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At least 19 recordsLinked to original sources
PubMed1976

Structure and function of the two heads of the myosin molecule. III. Cooperativity of the two heads of the myosin molecule, shown by the effect of modification of head A with rho-chloromercuribenzoate on the interaction of head B with F-actin.

Subfragment-1 of HMM was prepared by tryptic [EC 3.4.21.4] digestion of HMM, which had been modified with 1 mole of CMB per mole of HMM at a specific SH group, SHr. S-1(T) obtained from CMB-HMM retained almost all the CMB, and the amount of bound CMB was about 0.8-0.9 mole per 2 moles of S-1(T). S-2 of CMB-HMM contained no bound CMB. The ATPase [EC 3.6.1.3] activity of HMM increased gradually with increase in the concentration of FA, and the acto-HMM ATPase was inhibited by excess substrate or removal of Ca2+ ions in the presence of RP. The ATPase activity of CMB-HMM increased to a maximum level on adding a small amount of FA, and the acto-CMB-HMM ATPase showed neither substrate inhibition nor Ca2+ sensitivity in the presence of RP. On the other hand, the dependence on the concentration of FA of the ATPase activity of acto-S-1(T) was unaffected by modification of S-1 with CMB. The Ca2+ sensitivity of the ATPase activity of acto-S-1(T) in the presence of RP was also unaffected by the modification. Acto-S-1(T) dissociated almost completely, while acto-CMB-S-1(T) was only 50% dissociated on adding ATP. More than 80% of the bound CMB was contained in S-1(T) undissociated from FA. Furthermore, superprecipitation of actomyosin induced by ATP was completely inhibited by adding about 2 moles of CMB-S-1(T) per mole of actin monomer. On the other hand, about 90% of the burst size of Pi liberation was retained in S-1(T) dissociated from FA. It was concluded that the two heads of the myosin molecule are different: one shows the initial burst of Pi liberation, and does not contain the SHr group which binds CMB (head B), and the other does not show the initial burst and contains the SHr group (head A). It was also concluded that modification of head A of HMM or myosin with CMB increases its binding strength to FA, and consequently the substrate inhibition and Ca2+ sensitivity of acto-HMM or actomyosin ATPase at head B are lost on modification of head A with CMB. CMB-S-1(CT) was prepared by chymotryptic [EC 3.4.21.1] digestion of CMB-myosin, and separated into two fractions by ultracentrifugation of acto-CMB-S-1(CT) in the presence of ATP. Three components of CMB-S-1(CT) with molecular weights of 9, 2.4, and 1.2 X 10(4) were separated by SDS-polyacrylamide gel electrophoresis. The ratios of the peak areas of the three components in electrophoretograms were the same in CMB-S-1(CT) and in the two fractions (1 : 0.18 : 0.09), indicating that heads A and B have the same subunit structure.

Actins↗
PubMed1995

Anticipatory head direction signals in anterior thalamus: evidence for a thalamocortical circuit that integrates angular head motion to compute head direction.

Several regions in the rat brain contain neurons known as head-direction cells, which fire only when the rat's head is facing in a specific direction. Head-direction cells are influenced only by the direction of the head with respect to the static environmental surroundings, and not by the position of the head relative to the body. Each head-direction cell has its own preferred direction of firing, so that together, the population of cells provides a continuous signal of momentary directional heading. Here, head-direction cells were recorded from the post-subicular cortex (PSC) and anterodorsal nucleus (ADN) of the thalamus of freely moving rats. Cell activity was analyzed in relation to both momentary head direction, and the angular velocity of head turns. Head-direction cells in PSC maintained the same directional firing preference, regardless of the angular head velocity. By contrast, head-direction cells in ADN systematically shifted their directional firing preference, as a function of angular head velocity. The ADN cells always shifted their directional tuning peak to the left during clockwise head turns, and to the right during counterclockwise head turns. These results suggest that ADN neurons anticipate the future direction of the head, whereas PSC neurons encode the present direction of the head. Based on these findings, we hypothesize that neurons in PSC and ADN are reciprocally connected to form a thalamocortical circuit, which computes the directional position of the rat's head by integrating the angular motion of the head over time.

Animals↗
PubMed1999

[Electronystagmography of sudden head impulses (head-only impulsive rotational testing, head thrust test, head impulse test)].

The basis of the head-thrust test is when a head thrust is done in the horizontal plane, the ipsilateral semicircular canal is the only which gets the gaze stabilization, being the other HSC completely inhibited. In this paper are investigated the ENG records according the test and analyzed the main parameters obtained. As conclusion we admit that the main parameters are the compensating ocular movements amplitude and their latencies.

