PS1005 Provide an overview of the principal structures of the midbrain, hindbrain and forebrain and their functions. The human nervous system consists of many structures. We distinguish between the central- and the peripheral nervous system. The two major subdivisions of the central nervous system are the brain and the spinal cord, which is found in the spinal column. The peripheral nervous system consists of the nerves outside the brain and the spinal cord. The spinal cord communicates with all the sense organs and muscles except those of the head.
It is compromised of grey matter which is densely packed with cell bodies and dendrites and white matter which consists mostly of myelinated axons. According to the Bell-Magendie law, the entering dorsal roots carry sensory information and the exiting ventral roots carry motor information. Each segment sends sensory information to the brain and receives motor commands from the brain. The brain is the center of the nervous system and can be divided into three major subdivisions: Forebrain, Midbrain and Hindbrain. The number of neurons in the brain is estimated to range between 80 120 billion.
The purpose of this essay is to explain and analyse the three major subdivisions of the brain and their functions. The hindbrain, the posterior part of the brain, consists of the medulla, the pons and the cerebellum. The brainstem is constituted by certain central structures of the forebrain and by the medulla, pons and mitdbrain. The medulla could be regarded as an enlarged extension of the spinal cord but located in the skull. It controls vital reflexes such as breathing, vomiting, sneezing, coughing and heart rate.
The reflexes are controlled through the cranial nerves wh67ich control sensations from the head, muscle movements in the head, and much of the parasympathetic output to the organs. Damage to the medulla is frequently fatal and doses of opiates are life-threatening as they suppress activity of the medulla. For instance, injury here can cause numbness and paralysis of the palate and throat, difficulty swallowing, leading to drooling and disturbances of taste. The pons lies anterior and ventral to the medulla.
The term pons is Latin for bridge and reflects the fact that many nerves in the pons cross over from one side of brain to the other side of spinal cord. Hence the left hemisphere controls the muscles of the right side of the body and the right hemisphere controls the left side. Along with the medulla it contains recticular information, which is Complex network of neurones distributed through the core of the Medulla, Pons and Midbrain. It contains many discrete groups of neurones and acts as a sentry, alerting or blocking messages to higher centres of brain.
Its functions are pain gateway and general influence of arousal throughout the nervous systen with specialised nuclei involved in control of consciousness, attention, sleep and waking. The cerebellum was known for its contributions to the control of movement but the functions extend far beyond balance and coordination. Its is the dorsal outgrowth of the pons and it is similar in its structure to the cerebrum. Damage to the cerebellum causes difficulties in attention between auditory and visual stimuli.
Moreover people with cerebellar damage are clumsy and have difficulties with coordinated movements. As the name implies the midbrain is the middle of the brain. The roof of the midbrain is called tectum and is surrounded by the inferior and superior collicus. The superior Colliculi is part of the Visual System, primarily involved in visual reflexes to moving stimuli. The inferior Colliculi on the other hand is part of the audio system and receives info about sound from auditory pathways. Both are important for sensory processing.
Another midbrain structure is the substantia nigra, which gives rise to the dopamine-containing pathway that facilitates readiness for movement. Insufficient dopamine in substantia nigra results in Parkinson’s Disease, wich is characterised by rigidity, muscle tremors and slow movements. The forebrain is the most anterior and prominent part of the mammalian brain and consists of two cerebral hemispheres. Each hemisphere is organized to receive sensory information and controls motor movement from the cotralateral side of the body. It consists of the outer cortex and ubcortical regions. The outer portion is know as the cerebral cortex. A number of other interlinked structures, known as the limbic system form a border around the brainstem which are particularly important for motivations and emotions, such as eating, drinking, sexual activity, anxiety and aggression. Under the cerebral cortext are other structures including the thalamus, which is the main source of input to the cerebral cortex. The thalamus and hypothalamus form the diencephalon a section distinct from the telencephalon which is the rest of the forbrain.
All sensory information (except olfactory) passes through the thalamus and different nuclei receive different types of sensory information and relay it to appropriate primary cortex. The hypothalamus is a small area near the base of the brain just ventral to the thalamus. It contains several distinct nuclei involved in controlling behaviours related to survival of species, daily rhythms and sleep and autonomic regulation and homeostasis. Partly through nerves and partly through hypothalamic hormones, the hypothalamus conveys messages to the pituitary gland, altering ist release of hormones.
Any damage to the hypothalamic nucleus leads to abnormalities in motivated behaviours as mentioned earlier. The telencephalon includes three important structures, the basal ganglia, the limbic system and the cerebral cortex. A group of subcortical structures lateral to the thalamus form the basal ganglia and include three major structures: the caudate nucleus, the putamen and the globus pallidus. The basal ganglia have subdivisions that exchange information with different parts of the cerebral cortext.
The connections are most abundant with the frontal with the frontal areas of the cortex, which are responsible for planning sequences of behaviour and for certain aspects of memory and emotional expression (Graybiel, Aosaki, Flaherty, & Kimura, 1994). The cerebral cortex contains up to six distinct lamin, that are parallel to the surface of the cortex. Cells of the cortex are also divided into columns that lie perpendicular to the laminae, those within a column have similar properties to one another. The cells of the cerebral cortex are grey matter, andtheir axons extending inward are white matter.
Neurons in each hemisphere communicate with neurons in the corresponding part of the other hemisphere through two bundles of axons, the corpus callosm and the smaller anterior commissure. The occipital lobe, located at the posterior end of the cortex, is the main target for visual information. The posterior pole of the occipital lobe is known as the primary visual cortex. Damage to the optical lobe can result in cortical blindness in the related part of the visual field. The parietal lobe lies between the occipital lobe and the central sulcus, which is on eof the deepest grooves in the surface of the cortex.
The area just posterior to the central sulcus, the postcentral gyrus, is the main target for touch sensations and information from muscle-stretch receptors and joint receptors. The leteral portion of each hemisphere is the temporal lobe which is the primary cortical target for auditory information. Moreover it is essential for processing spoken language. The temporal lobe also contributes to complex aspects of vision, including perception of movement and recognition of faces. A tumor in the temporal lobe may give rise to elaborate auditory or visual hallucinations.
The frontal lobe which contains the primary motor cortex and the prefrontal cortex, extends from the central sulcus to then anterior limit of the brain. The prefrontal cortex ist he integration centre for all sensory information and other areas of the cortex. The dendrites in the prefrontal cortex have up to 16 times as many dendritic spines as neurons in other cortical areas. As a result the prefrontal cortex integrates an enormous amount of information. Rear to the frontal lobe and adjacent to the central sulcus ist the location of the primary motor cortex.
It controls fine voluntary body movements and different parts of motor cortex are connected to muscles in different parts of the body. The size of the areas assigned are proportional to complexity of the movement In conclusion, the frontal lobe is responsible for higher functions such as abstract thinking and planning. It allows for regulation of impulsive behaviours and the control of more complex behaviours. The Brain requires around 25% of glucose, lots of oxygen and other resources. A system of blood vessels surrounding the brain that block most chemicals from entering is called the blood-brain barrier.
It provides the necessary oxygen and disposes of CO2 resulting from cellular respiration. There are four fluid-filled cavities within the brain. Each hemisphere contains one of the two large lateral ventricles. The 4th ventricle connects to the central canal which runs through the spinal cord. CSF is formed inside the four ventricles and most is released into the subarachnoid space between the brain and the membranes surrounding it. Cerebrospinal fluid (CSF) provides “cushioning” for the brain during head movements. In addition, it is a reservoir of hormones and nutrition for the brain and spinal cord.
