Detailed essay questions

1. From sensation to movement, what neurophysiologic processes are involved in?

 

Reference to the answer: In general, it is a reflex process; however, the following points should be considered, 1) conversion of receptor potential to nerve pulses in the first order neurons; 2) synaptic transmission between primary afferent neurons and second order neurons in the spinal cord or brainstem; 3) Ascending neural pathways reach specific relay nuclei in the ventral posterior thalamus and nonspecific sensory nuclei in the internal medullary lamina; 4) the third neuron projects from thalamus in specific projection system to somatosensory cortex in the postcentral gyrus on the background of consciousness induced by nonspecific projection system; 5) integration of sensory signals in local neuronal circuit and communication between sensory cortex and motor cortex; 6) descending command from precentral gyrus in pyramidal tract or corticobulbar tract reaches motor neurons in the spinal cord and brainstem with the modulation of basal ganglion and cerebellum; 7) excitation of alpha-motor neurons causes contraction of muscles in the same motor unit by eliciting endplate potential and action potential with the assistance of gamma-neuronal activity.

 

2. What is the difference between nerve conduction and synaptic transmission?

 

Reference to the answer: Nerve conduction is the process of action potential propagation along axons. It has the following features: 1) structural and functional integrity; 2) insulation or relative independentability; 3) bidirectional capacity and one-way propagation; 4) relatively insensitive to environmental changes or relative indefatigability; and 5) all-or-none or relative infatigability.

In contrast, synaptic transmission occurs between the connect parts of two neurons through chemical and electrical synapses.

The chemical synapse involves a series of neurochemical events, which include, 1) arrival of action potential in neural terminals; 2) calcium mobilization in the synaptic button; 3). Release of neurotransmitters and their propagation through synaptic cleft; 4) activation of postsynaptic receptors and ion current at the postsynaptic membrane; 5) process of terminating transmitter actions. Correspondingly, it has different features from nerve conduction by 1) one-way conduction; 2) synaptic delay; 3) spatiotemporal summation; 4) changes in rhythmicity; and 5) sensitive to environmental changes and fatigability.

The electrical synapse performs neurotransmission through gap junctions and thus works more like EPSPs or local currents. It is bidirectional, low resistance, fast and can be summarized.

 

3. How does a somatic stimulus activate primary sensory cortex?

 

Reference to the answer: Adequate stimuli cause receptor potential at somatic sensory receptors, which is transduced to coded nerve pulses in the first order neurons directly or through the mediation of synapse, thereby producing sensory information in forms of action potential of corresponding neural fibers. All sensory information from the somatic segments of the body enters the spinal cord through the dorsal roots of the spinal nerves. However, from the entry point into the cord and then to the brain, the sensory signals are carried through one of two alternative sensory pathways: (1) the dorsal column–medial lemniscal system or (2) the anterolateral system. These two systems come back together partially at the level of the thalamus and then reach primary sensory cortex in the specific projection system. Here, let us use the Dorsal Column–Medial Lemniscal Pathway as an example to the details of sensory projection. 

When nerve fibers entering the dorsal columns pass uninterrupted up to the dorsal medulla, where they synapse in the dorsal column nuclei (the cuneate and gracile

nuclei). From there, second-order neurons decussate immediately to the opposite side of the brain stem and continue upward through the medial lemnisci to the thalamus. In this pathway through the brain stem, each medial lemniscus is joined by additional fibers from the sensory nuclei of the trigeminal nerve; these fibers subserve the same sensory

functions for the head that the dorsal column fibers subserve for the body. In the thalamus, the medial lemniscal fibers terminate in the thalamic sensory relay area, called the ventrobasal complex. From the ventrobasal complex, third-order nerve fibers project mainly to  and activate the postcentral gyrus of the cerebral cortex, which is called somatic sensory area I or primary sensory cortex.

