Monday, June 28, 2010

The brain and cranial nerves

Just as a recap, the nervous system is composed of two major parts: the central nervous system and the peripheral nervous system. The central nervous system is only composed of the brain and spinal cord, while the peripheral nervous system is made of all the nerves in the body.

While the brain is only about 2% of a person's body weight, it uses almost 20% of the body's glucose and oxygen.  Oxygen and glucose, remember, are major players in aerobic respiration to make ATP.  Blood flow increases to the parts of the brain which are most active.  This is the reason that we can look for problems within the brain on a cat scan.  If we are unable to get oxygen for four minutes, potentially lethal damage can start occurring. Without oxygen, lysosomes explode and release their low pH fluid, which destroys the brain.  The brain needs a constant flow of glucose because it doesn't store any.  The first signs of low blood sugar include dizziness and fainting.

Blood is brought to the brain by internal carotid arteries and basilar artery.  Blood is then drained into the dural sinuses and the jugular veins.  The dural sinuses sit beneath the skull and in between the dura mater layer of the meninges and looks kind of like a V.  There are several jugular veins that are located both on the posterior and anterior sides of the body.  The blood brain barrier prevents harmful substances and pathogens from passing into the brain.

There are three layers to the meninges of the brain.  They are the exact same as the spinal column.  From superficial to deep they are: dura mater, arachnoid and pia mater.  In the brain, the meninges help create the separations of the hemispheres and other differentiations in the brain. The Falx cerebri is a longitudinal fissure that separates the hemispheres of the cerebrum.  The falx cerebelli separates the two regions of the cerebellum.  The tentorium cerebelli separates the cerebellum from the cerebrum.   Meningitis (see I told you I'd talk about it again) is the inflammation of the pia mater, the arachnoid and the cerebral spinal fluid filled subarachnoid space.  Signs and symptoms include fever, chills, headaches, stiff neck, back, abdominal and extremity pains, nausea and vomiting.  These signs and symptoms are often pushed off as the flu until it is too late.  Bacterial meningitis may be caused by an infection  of the ear, upper respiratory tract, frontal sinus or carried through the blood from the lung.  These might include Pneumococcus or Menigococcus.  Viral meningitis is commonly caused my mumps, polio viruses, and occasionally from herpes simplex.

Brain bleeds are very dangerous because there isn't any room for extra fluid.  Brain bleeds increase intracranial pressure, compress and move the neurons and glia (which move them off tract from where they should be and messages don't get to where they are supposed to go).  If the bleeding occurs in the epidural space, it is an artery bleed and it occurs very rapidly.  If it is in the subdural space, the veins are bleeding and it slower.  Veinous bleeding can take days or even weeks to find.

Brain herniations occur when intracranial pressure pushes the brain out of position. These can include: a displacement of a cerebrum hemisphere under the faux cerebri to opposite side; downward displacement of the hemisphere, diencephalon and midbrain; temporal lob under dura can cause cerebral peduncle to be pinched; and brain tissue being compressed against the skull.

Cerebrospinal Fluid:
There are four deep ventricles that are fluid-filled spaces that form, contain and circulate cerebrospinal fluid.  The lateral ventricles are known as the first and second vents and look like ram horns.  The third ventricle is inferior and deep to the first and second vents.  The fourth ventricle runs with the brain stem. Cerebrospinal fluid is a clear liquid that contains glucose, proteins and ions.  CSF is used to circulate nutrients and waste products between brain and the blood stream.
CSF is formed in the choroid plexuses.  Choroid plexuses are capillaries covered by Ependymal cells (in ventricles).  Ependymal cells control substances that can enter the CSF from the brain.  The CSF is reabsorbed into blood through arachnoid villi in the dural sinuses.  IT then flows into jugular veins that drain out of the head.  We reabsorb CSF at the same rate we produce it: 20 ml/hour.
The CSF is formed in the choroid plexuses of each lateral ventricle.  From the lateral ventricles it flows into the third ventricle through interventricular foramina.  Then, the fluid moves  into the aqueduct of the midbrain which moves the CSF into the fourth ventricle.  The choroid process in the fourth ventricle adds more CSF.  CSF enters the subarachnoid space through three openings in the roof of the fourth ventricle.   Through those openings, the CSF circulates the central canal of the spinal cord and the subarachnoid space around the surface of the brain and spinal cord.

