Download Superior Frontal Gyrus Superior Longitudinal Fasciculus Superior

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Superior Frontal Gyrus
5. Munoz DP, Wurtz RH (1992) Role of the rostral superior
colliculus in active visual fixation and execution of
express saccades. J Neurophysiol 67:1000–1002
6. Krauzlis RJ, Basso MA, Wurtz RH (1997) Shared
motor error for multiple eye movements. Science
276:1693–1695
7. Keller EL, Gandhi NJ, Vijay Sekaran S (2000) Activity in
deep intermediate layer collicular neurons during interrupted saccades. Exp Brain Res 130:227–237
8. Lefevre P, Quaia C, Optican LM (1998) Distributed
model of control of saccades by superior colliculus and
cerebellum. Neural Netw 11:1175–1190
9. Aizawa H, Wurtz RH (1998) Reversible inactivation of
monkey superior colliculus. I. Curvature of saccade
trajectory. J Neurophysiol 79:2082–2096
10. Huerta MF, Harting JK (1984) The mammalian superior
colliculus: studies of its morphology and connections. In:
Vanegas H (ed) Comparative neurology of the optic
tectum. Plenum, New York, pp 687–773
The part of the fasciculus connecting the motor
(Broca's) speech center with the sensory (Wernicke's)
speech center is called the arcuate fasciclulus.
▶Pathways
Superior Oblique Muscle
Definition
Superior oblique is one of the six eye muscles.
▶Eye Orbital Mechanics
Superior Frontal Gyrus
Superior Olivary Nuclei
Synonyms
▶Gyrus front. sup.
T OM C. T. Y IN
Definition
Department of Physiology and Neuroscience Training
Program, University of Wisconsin-Madison, Madison,
WI, USA
In the area of the frontal gyrus close to the precentral
gyrus is situated the premotor cortex, which plays an
important role in planning effector voluntary movements and has close interaction with the cerebellum,
thalamic nuclei and basal ganglia.
At the level of the superior frontal gyrus is situated
the frontal eye field, which is involved in planning
voluntary eye movements. Hyperactivity of these
neurons due to hemorrhage or tumors causes conjugate
movements of both eyeballs (deviation conjugee).
Conversely, destruction of tissue causes ipsilateral
deviation conjugee, since now the activity of the
contralateral eye field no longer has an antagonist.
▶Telencephalon
Superior Longitudinal Fasciculus
Synonyms
▶Fasciculus longitudinalis sup.
Definition
With its two branches (anterior brachium and posterior
brachium), the superior longitudinal fasciculus establishes connections between virtually all cortical areas.
Synonyms
Nuclei of the superior olive; Superior olivary complex;
SOC
Definition
The superior olivary nuclei are a group of nuclei located
in the brainstem near the junction of the pons and
medulla. It is the first auditory relay after the cochlear
nucleus on the way to the auditory cortex and is the
major point at which information from the two ears is
integrated.
Characteristics
Introduction
The superior olivary nuclei or complex (SOC), as they
are more commonly called, occupy an important and
unique position in the ascending auditory pathway. The
SOC lies at the ponto-medullary border. Acoustic
information is conducted from the outer ear into the
inner ear where the cochlea transduces the mechanical
energy into neural impulses that are conveyed by the
auditory nerve fibers, which compose one component
of the VIII cranial nerve, into the central nervous
system. The other component of cranial nerve VIII is
the vestibular nerve which originates from the vestibular apparatus of the inner ear. All auditory nerve fibers
Superior Olivary Nuclei
synapse in the cochlear nuclei where there is some
initial processing of the afferent information. Cells in
the cochlear nuclei then project to the SOC of both sides
so that the SOC represents the first major point at which
cells combine the inputs from the two ears. Therefore
the SOC is a critical point for processing of binaural
information which is essential for accurate sound
localization. To understand the neural processing, we
must first consider what cues are needed for sound
localization.
Imagine walking down the street of a town late at
night and suddenly hearing a strange sound. In this
situation, there are two important tasks that our auditory
system must do. It has to identify the sound (a cat
meowing or the footsteps of a possible mugger) and it
has to tell us where the sound comes from. We
understand very little about how the auditory system
can identify sounds but considerably more about the
neural processes that underlie the ability to establish
where a sound originates.
Note that the problem of localizing a stimulus is quite
different for the auditory system than it is for the other
two major sensory systems, vision and somatosensation. In both of the latter systems the location of a
stimulus is naturally encoded in the location of the
sensory receptor since there is a map of the space in the
sensory epithelium, in the retina for the visual system
and on the body surface for the somatosensory system.
