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Special senses
PHYSIOLOGY
Special senses
Learning objectives
Chris JD Pomfrett
After reading this article, you should be able to: C explain how the size of a stimulus is encoded as the frequency of sensory action potentials C describe the way in which the sensors transduce light, sound, balance, taste and smell into afferent impulses, and the afferent pathways by which information reaches the central nervous system C outline the principles by which this sensory information is translated into conscious perception
Abstract Special senses are those regulated by the cranial nerves, and are of general interest for practitioners of anaesthesia and intensive care. This article gives a brief description of the neural mechanisms and pathways responsible for vision, hearing, smell, balance and taste.
Keywords Auditory; autonomic; cranial nerves; olfactory; vision Royal College of Anaesthetists CPD matrix: 1A01
remain closed spontaneously should be taped closed, even if drops or ointments are also used. A patient reporting a painful eye on waking should be investigated immediately to rule out serious complications, such as glaucoma. Humans perceive colour because they have three different receptors in the retina (cones), with sensitivities peaking at light wavelengths of 420 nm (blue), 531 nm (green) and 558 nm (red). The concept of red, green and blue as individual colours is a perceptual classification of electromagnetic wavelengths. Cones require more light to function than do rods, which are responsible for encoding light intensity. This explains why colour is not perceived at low light intensities; the colour is still reflected by objects, but is not perceived. Rods also exhibit more neural convergence in the retina, therefore they amplify the effect of available light at the cost of visual acuity, in contrast to the cones. The human retina is about two log units less sensitive to blue light than to red or green. Normal colour vision is trichromatic. About 5% of the male population exhibits congenital deuteranomaly, with an abnormality in the green cone pigment; 1.3% exhibit protanomaly, with abnormal red cone pigment, and 0.001% exhibit tritanomaly (blue cone pigment anomaly). The optic nerve relays information from the retina to the lateral geniculate nucleus (Figure 1), which in turn relays to the primary visual cortex (area V1). Increasing depth of anaesthesia progressively disrupts the visual system. Deep volatile anaesthesia removes the ability of the visual cortex to process complicated visual information, including the texture of an object. The moving edges of objects can still be coded by area V1 at moderate levels of volatile anaesthesia.
Sensory receptors A typical sensory receptor cell consists of a selectively sensitive transducer and a high-gain amplifier, contained within a structure that protects the receptor and amplifies the stimulus. Sensory receptors are the interface between the environment (external and internal) and the central nervous system (CNS). All that we see, hear, touch, smell and taste, along with our internal indications of balance and chemical environment, are represented as a result of a translation from energy into graded membrane potentials in sensory receptors. The simplest receptors are free nerve endings that respond to chemical agents. The most complex are involved in vision, where electromagnetic radiation is transduced into chemical electrical energy. In all sensory receptors, the magnitude of an appropriate sensory stimulus is proportional to the resultant amplitude of the graded receptor potential, which results in a changing frequency of action potentials relayed to the CNS. There are two main types of sensory receptor. Primary receptors generate action potentials within the receptors that propagate along axons into the CNS. An example is a stretch receptor within a muscle spindle. Secondary receptors generate graded receptor potentials that sum by reception of neurotransmitter from an interneuron. This causes some synaptic delay, but allows for integration and modulation of the signal. Rods and cones in the retina are examples of secondary receptors.
Visual system (cranial nerve II)
Auditory system (cranial nerve VIII)
The eye is a specialized structure that keeps the rods and cones of the retina in a protected environment while allowing appropriately focused light to reach the retina. All damage to the eye is serious. Corneal abrasion can be readily caused by surgical drapes, instruments and anaesthetic hardware. Masks and prone positioning occasionally result in orbital and periorbital compression leading to retinal detachment and retinal artery thrombosis. Eyes that are hidden from view or which do not
Auditory stimuli are encoded as a result of vibrations of frequency-tuned hair cells in the cochlea. The hair cells are paired and joined by a thin fibre that mechanically opens and closes ion channels in the hair cells as they move. The auditory information is relayed along the cochlear nerve and progressively through brainstem nuclei to the auditory cortices of the brain (Figure 2). Anaesthesia increases the time taken to process auditory information and reduces the magnitude of the auditory evoked potential (AEP), which has been used as an index of anaesthetic depth. Brainstem responses to auditory stimuli often remain during surgical levels of anaesthesia, therefore patients can still potentially hear during surgery. Whether the auditory stimuli are perceived appropriately is unknown.
