Riku Case Study
Riku is a 19-year-old college student. One morning, after a long night of studying, Riku woke up and made himself a hot cup of coffee and toast. Much to his surprise, when he brought the cup to his mouth to drink, the coffee spilt onto the table. Riku went to the bathroom mirror and noticed the left side of his face seemed to droop. He quickly got dressed and ran to the medical clinic on the college campus. As he ran, his left eye began to feel scratchy and dry, but he could not blink in response. The physician at the clinic listened to Riku’s story and then did a careful cranial nerve examination. She concluded that Riku had Bell palsy, an inflammatory condition of the facial nerve most likely caused by a virus.
1.What are an afferent neuron and efferent neuron? What are efferent components of the facial nerve and their actions?
2. Under certain circumstances, axons in the peripheral nervous system can regenerate after sustaining damage. Why is axonal regeneration in the central nervous system much less likely?
3. At a healthy myoneural junction, acetylcholine is responsible for stimulating muscle activity. What mechanisms are in place to prevent the continuous stimulation of a muscle fiber after the neurotransmitter is released from the presynaptic membrane?
Riku Case Study
Bell’s palsy as revealed in Rikus’s case is an inflammatory process caused most likely by his virus. Also referred to as acute peripheral facial palsy of an unknown cause, Bell’s palsy may occur at any age. It is believed to result from inflammation and nerve swelling that controls muscles on one side of an individual’s face (Yoo et al.,2020). It might as well be a reaction that usually transpires after a viral infection. Bell’s palsy provisionally paralyzes or weakens the facial muscles. Pinched facial nerves cause this particular palsy or paralysis. Individuals with this kind of facial nerve palsy develop a droopy appearance in one or at times on both sides of the face. Bell’s palsy condition is not that serious and frequently resolves in a few months without any treatment. The treatment for this condition may involve anti-inflammatory medications, steroids, or pain medications to reduce the bony passage through which the nerve runs. This condition usually is short-lived but might last long enough to demand a surgery for decompression that’s highly un-favored.
Afferent and Efferent Neuron
Afferent neurons, also known as sensory neurons, are nerve fibers responsible for carrying sensory information from the outside world to the brain. Sensory data may encompass distinct senses like vision, smell, hearing, taste, and a sense of pain, touch, and temperature. On the other hand, efferent neurons, also known as motor neurons, are nerve fibers accountable for carrying the signals from the brain to the peripheral nervous system to initiate a specific action (Hernández., n.d.). In simpler words, they are neurons that inform an individual’s body to complete an activity like removing the hand from the hot pan.
Afferent and Efferent neurons together create a feeling that usually senses stimulus and signals the central nervous systems (CNS) (Hernández., n.d.). The central nervous system completes the action by sending a specific signal through the nerve cells. The central nervous system plays a vital role in creating energy and protein synthesis. This is where the axons and dendrites act as part of the cell. They have an opposed connection in which dendrites carry the messages to the body cell, and axons carry the messages away from the cell. The transaction takes sensory afferent neurons to external stimulus connecting with an interneuron. Information is gathered, and the reaction is sent to efferent motor neurons and to the targeted body part. Afferent neurons connect data from stimulus to the brain or spinal cord. On the other hand, Efferent neurons carry data from the brain or the spinal cord to suitable body parts.
Efferent Components of Facial Nerve and Their Actions
Facial nerve is the seventh cranial nerve and rises from the brain stem spreading posteriorly towards the abducens nerve and then anteriorly towards the vestibulocochlear nerve. Generally, the facial nerve carries sensory and motor fibers. Motor axons innervate facial expression and stapedius muscle. The parasympathetic fibers move towards ganglia that usually supply oral cavity and lacrimal glands. The sensory component affords innervation into exterior hearing meatus, tympanic membrane, and ear pinna. Facial nerve similarly carries the sensation taste from the anterior two-thirds of the tongue.
A significant component of a facial nerve is Special visceral efferent (SVE) fibers. These remain key components of the facial nerve, and their purpose is innervated muscles of stylohyoid muscle, stapedius muscle, facial expression, and posterior digastric muscle belly (Hovland, Phuong & Lu., 2021). Fiber neurons are confined in the facial nucleus in the caudal pontine tegmentum. The other component is General visceral efferent (GVE) fibers. These leave the facial nerve as great chorda tympani and petrosal nerve. After synapsing into the pterygopalatine ganglion, the great petrosal nerve provides a postganglionic parasympathetic based innervation to the oral, nasal, and palatine glands.
