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Background: Lemniscus (Latin lēmniscus, ribbon) is a strap of second order nerve fibres which twist as they ascend to the brainstem.
Why do these these nerve fibres rotate? What could be the functional significance of such rotation?
I think that rotation is somewhat less beneficial because firstly it causes increase in length thereby increasing the conduction time , secondly it may decrease structural integrity of such a bundle of axons.
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Optic nerve, second cranial nerve, which carries sensory nerve impulses from the more than one million ganglion cells of the retina toward the visual centres in the brain. The vast majority of optic nerve fibres convey information regarding central vision.
The optic nerve begins at the optic disk, a structure that is 1.5 mm (0.06 inch) in diameter and is located at the back of the eye. The optic disk forms from the convergence of ganglion cell output fibres (called axons) as they pass out of the eye. When the nerve emerges from the back of the eye, it passes through the remainder of the posterior orbit (eye socket) and through the bony optic canal to emerge intracranially on the underside of the front of the brain. At this point the optic nerve from each eye comes together and forms an X-shaped structure called the optic chiasm. Here, approximately one-half of the nerve fibres from each eye continue on the same side of the brain, and the remaining nerve fibres cross over at the chiasm to join fibres from the opposite eye on the other side of the brain. This arrangement is essential for producing binocular vision. Posterior to the optic chiasm, the nerve fibres travel in optic tracts to various portions of the brain—predominantly the lateral geniculate nuclei. Fibres from the lateral geniculate nuclei form the optic radiations that course toward the visual cortex located in the occipital lobes in the back of the brain. Some nerve fibres leave the optic tract without entering the lateral geniculate nuclei and instead enter the brain stem to provide information that ultimately determines pupil size.
The retina, optic disk, optic nerve, optic chiasm, optic tracts, optic radiations, and visual centres of the brain are topographically organized to correspond to particular areas of the visual field. Therefore, damage to, or pressure on, particular portions of these structures can produce characteristic deficits in a person’s visual field (see visual field defect). The affected person may or may not notice these visual field defects.
Small fiber neuropathy is a common feature of Ehlers-Danlos syndromes
Objective: To investigate the involvement of small nerve fibers in Ehlers-Danlos syndrome (EDS).
Methods: Patients diagnosed with EDS underwent clinical, neurophysiologic, and skin biopsy assessment. We recorded sensory symptoms and signs and evaluated presence and severity of neuropathic pain according to the Douleur Neuropathique 4 (DN4) and ID Pain questionnaires and the Numeric Rating Scale (NRS). Sensory action potential amplitude and conduction velocity of sural nerve was recorded. Skin biopsy was performed at distal leg and intraepidermal nerve fiber density (IENFD) obtained and referred to published sex- and age-adjusted normative reference values.
Results: Our cohort included 20 adults with joint hypermobility syndrome/hypermobility EDS, 3 patients with vascular EDS, and 1 patient with classic EDS. All except one patient had neuropathic pain according to DN4 and ID Pain questionnaires and reported 7 or more symptoms at the Small Fiber Neuropathy Symptoms Inventory Questionnaire. Pain intensity was moderate (NRS ≥4 and <7) in 8 patients and severe (NRS ≥7) in 11 patients. Sural nerve conduction study was normal in all patients. All patients showed a decrease of IENFD consistent with the diagnosis of small fiber neuropathy (SFN), regardless of the EDS type.
Conclusions: SFN is a common feature in adults with EDS. Skin biopsy could be considered an additional diagnostic tool to investigate pain manifestations in EDS.
© 2016 American Academy of Neurology.
Figure. Small Fiber Neuropathy and Symptoms Inventory…
Figure. Small Fiber Neuropathy and Symptoms Inventory Questionnaire features
Optic (II) Nerve
The optic nerve (cranial nerve II) receives visual information from photoreceptors in the retina and transmits it to the brain.
Describe the optic nerve (cranial nerve II)
- The optic nerve is considered part of the central nervous system. The myelin on the optic nerve is produced by oligodendrocytes rather than Schwann cells and it is encased in the meningeal layers instead of the standard endoneurium, perineurium, and epineurium of the peripheral nervous system.
- The optic nerve travels through the optic canal, partially decussates in the optic chiasm, and terminates in the lateral geniculate nucleus where information is transmitted to the visual cortex.
- The axons responsible for reflexive eye movements terminate in the pretectal nucleus.
- oligodendrocyte: A type of neuroglia that provides support and insulation to axons in the central nervous system.
- retina: The thin layer of cells at the back of the eyeball where light is converted into neural signals sent to the brain.
- optic nerve: Either of a pair of nerves that carry visual information from the retina to the brain.
