Neuroanatomy - Journal Post (#2) - Overview

Well, I figured if I am going to finish these blogs, then they would have to be written about something I interested in. So, I am choosing to talk about neuroanatomy. This blog will initiate the beginning of a series of blogs related to neuroanatomy, and associated pathologies. I apologize in advance if the blogs get a bit technical, but such is neuroanatomy. This first blog will start with a general overview of neuroanatomy, such as categorization and functions. Following blogs will highlight specific structures, and associated pathologies.

                From a general functional anatomy standpoint the brain can be divided into three broad areas: the cerebrum, the cerebellum, and the brain stem. These three areas combine to form the structure we know as the brain.

               The cerebrum includes the cerebral hemispheres and the basal ganglia (more technically termed the basal nuclei). The cerebral hemispheres are separated by the falx cerebri within the longitudinal cerebral fissure. Each cerebral hemisphere is divided into four lobes. These four lobes are the frontal, temporal, parietal, and occipital lobes. Each of these lobes is primarily distinguished by its location and function. Each of which will be covered in greater detail later.

              The next structures contained within the cerebrum are the basal ganglia (nuclei). The basal ganglia consist of the Caudate, Putamen, Globus Pallidus, Substantia Nigra, and the Subthalamic nucleus. (The first three are also known collectively as the Striatum or Neostriatum). Each of these areas has a specific function.

             The mention of this topic is ground for mentioning a very important associated pathology with the basal ganglia; Parkinson’s disease. After having just completed a paper and presentation on the molecular basis of PD, this information is very fresh on my mind. To be very general, PD is a neurodegenerative disorder that is characterized clinically by certain symptoms cause by the production of Dopamine depleting cells. These symptoms include: tremors, bradykinesia (slowness in movement initiation), postural instability, and rigidity. There has been a multitude of research on the etiology of the disease, and this research indicates that this disorder, like many others, is multi-factorial in nature. This means that there is not a single cause for the development of PD. However, one of the more notable aspects of research that I have studied is by Chinea & Bezard (2010). Their research focused specifically on the action of dopamine depleting cells and their role in the initiation of PD. Although it has been known for many years that the main development of symptoms will occur after Dopamine depleting cells move into the synaptic cleft, it was not known how the action of these cells affects Dopamine itself. The following image is a schematic diagram of how depleted Dopamine leads to changes in other neurotransmitters that relay to the Thalamus, and how these changes can initiate the hallmark symptoms of PD.

   As mentioned earlier, another anatomical feature of the brain is the cerebellum. The cerebellum is section of the brain that coordinates sensory input with muscular responses, located inferior and posterior the cerebral hemispheres and superior the medulla oblongata. The cerebellum integrates nerve impulses from the labyrinths of the ear and from positional sensors in the muscles throughout the body. These cerebellar impulses then determine the extent and timing of contraction of individual muscle fibers to make fine adjustments in maintaining balance and posture and to produce smooth, coordinated movements of large muscle masses in voluntary motions. Like the cerebrum, the cerebellum is divided into two lateral hemispheres, which are connected by the vermis. Each of the hemispheres consists of a central core of white matter and a surface cortex of gray matter and is divided into three lobes. The flocculonodular lobe, the first section of cerebellum to evolve, receives sensory input from the vestibules of the ear; the anterior lobe receives sensory input from the spinal cord; and the posterior lobe, the last to evolve, receives nerve impulses from the cerebrum. All of these nerve impulses are integrated within the cerebellar cortex.

                An important associated pathology can be injuries or disease affecting the cerebellum that produces neuromuscular disturbances. These are categorically referred to as Ataxias, or disruptions of coordinated limb movements. The loss of integrated muscular control may cause tremors and difficulty in standing, or change in posture. The various types of cerebellar ataxia are related to the area of the cerebellum that is affected. For example, dysfunction of the vestibulocerebellum impairs the balance and the control of eye movements. Dysfunction of the spinocerebellum presents itself with a wide-based "drunken sailor" gait, characterized by uncertain starts and stops, lateral deviations, and unequal steps. This part of the cerebellum regulates body and limb movements. Finally, dysfunction of the cerebrocerebellum presents with disturbances in carrying out voluntary, planned movements.

References:

1.)      Moore Keith L., Dalley Arthur F., Agur Anne M.R., "Chapter 6: Upper Limb". Lippincott Williams & Wilkins. Clinically Oriented Anatomy, 6e. 2010

2.)      Larson, J.L., Hurston, M.T., & Felli, G.J., "Chapter 3: Cerebrum". McGraw-Hill Inc. Neuroanatomy Through Clinical Cases. 2011.
3.)      Waxman SG, "Chapter 4. The Relationship Between Neuroanatomy and Neurology" (Chapter). Waxman SG: Clinical Neuroanatomy, 26e: http://www.accessmedicine.com/content.aspx?aID=5271560. 4.)      Lomen-Hoerth Catherine, Messing Robert O, "Chapter 7. Nervous System Disorders" (Chapter). McPhee SJ, Hammer GD: Pathophysiology of Disease, 6e: http://www.accessmedicine.com/content.aspx?aID=5368376.

5.)     Ropper AH, Samuels MA, "Chapter 28. Normal Development and Deviations in Development of the Nervous System" (Chapter). Ropper AH, Samuels MA: Adams and Victor's Principles of Neurology, 9e: http://www.accessmedicine.com/content.aspx?aID=3634622.

              Finally, the last structure of the big picture of the brain is the brain stem. As seen in a previous image the brainstem is composed of the Pons, the Midbrain, and the Medulla Oblongata (in most neuroscience circles the structure is now referred to as simply the Medulla). The brainstem is area at the base of the brain that lies between the deep structures of the cerebral hemispheres and the cervical spinal cord. The brainstem houses many of the control centers for vital body functions, such as swallowing, breathing, and vasomotor control. All of the cranial nerve nuclei, except those associated with olfaction and vision, are located in the brainstem, providing motor and sensory function to structures of the cranium, including the facial muscles, tongue, pharynx, and larynx, as well as supplying the senses of taste, equilibrium, and hearing. The brainstem also has nuclei important for sympathetic and parasympathetic autonomic functions. All efferent and afferent pathways between the cerebrum and cerebellum course through the brainstem, and many of them decussate within this structure.

                  Because of the important neural structures concentrated in this small portion of the nervous system, even very small lesions of the brainstem may have intense effects. Disorders involving the brainstem include trauma, tumors, strokes, infections, and demyelination (like multiple sclerosis). It should be noted that the most common of these disorders is the trauma related events. In general there is a division between primary and secondary injuries of the brainstem. These are also known as Coux and Contra-coux. The coux is the first impact, and the contra-coux is any event that is related to the recoil of the brainstem back into place. Often these events are very damaging and irreparable.