Monday, October 15, 2012

Medical Monday - The Brain and Down syndrome: Part 1: How the Brain Works (31 for 21, Day 15)

As I was wondering how to approach this topic, it occurred to me that reviewing a bit of anatomy and physiology might be helpful for readers to better understand how the brain functions in typical persons and the differences that an extra 21st chromosome brings.  With that in mind, I have divided this post into two... hemispheres, if you like.  This week we will explore the brain, while next week we will examine differences in the brains of those with Trisomy 21 or Down syndrome.


Image courtesy of http://www.tutorvista.com/
The 'Typical' Human Brain

When regarded as a whole, the human brain is a large, beige-ish wrinkly organ that approximately 3 lbs of mostly neurons.  It is housed by the bones of the skull, cushioned by several layers of tough meninges and immersed in cerebrospinal fluid.  There are three major sections of the brain:  the brain stem, the cerebellum and the cerebrum.

The brainstem (at the base of the brain) contains the pons, the medulla oblongata and the midbrain (mesencephalon).  The brainstem controls the central nervous system, including the regulation of sleep, the ability to remain conscious and rudimentary body functions such as heart rate, digestion and breathing.  It also connects the brain to the spinal column and it is through here that the cranial nerves pass, providing movement and sensation to the face and neck.  Also passing through this area are nerves that regulate motor skills, temperature, pain, itch, fine and crude touch, vibration and proprioception.

Most motor skills are controlled by the cerebellum, as is balance, co-ordination, timing, precision and posture.  Cognitive functions like attention, language, emotions (such as fear and pleasure) and procedural memory are also controlled by the cerebellum. 

The majority of the brain is comprised of the cerebrum (forebrain).  It is often referred to as having hemispheres, that is a right and left side, which is created by the longitudinal fissure that runs the length of the cerebrum.  The two hemispheres are linked by small connections known as commissures, but mainly by a large structure known as the corpus collosum.  Each side of the cerebrum has it's own temporal lobe, as well as it's own hippocampus (however, we refer to both in the singular)  The right side of the cerebrum controls the left side of the body, and the left side controlls the right side of the body.

Ventricles are pockets located throughout the brains structures, through which cerebrospinal fluid (CSF) flows. 


The lobes of the brain:  Frontal (blue), Temporal (green),
Parietal (yellow) and Occipital (pink)
The cerebral cortex is the outermost layer of the cerebrum and contains most of the brain's neurons or nerve cells.   As a result, it is responsible for much of thought, perceptual awareness, memory, attention, consciousness and language. It also encases structures such as the thalamus, the hypothalamus and the pituitary gland.  It has many folds;  this way it fits a lot of surface area into the small space of the skull.  Many of the major folds have names that are used for landmarking.   The cerebral cortex is divided into lobes:  the frontal lobe, the temporal lobe, the parietal lobe and the occipital lobe.

The frontal lobe is responsible for conscious thought including decision making;  this is mainly done in a structure within the frontal lobe known as the prefrontal cortex.  It is also responsible for memories, specifically the processing of short term memories and the eventual creation of long term ones.  The temporal lobe processes speech and vision (including such complex things as faces) as well as being involved in smell and sound and thus is involved in the creation of long term memory.  The parietal lobe integrates sensory information and determines spacial sense and allows navigation.  The occipital lobe is primarily dedicated to sight.

The Limbic System and Basal Ganglia
The Limbic System and Basal Ganglia
(courtesy of How Stuff Works http://people.howstuffworks.com/)
Within the temporal lobe is the medial temporal lobe, which is considered to be the area of the brain involved in episodic and declarative memory formation.  Contained within the medial temporal lobe is the Limbic System, which includes highly specialized structures such as the hippocampus, the amygdala, the cingulate gyrus, the thalamus, hypothalamus and epithalamus and the mammillary body

The hippocampus in particular is important for short term memory, especially in the encoding of long term memories, spacial memory and related behaviour.  The amygdala processes memory related to
emotionality, social and sexual behaviours while helping to regulate the sense of smell.

The Basal Ganglia system is also located within the cerebral cortex; this area is important in the formation and usage of procedural memory.  

Neurons and Nerve Transmission

Nerve cell or Neuron
The brain is mainly made up of two different type of cells:  glia and neuronsGlial cells are primarily tasked with providing structure, insulation, structural support and metabolic support.  Neurons, by contrast, are highly specialized cells that transmit information across the body in the form of electrical impulses known as action potentials.

Neurons transmit via their axons at synapses, specialized points where an axon of one cell meets other cells.  When an action potential reaches a synapse, it releases chemicals known as neurotransmitters that bind to specialized receptors on the target neuron.  That cells electrical activity is then altered.  Brain activity then, is controlled by cell to cell transmission at the synapses.  Synapses are known as excitatory or inhibitory, depending on their action towards the target cell.  Some are highly adaptable which is thought to be how the brain learns and creates memories.


[Next week:  The Brain and Trisomy 21]


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Gray, Henry. Anatomy of the Human Body. Philadelphia: Lea & Febiger, 1918; Bartleby.com, 2000. www.bartleby.com/107/

Mastin, Luke. "Memory & the Brain - The Human Memory." Memory & the Brain - The Human Memory. The Human Memory, 2010. Web. <http://www.human-memory.net/brain.html>.

4 comments :

  1. Very cool! Looking forward to the next installment!

    ReplyDelete
  2. Thanks for all the info on the human brain’s functions. This will be a helpful source for my research and teaching. The illustrations are helpful tools for easy comprehension because of its simplicity and clarity.

    Brooke Arredondo

    ReplyDelete

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