| Myelencephalon Afterbrain | |
|---|---|
 | |
| Details | |
| Identifiers | |
| Latin | myelencephalon |
| MeSH | D054024 |
| NeuroNames | 698 |
| TA98 | A14.1.03.003 |
| TA2 | 5983 |
| Anatomical terms of neuroanatomy | |
The myelencephalon or afterbrain[ citation needed ] is the most posterior region of the embryonic hindbrain, from which the medulla oblongata develops. [1]
During fetal development, divisions of the neural tube that give rise to the hindbrain (rhombencephalon) and the other primary vesicles (forebrain and midbrain) occur at 28 days after conception. With the exception of the midbrain, these primary vesicles undergo further differentiation at 5 weeks after conception to form the myelencephalon and the other secondary vesicles. [2]
Final shape differentiation of the myelencephalon into the medulla oblongata can be observed at 20 weeks gestation. [2]
| Neural Tube | Primary Vesicles | Secondary Vesicles | Adult Structures |
| Brain | Forebrain | Telencephalon | Rhinencephalon, Amygdala, Hippocampus, Cerebrum(Cortex), Basal Ganglia,Lateral ventricles |
| Diencephalon | Epithalamus, Thalamus, Hypothalamus, Subthalamus, Pituitary, Pineal, Third ventricle | ||
| Midbrain | Mesencephalon | Tectum, Cerebral peduncle, Pretectum, Cerebral aqueduct | |
| Hindbrain | Metencephalon | Pons, Cerebellum | |
| Myelencephalon | Medulla Oblongata | ||
| Spinal cord | |||
The medulla oblongata is part of the brain stem that serves as the connection of the spinal cord to the brain. It is situated between the pons and the spinal cord.
The medulla oblongata is responsible for several functions of the autonomic nervous system. These functions include: [5]
1) Respiration: monitors the acidity of the blood and sends electrical signals to intercostal muscle tissue to increase their contraction rate in order to oxygenate the blood as needed.
2) Cardiac & Vasomotor Center: [6] monitors and regulates cardiovascular activities by:
3) Reflexes
Because of its location in the brainstem and its many important roles in the autonomic nervous system, damage to the medulla oblongata is usually fatal.
The central nervous system (CNS) is the part of the nervous system consisting of the brain and spinal cord, the retina and optic nerve, and the olfactory nerve and epithelia. The CNS is so named because the brain integrates the received information and coordinates and influences the activity of all parts of the bodies of bilaterally symmetric and triploblastic animals—that is, all multicellular animals except sponges and diploblasts. It is a structure composed of nervous tissue positioned along the rostral to caudal axis of the body and may have an enlarged section at the rostral end which is a brain. Only arthropods, cephalopods and vertebrates have a true brain, though precursor structures exist in onychophorans, gastropods and lancelets.
The autonomic nervous system (ANS), formerly referred to as the vegetative nervous system, is a division of the nervous system that operates internal organs, smooth muscle and glands. The autonomic nervous system is a control system that acts largely unconsciously and regulates bodily functions, such as the heart rate, its force of contraction, digestion, respiratory rate, pupillary response, urination, and sexual arousal. This system is the primary mechanism in control of the fight-or-flight response.
The medulla oblongata or simply medulla is a long stem-like structure which makes up the lower part of the brainstem. It is anterior and partially inferior to the cerebellum. It is a cone-shaped neuronal mass responsible for autonomic (involuntary) functions, ranging from vomiting to sneezing. The medulla contains the cardiac, respiratory, vomiting and vasomotor centers, and therefore deals with the autonomic functions of breathing, heart rate and blood pressure as well as the sleep–wake cycle.
In the developing chordate, the neural tube is the embryonic precursor to the central nervous system, which is made up of the brain and spinal cord. The neural groove gradually deepens as the neural fold become elevated, and ultimately the folds meet and coalesce in the middle line and convert the groove into the closed neural tube. In humans, neural tube closure usually occurs by the fourth week of pregnancy.
The brainstem is the stalk-like part of the brain that interconnects the cerebrum and diencephalon with the spinal cord. In the human brain, the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is continuous with the thalamus of the diencephalon through the tentorial notch.
In neuroanatomy, the ventricular system is a set of four interconnected cavities known as cerebral ventricles in the brain. Within each ventricle is a region of choroid plexus which produces the circulating cerebrospinal fluid (CSF). The ventricular system is continuous with the central canal of the spinal cord from the fourth ventricle, allowing for the flow of CSF to circulate.