 

4. How do retinal photoreceptors convert light into action potentials along optic nerves?

 

Reference to the answer: The retina is the light-sensitive portion of the eye that contains (1) the cones, which are responsible for color vision, and (2) the rods, which can detect dim light and are mainly responsible for black and white vision and vision in the dark. When either rods or cones are excited, signals are transmitted first through successive layers of neurons in the retina and, finally, into optic nerve fibers and the cerebral cortex. The conversion of light energy into action potentials along optic nerves has been well studied in rods.

Light entering the eye is refracted and project an inverted image onto the retina as it passes through the cornea and lens. When the rod is exposed to light, light is absorbed by the rhodopsin, causing photoactivation of the electrons in the retinal portion; the activated rhodopsin stimulates a G protein called transducin, which then activates cGMP phosphodiesterase, an enzyme that catalyzes the breakdown of cGMP to 5′-cGMP; the reduction in cGMP closes the cGMP-gated sodium channels and reduces the inward sodium current in the outer segment of the rod. Sodium ions continue to be pumped outward through the membrane of the inner segment by sodium pump. Thus, more sodium ions leave the rod than leak back in. As a result, hyperpo­larization of the intrarod membrane potential occurs.

 

When hyperpolarization occurs in the outer segment of a rod or a cone, almost the same degree of hyperpolarization is conducted by direct electric current flow in the cytoplasm all the way to the synaptic body, and no action potential is required. Then, this hyperpolarization inhibits the release of glutamate that excites or inhibits bipolar cells depending on the receptors expressed and being modulated by horizontal cells. Changes in the membrane potential in bipolar cells are again transmitted from the input to the output by direct electric current flow, not by action potentials. Transmitters release by bipolar cells can cause generator potential in ganglion cell, which can be transduced into action potential and modulated by amacrine cells. Through these processes, light is converted to action potentials of optic nerves.


5. Please describe the processes of hearing sensation in detail.


In the sensation of hearing, the ear receives sound waves, discriminates their frequencies, and transmits auditory information into the CNS, where its meaning is deciphered.

1) To receive sound waves, the auricle has to direct sound into the external ear canal that further conducts the sound to the middle ear; the tympanic membrane and the ossicles, which conduct sound from the tympanic membrane through the middle ear to the cochlea (the inner ear). Attached to the tympanic membrane is the handle of the malleus. The malleus is bound to the incus by minute ligaments, so whenever the malleus moves, the incus moves with it. The opposite end of the incus articulates with the stem of the stapes, and the faceplate of the stapes lies against the membranous labyrinth of the cochlea in the opening of the oval window. The tip end of the handle of the malleus is attached to the center of the tympanic membrane, and this point of attachment is constantly pulled by the tensor tympani muscle, which keeps the tympanic membrane tensed. This tension allows sound vibrations on any portion of the tympanic membrane to be transmitted to the ossicles. The ossicles of the middle ear are suspended by ligaments in such a way that the combined malleus and incus act as a single lever, having its fulcrum approximately at the border of the tympanic membrane. The articulation of the incus with the stapes causes the stapes to (i) push forward on the oval window and on the cochlear fluid on the other side of window every time the tympanic membrane moves inward and (ii) pull backward on the fluid every time the malleus moves outward.

In the absence of the ossicular system and tympanic membrane, sound waves can still travel directly through the air of the middle ear and enter the cochlea at the oval window. However, the sensitivity for hearing is then 15 to 20 decibels less than for ossicular transmission.

2) To discriminate the frequencies of sound waves, traveling waves along the basilar membrane in the cochlea are necessary. Sound vibrations enter the scala vestibuli from the faceplate of the stapes at the oval window. The faceplate covers this window and is connected with the window’s edges by a loose annular ligament so that it can move inward and outward with the sound vibrations. Inward movement causes the fluid to move forward in the scala vestibuli and scala media, and outward movement causes the fluid to move backward. The scala tympani and scala media are separated from each other by the basilar membrane. Different parts of the basilar membrane sense to sounds with different frequencies. High-frequency resonance of the basilar membrane occurs near the base, where the sound waves enter the cochlea through the oval window. However, low-frequency resonance occurs near the helicotrema, mainly because of the less stiff fibers but also because of increased “loading” with extra masses of fluid that must vibrate along the cochlear tubules. On the surface of the basilar membrane lies the organ of Corti, which contains a series of electromechanically sensitive cells, the hair cells. They are the receptive end organs that generate nerve impulses in response to sound vibrations.