Adult↗
PubMed1998

Firing properties of rat lateral mammillary single units: head direction, head pitch, and angular head velocity.

Many neurons in the rat anterodorsal thalamus (ADN) and postsubiculum (PoS) fire selectively when the rat points its head in a specific direction in the horizontal plane, independent of the animal's location and ongoing behavior. The lateral mammillary nuclei (LMN) are interconnected with both the ADN and PoS and, therefore, are in a pivotal position to influence ADN/PoS neurophysiology. To further understand how the head direction (HD) cell signal is generated, we recorded single neurons from the LMN of freely moving rats. The majority of cells discharged as a function of one of three types of spatial correlates: (1) directional heading, (2) head pitch, or (3) angular head velocity (AHV). LMN HD cells exhibited higher peak firing rates and greater range of directional firing than that of ADN and PoS HD cells. LMN HD cells were modulated by angular head velocity, turning direction, and anticipated the rat's future HD by a greater amount of time (approximately 95 msec) than that previously reported for ADN HD cells (approximately 25 msec). Most head pitch cells discharged when the rostrocaudal axis of the rat's head was orthogonal to the horizontal plane. Head pitch cell firing was independent of the rat's location, directional heading, and its body orientation (i.e., the cell discharged whenever the rat pointed its head up, whether standing on all four limbs or rearing). AHV cells were categorized as fast or slow AHV cells depending on whether their firing rate increased or decreased in proportion to angular head velocity. These data demonstrate that LMN neurons code direction and angular motion of the head in both horizontal and vertical planes and support the hypothesis that the LMN play an important role in processing both egocentric and allocentric spatial information.

Action Potentials↗
PubMed2002

Comparison of head thrust test with head autorotation test reveals that the vestibulo-ocular reflex is enhanced during voluntary head movements.

OBJECTIVES: To compare 2 clinical tests of vestibular function, the head autorotation test (HART) and the head thrust test (HTT), and to determine why they give disparate results in patients with known unilateral vestibular deficiency (UVD) due to labyrinthectomy. METHODS: We used scleral coils to measure the horizontal (yaw) vestibulo-ocular reflex (VOR) in 5 healthy human subjects and in 11 patients who underwent labyrinthectomy. We used 2 paradigms. Using HART, subjects visually fixated a target during self-generated, swept-frequency, sinusoidal, horizontal head rotations. Using HTT, patients fixated the target during horizontal head thrusts delivered randomly in direction and time. RESULTS: In subjects without UVD, eye movements were almost perfectly compensatory for both paradigms. In subjects with UVD, VOR gain for ipsilesional head thrusts was low for both paradigms, but significantly (P<.001) higher (less abnormal) for HART (0.60 +/- 0.13) than for HTT (0.14 +/- 0.13). Contralesional gain was reduced for both, to 0.64 +/- 0.20 for HART and to 0.57 +/- 0.17 for HTT. Because ipsilesional and contralesional gains were not statistically different for HART (P =.69), comparison of VOR gains for half-cycle responses to the HART stimulus could not reliably identify the side of the known lesion. In contrast, HTT consistently identified the side of the lesion for all subjects with UVD. To investigate whether preprogramming contributes to the boost in VOR as measured by HART, we compared the gain and response delay of eye movements during actively self-generated and passively received head thrusts. For subjects without UVD, response delays were shorter for active (6 +/- 1 milliseconds) than for passive (12 +/- 1 milliseconds) HTT. For ipsilesional rotations of subjects with UVD, active HTT yielded a significantly higher gain (0.44 +/- 0.20) (P<.001) and a shorter delay (15 +/- 6 milliseconds) (P<.001) than did passive HTT (0.14 +/- 0.13 and 37 +/- 15 milliseconds, respectively). Contralesional test results revealed a similar performance boost for active head movements. Data are given as mean +/- SD. CONCLUSION: When comparison of half-cycle gains is used to identify the lesion side, self-generated predictable head movement paradigms, such as HART and active HTT, are less accurate than passive HTT in the characterization of UVD, in part because preprogramming can augment the VOR during voluntary head movements.

Adult↗
PubMed2001

New dinuclear catalysts Rh(2)(N-O)(2)[(C6H4)P(C6H5)2]2 with imidate ligands: synthesis and isomerization from head-to-tail to head-to-head configuration of the imidate ligands.