By contrast, the inner ear contains a map of the
frequency, not location, of the sound. The location of
the sound must then be computed by the nervous system
by analyzing the small differences between the sounds
at the two ears, ▶interaural time differences (ITDs) and
▶interaural level differences (ILDs, also called interaural intensity differences, IIDs). A remarkable feature
of the auditory system is its sensitivity to these
interaural disparities: the maximum ITD for the human
head when a sound is opposite one ear is about 800 μs
while human subjects can detect ITDs as small as 10 μs.
The maximum ILD is heavily dependent upon the
frequency of the sound since the head acts as an
effective acoustic shadow only for sounds with
wavelengths that are shorter than the dimensions of
the head, i.e. for high frequency sounds. Thus maximal
ILDs are on the order of 20–30 dB at 15–20 kHz at the
upper end of human hearing and only a few dB at
the lower end of human hearing. Therefore, we would
expect ILDs to be an effective cue only at high
frequencies. The width of an average human head is
around 15 cm which corresponds to the wavelength of a
2,000 Hz tone. Therefore ILDs should be effective for
frequencies above 2 kHz and ineffective for lower
frequencies. On the other hand the phase-locking that
encodes temporal patterns in the cochlea is also
frequency dependent: in mammals auditory nerve fibers
will only phase-lock to tones below about 2–3 kHz.
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Therefore timing information about the fine structure of
sounds is only preserved at low frequencies and ITDs
would only be effective at those frequencies. The
frequency dependence of ITDs and ILDs is the basis for
the classical duplex theory of sound localization [1].
Where in the nervous system are these cues encoded?
The most likely candidates are the nuclei in the superior
olivary complex (SOC) which occupy a unique position
in the ascending auditory pathway: they represent the
first major point at which cells in the auditory system
combine the inputs from the two ears. These inputs
arrive from the anteroventral cochlear nucleus of both
sides and they are shown in Fig 1 from a classical
drawing of the left SOC [2] from Golgi stained sections
of the neonatal cat. In Fig. 1 the midline is to the right
and the three major nuclei can be discerned from lateral
(left) to medial (right): ▶lateral superior olive the
(LSO), ▶medial superior olive (MSO) and the ▶medial
nucleus of the trapezoid body (MNTB). In the cat in
coronal sections the LSO takes the appearance of a
prominent S-shape while the MSO is a narrow nucleus.
The LSO and MSO are the key players in the encoding
of the two interaural cues of ITDs and ILDs.
Processing of Interaural Time Differences
Fig. 2 shows simplified versions of the circuits that are
believed to be important for encoding the interaural
cues of ITDs and ILDs. ITDs are believed to be encoded
by cells in the medial superior olive (MSO). Anatomically, cells in the MSO receive excitatory inputs from
the spherical bushy cells of the anteroventral cochlear
S
Superior Olivary Nuclei. Figure 1 Drawings of the
terminal arborizations from Golgi stains of afferents to
the superior olivary nuclei of the neonatal cat. The three
major nuclei are labeled: (A) medial nucleus of the
trapezoid body, (C) medial superior olive, and (D) lateral
superior olive. In addition two periolivary nuclei are also
shown: (B) ventromedial periolivary nucleus and (E)
lateral periolivary nucleus. The fibers of the trapezoid
body (F) are also labeled.
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Superior Olivary Nuclei
Superior Olivary Nuclei. Figure 2 Schematic drawings of the two circuits in the superior olivary complex that
encode interaural time differences (ITDs) (above) and interaural level differences (ILDs) (below). Abbreviations:
AN, auditory nerve fibers; AVCN, anteroventral cochlear nucleus; MSO, medial superior olive; LSO, lateral superior
olive; MNTB, medial nucleus of the trapezoid body; SBC, spherical bushy cell; GBC globular bushy cell. To the
right are drawings of typical responses of a cell in the MSO (top right) to variations in ITDs of pure tones and
responses of a cell in the LSO (bottom right) as a function of the ILD in dB. The response to ITDs is periodic at the
period of the stimulus tone and reflects the fact that the cells in the MSO are sensitive to interaural phase differences.
nucleus of each side. MSO cells are unusual cytoarchitecturally in having prominent dendrites that extend to
the lateral and medial side where they meet the afferents
from the left and right side (Fig. 1). The afferents from
the ipsilateral cochlear nucleus synapse on the lateral
dendrites while those from the contralateral side
synapse on the medial dendrites. Physiologically, the
cells in the MSO have been shown to be binaurally
excited and exquisitely sensitive to the time of arrival
of sounds to the two ears [3,4]. MSO cells behave
like coincidence detectors with very sharp temporal
windows so that they respond only when the inputs
from the two sides arrive coincidentally or nearly so.
