Part A: Cranial Nerves:
There are 12 pairs of cranial nerves emerging from the brain’s underside passing via cranial openings leading to various parts of the head, neck and body trunk. These nerves are naming, and numbering of these nerves is based on their position i.e. from the anterior to the posterior of the brain. In this case, the olfactory nerve is CN1 while CN12 is the hypoglossal nerve (Wissmann, 2013). In this paper, only four of the twelve cranial nerves shall be discussed i.e. the Olfactory nerve (CN1), Oculomotor Nerve (CN-III) which are sensory nerves, and the Trigeminal Nerve (CN-V) and Facial Nerve (CN-VII) which are mixed nerves (sensory/motor).
The Olfactory nerve (CN1) is the sensory nerve for smell and has its nerve bundles originating from the olfactory region i.e. olfactory mucosa of the nasal cavity. The nerve passes via the cribriform plate on the ethmoid bone and is attached to the brain on the cerebral hemispheres. The oculomotor nerve (CN-III) is a motor nerve that serves muscles responsible for eye movement except lateral rectus muscles and the superior oblique muscles. It also serves the levator palpebrae superioris muscle which elevates eyelids, ciliary muscles, and the sphincter pupillae responsible for pupil constriction. The nerve enters the cranium via the superior orbital fissure to the midbrain (Majeed, Fatima, & Fatima, 2016; Wissmann, 2013).
The Trigeminal nerve (CN-V) is a mixed (sensory/motor) nerve and is also the largest cranial nerve. The nerve is responsible for facial sensation serving the greater part of teeth, scalp, and nasal and oral cavities. For motor functions, the CN-V supplies the masticatory and other muscles. It also has proprioceptive fibers originating from the extraocular and masticatory muscles. It has three major branches i.e. maxillary, ophthalmic, and mandibular which pass via the foramen rotundum, superior orbital fissure, and foramen ovale respectively. It originates from the pons with a small motor and large sensory root. The facial nerve (CN-VII) is another mixed (sensory/motor) nerve passing via the stylomastoid foramen with its motor root supplying the facial and scalp muscles such as the buccinators and platysma. It is also responsible for motor control in the lacrimal, sublingual and submandibular glands. The sensory root carries sensory information from the chorda tymphani gustatory fibers, and also from the taste fibers on the soft palate, and the palatine and greater petrosal nerves. The two roots are attached to the caudal border of the pons (Majeed, Fatima, & Fatima, 2016; Wissmann, 2013).
Part B: Pathways and tracts:
At any one time, there are millions of sensory neurons delivering sensory information to the CNS, and similarly, there are millions of motor neurons that cause the body to respond in various ways. These motor and sensory neurons follow various pathways/tracts within the spinal cord. These tracts are then responsible for channeling information to the brain and vice versa. There are two types of tracts i.e. ascending and descending. Ascending tracts carry information to the brain and are thus sensory tracts while descending tracts carry information from the brain and are thus motor tracts (Martini, Timmons, & Tallitsch, 2012).
The spinothalamic tract is a sensory tract transmitting temperature, pain, crude touch and pressure sensations to the thalamus, then to the cerebrum. The corticospinal tract is a motor tract responsible for conscious control of skeletal muscles (Martini, Timmons, & Tallitsch, 2012). The posterior/dorsal column-medial lemniscus tract is an ascending sensory tract of the CNS that carries information on localized sensations such as tactile two-point discrimination (fine touch), vibration and proprioception (sense of limb position). It transmits this information from the body trunk to the postcentral gyrus in the cerebral cortex (Stoney, 2016).
If the spinothalamic, corticospinal and posterior/dorsal column-medial lemniscus tract are damaged by a viral infection, several symptoms might be observed on the patient. First, a viral infection is likely to cause lesions on the spinal cord. Depending on the location of this lesion, damage to the spinothalamic tract on one side leads to loss of temperature and pain sensation on the other side (contralateral part) of the body. On the same note, damage to the corticospinal tract on one side of the spinal cord will exhibit UMN (Upper Motor Neuron) syndrome on the contralateral side of the patient’s body. The signs for UMN syndrome include muscle weakness, reduced motor control on voluntary muscles, involuntary and successive muscle contractions and relaxations as well as the inability to perform fine motor movements. Finally, a patient with a viral infection lesion on the posterior column-medial lemniscus tract damage would exhibit signs such as loss of vibration sense, reduced tactile discrimination, and loss of sense and position of limbs (Strominger, Demarest, & Laemle, 2012).
References:
Majeed, N., Fatima, S., & Fatima, H. (2016). The Cranial Nerves: Origins, Pathways & Basic Applied Anatomy (1st ed., pp. 1-10). Trend Setter Doctors (TSDocs). Retrieved from http://www.tsdocs.org/downloads/CranialNerves.pdf
Martini, F., Timmons, M., & Tallitsch, R. (2012). Human anatomy (7th ed.). Boston: Pearson Education.
Stoney, S. (2016). Sect. 8, Ch. 5: The Dorsal Column - Medial Lemniscal (DC-ML) System and its Trigeminal Analogues. Humanphysiology.tuars.com. Retrieved 11 May 2016, from http://humanphysiology.tuars.com/program/section8/8ch5/s8ch5_22.htm
Strominger, N., Demarest, R., & Laemle, L. (2012). Noback's Human Nervous System, Seventh Edition (pp. 297-300). Totowa, NJ: Humana Press.
Wissmann, P. (2013). THE CRANIAL NERVES (1st ed., pp. 1-4). Santa Monica, CA: Santa Monica College (SMC). Retrieved from http://homepage.smc.edu/wissmann_paul/physiology/THECRANIALNERVES.pdf
