eyes to be coordinated, and assist in maintaining balance by influencing muscle tone of the trunk and limbs. HIPPOCAMPUS Defination The hippocampus is part of the limbic system, and plays a role in the control of emotion, behaviour, drive and memory. It is a curved elevation of gray matter that extends throughout the entire length of the floor of the inferior horn of the lateral ventricle, in each hemisphere. It is called the hippocampus because, in coronal section, it resembles a seahorse! Anatomy The ventricular surface of the hippocampus does have a layer of white matter, and the fibres from here converge medially in a bundle known as the fimbria. The fimbria leaves the posterior end of the hippocampus, forming the crus of the fornix. The two crura then join together beneath the corpus callosum to form the body of the fornix. The body courses anteriorly, and divides into two again, forming the two columns of the fornix. Each column then curves forward and downward to terminate in the mamillary body Read More