There are four major regions of the brain.  They are the cerebral hemisphere, diencephelon, cerebellum, and the brain stem.  Each of the four regions of the adult brain come from the development of the embryonic brain.  At three-four weeks, there are three primary vesicles.  There is the prosencephalon (the forebrain), the mesencephalon (the mid-brain), and the rhombencephalon.  These three regions change into five regions at the five-week stage. The wall of the three-four week embryo creates the following:  The prosencephalon splits into the telencephalon (cerebrum) and the diencephelon (the thalamus, hypothalamus and epithalamus).  The mesencephalon remain the midbrain.  The rhombencephalon splits into the metencephalon (which creates both the pons and the cerebellum) and the myelencephalon (medulla oblongata).  The cavities of the three-four week embryo creates the following:  The prosencephalon splits into the telencephalon which makes the lateral ventricles and the diencephelon which creates the third ventricle.  The mesencephalon makes the aqueduct of the midbrain.  The rhombencephalon splits into the metencephalon makes the upper part of the fourth ventricle and the myelencephalon makes the lower part of the fourth ventricle.

Just a few key words to remember:
Sulcus: a valley.  sulking-down in the dumps...
Gyrus: a bump.  Gyrus kinda sounds like joyous which is an up
Corpus callosum: the connection for communication between the left and right hemispheres of the cerebrum.

There are several functions of the cerebrum.  It interprets impulses, initiatives voluntary movement, stores memories, retrieves memories, reasons, and is the seat of intelligence and personality.
The cerebrum is made of the frontal lobe, parietal lobes, temporal lobes, occipital lobe and the insula.  A few landmarks include the Central Sulcus and the cerebri falx.  The gray matter of the cerebrum is approximately 2 millimeters thick.  Gray matter is also known as the cerebral cortex.  It is unmylinated axons, dendrites and cell bodies.  It covers the largerst portion of the brain. The cerebral cortex is the thin layer of gray matter that contains 75% of al neurons in the entire nervous system.  White matter is known as cerebral medulla.  It is deep to the cerebral cortex.  It includes the asociation fibers between gyri in the same hemisphere.  Commissoral fibers connect from one hemisphere to the other (they make up the corpus collosum).  Projection fibers form ascending and descending tracts.

There are two major areas that control speech in the brain.  The Broca area controls for the motor area and is in the frontal lobe.  The Wenike area is for speech interpretation and is located on the temporal lobe.  The are both only found on the left side of the brain.

The longitudinal fissure separates the brain into left and right hemispheres.  The left hemisphere is more for verbal, logical, analytical and rational functions; while the right side is more for nonverbal, intuitive and creative functions.  Males have more specialized hemispheres than females; males tend to use more side more than females.  Females have a larger corpus callosum.  If someone has a stroke on the left side, it is called aphasia and on the right side it is causes speech with no emotion or inflection.

The Limbic System consists of the frontal and temporal lobes, the hypothalamus, the thalamus,  the basal nuclei and other deep nuclei.  The functions of the limbic system include controlling emotions, producing feelings and interpreting sensory impulses. 

The Diencephelon consists of the thalamus (a relay station for the sensory information.  It is kind of like the post office of the brain); the hypothalamus (this part of the brain regulates basic functions: temperature, water, balance, metabolism, sex, appetite, emotions, pituitary gland and the autonomic nervous system); the epithalamus (which contains or is also called the pineal gland. this produces melatonin [the sleep horomone]); and the optic tracts, optic chiasm, infundibulum and the pituitary gland.  The diencephelon surrounds the third ventricle.  The superior portion is the thalamus, while the inferior part of walls and floor is the hypothalamus. 

The cerebellum integrates sensory input from the eyes, ears, joints and muscles about the present position of body parts.  The cerebellum helps us maintain balance and posture, have smoothly coordinated voluntary movements and learn new motor skills such as playing the piano or hitting a baseball. The cerebellum is posterior to the mid-brain, pons and medulla oblongata (aka the brain stem). 

The brain stem consists of the mid-brain, pons and medulla oblongata. 
The midbrain is the station for relaying messages and reflexes.  It contains the cerebral aqueduct between the third and fourth ventricles.  It also has the cerebral peduncles between the bundles of fibers between the cerebellum and cord.  The corpora quadrigemina has two parts: the superior colliculi which is involved in visual reflexes and the inferior colliculi which deals with auditory reflexes. 
The pons is a bridge of axons traveling between the cerebellum and the rest of the central nervous system. The pons helps regulate rate and depth for breathing. 
The medulla oblongata is a vital center for regulating heart beat, breathing and blood pressure.  It is a nonvital center for reflexes such as coughing, sneezing, swallowing and vomiting. 
The reticular formation is a network of nerve fibers scattered througout the brain stem into the diencephalon.  It is the center for reticular activating system.  The reticular formation connects centers of the hypothalamus, cerebellum and cerebrum.  The reticular formation filters incoming sensory information and arouses the cerebral cortex into state of wakefulness. 

