Cranial nerves are the nerves that emerge directly from the brain, in contrast to spinal nerves. 10 of the cranial nerves originate in the brainstem. Cranial nerves relay information between the brain and parts of the body to and from regions of the head and neck. Spinal nerves emerge sequentially from the spinal cord with the spinal nerve closest to the head emerging in the space above the first cervical vertebra; the cranial nerves, emerge from the central nervous system above this level. Each cranial nerve is present on both sides. Depending on definition in humans there are twelve or thirteen cranial nerves pairs, which are assigned Roman numerals I–XII, sometimes including cranial nerve zero; the numbering of the cranial nerves is based on the order in which they emerge from the brain, front to back. The terminal nerves, olfactory nerves and optic nerves emerge from the cerebrum or forebrain, the remaining ten pairs arise from the brainstem, the lower part of the brain; the cranial nerves are considered components of the peripheral nervous system, although on a structural level the olfactory and trigeminal nerves are more considered part of the central nervous system.
Most humans are considered to have twelve pairs of cranial nerves, with the terminal nerve more canonized. They are: the olfactory nerve, the optic nerve, oculomotor nerve, trochlear nerve, trigeminal nerve, abducens nerve, facial nerve, vestibulocochlear nerve, glossopharyngeal nerve, vagus nerve, accessory nerve, hypoglossal nerve. Cranial nerves are named according to their structure or function. For example, the olfactory nerve supplies smell, the facial nerve supplies motor innervation to the face; because Latin was the lingua franca of the study of anatomy when the nerves were first documented and discussed, many nerves maintain Latin or Greek names, including the trochlear nerve, named according to its structure, as it supplies a muscle that attaches to a pulley. The trigeminal nerve is named in accordance with its three components, the vagus nerve is named for its wandering course. Cranial nerves are numbered based on their rostral-caudal position. If the brain is removed from the skull the nerves are visible in their numeric order, with the exception of the last, CN XII, which appears to emerge rostrally to CN XI.
Cranial nerves have paths outside the skull. The paths within the skull are called "intracranial" and the paths outside the skull are called "extracranial". There are many holes in the skull called "foramina" by. All cranial nerves are paired, which means that they occur on both the right and left sides of the body; the muscle, skin, or additional function supplied by a nerve on the same side of the body as the side it originates from, is referred to an ipsilateral function. If the function is on the opposite side to the origin of the nerve, this is known as a contralateral function. Intracranial course of cranial nerves is important regarding diagnosis of various intracranial lesions like brain tumors and intracranial arterial aneurysms. Dysfunction of one or more cranial nerves indicates stimulation by some lesion. For example an acoustic schwanoma may cause disturbance in hearing but with further growth of tumor it may involve other cranial nerves and the patient may present with pain resembling trigeminal neuralgia when the tumor involves trigeminal nerve or diplopia due to abducent nerve involvement facial palsy with facial nerve compression.
These findings along with cerebellar signs will suggest the diagnosis of a cerebellopontine angle lesion. A patient presenting with ptosis may have a posterior communicating artery aneurysm compressing the oculomotor nerve during its intracranial course. Facial pain in the distribution of any one or all divisions of trigeminal nerve suggests stimulation of trigeminal nerve roots by a near by vessel; the cell bodies of many of the neurons of most of the cranial nerves are contained in one or more nuclei in the brainstem. These nuclei are important relative to cranial nerve dysfunction because damage to these nuclei such as from a stroke or trauma can mimic damage to one or more branches of a cranial nerve. In terms of specific cranial nerve nuclei, the midbrain of the brainstem has the nuclei of the oculomotor nerve and trochlear nerve; the fibers of these cranial nerves exit the brainstem from these nuclei. Some of the cranial nerves have sensory or parasympathetic ganglia of neurons, which are located outside the brain.
