The skull has the function of housing and protecting the brain and is made up of 22 different bones, 14 of which are the facial bones while 8 are the cranial bones. Some major ones include the frontal bone (forehead), parietal bone (side and roof), sphenoid bone (base), temporal bone (sides and base), occipital bone (bottom), nasal bone (nose), zygomatic bone (cheek), maxilla (upper jaw), and mandible (lower jaw). Sutures are where two bones meet with each other to form synarthrodial joints. They normally only fuse in adulthood, allowing the child and teenage brain to continue growing.
There are four lobes to our brain—the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe. The frontal lobe is responsible for vital cognitive functions and aspects of our personality. The parietal lobe is responsible for processing sensory information and understanding language. The temporal lobe is responsible for processing auditory stimuli and integrating information from all of our senses. The occipital lobe is responsible for visual processing, which entails recognizing colors and objects.
It is also worth examining specific structures of the brain. The cerebellum is responsible for managing our coordination, balance, and posture. The brain stem, made up of the midbrain, pons, and medulla, takes care of vital unconscious functions like breathing, relaying neuronal information to the brain, and voluntary motor functions. The hippocampus stores short-term memory and then plays an important role in the conversion into long-term memory. The amygdala is responsible for our emotions, but, more specifically, fear. The hypothalamus maintains homeostasis in the body, linking the nervous system to the endocrine system by signaling the pituitary gland to release certain hormones. Thus, it is responsible for our sleep-wakefulness and reproductive behavior among other functions.
The nerve is a cell that transmits electrical signals throughout the body, spreading information. The cell body is made up of a nucleus and dendrites, which receive information from other nerves. The signal is carried along the axon, the long tail of the neuron, which is covered in myelin sheaths, fatty layers that help improve conduction. The Schwann cells produce the myelin, and the gaps between the myelin sheaths are the nodes of Ranvier, which is where the action potential actually occurs (saltatory conduction). Finally, the signal reaches the axon terminal, which then transmits the signal to the next nerve’s dendrites. There are three main types of neurons—sensory, inter, and motor neurons. Sensory neurons receive information from the outside world via sensory receptors. Interneurons interpret this sensory input, processing it and giving instruction to the motor neurons, which then elicit a response or movement.
There are twelve important cranial nerves that can be remembered with the acronym “Oh, Oh, Oh, To Touch And Feel Very Green Vegetables, Ah Heaven!” The first three nerves are the olfactory (smell), optic (sight), and oculomotor (eye movement). They can be checked in a neurophysical exam with smelling scents, vision card, and tracking a finger respectively. The second three nerves are the trochlear (eye movement down and out), trigeminal (ophthalmic, maxillary, mandibular), and abducens (moving eyes out) nerves. They can be checked in a neurophysical exam with tracking a finger, poking in the eye/biting down/jaw jerk reflex, and tracking a finger respectively. The penultimate three nerves are the facial (face movement and anterior taste), vestibulocochlear (hearing), and glossopharyngeal (posterior taste and swallowing) nerves. They can be checked in a neurophysical exam with raising eyebrows/smiling, hearing exam, and swallowing/palate elevation respectively. The final three nerves are the vagus (pharynx), spinal accessory (sternocleidomastoid and trapezius), and hypoglossal (tongue) nerves. They can be checked in a neurophysical exam by saying “ah,” shrugging/turning hear, and sticking out the tongue respectively.
The abdominal aorta is the main blood vessel from which all other vessels that feed the abdominal organs come from. The order of them can be remembered with the acronym “Canned Soup Really Good in Cans” for celiac trunk, superior mesenteric artery, renal artery, gonadal artery, inferior mesenteric artery, and common iliac artery. The first to branch off the abdominal aorta is the celiac trunk, which feeds the stomach. The superior mesenteric artery (SMA) is the next one to branch off, feeding the top half of the intestines. The renal artery feeds the kidneys, and the gonadal artery feeds the gonads. In men, this is the testes, and in women, this is the ovaries. The inferior mesenteric artery branches off afterward, feeding the bottom half of the intestines. Lastly, the common iliac artery branches off to feed the legs.
The digestive system begins as food enters the mouth, being chewed up and mixed with saliva, making a moist mush known as the bolus. The food then makes its way into the esophagus with the epiglottis folding over the larynx (windpipe) to prevent the aspiration of food. Voluntary swallowing as well as involuntary peristalsis, the contraction and relaxation of muscles, help move the food into the stomach. The food then passes through the cardiac, or lower esophageal, sphincter, which is at the junction of the esophagus and the stomach. Usually, this ring muscle stays closed, only opening to let food inside. If it does not work properly, gastrointestinal reflux disease (GERD) can occur as acidic contents of the stomach go up into the esophagus. Not only does this cause heartburn, but also it can cause esophageal cancer as the cells become more vulnerable due to the high acidity. The food goes through the cardia, fundus, body, and pylorus of the stomach while digestive juices produced by stomach glands mix and break the food down, making chyme (partially digested food). It is worth noting that most of the digestion does not happen in the stomach; the stomach is more of a storage facility.
The chyme is then carefully let into the small intestine via the pyloric sphincter. Only a little chyme is pushed through at a time to allow the small intestine, which does some 90-95% of the digestion, to properly do its job. A fairly common condition actually is pyloric stenosis where the food never gets digested since the muscular valve to the small intestine is blocked. Back to the small intestine, digestive enzymes, secreted by the pancreas, like trypsin, lipase, and amylase help to hydrolyze down macromolecules like proteins, lipids, and carbohydrates respectively. Bile, made by the liver, is stored in the gallbladder before being secreted into the small intestine, also playing a crucial role in the digestion of fats and fat-soluble compounds like vitamins, helping the lipase do its job. Most of the digestion occurs in the duodenum, the first part of the small intestine. From there the food goes into the jejunum and ileum respectively where some additional breaking down is done. However, nutrient absorption through the thin intestinal walls into the bloodstream is the main role of these latter two portions of the small intestine with the help of the villi, tiny finger-like projections.
From the small intestine, the remaining liquid passes through the ileocecal junction, another muscle valve, into the large intestine. Water and salt (along with any other residual nutrients) are absorbed here while mucus and bacteria are secreted to help produce compact stool for secretion. This is done as the matter goes through the ascending colon, transverse colon, descending colon, and sigmoid colon. Like with the esophagus and small intestine, peristalsis keeps the food moving through the large intestine as well into the rectum where the stool is then stored for a while. During a bowel movement, the stool exits the rectum, passing through the exterior and interior sphincters, and is pushed out of the anus.
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