Cranial Nerves:
There are twelve cranial nerves and they are labled C-I through C-XII.  The only two that we have to identify on a model are I and II, the olfactory and optic nerves.
Here is a brief list:
I: Olfactory: from olfactory receptors
II: Optic: from eyes of retina
III: Oculomotor: to eye muscles
IV: Trochlear: to eye muscles
V: Trigerminal: from mouth and to jaw muscles
VI: Abducens: to eye muscles
VII: Facial: from taste buds and to facial muscles and glands
VIII: Glossopharyngeal: from inner ear
IX: Accessory: from pharynx and to pharyngeal muscles
X: Vagus: to and from internal organs
XI: Hypoglossal: to and from back and neck muscles
XII: Vestibulocochlear: to tongue muscles

Now, we'll go into each of the nerves just a little bit deeper and in reverse :) When I refer to a mixed nerve, it means that it is both a motor nerve and a sensory nerve. 

 XII: Hypoglossal nerve
The hypoglossal nerve is 5 cranial nevers that arise from the medulla (8-12).  The hypoglossal nerve controls muscles of the tongue during speech and swallowing.  If your hypoglossal nerve is injured, your tongue will fall to the side with the injury when you protrude it.  The hypoglossal nerve is mixed; however it is primarily motor. 

XI: Spinal Accessory Nerve
There are two portions: cranial and spinal.  The cranial portion arises from the medulla and controls skeletal muscle of the through and soft palate.  The spinal portion arises from the cervical spinal cord and controls the sternocleidomastoid and the trapezius.  The spinal accessory nerve is mixed. 

X: Vagus Nerve
The vagus nerve receives sensations from the viscera.  It controls cardiac muscle as well as the smooth muscle of the viscera.  The vagus also controls secretion of digestive fluids.  The vagus nerve can slow down heart while resting by inhibition. 

IX: Glossopharyngeal Nerve
The glossopharyngeal nerve controls the sternocloidomastoid.  It lifts the throat during swallowing, secretions from the parotid gland, salivary gland, aids in somatic sensations and taste on posterior 1/3 of tongue.  It is a mixed nerve. 

VIII: Vestibulocochlear Nerve
The vestibulocohlear nerve has two branches: cochlear and vestibular.  The cochlear branch begins in the medulla receptors of the cochlea.  It aids in hearing and if it is damaged, deafness or tinnitus is produced.  The vestibular branch begins in the pons receptors in the vestibular apparatus. It aids in the sense of balance, vertigo and ataxia.  The glossopharyngeal nerve is mixed, but mainly sensory.  

VII: Facial Nerve
The facial nerve has a motor portion and a sensory portion.  The motor portion controls facial muscles, salivary, nasal, and oral mucous glands and tears.  The sensory portion of the facial nerves are the taste buds on the anterior 2/3's of the tongue.  The facial nerve is a mixed nerve. 

XI: Abducens Nerve
The abducens nerve controls the lateral rectus eye muscle.  The abducens nerve is a mixed nerve, but mainly motor. 

V: Trigeminal Nerve
The trigeminal nerve has a motor portion and a sensory portion.  The motor portion works on mastication. The sensory portion receives touch, pain and temperature of the fave.  The three branches are ophthalmic, maxillary and mandibular.  The trigeminal nerve is mixed. 

IV: Trochlear nerve
The trochlear nerve controls the superior oblique eye muscle.  The trochlear nerve is mixed, but mianly motor. 

III: The oculomotor nerve
The oculomotor nerve conttrols many muscles of the eye. These include the levator palpabrae, four of six extrinsic eye muscles (superior, inferior and medial recti and inferior oblique).  It is a motor nerve of the autonomic nervous system and works with two intrinsic eye muscles of the lens and pupil.  It is also the part that accomadates and constricts the pupil. 

II: The Optic Nerve
The optic nerve connects to the retina to supply vision.  It is strictly a sensory nerve. 

I: The Olfactory Nerve
The olfactory nerve extends from the olfactory mucosa of the nasal cavity to the olfactory bulb.  It aids in the sense of smell.  The olfactory nerve is strictly a sensory nerve. 

No comments: