[Fundamental of Neuroscience] Early brain development

At the earliest stage, human brain is developed into three divisions, ProsencephalonMesencephalon, and Rhombencephalon. Rhombencephalon is located rostrally to spinal cord. These early three divisions will further developed into more funcional brain structures.

At a relative later stage, prosencephalon will develop into Telencephalon and Diencephalon. Mesencephalon will continue to grow but will not differentiate as prosencephalon does. Meanwhile, rhombencephalon continues to divide into Metencephalon and Myelencephalon

Telencephalon, or commonly called cerebrum, grows into the largest part of the brain. Telencephalon refers to the embryonic structure, from which the mature cerebrum develops.The dorsal telencephalon, or pallium, develops into the cerebral cortex, and the ventral telencephalon, or subpallium, becomes the basal ganglia. The cerebrum is also divided into symmetric left and right cerebral hemispheres. Diencephalon includes thalamus, hypothalamus, subthalamus and several other structures,  underlying the cortex. The metencephalon is composed of the pons and the cerebellum; The myelencephalon differentiates primarily into the medulla oblongata and a caudal portion of the fourth ventricle, but will also contain portions of cranial nerves.

[Fundamental of Neuroscience] Basal Ganglia

(These awesome figures are from <Neuroscience> by Dale Purves)

Basal ganglia is a group of nucleus hiding beneath the cortex but above the brainstem, roughly. These nucleus, including caudate nucleus, putamen (these two are also called striatum), globus pallidus (internal and external), substantial nigra (SNr) and subthalamic nucleus (STN),  function more or less like a “radio station” for our brain. They receive almost all the signals coming down from cortex and tune them into a proper band of frequency and firing rates back to cortex via thalamus. Malfunction in basal ganglia can leads to widely known motor deficits such as Parkinson’s disease and Huntington’s disease.

There is a classic model of basal ganglia. This model depicts two pathways used for modification of firing rates, direct pathway and indirect pathway. Direct pathway sends signals directly to internal globus pallidus/substantial nigra par reticulata. The indirect pathway sends signal to external globus pallidus first and then subthalamic nucleus. Simply speaking, direct pathway can facilitate wanted movements and indirect pathway will inhibit unwanted movements. The two pathways are working together so certain movements like reaching can be accurately executed. The differences in the effect of two pathways lie in the fact that they have two distinctive dopamin receptors in Medium Spiny Neurons, D1 and D2 receptors, respectively. These enables the pathway can function differently and efficiently. Another important component is substantial nigra pars compacta, which releases dopamin into striatum. Deterioration of the part will cause Parkinson’s disease. 

However, this model has been questioned frequently because it can only partly explain the clinical cases. And the use of Deep Brain Stimulation (DBS) in STN which is theoretically supposed to worsen the Parkinson’s disease according to this model, turns out to ameliorate this disease and reduce the tremor. It has been proposed that not only the firing rates are important in this model, but firing pattern also determines the effect. And DBS is proposed to change the pattern instead of rate. 

Unfortunately, even today, we are still not very clear about basal ganglia. But DBS is still widely used in clinic. Conventional medications such as Levodopa are also taken by patients with Parkinson’s disease. 

[Fundamental of Neuroscience] Glial Cells

Glial cells are non-neuronal cells in brain. There are four different types of glial cells, 3 in the Central Nervous System (CNS) and 1 in the Peripheral Nervous System (PNS). 90% of brain tissue in human is made up of glial cells. Equally striking is we are still not clear why our brian is composed of a huge portion of such cells. 

The four types of glial cells are:

Microglia: Most serve as representatives of the immune system in the brain. Microglia protect the brain from invading microorganisms and are thought to be similar in nature to microphages in the blood system.

Astrocyte: Astrocytes are star shaped glial cells that perform a variety of functions in the CNS. Astrocytes provide physical support to neurons and clean up debris within the brain. They also provide neurons with some of the chemicals needed for proper functioning and help control the chemical composition of fluid surrounding neurons. Finally, astrocytes play a role in providing nourishment to neurons.

Oligodendrocyte: The principle function of oligodendrocytes is to provide support to axons and to produce the Myelin sheath, which insulates axons. Myelin is 80% lipid and 20% protein and allows for the efficient conduction of action potentials down the axon. Oligodendrocytes unlike Schwann cells of the PNS, form segments of myelin sheaths of numerous neurons at once.

Schwann cells: Schwann cells are the supporting cells of the PNS. Like oligodendrocytes Schwann cells wrap themselves around nerve axons, but the difference is that a single Schwann cell makes up a single segment of an axon’s myelin sheath. Oligodendrocytes on the other hand, wrap themselves around numerous axons at once.

Recommended Reading:

The Other Brain: From Dementia to Schizophrenia, How New Discoveries about the Brain Are Revolutionizing Medicine and Science

by R. Douglas Fields

[Fundamental of Neuroscience] Amyotrophic Lateral Sclerosis

 

"I wouldn’t compare [the joys of scientific discovery] to sex. But it lasts longer."

                                                                                —-Stephen Hawking

ALS is constantly called “the Beast inside body” because it “eats up” every motor neurons unitl the death. The loss of motor neurons may cause weakness, muscle atrophy, fasciculations. It is named after the famous New York Yankee baseball player Lou Gehrig, who has been dignosed with this disease in 1939. Most die of pneumonia or suffocation due to the weak muscles responsible for breathing. 

ALS is progressive. Soon after dignosis, patients can barely walk, stand or get up from bed. It is extremely frustrating that they cannot smile, swallow or chew, and sometimes they cannot speak. But the eye control can be preserved. ALS patients can live longer if they can receive a good medical care, like Stephen Hawking, who had this disease for over 48 years.

The exact cause to ALS is unknown, at least to 95% cases. Scientists frequently linked the cause to neurotoxins and other environmental factors.

However, the sensory system is almost entirely preserved. Patients do not have problem in smelling, sensing touch and limb position. They also do not have problem in memory or other cognitive function. 

[Fundamental of Neuroscience] Tourette Syndrome

Tourette syndrome is mostly characterized as motor tics, voice tics and abrubt socially inappropriate body movement. These motor tics cannot be voluntarilly controled and impossible to be predicted by patients. It typically affects children and tends to decrease as they pass through adolescence. It does not adversly affect intelligence and life expendency, however, the inappropriate and uncontrollable movements can affect daily life.

The exact cause of Tourette is unknown, but it is certain that Tourette is genetic and environment associated. A person who has tourette have a 50% to pass their gene to their kids. Males are affected three to four times more than females. 

[Fundamental of Neuroscience] Multiple Sclerosis

"It feels like having a hangover forever without having the fun of being drunk". 

Multiple Sclerosis (MS) affects 400,000 people in the US, mostly young aduclts and female especially. Patients dignosed with MS can possibly have all kinds of neurological disorder, and their life expectancy is usually 5 to 10 years lower than that of the unaffected population.

MS is basically caused by loss of myelination, which is formed by one kind of glia cells—-oligodendrocyte. Large axons need those myelinations to help neural signal transimission in the Central Nervous System. Without myelinations, not only the transmission is less efficient, the axons will also lose the protection and be exposed to the environment. After a period of time, the axons will disconnect with others and start to deteriorate.

period by KRUNK Interactive