In anatomy, the extrapyramidal system is a part of the motor system network causing involuntary actions. The system is called extrapyramidal to distinguish it from the tracts of the motor cortex that reach their targets by traveling through the pyramids of the medulla. The pyramidal tracts may directly innervate motor neurons of the spinal cord or brainstem, whereas the extrapyramidal system centers on the modulation and regulation of anterior (ventral) horn cells.
In the human brain, the diencephalon is a division of the forebrain. It is situated between the telencephalon and the midbrain. The diencephalon has also been known as the 'tweenbrain in older literature. It consists of structures that are on either side of the third ventricle, including the thalamus, the hypothalamus, the epithalamus and the subthalamus.
Neuromeres are morphologically or molecularly defined transient segments of the early developing brain. Rhombomeres are such segments that make up the rhombencephalon or hindbrain. More controversially, some argue that there exist early developmental segments that give rise to structures of the midbrain (mesomeres) and forebrain (prosomeres).
The cardiovascular centre is a part of the human brain which regulates heart rate through the nervous and endocrine systems. It is considered one of the vital centres of the medulla oblongata.
The baroreflex or baroreceptor reflex is one of the body's homeostatic mechanisms that helps to maintain blood pressure at nearly constant levels. The baroreflex provides a rapid negative feedback loop in which an elevated blood pressure causes the heart rate to decrease. Decreased blood pressure decreases baroreflex activation and causes heart rate to increase and to restore blood pressure levels. Their function is to sense pressure changes by responding to change in the tension of the arterial wall The baroreflex can begin to act in less than the duration of a cardiac cycle and thus baroreflex adjustments are key factors in dealing with postural hypotension, the tendency for blood pressure to decrease on standing due to gravity.
The hindbrain, rhombencephalon or lower brain is a developmental categorization of portions of the central nervous system in vertebrates. It includes the medulla, pons, and cerebellum. Together they support vital bodily processes.
The metencephalon is the embryonic part of the hindbrain that differentiates into the pons and the cerebellum. It contains a portion of the fourth ventricle and the trigeminal nerve, abducens nerve, facial nerve, and a portion of the vestibulocochlear nerve.
The neural groove is a shallow median groove of the neural plate between the neural folds of an embryo. The neural plate is a thick sheet of ectoderm surrounded on either side by the neural folds, two longitudinal ridges in front of the primitive streak of the developing embryo.
The lateral grey column is one of the three grey columns of the spinal cord ; the others being the anterior and posterior grey columns. The lateral grey column is primarily involved with activity in the sympathetic division of the autonomic motor system. It projects to the side as a triangular field in the thoracic and upper lumbar regions of the postero-lateral part of the anterior grey column.
Neuroectoderm consists of cells derived from the ectoderm. Formation of the neuroectoderm is the first step in the development of the nervous system. The neuroectoderm receives bone morphogenetic protein-inhibiting signals from proteins such as noggin, which leads to the development of the nervous system from this tissue. Histologically, these cells are classified as pseudostratified columnar cells.
Noradrenergic cell group A5 is a group of cells in the vicinity of the superior olivary complex in the pontine tegmentum that label for norepinephrine in primates, rodents and other mammals. The noradrenergic A5 pontine cell group receives inputs from the hypothalamus and several brainstem areas extending from the midbrain to the medulla. It has reciprocal connections with these areas. Its major output is to the spinal cord. These anatomical connections suggest a role in autonomic regulation and A5 has been implicated in control of heart rate, blood pressure and respiration. It may also be involved in modulation of pain as part of the descending mesencephalic to spinal cord system.
Neurocardiology is the study of the neurophysiological, neurological and neuroanatomical aspects of cardiology, including especially the neurological origins of cardiac disorders. The effects of stress on the heart are studied in terms of the heart's interactions with both the peripheral nervous system and the central nervous system.
Brain vesicles are the bulge-like enlargements of the early development of the neural tube in vertebrates, which eventually give rise to the brain.
Three flexures form in the part of the embryonic neural tube that develops into the brain. At four weeks gestational age in the human embryo the neural tube has developed at the cranial end into three swellings – the primary brain vesicles. The space into which the cranial part of the neural tube is developing is limited. This limitation causes the neural tube to bend, or flex, at two ventral flexures – the rostral cephalic flexure, and the caudal cervical flexure. It also bends dorsally into the pontine flexure. These flexures have formed by the time that the primary brain vesicles have developed into five secondary brain vesicles in the fifth week.
