The brain is the central processing organ of our body. It commands and controls all the functions of the body. This is because all systems in the body follow the commands given to them by the brain. The commands are delivered through the nerves. The brainstem is composed of the midbrain, hindbrain, pons, and medulla oblongata. Together all these structures execute different functions such as regulating breathing, heart rate, blood pressure, and other functions. This forms the brainstem and this is the basic brainstem definition.
The brainstem structures and the brainstem location is in the brain so we will understand a bit about the structure of the human brain and then move on to the parts the brain stem is formed by. It has a soft and delicate structure and looks like a walnut. It has a wrinkled surface. The bony skull protects the human brain. Cranial and facial bones are present in the skull. Cranial bones are eight in number and they form the hard protective outer covering which is known as the cranium. Inside the skull, the brain is covered by protective layers called the cranial meninges which are three in number and are the Dura mater, Arachnoid, and Pia mater. All these three covers protect the brain from external forces and the brain is floated in a fluid known as cerebrospinal fluid in which the weight of the brain is reduced and this fluid acts as a shock absorber.
The human brain is divided into three divisions that are forebrain, midbrain, and hindbrain. As we read above about the brainstem and what the brainstem consists of the midbrain, hindbrain, pons, and medulla oblongata so we will learn more about brain stem parts and functions.
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Figure: Brain structure and brainstem
The brain stem is formed by the midbrain. The midbrain is present between the thalamus/hypothalamus of the forebrain and pons of the hindbrain. The dorsal portion of the midbrain consists mainly of four round swellings or lobes. These four lobes are collectively called the corpora quadrigemina. These four lobes in the midbrain are arranged such that two are present on the upper side and two are present on the lower side. The two upper lobes are called the superior colliculi whereas the two inferior lobes are called inferior colliculi. Sensory impulses from the eyes and muscles of the head are received from super colliculi and control visual reflexes. They control and coordinate the movement of the head and eye at the same time to fix and focus on the object. Sensory impulses from the ears and muscles of the head are received from the inferior colliculi and control the auditory reflexes. The auditory reflexes are related to the ears. Inferior colliculi control and coordinate the movement of the head to locate and detect the source of the sound. Cerebral aqueduct passes through the midbrain. The canal is a part of the ventricle system of the brain. Ventricles are the cavities present within the brain that are called cerebral ventricles. The cerebral aqueduct is a canal that passes through the midbrain and connects the third ventricle with the fourth ventricle of the brain. The cerebral aqueduct is also called iter.
The brainstem is made up of the hindbrain. This is the posteriormost part of the brain that is why it is called the hindbrain. The hindbrain comprises the pons, cerebellum, and medulla oblongata.
The brain stem is formed by the pons also. Pons consist of fibre tracts. Tracts are a bundle of nerve fibres in the CNS. These fibre tracts interconnect different regions of the brain. As the tract of the brain connects various regions of the brain, this is the reason for its name pons that means a bridge. Pons mainly acts as a neuronal link between the cerebral cortex and the cerebellum. A centre present in pons called the pneumatic centre can moderate the functions of the respiratory rhythm centre located in the medulla oblongata. Neural signals from the pneumatic centre can reduce the duration of the inspiration phase and thereby can alter the rate of respiration.
The brainstem anatomy also consists of a very important part that is the cerebellum. It is the second-largest part of the brain after the cerebrum. The word cerebellum means little cerebrum. The cerebellum is also made up of two cerebellar hemispheres and a vermis and has a grey matter on the outer side and white matter on the inner side. Like the cerebrum, the cerebellum has its grey matter on the outside comprising three layers of cells and fibres. Purkinje cells are present in the middle layer. Tree-like themselves with a myriad of dendrites, Purkinje cells rank among the most complex of all neurons. The white and grey matter form arbour vitae. The central portion of the cerebellum has a worm-like appearance as it is narrowed and furrowed. It is called the vermis. Cerebellar peduncles are the three paired bundles that are formed by myelinated nerve fibres. They help in providing the communication pathways which are formed between the cerebellum and some parts of the CNS. The superior cerebellar peduncles are responsible for connecting the cerebellum to the midbrain, the middle cerebellar peduncles help in communicating with the pons, and the inferior cerebellar peduncles possess pathways that are present between the cerebellum and the medulla oblongata, as well as the spinal cord.
The medulla oblongata is also an important part of brainstem anatomy. It is the posterior-most part that connects various parts of the brain and spinal cord. Though small in size, it is essential to life. The medulla has various centres that help in controlling respiration, cardiovascular reflexes, gastric secretions. To control respiration there is a centre present in the medulla which is called the respiratory rhythm centre. The function of this centre is to maintain the respiratory rhythm to suit the demands of the body.
The brainstem is not composed of one single unit but brainstem anatomy is made up of various parts of the midbrain and hindbrain. The medulla oblongata, the pons, and the cerebellum make the brainstem. The brainstem functions comprise controlling the rate of breathing, regulating the heartbeat and blood pressure.