A widely accepted model of this circuit was first
proposed by Jeffress [5] who hypothesized that the
timing of the arrival of inputs from each side is
governed by the delays associated with differences in
axonal length and that there is a systematic change in
axonal delays from one end of the MSO to the other,
which would result in a map of ITDs across one axis of
the MSO. Since the pathlength to the MSO is naturally
longer from the contralateral ear, then a natural
consequence of the coincidence mechanism is that
MSO cells respond best when the sound source is in the
contralateral sound field where the sound can reach the
contralateral ear before the ipsilateral ear and thus
compensate for the longer axonal path from the
contralateral side.
Recent studies of the anatomy [6] and physiology [7]
of the MSO have shown the existence of inhibitory
inputs which originate from the medial nucleus of
the ▶trapezoid body (MNTB) and ▶lateral nucleus
of the trapezoid body (LNTB). The MNTB receives
input from the spherical bushy cells of the anteroventral
cochlear nucleus of the contralateral side while the
LNTB receives input from the same cells on the
ipsilateral side. The function of these inhibitory circuits
is still controversial [8].
Processing of Interaural Level Differences
A parallel circuit in the superior olivary complex is
believed to be responsible for encoding interaural level
differences (ILDs) (Fig. 2, bottom). Cells in the ▶lateral
superior olive (LSO) receive excitatory input from the
spherical bushy cells of the ipsilateral side and
inhibitory input from the contralateral side that is
relayed via an inhibitory interneuron in the medial
nucleus of the trapezoid body (MNTB) [9]. The MNTB
cells received input from the globular bushy cells of
the contralateral side by way of a very specialized
synaptic ending, the calyx of Held. In accordance with
this circuit, cells in the LSO are excited by stimulation
of the ipsilateral ear and inhibited by stimulation of the
contralateral ear.
For binaural stimuli, LSO cells respond to the
difference in intensity (ipsilateral intensity – contralateral
Superior Olive
intensity) of the sounds to the two ears. Thus for free-field
sounds, LSO cells respond well to stimuli presented in the
ipsilateral sound field where the level of the sound is
greater in the ipsilateral than the contralateral ear and
poorly when the sound is in the contralateral sound field.
The large calyceal synapses between the globular
bushy axons and the MNTB cells are very unusual one
and can be seen prominently in Fig. 1. It is often said to
be the largest synapse in the brain. The presynaptic
element is so large that recordings can be made from
both the pre- and post-synaptic neurons so that this
synapse has become a model for biophysical studies of
synaptic transmission.
It is well-known that there is a systematic crossed
relationship between the representation in the cerebral
cortex and body part or sensory field in all sensory and
motor systems. The right motor cortex controls muscles
on the left side of the body, cells in the left visual cortex
have receptive fields in the right visual field, and cells in
the left somatosensory cortex respond to touch or pain
to the right side of the body. Note that cells in the MSO
respond preferentially to sounds in the contralateral
sound field whereas cells in the LSO respond to sounds
in the ipsilateral sound field. This apparent paradox is
resolved by having MSO project to the ipsilateral
inferior colliculus while most LSO cells project to the
contralateral inferior colliculus (Fig. 2). The subsequent
projections of the inferior colliculus to the medial
geniculate and then onto the cortex are all predominantly uncrossed which then makes cells in the auditory
cortex respond to sounds in the contralateral sound
field, as with the other sensory systems.
All of the nuclei in the superior olivary complex, like
those of other ascending auditory nuclei are tonotopically organized, i.e. there is a systematic map of
frequency along one axis of the nucleus. In accordance
with the expectations of the classical duplex theory,
both the MNTB and LSO have a disproportionate
representation of high frequencies, which are associated
with ILDs, while the MSO is biased toward low
frequencies, which are useful for encoding ITDs. In
animals with different head sizes, the ability to localize
low frequency tones appears to be correlated to the size
of the MSO. Interestingly, humans have a very
prominent MSO, which is correlated with the importance of ITDs for sound localization but a very small
LSO, even though we are clearly able to encode ILDs.