Chris JD Pomfrett BSc PhD is a Technical Adviser at the National Institute for Health & Care Excellence (NICE), UK. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 18:4
202
Ó 2017 Elsevier Ltd. All rights reserved.
PHYSIOLOGY
Vision Ciliary ganglia (left and right)
Right eye
Edinger–Westphal nuclei (left and right)
III III Pupilloconstrictor muscles (left and right)
Left eye
II VI primary visual cortex
Nucleus of the posterior commissure (left)
Retinal ganglion cell (left)
Roman numerals denote cranial nerves Pupillodilatation is via sympathetic control originating from the hypothalamus via sympathetic ganglia at T1, T2 and T3
Figure 1
Auditory and vestibular system Semicircular canals Malleus
Vestibular nerve VIII
Auricle
Vestibular nuclei
Cerebellum
Muscles controlling eye movement
Cochlear nerve VIII
Cochlear nuclei (Medulla)
Superior olivary nuclei (Pons)
Inferior collicular nucleus (Midbrain)
Medial geniculate nucleus (Midbrain)
Primary auditory cortex
Tympanum Stapes Eustachian tube Cochlea
Figure 2
Vestibular system (cranial nerve VIII)
Olfactory receptors (cranial nerve I)
Hair cells detect the movement of endolymph in the semicircular canals, and translate this into receptor potentials. These initiate action potentials in neurons of the vestibular nerve, which relay information on movement to regions such as the oculomotor nucleus, responsible for involuntary movement of the eyes, and the cerebellum.
Odours initiate a second messenger cascade, which opens cyclic nucleotide-gated ion channels and depolarizes the olfactory cell membrane, and can elicit action potentials when enough channels are opened. The olfactory information is relayed to the anterior olfactory nucleus and centres including the amygdala and thalamus.
Taste (cranial nerves VII, IX)
Interpretation of senses
Five different basic tastes (bitter, salty, umami, sour and sweet) are encoded by taste buds on the tongue. These relay via the chorda tympani and glossopharyngeal nerves to the solitary nucleus of the medulla oblongata, and then on to the thalamus.
The time taken to translate environmental stimuli from energy into perceived events is neither instantaneous nor consistent for different sensory modalities. The visual evoked potential (VEP) is the averaged electroencephalograph (EEG) response to
ANAESTHESIA AND INTENSIVE CARE MEDICINE 18:4
203
Ó 2017 Elsevier Ltd. All rights reserved.
PHYSIOLOGY
thousands of visual stimuli. The first major VEP peak occurs 100 ms after a flash stimulus, and originates in the primary visual cortex (area V1) at the back of the brain. The primary visual cortex comprises different types of interneurons sensitive to the direction, thickness, width, velocity, colour and binocular disparity of everything seen. This means that everything seen is delayed by at least a tenth of a second from reality. Area V1 is only an early stage of visual processing, because the information from the retina and lateral geniculate nucleus contains raw, largely unprocessed visual information, and the completed cognitive response to a visual event occurs some 300 ms after a stimulus. The AEP is faster than the VEP, indicating that sounds evoke a response in the primary auditory cortex about 40 ms after the stimulus. Resolving this difference in the temporal characteristics of different sensory inputs is a major task for the
ANAESTHESIA AND INTENSIVE CARE MEDICINE 18:4
brain, which has to bind these varied virtual impressions together to make an internal projection of the world. Disorientation at recovery from anaesthesia, and with the use of hallucinogenic agents, can be partly explained by the partial breakdown of binding between these sensory inputs. A FURTHER READING Woolsey TH, Hanaway J, Gado MH. The brain atlas: a visual guide to the human nervous system. 4th edn. New York: Wiley, 2017. Kandel ER, Schwartz JH, Jessell TM, Siegelbaum SA, Hudspeth AJ, eds. Principles of neural science. 5th edn. London: Elsevier, 2012. Taylor TH, Major E, eds. Hazards and complications of anaesthesia. 2nd edn. London: Churchill Livingstone, 1993.
204
Ó 2017 Elsevier Ltd. All rights reserved.
