Brainstem, Cerebellum, Cerebrum, Cerebral Cortex, Brain Plasticity, etc

brainstem

• lowest part of the brain (just below the thalamus), connecting it with the spinal cord, which carries essential basic functions such as respiratory or heart regulation

• has 3 main parts: midbrain, pons, and medulla oblongata

midbrain

Contains different nuclei, including:

• the superior colliculi - receives the visual input and are important for eye gaze

• the inferior colliculi - receives auditory input and are important for the startle response and orienting to novelty

• the respiratory nuclei

• the tracts to and from the cerebellum

superior colliculi vs the inferior colliculi

• the superior colliculi - receives the visual input and are important for eye gaze

• the inferior colliculi - receives auditory input and are important for the startle response and orienting to novelty

pons

• means the “bridge” bc it contains tracts that transmit signals between the cerebrum and cerebellum

• includes the nuclei responsible for breathing, sleep cycles (REM sleep), arousal, etc

• responsible for pontine strokes

pontine stroke

• very rare, but can result in paralysis and a “locked in” syndrome whereby people are completely paralyzed except for eye movements

• looks like they are in a coma, but they are fully conscious and can only communicate via eye movements

medulla oblongata

• most caudal (inferior) portion of the brainstem and is continuous with the spinal cord

• controls a range of vital functions, ie respiration, heart rate, blood pressure, reflexes (such as vomiting coughing, sneezing, etc), and more

• the blood-brain barrier is very weak/missing in the area postrema, which makes it possible to detect any poisonous/toxic content in the blood - can be life-saving when the vomiting center is activated and the stomach is emptied of the toxic substance

cerebellum

• located ventrally to the cerebrum

• means “little brain” in Latin

• contains most neurons in the brain because they are primarily tiny granule cells

• represents ~10% of the volume of the entire brain

• neurons are tightly packed in the cerebellar cortex, which is highly folded

• plays a vital role in motor control, coordination (walking, posture, fine-tuned movements), and cognitive functions such as attention and language

• contains specialized Purkinje cells

Purkinje cells

• found in the cerebellum

• beautiful, 2-dimensional, GABA-ergic (inhibitory) cells involved in movement regulation

cerebrum

contains the 4 lobes: frontal, parietal, temporal, and occipital

How are the lobes separated?

• the frontal and parietal lobes are separated by the central (Rolandic) sulcus

• the frontal/parietal and temporal lobes are separated by the Sylvian fissure (lateral sulcus)

cerebral cortex

• Brodmann’s anatomical areas of the cerebral cortex were defined at teh beginning of the 20th century based on cell architecture (cytoarchitectonics) of different cortical regions and gross anatomy

• Brodmann came up with 52 cortical areas that have specific cell characteristics and functions

Note that the nomenclature (terminology) is not always standardized, so the same cortical area can be referred to in different ways, for example the primary visual cortex can be labeled as…

• V1 - the primary visual cortex

• BA17 - Brodmann’s area 17

• located in the calcarine fissure (seen in the medial view of the occipital cortex)

• also defined as the striate cortex

Why is it called the striate cortex?

Because the line (“stria”) of Gennari is visible and it defines V1

Do congenitally blind people have the line (“stria”) of Gennari? What does this mean?

Yes. This means that the line of Gennari doesn’t require visual input to develop, and it isn’t degraded by deprivation of the visual input. It is predetermined genetically. This brings up the question of brain plasticity.

brain plasticity

• deals with questions such as: is neural wiring fixed in place? to what degree are neural structures genetically predetermined? conversely, to what degree are they shaped by experience?

• based on the idea that while genes provide a blueprint for brain development, experience is necessary, as it allows for the tuning, especially during the critical period

What is the relationship between brain plasticity and the critical period?