When the basilar fibers bend toward the scala vestibuli, the hair cells depolarize, and in the opposite direction they hyperpolarize, thereby generating an alternating hair cell receptor potential that, in turn, stimulates the cochlear nerve endings that synapse with the bases of the hair cells. It is believed that a rapidly acting neu­rotransmitter is released by the hair cells at these synapses during depolarization.

3) To transmit auditory information into the CNS the major auditory pathways. Nerve fibers from the spiral ganglion of Corti enter the dorsal and ventral cochlear nuclei located in the upper part of the medulla. At this point, all the fibers synapse, and second-order neurons pass mainly to the opposite side of the brain stem to terminate in the superior olivary nucleus. A few second-order fibers also pass to the superior olivary nucleus on the same side. From the superior olivary nucleus, the auditory pathway passes upward through the lateral lemniscus. Some of the fibers terminate in the nucleus of the lateral lemniscus, but many fibers bypass this nucleus and travel on to the inferior colliculus, where all or almost all the auditory fibers synapse. From there, the pathway passes to the medial geniculate nucleus, where all the fibers do synapse. Finally, the pathway proceeds by way of the audi­tory radiation to the auditory cortex, located mainly in the superior gyrus of the temporal lobe. Then hearing is generated.

 

6. How does a command of movements in the precentral gyrus cause the contraction of somatic skeletal muscles?


Motor signals are transmitted directly from the cortex to the spinal cord through the corticospinal tract and indirectly through multiple accessory pathways that involve the basal ganglia, cerebellum, and various nuclei of the brain stem. In general, the direct pathways are concerned more with discrete and detailed movements, especially of the distal segments of the limbs, particularly the hands and fingers.

The most important output pathway from the motor cortex is the corticospinal tract, also called the pyramidal tractThe corticospinal tract originates about 30 percent from the primary motor cortex, 30 percent from the premotor and supplementary motor areas, and 40 percent from the somatosensory areas posterior to the central sulcus.

After leaving the cortex, it passes through the posterior limb of the internal capsule (between the caudate nucleus and the putamen of the basal ganglia) and then downward through the brain stem, forming the pyramids of the medulla. The majority of the pyramidal fibers then cross in the lower medulla to the opposite side and descend into the lateral corticospinal tracts of the cord, finally termi­nating principally on the interneurons in the intermediate regions of the cord gray matter; a few terminate on sensory relay neurons in the dorsal horn, and a very few terminate directly on the anterior motor neurons that cause muscle contraction.

 

A few of the fibers do not cross to the opposite side in the medulla but pass ipsilaterally down the cord in the ventral corticospinal tracts. Many, if not most, of these fibers eventually cross to the opposite side of the cord either in the neck or in the upper thoracic region. These fibers may be concerned with control of bilateral postural movements by the supplementary motor cortex.

 

7. What are the functional features of sympathetic and the parasympathetic nervous system? Please explain with examples.

 

The sympathetic nervous system and the parasympathetic nervous system are the two major subdivisions of the autonomic nervous system that controls most visceral functions of the body in the nervous systemThis system helps to control arterial pressure, gastrointestinal motility, gastrointestinal secretion, urinary bladder emptying, sweating, body temperature, and many other activities. Some of these activities are controlled almost entirely and some only partially by the autonomic nervous system. Thus, there is no generalization one can use to explain whether sympathetic or parasympathetic stimulation will cause excitation or inhibition of a particular organ. Therefore, to understand sympathetic and parasympathetic function, one must learn all the separate functions of these two nervous systems on each organ. However, some functional features of the two systems can help understandings of the autonomic system.