The sensory ganglia are directly correspondent to dorsal root ganglia of spinal nerves and are known as cranial sensory ganglia. Sensory ganglia exist for nerves with sensory function: V, VII, VIII, IX, X. There are parasympathetic ganglia, which are part of the autonomic nervous system for cranial nerves III, VII, IX and X; the trigeminal ganglia of the trigeminal nerve occupies a space in the dura mater called Trigeminal cave. This ganglion contains the cell bodies of the sensory fibers of the three branches of the trig
In physiology, an action potential occurs when the membrane potential of a specific axon location rises and falls: this depolarisation causes adjacent locations to depolarise. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, endocrine cells, in some plant cells. In neurons, action potentials play a central role in cell-to-cell communication by providing for—or with regard to saltatory conduction, assisting—the propagation of signals along the neuron's axon toward synaptic boutons situated at the ends of an axon. In other types of cells, their main function is to activate intracellular processes. In muscle cells, for example, an action potential is the first step in the chain of events leading to contraction. In beta cells of the pancreas, they provoke release of insulin. Action potentials in neurons are known as "nerve impulses" or "spikes", the temporal sequence of action potentials generated by a neuron is called its "spike train".
A neuron that emits an action potential, or nerve impulse, is said to "fire". Action potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane; these channels are shut when the membrane potential is near the resting potential of the cell, but they begin to open if the membrane potential increases to a defined threshold voltage, depolarising the transmembrane potential. When the channels open, they allow an inward flow of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential; this causes more channels to open, producing a greater electric current across the cell membrane and so on. The process proceeds explosively until all of the available ion channels are open, resulting in a large upswing in the membrane potential; the rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, the ion channels rapidly inactivate. As the sodium channels close, sodium ions can no longer enter the neuron, they are actively transported back out of the plasma membrane.
Potassium channels are activated, there is an outward current of potassium ions, returning the electrochemical gradient to the resting state. After an action potential has occurred, there is a transient negative shift, called the afterhyperpolarization. In animal cells, there are two primary types of action potentials. One type is generated by the other by voltage-gated calcium channels. Sodium-based action potentials last for under one millisecond, but calcium-based action potentials may last for 100 milliseconds or longer. In some types of neurons, slow calcium spikes provide the driving force for a long burst of emitted sodium spikes. In cardiac muscle cells, on the other hand, an initial fast sodium spike provides a "primer" to provoke the rapid onset of a calcium spike, which produces muscle contraction. In the Hodgkin–Huxley membrane capacitance model, the speed of transmission of an action potential was undefined and it was assumed that adjacent areas became depolarised due to released ion interference with neighbouring channels.
Measurements of ion diffusion and radii have since shown this not to be possible. Moreover, contradictory measurements of entropy changes and timing disputed the capacitance model as acting alone. Nearly all cell membranes in animals and fungi maintain a voltage difference between the exterior and interior of the cell, called the membrane potential. A typical voltage across an animal cell membrane is −70 mV; this means that the interior of the cell has a negative voltage of one-fifteenth of a volt relative to the exterior. In most types of cells, the membrane potential stays constant; some types of cells, are electrically active in the sense that their voltages fluctuate over time. In some types of electrically active cells, including neurons and muscle cells, the voltage fluctuations take the form of a rapid upward spike followed by a rapid fall; these up-and-down cycles are known as action potentials. In some types of neurons, the entire up-and-down cycle takes place in a few thousandths of a second.
In muscle cells, a typical action potential lasts about a fifth of a second. In some other types of cells, in plants, an action potential may last three seconds or more; the electrical properties of a cell are determined by the structure of the membrane that surrounds it. A cell membrane consists of a lipid bilayer of molecules in which larger protein molecules are embedded; the lipid bilayer is resistant to movement of electrically charged ions, so it functions as an insulator. The large membrane-embedded proteins, in contrast, provide channels through which ions can pass across the membrane. Action potentials are driven by channel proteins whose configuration switches between closed and open states as a function of the voltage difference between the interior and exterior of the cell; these voltage-sensitive proteins are known as voltage-gated ion channels. All cells in animal body tissues are electrically polarized – in other words, they maintain a voltage difference across the cell's plasma membrane, known as the membrane potential.