- visual cortex: The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain.
- pretectal nucleus: This mediates behavioral responses to acute changes in ambient light, such as the pupillary light reflex and the optokinetic reflex.
The optic nerve is also known as cranial nerve II. It transmits visual information from the retina to the brain.
Each human optic nerve contains between 770,000 and 1.7 million nerve fibers. The eye’s blind spot is a result of the absence of photoreceptors in the area of the retina where the optic nerve leaves the eye.
Optic nerve: An illustration of the brain highlighting the optic nerve and optic tract.
The optic nerve is the second of twelve paired cranial nerves. It is considered by physiologists to be part of the central nervous system, as it is derived from an outpouching of the diencephalon during embryonic development.
As a consequence, the fibers are covered with myelin produced by oligodendrocytes, rather than Schwann cells that are found in the peripheral nervous system. The optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium, perineurium, and endoneurium found in the peripheral nerves.
The fiber tracks of the mammalian central nervous system are incapable of regeneration. As a consequence, optic nerve damage produces irreversible blindness.
The optic nerve leaves the orbit, which is also known as an eye socket, via the optic canal, running posteromedially toward the optic chiasm, where there is a partial decussation (crossing) of fibers from the nasal visual fields of both eyes.
Most of the axons of the optic nerve terminate in the lateral geniculate nucleus (where information is relayed to the visual cortex), while other axons terminate in the pretectal nucleus and are involved in reflexive eye movements.
The optic nerve transmits all visual information including brightness perception, color perception, and contrast. It also conducts the visual impulses that are responsible for two important neurological reflexes: the light reflex and the accommodation reflex.
The light reflex refers to the constriction of both pupils that occurs when light is shone into either eye the accommodation reflex refers to the swelling of the lens of the eye that occurs when one looks at a near object, as in reading.
This system connects the brain stem and spinal cord with internal organs and regulates internal body processes that require no conscious effort and that people are thus usually unaware of (see Overview of the Autonomic Nervous System). Examples are the rate and strength of heart contractions, blood pressure, the rate of breathing, and the speed at which food passes through the digestive tract.
The autonomic nervous system has two divisions:
Sympathetic division: Its main function is to prepare the body for stressful or emergency situations—for fight or flight.
Parasympathetic division: Its main function is to maintain normal body functions during ordinary situations.
These divisions work together, usually with one activating and the other inhibiting the actions of internal organs. For example, the sympathetic division increases pulse, blood pressure, and breathing rates, and the parasympathetic system decreases each of them.
Typical Structure of a Nerve Cell
A nerve cell (neuron) consists of a large cell body and nerve fibers—one elongated extension (axon) for sending impulses and usually many branches (dendrites) for receiving impulses.
Each large axon is surrounded by oligodendrocytes in the brain and spinal cord and by Schwann cells in the peripheral nervous system. The membranes of these cells consist of a fat (lipoprotein) called myelin. The membranes are wrapped tightly around the axon, forming a multilayered sheath. This myelin sheath resembles insulation, such as that around an electrical wire. Nerve impulses travel much faster in nerves with a myelin sheath than in those without one.
If the myelin sheath of a nerve is damaged, nerve transmission slows or stops. The myelin sheath may be damaged by various conditions that damage the brain or peripheral nerves including
Certain autoimmune disorders (such as Guillain-Barré syndrome)
Certain hereditary disorders
Representation of Action Potential
As the nerve impulse moves along the axon as represented in the image above, it's possible to see the change in ion movement in and out of the cell. However, once the impulse passes, the part behind the impulse on the axon starts reverting back to the resting membrane potential.
Although the image above gives a general representation of action potential, it does not show the myelin sheath and nodes of Ranvier. In a normal nerve cell, these structures are present and enhance the propagation of action potential.
The areas covered with the myelin sheath prevent the exchange of ions along the axon. However, at the nodes of Ranvier, which are the uncovered gaps, ion exchange takes place which allows for faster propagation.
This is due to the fact that the process jumps from one node to the next rather than the transmission occurring along the entire length of the axon.
Transmission that occurs due to the presence of the myelin sheath cells (with discrete jumps) is known as saltatory conduction
Image with Myelin Sheath cells:
Why can nerve impulses travel only in one direction?
Because of the chemical nature of impulse and the axon-dendrite structure.
Because of the chemical nature of impulse and the axon-dendrite structure.
http://www.biologymad.com/nervoussystem/nerveimpulses.htm (Best technical explanation and excellent graphics and animations)
The best generic answer from a 2008 post follows (with my own edits for clarity):
A Nerve electrical impulse only travels in one direction. There are several reasons nerve impulses only travel in one direction. The most important is synaptic transport.