The Periolivary Nuclei
In addition to the major nuclei of the SOC, the MSO, LSO
and MNTB, there are also some smaller nuclei that
collectively are usually referred to as the periolivary
nuclei. The size and prominence of the periolivary nuclei
vary somewhat from one species to another and the identification of individual nuclei vary from one investigator
to another. An interesting aspect of these nuclei is that in
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some animals they are the source of the olivo-cochlear
efferents [10]. These efferent fibers project from to the
cochlea and innervate primarily the outer hair cells,
though there are also fibers that end on the inner hair cells.
Since 90% of the auditory nerve fibers innervate a single
inner hair cells, the action of the olivo-cochlear efferents
must be indirect. Current theories center on the fact that
the outer hair cells are motile and can contract which
could affect the micromechanics of the basilar membrane
motion and consequently modulate the inner hair cell
response. In rodents the olivo-cochlear efferents originate
from cells that are located within the lateral superior olive.
References
1. Rayleigh LJS (1907) On our perception of sound
direction. Philos Mag 6 Ser:214–232
2. Ramon y Cajal S (1909) Histologie du Systeme Nerveux
de l’Homme et des Vertebrates, vol 1. Instituto Ramon y
Cajal, Madrid, pp 754–838
3. Goldberg JM, Brown PB (1969) Response of binaural
neurons of dog superior olivary complex to dichotic tonal
stimuli: some physiological mechanisms of sound
localization. J Neurophysiol 32:613–636
4. Yin TCT, Chan JC (1990) Interaural time sensitivity in
medial superior olive of cat. J Neurophysiol 64:465–88
5. Jeffress LA (1948) A place theory of sound localization.
J Comp Physiol Psychol 41:35–39
6. Cant NB, Hyson RL (1992) Projections from the lateral
nucleus of the trapezoid body to the medial superior
olivary nucleus in the gerbil. Hear Res 58:26–34
7. Grothe B, Sanes DH (1993) Bilateral inhibition by
glycinergic afferents in the medial superior olive.
J Neurophysiol 69:1192–1196
8. Joris PX, Yin TCT (2007) A matter of time: internal
delays in binaural processing. Trends Neurosci 30:70–78
9. Boudreau JC, Tsuchitani C (1968) Binaural interaction
in the cat superior olive S segment. J Neurophysiol
31:442–454
10. Warr WB (1992) Organization of olivocochlear efferent
systems in mammals. In: Fay RR, Popper A (eds) The
mammalian auditory pathway: neuroanatomy. Springer,
New York, pp 410–448
Superior Olive
Synonyms
▶Nucl. olivaris sup.; ▶Superior olivary nucleus
Definition
The superior olivary complex comprises the nuclei:
. Nucleus of the trapezoid body
. Nucleus of the superior lateral olive
S
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Superior Parietal Lobule
. Medial nucleus of the superior olive
and is thus a vital synaptic center in the auditory tract,
playing an important role in acoustic reflexes (reflex
eye movements towards the source of noise, fright
movements).
▶Mesencephalon
Superior Parietal Lobule
Superior Semicircular Canal
Dehiscence Syndrome
Definition
Disorder of the labyrinth caused by a dehiscence
(opening) in the bone that covers the superior canal.
Patients can develop vestibular and/or auditory symptoms and signs. The effect of the dehiscence is to create
a third mobile window into the inner ear.
▶Disorders of the Vestibular Periphery
Synonyms
▶Lobulus parietalis sup.
Superior Temporal Gyrus
Definition
In the direction of the occipital pole, the inferior and
superior lobules unite at the postcentral gyrus.
Analogous to the secondary motor cortex there is also
a secondary sensory cortex for the somatosensory
control; this is believed to stretch across both lobules
and to be responsible for analysis, recognition and
assessment of tactile information.
▶Telencephalon
▶Visual Space Representation for Reaching
Definition
The superior temporal gyrus is the cerebral cortical fold
immediately ventral to the lateral fissure. Its posterior
portion is part of the language cortex.
Superior Vestibular Nucleus
Synonyms
▶Nucl vestibularis sup.
Superior Prefrontal Gyrus
▶Vestibular Nuclei
▶Pons
Definition
Part of the frontal lobe; involved in orchestrating
executive function.
Supernormal Stimulus
Definition
Stimulus with a releasing value that is higher than the
releasing value of the natural key stimulus.
Superior Rectus Muscle
Definition
SuperSAGE
Superior rectus is one of the six eye muscles.
▶Eye Orbital Mechanics
▶Serial Analysis of Gene Expression