Special senses
Learning objectives
Chris JD Pomfrett
After reading this article, you should be able to: C explain how the size of a stimulus is encoded as the frequency of sensory action potentials C describe the way in which the sensors transduce light, sound, balance, taste and smell into afferent impulses, and the afferent pathways by which information reaches the central nervous system C outline the principles by which this sensory information is translated into conscious perception
Abstract Special senses are those regulated by the cranial nerves, and are of general interest for practitioners of anaesthesia and intensive care. This article gives a brief description of the neural mechanisms and pathways responsible for vision, hearing, smell, balance and taste.
Keywords Auditory; autonomic; cranial nerves; olfactory; vision Royal College of Anaesthetists CPD matrix: 1A01
remain closed spontaneously should be taped closed, even if drops or ointments are also used. A patient reporting a painful eye on waking should be investigated immediately to rule out serious complications, such as glaucoma. Humans perceive colour because they have three different receptors in the retina (cones), with sensitivities peaking at light wavelengths of 420 nm (blue), 531 nm (green) and 558 nm (red). The concept of red, green and blue as individual colours is a perceptual classification of electromagnetic wavelengths. Cones require more light to function than do rods, which are responsible for encoding light intensity. This explains why colour is not perceived at low light intensities; the colour is still reflected by objects, but is not perceived. Rods also exhibit more neural convergence in the retina, therefore they amplify the effect of available light at the cost of visual acuity, in contrast to the cones. The human retina is about two log units less sensitive to blue light than to red or green. Normal colour vision is trichromatic. About 5% of the male population exhibits congenital deuteranomaly, with an abnormality in the green cone pigment; 1.3% exhibit protanomaly, with abnormal red cone pigment, and 0.001% exhibit tritanomaly (blue cone pigment anomaly). The optic nerve relays information from the retina to the lateral geniculate nucleus (Figure 1), which in turn relays to the primary visual cortex (area V1). Increasing depth of anaesthesia progressively disrupts the visual system. Deep volatile anaesthesia removes the ability of the visual cortex to process complicated visual information, including the texture of an object. The moving edges of objects can still be coded by area V1 at moderate levels of volatile anaesthesia.
Sensory receptors A typical sensory receptor cell consists of a selectively sensitive transducer and a high-gain amplifier, contained within a structure that protects the receptor and amplifies the stimulus. Sensory receptors are the interface between the environment (external and internal) and the central nervous system (CNS). All that we see, hear, touch, smell and taste, along with our internal indications of balance and chemical environment, are represented as a result of a translation from energy into graded membrane potentials in sensory receptors. The simplest receptors are free nerve endings that respond to chemical agents. The most complex are involved in vision, where electromagnetic radiation is transduced into chemical electrical energy. In all sensory receptors, the magnitude of an appropriate sensory stimulus is proportional to the resultant amplitude of the graded receptor potential, which results in a changing frequency of action potentials relayed to the CNS. There are two main types of sensory receptor. Primary receptors generate action potentials within the receptors that propagate along axons into the CNS. An example is a stretch receptor within a muscle spindle. Secondary receptors generate graded receptor potentials that sum by reception of neurotransmitter from an interneuron. This causes some synaptic delay, but allows for integration and modulation of the signal. Rods and cones in the retina are examples of secondary receptors.
Visual system (cranial nerve II)
Auditory system (cranial nerve VIII)
The eye is a specialized structure that keeps the rods and cones of the retina in a protected environment while allowing appropriately focused light to reach the retina. All damage to the eye is serious. Corneal abrasion can be readily caused by surgical drapes, instruments and anaesthetic hardware. Masks and prone positioning occasionally result in orbital and periorbital compression leading to retinal detachment and retinal artery thrombosis. Eyes that are hidden from view or which do not
Auditory stimuli are encoded as a result of vibrations of frequency-tuned hair cells in the cochlea. The hair cells are paired and joined by a thin fibre that mechanically opens and closes ion channels in the hair cells as they move. The auditory information is relayed along the cochlear nerve and progressively through brainstem nuclei to the auditory cortices of the brain (Figure 2). Anaesthesia increases the time taken to process auditory information and reduces the magnitude of the auditory evoked potential (AEP), which has been used as an index of anaesthetic depth. Brainstem responses to auditory stimuli often remain during surgical levels of anaesthesia, therefore patients can still potentially hear during surgery. Whether the auditory stimuli are perceived appropriately is unknown.