Why Peripheral Nervous System Axons can regenerate after Damage whereas those
In Central Nervous System are less likely to regenerate
In the peripheral nervous system, bundles of axons or nerve ﬁbers conduct data to and from the central nervous system. A neuron contains a cell body, dendrites that usually carry the electrical impulses, and a long axon that carries impulses away from the cell. In Central Nervous System, bundles of the myelinated axons create nerve tracts. These particular tracts cross the brain’s midline to join opposite regions, known as commissures. The largest is the corpus callosum, which joins the two cerebral hemispheres and has around twenty million axons. Several Central Nervous System axons post less or no regenerative response after getting cut, contrastingly to the peripheral axons that usually make a vital effort to regrow. The embryonic axons renew more strongly than the adult axons. Individuals have established a tissue culture model to see developmental change that young axons regenerate whenever cut, but mature axons usually fail.
Axons in the peripheral nervous system thus regenerate after sustaining damage, whereas axonal regeneration in the central nervous system doesn’t since peripheral nerves axons are involved with myelin and are blanketed by the endoneurium (Varier et al., 2022). This loose connection tissue makes the guiding tube to the proposed organ. If endoneurium is integral after the axonal injury, axonal “bud” can cultivate in the endoneurium tube to its initial target. In the central nervous system, there’s lack of endometrial tissue. Axonal restoration is therefore restricted in this part of the nervous system.
Mechanisms at Place to Prevent Continuous Stimulation of Muscle Fiber after Releasing
Neurotransmitter from Presynaptic Membrane
At the neuromuscular junction, presynaptic axons end close to the plasma membrane of the muscle fibers (Colombo & Francolini., 2019). The presynaptic axons relieve acetylcholine, a neurotransmitter that encourages muscle movement. However, several mechanisms are present to stop constant muscle fiber stimulation due to releasing acetylcholine. These include: First, downregulation of the acetylcholine receptors in muscle fiber sarcolemma. Here, a form of ligand-gated ion passage known as nicotinic acetylcholine receptor forms a binding site for the acetylcholine. After the acetylcholine has been released and its influence executed by muscle fibers, the binding sites are down controlled from the sarcolemma of the muscle fibers by being detached to reduce their number to prevent sustained acetylcholine action. Secondly, theres flow of released acetylcholine back to presynaptic axons vesicles. Here, acetylcholine similarly spreads inactively back to the presynaptic axons after releasing acetylcholine and exerting its effect on the muscle fibers. Besides, there is a breakdown by the cholinesterase. To prevent sustained action of the acetylcholine on muscle fibers, an enzyme known as cholinesterase breaks down the acetylcholine to choline and acetic acid through hydrolysis, choline then is reabsorbed back to presynaptic axons, and the acetic acid is applied to synthesize more acetyl.
Colombo, M. N., & Francolini, M. (2019). Glutamate at the vertebrate neuromuscular
junction: from modulation to neurotransmission. Cells, 8(9), 996. https://doi.org/10.3390/cells8090996
Hernández. (n.d.). Afferent vs. Efferent Neurons. Cloudflare. https://www.osmosis.org/answers/afferent-vs-efferent-neurons#
Hovland, N., Phuong, A., & Lu, G. N. (2021). Anatomy of the facial nerve. Operative Techniques in Otolaryngology-Head and Neck Surgery, 32(4), 190-196. https://doi.org/10.1016/j.otot.2021.10.009
Varier, P., Raju, G., Madhusudanan, P., Jerard, C., & Shankarappa, S. A. (2022). A Brief Review of In Vitro Models for Injury and Regeneration in the Peripheral Nervous System. International Journal of Molecular Sciences, 23(2), 816. https://doi.org/10.3390/ijms23020816
Yoo, M. C., Soh, Y., Chon, J., Lee, J. H., Jung, J., Kim, S. S., … & Yeo, S. G. (2020). Evaluation of factors associated with favorable outcomes in adults with Bell palsy. JAMA Otolaryngology–Head & Neck Surgery, 146(3), 256-263. doi:10.1001/jamaoto.2019.4312