Which of the following cranial nerves is purely afferent?
E. The vestibulocochlear nerve is purely sensory. It has two components, the vestibular and the cochlear, both of which are sensory. The vestibular component transmits information on position and balance received from the semicircular canals and the cochlear component serves the sense of hearing. The hypoglossal nerve innervates the ipsilateral side of the tongue. It is a purely motor efferent nerve. In unilateral lower motor neurone palsy of the eight nerve, when protruded, the tongue deviates toward the side of weakness. The trochlear nerve is unique among cranial nerves in that it decussates to the contralateral side and its point of exit is through the dorsal surface of the brain. The trochlear nerve thus innervates the superior oblique muscle of the contralateral eye. It is a purely efferent nerve. The facial nerve innervates the muscles of the face. It has a sensory component, innervating the anterior two-thirds of the tongue, via the chorda tympani. Facial nerve palsy causes deviation of the angle of mouth to the normal side. As a mnemonic, ‘the rule of 17’ applies to deviations in cranial nerve palsies. Palsy of tenth (vagus and hence palate) and seventh (facial) nerve leads to deviation to the normal side. Palsy of the fi fth (trigeminal and hence the jaw muscles) and twelfth (hypoglossal, tongue) nerve leads to deviation to the paralytic side.
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The light reflex pathway includes all of the following except:
C. The pathway that enables normal vision starts from rods and cones in the retina, which are receptors of the ganglion cells. The axons of ganglion cells extend as the optic nerve, through the optic chiasma, via the optic tract, and synapse at the lateral geniculate body of thalamus. From the lateral geniculate body, second-order neurones go through the optic radiation to the visual cortex in the occipital lobe. In contrast, the pathway constituting the papillary light refl ex digresses from the visual pathway before it joins the lateral geniculate body to reach the dorsal midbrain. They synapse at the pretectal nuclei, from where second-order neurones go to the Edinger–Westphal nucleus on both sides. Edinger–Westphal nuclei, via the third nerve, control the pupillary constrictors that constitute the response to light. So, pupillary light refl ex do not involve the occipital cortex. Hence even in those with cortical blindness due to bilateral occipital cortex damage, light refl ex may be intact.
All of the following carry parasympathetic fibres in them except:
C. The hypoglossal nerve is a purely somatic motor nerve. The autonomic nervous system has two parts, the sympathetic and parasympathetic. Sympathetic output from the CNS is mainly through thoracic and lumbar spinal nerves. Sympathetic preganglionic nerves are short and form synapses in paired ganglia adjacent to the spinal cord. The parasympathetic system has a craniosacral output, that is it operates through some cranial nerves and sacral spinal nerves. These have long preganglionic nerves which form synapses at ganglia near or on the organ innervated. Among the cranial nerves, the vagus is the chief parasympathetic nerve. It supplies parasympathetic efferents to heart and most of the abdominal viscera and the gastrointestinal tract, but oculomotor (III), facial (VII), and glossopharyngeal nerves (IX) also carry parasympathetic fibres. The neurotransmitter at the preganglionic nerve ending is acetylcholine in both sympathetic and parasympathetic systems. At the post ganglionic nerve ending, the neurotransmitter is acetylcholine in the parasympathetic system and mostly norepinephrine in the sympathetic system.
Which of the following is a result of parasympathetic activity?
D. The sympathetic system is responsible for the ‘flight and fight’ and the parasympathetic system for the ‘rest and digest’ reactions. Generally, they are considered to have opposing actions. The sympathetic system is activated in emergency situations, where the body requires more energy. This response includes increased cardiac output, dilatation of bronchioles, routing blood to the muscles, glycogen and fat breakdown leading to a rise in the blood glucose and fatty acids, and slowing down of digestion and renal filtration. This also leads to a decrease in gastrointestinal secretion and motility (leading to dryness of the mouth). In addition, sympathetic activity causes constriction of bladder and bowel sphincters and relaxation of the smooth muscles of the viscera. The pupils dilate due to action on the dilators. In contrast, parasympathetic stimulation leads to pupillary constriction and accommodation for close vision, reduces heart rate, constricts bronchioles, and increases gastrointestinal secretions with relaxation of sphincters. Parasympathetic stimulation is necessary for erection and sympathetic stimulation for ejaculation.
The posterior column of the spinal cord is responsible for all of the following except:
D. The posterior column is responsible for transmission of proprioception, light touch, tactile localization, and vibration senses. Posterior column dysfunction can result in disturbances in the knowledge of extremity movement and position. This presents as sensory ataxia (noted first in the dark as visual input does not compensate for the lost position sense) and a positive Romberg’s sign. Pain and temperature is transmitted to the central nervous system through the spinothalamic tract. Spinothalamic tracts cross over two segments above the level of entry of the root at the spinal cord. Posterior column tracts cross over only at the midbrain level, where they synapse with the cuneate and gracilis nuclei. Hence, if hemisection of the cord takes place, ipsilateral posterior column senses are lost below the level of section; contralateral spinothalamic sensations are lost from two levels below the site of section.
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