Chris JD Pomfrett BSc PhD is a Technical Adviser at the National Institute for Health & Care Excellence (NICE), UK. Conflicts of interest: none declared.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 18:4
202
Ó 2017 Elsevier Ltd. All rights reserved.
PHYSIOLOGY
Vision Ciliary ganglia (left and right)
Right eye
Edinger–Westphal nuclei (left and right)
III III Pupilloconstrictor muscles (left and right)
Left eye
II VI primary visual cortex
Nucleus of the posterior commissure (left)
Retinal ganglion cell (left)
Roman numerals denote cranial nerves Pupillodilatation is via sympathetic control originating from the hypothalamus via sympathetic ganglia at T1, T2 and T3
Figure 1
Auditory and vestibular system Semicircular canals Malleus
Vestibular nerve VIII
Auricle
Vestibular nuclei
Cerebellum
Muscles controlling eye movement
Cochlear nerve VIII
Cochlear nuclei (Medulla)
Superior olivary nuclei (Pons)
Inferior collicular nucleus (Midbrain)
Medial geniculate nucleus (Midbrain)
Primary auditory cortex
Tympanum Stapes Eustachian tube Cochlea
Figure 2
Vestibular system (cranial nerve VIII)
Olfactory receptors (cranial nerve I)
Hair cells detect the movement of endolymph in the semicircular canals, and translate this into receptor potentials. These initiate action potentials in neurons of the vestibular nerve, which relay information on movement to regions such as the oculomotor nucleus, responsible for involuntary movement of the eyes, and the cerebellum.
Odours initiate a second messenger cascade, which opens cyclic nucleotide-gated ion channels and depolarizes the olfactory cell membrane, and can elicit action potentials when enough channels are opened. The olfactory information is relayed to the anterior olfactory nucleus and centres including the amygdala and thalamus.
Taste (cranial nerves VII, IX)
Interpretation of senses
Five different basic tastes (bitter, salty, umami, sour and sweet) are encoded by taste buds on the tongue. These relay via the chorda tympani and glossopharyngeal nerves to the solitary nucleus of the medulla oblongata, and then on to the thalamus.
The time taken to translate environmental stimuli from energy into perceived events is neither instantaneous nor consistent for different sensory modalities. The visual evoked potential (VEP) is the averaged electroencephalograph (EEG) response to
ANAESTHESIA AND INTENSIVE CARE MEDICINE 18:4
203
Ó 2017 Elsevier Ltd. All rights reserved.
PHYSIOLOGY
thousands of visual stimuli. The first major VEP peak occurs 100 ms after a flash stimulus, and originates in the primary visual cortex (area V1) at the back of the brain. The primary visual cortex comprises different types of interneurons sensitive to the direction, thickness, width, velocity, colour and binocular disparity of everything seen. This means that everything seen is delayed by at least a tenth of a second from reality. Area V1 is only an early stage of visual processing, because the information from the retina and lateral geniculate nucleus contains raw, largely unprocessed visual information, and the completed cognitive response to a visual event occurs some 300 ms after a stimulus. The AEP is faster than the VEP, indicating that sounds evoke a response in the primary auditory cortex about 40 ms after the stimulus. Resolving this difference in the temporal characteristics of different sensory inputs is a major task for the
ANAESTHESIA AND INTENSIVE CARE MEDICINE 18:4
brain, which has to bind these varied virtual impressions together to make an internal projection of the world. Disorientation at recovery from anaesthesia, and with the use of hallucinogenic agents, can be partly explained by the partial breakdown of binding between these sensory inputs. A FURTHER READING Woolsey TH, Hanaway J, Gado MH. The brain atlas: a visual guide to the human nervous system. 4th edn. New York: Wiley, 2017. Kandel ER, Schwartz JH, Jessell TM, Siegelbaum SA, Hudspeth AJ, eds. Principles of neural science. 5th edn. London: Elsevier, 2012. Taylor TH, Major E, eds. Hazards and complications of anaesthesia. 2nd edn. London: Churchill Livingstone, 1993.
204
Ó 2017 Elsevier Ltd. All rights reserved.