Two new dirhodium(II) catalysts of general formula Rh(2)(N-O)(2)[(C(6)H(4))P(C(6)H(5))(2)](2) (N-O = C(4)H(4)NO(2)) are prepared, starting from Rh(2)(O(2)CCH(3))(2)(PC)(2)L(2) [PC = (C(6)H(4))P(C(6)H(5))(2) (head-to-tail arrangement); L = HO(2)CCH(3)]. The thermal reaction of Rh(2)(O(2)CCH(3))(2)(PC)(2).L(2) with the neutral succinimide stereoselectively gives one compound that according to the X-ray structure determination has the formula Rh(2)(C(4)H(4)NO(2))(2)[(C(6)H(4))P(C(6)H(5))(2)](2) (1). It corresponds to the polar isomer with two bridging imidate ligands in a head-to-head configuration. However, stepwise reaction of Rh(2)(O(2)CCH(3))(2)(PC)(2).L(2) with (CH(3))(3)SiCl and potassium succinimidate yields a mixture of 1 and one of the two possible isomers (structure B) with a head-to-tail configuration of the imidate ligands, Rh(2)(C(4)H(4)NO(2))(2)[(C(6)H(4))P(C(6)H(5))(2)](2) (2), also characterized by X-ray methods. In solution, compound 2 undergoes slow isomerization to 1; the rate of this process is enhanced by the presence of acetonitrile. Compounds 1 and 2 are obtained as pure enantiomers starting from (M)- and (P)-Rh(2)(O(2)CCH(3))(2)(PC)(2).L(2) rather than from the racemic mixture. Their enantioselectivities in cyclopropanation of 1-diazo-5-penten-2-one are similar to those reported for the dirhodium amidate catalysts.

Journal Article↗
PubMed2000

Performance and standards for the process of head and neck cancer care: South and West audit of head and neck cancer 1996-1997 (SWAHN I). South and West Regional Cancer Organisation Tumour Panel for Head and Neck Cancer.

Evidence suggests wide variation in cancer care between different hospitals in the UK. To establish bench-marking data, we designed a prospective, 1 year regional study comparing key performance measures with established standards for the 28 hospital Trusts in the South and West of England involved in head and neck cancer care. 566 sequential patients with a newly-diagnosed head and neck cancer were included. Numbers referred and treated per hospital Trust were 1-58 and 1-65 respectively. 59% of patients received a pretreatment chest X-ray (standard 95%). 45% of patients were seen in a multidisciplinary clinic pretreatment (standard 95%), and this was proportional to the frequency of clinics held (P<0.0001). Median number of cases treated per surgeon was 4 (1-26), and by radiotherapist was 10 (1-51). Times between parts of the process of oral cancer care were closer to the standards than those for laryngeal cancer. Two patients were entered into a clinical trial. One had a quality-of-life score. Thus, in 1996-1997, in the South and West of England, there were major discrepancies between actual performance and established standards in many fundamental aspects of head and neck cancer care. Re-audit is essential to determine if the implementation of the Calman-Hine report has resulted in improvements.

Female↗
PubMed1988

Production and characterization of monoclonal antibodies recognizing head activator in precursor form and immunocytochemical localization of head activator precursor and head activator peptide in the neural cell line NH15-CA2 and in hydra.

A synthetic gene for the hydra neuropeptide head activator (HA) was used to produce large amounts of an HA bacterial fusion protein. From this protein an HA-containing fragment was cleaved out, attached in high copy number to carrier proteins, and used as an immunogen to produce monoclonal antibodies able to recognize head activator in precursor form. Using such antibodies and others with different specificities for HA epitopes in combination with different fixation procedures, we detected HA immunoreactivity in three locations in the HA-rich neural cell line NH15-CA2. A precursor-like HA immunoreactivity was present in the cytoplasm of cells and detected, independent of fixation procedure, by monoclonal antibodies characterized as HA-precursor-specific. With antibodies specific for the HA peptide, two immunoreactivities could be distinguished, one within cells and one at the outer cell membrane. HA was detected within differentiated cells with long processes when crosslinkers such as carbodiimide or glutaraldehyde were applied together with agents like methanol. HA peptide bound to target cells was restricted to small round cells with an undifferentiated morphology, especially to those in the process of cell division. In hydra HA precursor immunoreactivity was localized in interstitial cells and in developing nerve cells. HA peptide immunoreactivity was present in nerve cells, but was more concentrated on and in target cells such as interstitial cells and epithelial cells. In tissue sections immunoreactive cells were especially abundant in regions of high HA content such as hypostome, subhypostomal region, and the future head region of developing buds.