• sensory deprivation during the critical period (the first couple of years) can result in sensory deficits

• after the critical period, connections are less numerous (the unused ones are pruned and eliminated), so that the remaining one are more permanent, robust, and precise

  • synapses get stronger through repeated activation (“use it or lose it”)

amblyopia

• an example of brain plasticity

• a central disorder of vision - NOT a peripheral disorder (ie nothing is wrong with the eye as an organ of vision)

• the eye may not transmit the visual input correctly due to various causes, such as strabismus (eye misalignment) or blockage - bc the brain constructs an image of the visual world based on the input from both eyes, if one eye is sending a blurry or wrong image, the brain will suppress that incorrect image

• after a while, this will result in permanent vision loss in one eye (ie amblyopia)

  • therefore, it’s essential to diagnose any potential problems with the visual input asap so that proper treatment can be applied to enable the brain to get clear images from both eyes (ie glasses, cataract removal, eye muscle surgery, eye patches to force the “weaker” eye to be used, etc)

cortical gyrification

• cortex is highly convoluted (folded, “wrinkled”) - evolutionary process results in the packing of more gray matter (neurons) within the skull, this way, more cortical surface area can fit into a relatively small skull, which can also optimize connectivity

• the size of the cortex has increased disproportionally compared to other parts of teh brain, especially in primates

cortical folding is reflected in the following features:

• gyrus (singular) or gyri (plural) - “circle”; the “crown” of the fold

• sulcus (singular) or sulci (plural) - “groove, ditch”; the hidden “valley” between folds

the cortex

• essential for higher-level neural functions, including perception, reasoning, decision-making, learning, planning, etc

• visible from the outside and consists mainly of neurons (cell bodies and dendrites) and is known as the gray matter

• comprises 2 hemispheres

the 2 hemispheres are separated by the ___

interhemispheric (longitudinal) fissure

How has cortical gyrification changed across different mammal species?

• gradual increase in cortical gyrification

• the complexity of the sulci increases, allowing for an increased cortical surface

How do dolphin brains look? Why?

Dolphins have a highly folded brain

• Primarily due to more glia (glial cells support neurons but do not generate action potentials). One of those support functions is producing heat to allow neurons to function in cold temperatures in the ocean

Phylogenetically, there has been more significant growth in the ____ than in ____, which resulted in increased gyrification (cortical folding increases the surface area that can fit inside the skull)

• cortical surface area

• cortical thickness

Folding is an evolutionary solution to what problem and how?

• The folding is an evolutionary solution to the problem of increasing cortical surface area

• To fold, the cortex can’t be thick; it needs to be pliable. Hence, the cortex is relatively thin

  • pruning may be necessary for optimal gyrification

• The human cortical sheet is ___ times as large as the inner skull surface

• The surface of the cerebral cortex is ___ cm², which is approximately ___ square feet. It is only ___ cm² in the rat (less than ___ square inch)

• 3

• 2500; 2.7; 6; 1

There is a remarkable variability of brain size among different mammalian species. However, the ___ varies very little

thickness of the cortex

The human cortical surface is around ___ times greater than in the macaque, but its thickness is only ___ times greater

• 10

• 2

As a result of increased number of ___, the cortical surface area has increased a lot, but the ___ has remained very similar.

neurons; cortical thickness

The cortex expanded ___, which resulted in increased ___

“laterally”; cortical folding

Human cortex: ___ mm thickness on average (only ___ inch thick)

• ~2.5

• 1/10

Benefits of having a thinner cortex

the thinner cortex remains more pliable and has a greater chance for future expansion

___ of the cortex is preserved across mammalian species

Laminar organization

The number of laminae (layers) in the cortex is ___. The neocortex occupies ___ of the cortex.

• 6

• ~90%

• There are ___ layers in older parts of the brain. For example, there are only ___ layers in the oldest parts of the brain: ___ (such as the hippocampus and olfactory cortex)

• Reptiles have a __-layer cortex

• Even though the neocortex has 6 layers across mammalian species, note that the proportions of the ___ vs ___ layers differ. For example, the upper layers occupy almost ___ of the gray matter thickness in primates but account for only ___ in rodents

• Upper layers contain widespread ___, which are important for synchronizing ___ and are essential for ___

• fewer; 3; archicortex

• 3

• upper; lower; half; 20%

• cross-cortical connections; activation; consciousness