This electrical polarization results from a complex interplay between protein structures embedded in the membrane called ion pumps and ion channels. In neurons, the types of ion channels in the membrane vary across different parts of the cell, giving the dendrites and cell body different electrical properties; as a result, some parts of the membrane of a neuron may be excitable (capable of generating action potentia
The ear canal is a pathway running from the outer ear to the middle ear. The adult human ear canal extends from the pinna to the eardrum and is about 2.5 centimetres in length and 0.7 centimetres in diameter. The human ear canal is divided into two parts; the elastic cartilage part forms the outer third of the canal. The cartilage is the continuation of the cartilage framework of pinna; the cartilaginous portion of the ear canal contains small hairs and specialized sweat glands, called apocrine glands, which produce cerumen. The bony part forms the inner two thirds; the bony part is only a ring in the newborn. The layer of epithelium encompassing the bony portion of the ear canal is much thinner and therefore, more sensitive in comparison to the cartilaginous portion. Size and shape of the canal vary among individuals; the canal is 2.5 centimetres long and 0.7 centimetres in diameter. It runs from behind and above downward and forward. On the cross-section, it is of oval shape; these are important factors to consider.
Due to its relative exposure to the outside world, the ear canal is susceptible to diseases and other disorders. Some disorders include: Atresia of the ear canal Cerumen impaction Bone exposure, caused by the wearing away of skin in the canal Auditory canal osteoma Cholesteatoma Contact dermatitis of the ear canal Fungal infection Ear mites in animals Ear myiasis, an rare infestation of maggots Foreign body in ear Granuloma, a scar caused by tympanostomy tubes Otitis externa, bacteria-caused inflammation of the ear canal Stenosis, a gradual closing of the canal Earwax known as cerumen, is a yellowish, waxy substance secreted in the ear canals, it plays an important role in the human ear canal, assisting in cleaning and lubrication, provides some protection from bacteria and insects. Excess or impacted cerumen can press against the eardrum and/or occlude the external auditory canal and impair hearing, causing conductive hearing loss. If left untreated, cerumen impaction can increase the risk of developing an infection within the ear canal.
List of specialized glands within the human integumentary system Veterans Health Administration web site OSHA web site Continuing Medical Education Ear Photographs Otoscopy Tutorial w/ Images "Anatomy diagram: 34257.000-1". Roche Lexicon - illustrated navigator. Elsevier. Archived from the original on 2014-01-01
The antitragus is a feature of mammalian ear anatomy. In humans, it is a small tubercle on the visible part of the ear; the antitragus is located just above the earlobe and points anteriorly. It is separated from the tragus by the intertragic notch; the antitragicus muscle, an intrinsic muscle of the ear, arises from the outer part of the antitragus. The antitragus can be most notably bats. Antitragus piercing This article incorporates text in the public domain from page 1034 of the 20th edition of Gray's Anatomy Anatomy photo:30:01-0105 at the SUNY Downstate Medical Center lesson3 at The Anatomy Lesson by Wesley Norman Diagram at bodymodforums.com
The helix is the prominent rim of the auricle. Where the helix turns downwards posteriorly, a small tubercle is sometimes seen, namely the auricular tubercle of Darwin; this article incorporates text in the public domain from page 1033 of the 20th edition of Gray's Anatomy
An axon, or nerve fiber, is a long, slender projection of a nerve cell, or neuron, in vertebrates, that conducts electrical impulses known as action potentials away from the nerve cell body. The function of the axon is to transmit information to different neurons and glands. In certain sensory neurons, such as those for touch and warmth, the axons are called afferent nerve fibers and the electrical impulse travels along these from the periphery to the cell body, from the cell body to the spinal cord along another branch of the same axon. Axon dysfunction has caused many inherited and acquired neurological disorders which can affect both the peripheral and central neurons. Nerve fibers are classed into three types – group A nerve fibers, group B nerve fibers, group C nerve fibers. Groups A and B are myelinated, group C are unmyelinated; these groups include both sensory fibers and motor fibers. Another classification groups only the sensory fibers as Type I, Type II, Type III, Type IV. An axon is one of two types of cytoplasmic protrusions from the cell body of a neuron.
