Cerebrum

Also known as grey matter and constitutes the largest volume of tissue in the brain. It is divided into the right and left hemispheres, and the surface is covered with fissures called sulci and convolutions called gyri these increase surface area and allow for greater numbers of neurons. The right side of the brain is responsible for control of the left side of the body and vice versa. The cerebral cortex is divided into 4 main areas frontal lobe temporal lobe parietal lobe and occipital lobe.

brainlandmarks

Image retrieved from http://www2.highlands.edu/academics/divisions/scipe/biology/faculty/harnden/2121/images/brainlandmarks.jpg

The motor cortex is located anterior to the central sulcus. The somatosensory cortex is located directly posterior to the central sulcus in the darker purple area. The motor cortex is responsible for voluntary motor movements while the somatosensory cortex is responsible for receiving and interpreting sensory information.

Language generation and comprehension usually occur on the left side of the brain in areas called Broca’s area and Wernicke’s area respectively. Damage to these areas via stroke, trauma or alcoholism results in loss of speech and/or comprehension (Aphasia).

Brainstem and diencephalon

The brain stem consists of the

  • Midbrain – Responsible for movements in the eye, movements to improve hearing, production of dopamine and the origin of cranial nerves iii oculomotor (eye movement) and iv trochlear (muscle movement)
  • Pons – Responsible for communication between cerebellum, the brain stem, and the cerebral cortex. Also the cranial nerves v trigeminal (biting, chewing, and facial sensation) and viii vestibulocochlear (sound and ballance)
  • Medulla oblongata – Responsible for autonomic activities such as vomiting, breathing, blood pressure control, sneezing and coughing, heart rate, and swallowing. Also the nuclei for cranial nerves ix the glossopharyngeal (has both afferent and efferent divisions, swallowing, tongue, and salivary glands) and xii hypoglossal (tongue)

brain-stem-ad-diencephalon

Image retrieved from http://www.edoctoronline.com/medical-atlas.asp?c=4&m=1&p=7&cid=1042&s=

The diencephalon consists primarily of the thalamus and the hypothalamus. The thalamus is essential for perceiving and trafficking afferent impulses to the cerebral cortex for interpretation for example, pain. The hypothalamus is the major centre for maintenance of homeostasis, temperature control, sexual urges, eating, thirst, emotional responses, sympathetic or parasympathetic nervous responses hormone synthesis, among many others.

Cerebrospinal fluid

Cerebrospinal fluid (CSF) is responsible for stabilising and protecting the structures inside the skull, and achieves this by exerting pressure on the brain. This pressure is dynamic as the position of the person changes. Further, CSF reduces friction between structures such as the brain and the meninges. The choroid plexuses manufacture around 600ml CSF per day however at any one time there is only around 135ml circulating in the central nervous system including the brain and spinal tract. CSF is reabsorbed into the venous system.

Cerebral blood supply and the blood brain barrier

Cerebral blood supply consists of 15-20% of cardiac output up to 1000ml per minute. Carbon dioxide levels dictate the blood supply to the brain, increased levels increase blood flow. Thus, hyperventilation which increases oxygen levels reduces blood supply leading to a feeling of light-headedness.

The Circle of Willis located inferior to the brain is a group of vessels that afford an alternative route for blood supply in the event of obstruction.

Images retrieved from http://cnx.org/contents/1aa6075b-8678-43f0-891d-cff6252fb2eb@3

Image retrieved [Circle of Willis] from http://clinicalgate.com/anatomy-and-physiology-of-cerebral-and-spinal-cord-circulation/

Intracranial pressure (ICP) regulation and compensation

Pressure is maintained via a balance between a triad of brain tissue, blood, and CSF volumes. Monro-Kellie doctrine states the triad must remain constant within the closed structure of the skull for homeostasis. Displacement occurs if one component increases in order to maintain ICP. Normal ICP is maintained between 5 to 15mmHg Sustained pressure of 20mmHg requires treatment. In normal circumstances ICP is influenced by arterial pressure, venous pressure, intraabdominal, and thoracic pressure, posture, temperature, and blood gasses especially Co2.

Compensatory mechanisms include CSF volume changes followed by blood volume changes via collapse of blood vessels, dural sinuses, reginal vasoconstriction or dilation, distention of the dura mater, compression of cerebral cortex can also occur. These mechanisms are of course limited due to the space available.

Maintenance of cerebral blood flow is essential, as the brain requires 20% of the body’s total oxygen intake and 25% of its glucose.

 

References:

Craft, J., & Gordon, C. (2013) The structure and function of the neurological system. In        J. Craft, C. Gordon, & A. Tiziani (Eds.). Understanding Pathophysiology. (pp. 93-     139). Sydney, Australia: Mosby Elsevier.

Olson, D.  (2010). Nursing management: Nervous system. In D. Brown & H. Edwards (Eds.), Lewis’s medical-surgical nursing: Assessment and management of clinical problems (pp. 1335-1355). Sydney, Australia: Mosby Elsevier.

 

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