1) Both the sympathetic and parasympathetic nervous systems show base tone; the biological effects are largely determined by the neurotransmitter released and the corresponding receptors on the target organs. For example, acetylcholine release by sympathetic postganglionic nerves causes sweating via M receptor while their released noradrenaline causes iris dilation and intestine relaxation; acetylcholine from parasympathetic nerve terminals decreases force of myocardial contraction via M receptor.

2) Two systems possess different functions that are often opposite but coordinative in a specific process. For example, in near reflex, activation of parasympathetic nerve causes contraction of the pupil constrictor muscle that is accompanied with inhibition of sympathetic nerve outflow to inhibit the contraction of pupil dilator muscle, thereby allowing the reduction of pupil size to occur.

3) Their physiological effects are largely modulated by functional state of the effectors. This can be seen in the effect of sympathetic excitation on non-pregnant uterus and pregnant uterus, i.e. relaxation in the former but contraction in the latter.

4) Sympathetic system often responds by mass discharge, and makes the body adapt to a stressful environment while parasympathetic system usually causes specific and localized response, promotes digestion, excretion and reproduction, stores energy and thus makes the body rest, recover and protected.

 

8. How does the hypothalamus regulate visceral activity?


The hypothalamus is a key structure in neural network that integrates visceral sensory input and higher order visceral motor signals, and thus is the higher regulating center of the visceral activity. The functions of hypothalamic involvement include at least:

1) the control of blood flow by promoting adjustments in cardiac output, vasomotor tone, blood osmolarity, and renal clearance, and by motivating drinking and salt consumption; 2) the regulation of energy metabolism by monitoring blood glucose levels and regulating feeding behavior, digestive functions, metabolic rate, and temperature;

3) the regulation of reproductive activity by influencing gender identity, sexual orientation and mating behavior and, in females, by governing menstrual cycles, pregnancy, and lactation; and

4) the coordination of responses to threatening conditions by governing the release of stress hormones, modulating the balance between sympathetic and parasympathetic tone, and influencing the regional distribution of blood flow.

 

Signals from the hypothalamus can affect the activities of almost all the brain stem autonomic control centers as well as the secretion of pituitary hormones. On the one hand, stimulation in the posterior hypothalamus can activate the medullary cardiovascular control centers strongly enough to increase arterial pressure to more than twice normal. Likewise, other hypothalamic centers control body temperature, increase or decrease salivation and gastrointestinal activity, and cause bladder emptying. To some extent, therefore, the autonomic centers in the brain stem act as relay stations for control activities initiated at higher levels of the brain, especially in the hypothalamus. Since the autonomic nervous system is the dominant regulatory approach of neural modulation of visceral activities, by controlling the outflows of the sympathetic nervous system and the parasympathetic nervous system, the hypothalamus can extensively regulate visceral activities in rapid and accurate fashion.

 

On the other hand, the hypothalamus controls almost all secretion by the pituitary via either hormonal (anterior lobe) or neural (posterior lobe) signals. By releasing hypothalamic releasing and hypothalamic inhibitory hormones, the hypothalamus can control the secretion of pituitary hormones from the anterior pituitary via hypothalamic-hypophysial portal vessels. Consequently, changes in hypothalamic activity can change the secretion of glucocorticoids, thyroid hormone, sex steroid hormone, growth hormone and prolactin, thereby extensively change visceral functions along with alterations of physical features. By discharging neural pulses, the hypothalamus can directly release oxytocin and vasopressin into the blood from the posterior pituitary. Vasopressin can directly increase water reabsorption in the kidneys and vasoconstriction while oxytocin triggers milk letdown and uterus contraction. As a whole, the hypothalamus plays a dominant role in neural regulation of visceral activities via both neural and endocrine approaches.

2016年12月11日

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