Axons are distinguished from dendrites by several features, including shape and function. Some types of neurons have no transmit signals from their dendrites. In some species, axons can emanate from dendrites and these are known as axon-carrying dendrites. No neuron has more than one axon. Axons are covered by a membrane known as an axolemma. Most axons branch, in some cases profusely; the end branches of an axon are called telodendria. The swollen end of a telodendron is known as the axon terminal which joins the dendron or cell body of another neuron forming a synaptic connection. Axons make contact with other cells—usually other neurons but sometimes muscle or gland cells—at junctions called synapses. In some circumstances, the axon of one neuron may form a synapse with the dendrites of the same neuron, resulting in an autapse. At a synapse, the membrane of the axon adjoins the membrane of the target cell, special molecular structures serve to transmit electrical or electrochemical signals across the gap.
Some synaptic junctions appear along the length of an axon as it extends—these are called en passant synapses and can be in the hundreds or the thousands along one axon. Other synapses appear as terminals at the ends of axonal branches. A single axon, with all its branches taken together, can innervate multiple parts of the brain and generate thousands of synaptic terminals. A bundle of axons make a nerve tract in the central nervous system, a fascicle in the peripheral nervous system. In placental mammals the largest white matter tract in the brain is the corpus callosum, formed of some 20 million axons in the human brain. Axons are the primary transmission lines of the nervous system, as bundles they form nerves; some axons can extend up to more while others extend as little as one millimeter. The longest axons in the human body are those of the sciatic nerve, which run from the base of the spinal cord to the big toe of each foot; the diameter of axons is variable. Most individual axons are microscopic in diameter.
The largest mammalian axons can reach a diameter of up to 20 µm. The squid giant axon, specialized to conduct signals rapidly, is close to 1 millimetre in diameter, the size of a small pencil lead; the numbers of axonal telodendria can differ from one nerve fiber to the next. Axons in the central nervous system show multiple telodendria, with many synaptic end points. In comparison, the cerebellar granule cell axon is characterized by a single T-shaped branch node from which two parallel fibers extend. Elaborate branching allows for the simultaneous transmission of messages to a large number of target neurons within a single region of the brain. There are two types of axons in the nervous system: unmyelinated axons. Myelin is a layer of a fatty insulating substance, formed by two types of glial cells Schwann cells and oligodendrocytes. In the peripheral nervous system Schwann cells form the myelin sheath of a myelinated axon. In the central nervous system oligodendrocytes form the insulating myelin.
Along myelinated nerve fibers, gaps in the myelin sheath known as nodes of Ranvier occur at evenly spaced intervals. The myelination enables an rapid mode of electrical impulse propagation called saltatory conduction; the myelinated axons from the cortical neurons form the bulk of the neural tissue called white matter in the brain. The myelin gives the white appearance to the tissue in contrast to the grey matter of the cerebral cortex which contains the neuronal cell bodies. A similar arrangement is seen in the cerebellum. Bundles of myelinated axons make up the nerve tracts in the CNS. Where these tracts cross the midline of the brain to connect opposite regions they are called commissures; the largest of these is the corpus callosum that connects the two cerebral hemispheres, this has around 20 million axons. The structure of a neuron is seen to consist of two separate functional regions, or compartments – the cell body together with the dendrites as one region, the axonal region as the other.
The axonal region or compart
The antihelix is a part of the visible ear. The antihelix is a curved prominence of cartilage parallel with and in front of the helix on the pinna; the antihelix divides above into crura. Lesson3 at The Anatomy Lesson by Wesley Norman