In order for a "nerve impulse" to pass from cell to cell, it must cross synaptic junctions. The nerve cells are lined up head to tail all the way down a nerve track, and are not connected, but have tiny gaps between them and the next cell. These tiny gaps are called synapses.
When you get a nerve firing, you have probably heard that it is an electrical impulse that carries the signal. This is true, but it is not electrical in the same way your wall outlet works. This is electrochemical energy. Neurotransmitters are molecules that fit like a lock and key into a specific receptor. The receptor is located on the next cell in the line. When the neurotransmitter hits the receptor on the next cell in line, it signals that cell to begin a firing as well.
This will continue all the way down the length of the nerve track. In a nutshell, a nerve firing results in a chain reaction down the nerve cell's axon, or stemlike section. Sodium (Na+) ions flow in, potassium (K+) ions flow out, and we get an electrochemical gradient flowing down the length of the cell. You can think of it as a line of gunpowder that someone lit, with the flame traveling down the length of it. Common electrical power is more like a hose full of water, and when you put pressure on one end, the water shoots out the other.
Therefore, nerve impulses cannot travel in the opposite direction, because nerve cells only have neurotransmitter storage vesicles going one way, and receptors in one place.
A comparison of the speed of conduction of nerve impulse through an unmyelinated and myelinated Neuron (Image Source: CC Wikipedia)
|Sl. No.||Myelinated Neurons||Unmyelinated Neurons|
|1||Myelinated nerve fibres contain the myelin sheath.||Unmyelinated nerve fibres do not have the myelin sheath|
|2||The Schwann cells wrap tightly around the nerve axon and form the myelin sheath.||Schwann cells are not wound around the axons but simply form a groove.|
|3||The axis cylinder of the myelinated nerve fibres has two sheaths.||The axis cylinder of unmyelinated nerve fibres has only one sheath.|
|4||The speed of transfer of nerve impulses through myelinated nerve fibres is much faster when compared to unmyelinated fibres||The speed of transfer of nerve impulse through unmyelinated nerve fibres is much slower.|
|5||Myelinated nerve fibres appear as white in the fresh state.||Unmyelinated nerve fibres appear as grey in the fresh state.|
|6||Myelinated fibres show nodes and internodes.||Unmyelinated nerve fibres do not show notes and internodes.|
|7||Myelinated fibres possess notes of Ranvier.||Unmyelinated nerve fibres do not possess the Notes of Ranvier.|
|8||Myelinated nerve fibres occur in the white matter of the brain, spinal cord and in the central and cranial nervous system.||Unmyelinated nerve fibres occur in the autonomic nervous system.|
|9||Myelinated nerve fibres may give off collateral nerve fibres.||Collateral fibres are not produced in unmyelinated nerves.|
Similarities between Myelinated and Unmyelinated Nerve Cells
Ø Both myelinated and unmyelinated cells are nerve cells.
Ø Both can conduct impulses as electric signals.
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Generating Nerve Impulses
A nerve impulse, like a lightning strike, is an electrical phenomenon. A nerve impulse occurs because of a difference in electrical charge across the plasma membrane of a neuron. How does this difference in electrical charge come about? The answer involves ions, which are electrically charged atoms or molecules.
Figure (PageIndex<2>): The sodium-potassium pump maintains the resting potential of a neuron. There is more negative charge inside than outside the cell membrane. ATP is used to pump sodium out and potassium into the cell. There is more concentration of sodium outside the membrane and more concentration of potassium inside the cell due to the unequal movement of these ions by the pump
When a neuron is not actively transmitting a nerve impulse, it is in a resting state, ready to transmit a nerve impulse. During the resting state, the sodium-potassium pump maintains a difference in charge across the cell membrane of the neuron. The sodium-potassium pump is a mechanism of active transport that moves sodium ions out of cells and potassium ions into cells. The sodium-potassium pump moves both ions from areas of lower to higher concentration, using energy in ATP and carrier proteins in the cell membrane. Figure (PageIndex<3>)shows in greater detail how the sodium-potassium pump works. Sodium is the principal ion in the fluid outside of cells, and potassium is the principal ion in the fluid inside of cells. These differences in concentration create an electrical gradient across the cell membrane, called resting potential. Tightly controlling membrane resting potential is critical for the transmission of nerve impulses.
In cases of musculocutaneous nerve damage, some people experience spontaneous recovery, but even when that happens, it generally takes several months.
Many cases can be treated conservatively, such as with rest, ice, anti-inflammatory medications, and physical therapy. However, if that approach isn’t successful, surgical decompression may become necessary.
In some cases, nerve grafting or nerve transfer may be necessary for restoring function.