Animals↗
PubMedDate not supplied

Head and neck sarcoma: report of the Head and Neck Sarcoma Registry. Society of Head and Neck Surgeons Committee on Research.

A Head and Neck Sarcoma Registry was established by the Society of Head and Neck Surgeons to review treatment results of a rare tumor by surgeons with special interest in this anatomic site. Two hundred fourteen patients were analyzed. There were 194 adult tumors and 20 pediatric tumors. The major sites included parotid and neck, 20%; face and forehead, 18%; maxilla and palate, 13%; scalp, 12%; mandible, 11%; paranasal sinuses, 7%; larynx, 2%; and oral cavity, 5%. Eighty-four percent were resectable. The disease-free survival was 56%; overall survival was 70% at 5 years. Major determinants of survival were adequacy of resection (margins free of tumor) and tumor type. Survival differed according to tumor cell type (tumor grade was not available). Patients with chondrosarcoma and dermatofibrosarcoma had survival approaching 100%. Patients with malignant fibrous histiocytoma (MFH) and fibrosarcoma (FSA) had intermediate survival of 60% to 70%. The worst survival, less than 50% at 5 years, occurred in patients with osteosarcoma, angiosarcoma, and rhabdomyosarcoma in decreasing order. This suggests a rationale for identifying high-risk patients for prospective adjuvant protocols. This study emphasizes the value of recording uncommon tumors to provide relevant information for future study and possibly therapy.

Adolescent↗
PubMed2003

The organisation of head and neck oncology services in the UK: The Royal College of Surgeons of England and British Association of Head and Neck Oncologists' preliminary multidisciplinary head and neck oncology audit.

This study was a collaboration between The Royal College of Surgeons of England Clinical Effectiveness Unit and the British Association of Head and Neck Oncologists (BAHNO). We created a multidisciplinary database through an enquiry to all 49 UK radiotherapy centres. A questionnaire audit identified teams and individuals in the UK involved with treatment of head and neck cancer. A questionnaire on their organisation, and intentions for change was sent to the 108 teams (90% response) and 11 sole practitioners (45% response) identified. Overall, 335 surgical consultants were involved in the treatment of 7500 cases per annum, with large variations in size of catchment populations served by teams. Mean length of time spent with each out-patient was 11 min. Of respondents, 58% were already using the BAHNO basic dataset and more indicated intention to use it, but only 32% could actually deliver information on their work-load. More computerisation of data collection is essential, and national audit may bridge the data gap.

Databases, Factual↗
PubMed1996

The dynamics of head activation changes during proportioning in Hydra oligactis with altered head-body ratios.

Normal hydra head-body proportions were altered by axially grafting a second head in place of the lower body column. The resulting animals had double the head tissue and one-quarter the normal body column. Changes in the head activation potential of tissue subjacent to both heads were monitored by assaying the ability of these animals to regenerate heads. The host head, the grafted head, or both heads were removed at varying times following graft construction and the animals were scored for head regeneration and/or the ability to express a head-specific antigen recognized by monoclonal antibody, CP8. In the presence of the grafted head, tissue subjacent to the host head lost the ability to regenerate a head or express the head-specific antigen over a 48-hr period. In the presence of the host head, tissue subjacent to the grafted head regenerated heads at a very low frequency and lost the ability to express the head-specific antigen over the same 48-hr period. Following simultaneous removal of both heads, animals initially regenerated both heads for the first 48 hr after graft construction. Then, both head regeneration and expression of the head-specific antigen declined gradually over the next 3 days, though not to the very low levels observed when one head remained. These data, especially the loss of CP8 labeling, support the hypothesis that loss of regeneration ability was due to a loss of head activation potential in tissues subjacent to the heads. We propose that this reflected the attempt of grafted animals to compensate for the altered head-body proportions through reproportioning. In keeping with this hypothesis, feeding the grafted animals to stimulate growth of body column tissue and a shift toward more normal head-body proportions prior to decapitation resulted in animals which were capable of regenerating heads when decapitated. Several interpretations of the results based on the Gierer-Meinhardt reaction-diffusion model of pattern formation are discussed.

Animals↗
PubMed2004

Burden of infection among heads and non-head of rural households in Rakai, Uganda.

The objective was to determine HIV prevalence, symptomatology and mortality among adult heads and non-heads of households, in order to assess the burden of HIV on households. It was a community study of 11,536 adults aged 15-59, residing in 4,962 households in 56 villages, Rakai district, Uganda. First, 4,962 heads and 6,574 non-heads of households were identified from censuses. Interviews were then used to determine socio-demographic/behavioural characteristics. HIV seroprevalence was diagnosed by two EIAs with Western blot confirmation. The adjusted odds ratio (OR) and 95% confidence intervals (CI) of HIV infection in household heads and non-heads were estimated by multivariate logistic regression. Age-adjusted mortality was also assessed. HIV prevalence was 16.9% in the population, and 21.5% of households had at least one HIV-infected person (<0.0001). HIV prevalence was higher among heads than non-heads of households (21.5 and 13.3%, respectively, OR=1.79; CI 1.62-1.97). Most household heads were males (70.5%), and HIV prevalence was 17.8% among male heads compared with 6.6% in male non-heads of households (OR=2.31; CI 1.65-2.52). Women heading households were predominantly widowed, separated or divorced (64.4%). HIV prevalence was 30.5% among female heads, compared with 15.6% in female non-household heads (OR=1.42; CI 1.15-1.63). Age-adjusted mortality was significantly lower among male household heads than non-heads, both for the HIV-positive (RR=0.68) and HIV-negative men (RR=0.63). Among women, HIV-negative female household heads had significantly higher mortality than HIV-uninfected female non-heads (RR=1.72). HIV disproportionately affects heads of households, particularly males. Mortality due to AIDS is likely to increase the proportion of female-headed households, and adversely affect the welfare of domestic units.

Adolescent↗
PubMed2001

Heading and head injuries in soccer.

In the world of sports, soccer is unique because of the purposeful use of the unprotected head for controlling and advancing the ball. This skill obviously places the player at risk of head injury and the game does carry some risk. Head injury can be a result of contact of the head with another head (or other body parts), ground, goal post, other unknown objects or even the ball. Such impacts can lead to contusions, fractures, eye injuries, concussions or even, in rare cases, death. Coaches, players, parents and physicians are rightly concerned about the risk of head injury in soccer. Current research shows that selected soccer players have some degree of cognitive dysfunction. It is important to determine the reasons behind such deficits. Purposeful heading has been blamed, but a closer look at the studies that focus on heading has revealed methodological concerns that question the validity of blaming purposeful heading of the ball. The player's history and age (did they play when the ball was leather and could absorb significant amounts of water), alcohol intake, drug intake, learning disabilities, concussion definition and control group use/composition are all factors that cloud the ability to blame purposeful heading. What does seem clear is that a player's history of concussive episodes is a more likely explanation for cognitive deficits. While it is likely that the subconcussive impact of purposeful heading is a doubtful factor in the noted deficits, it is unknown whether multiple subconcussive impacts might have some lingering effects. In addition, it is unknown whether the noted deficits have any affect on daily life. Proper instruction in the technique is critical because if the ball contacts an unprepared head (as in accidental head-ball contacts), the potential for serious injury is possible. To further our understanding of the relationship of heading, head injury and cognitive deficits, we need to: learn more about the actual impact of a ball on the head, verify the exposure to heading at all ages and competitive levels, determine stable estimates of concussive injury rates across the soccer spectrum, conduct prospective longitudinal studies on soccer players focusing on exposure, injury and cognition, and determine the minimum safe age to begin instruction on the skill of heading. Only then will we be able to speak with some authority on the issue of heading and head injuries in soccer.

Biomechanical Phenomena↗
PubMedDate not supplied

Effects of vestibular loss on head stabilization in response to head and body perturbations.

Control of head position during postural responses is important to facilitate both the interpretation of vestibular signals and the stabilization of gaze. In these experiments, we compared head stabilization for two different postural tasks: 1) in response to perturbations at the head, and 2) in response to perturbations induced at the support surface, which perturb both body and head position. To determine whether normal vestibular function is necessary for head stabilization in these two tasks, responses to forward and backward mechanical perturbations of the head and body were compared for 13 normal subjects and 4 patients with profound bilateral vestibular loss (two with vestibular loss in adulthood and two in infancy). Normal subjects showed little neck muscle activity for body perturbations, but large, early activations in both neck extensors and flexors for head perturbations. In contrast, vestibular patients showed excessive neck muscle activation for body perturbations and reduced or absent neck muscle activity for head perturbations. Patients with vestibular loss in adulthood also showed increased head acceleration in response to both head and body perturbations, but patients with vestibular loss in infancy showed more normal head accelerations. For body perturbations, the differences in head acceleration between patients and normals were greater for later head acceleration peaks, indicating poor head control during the execution of the postural response. Trunk angle changes were also higher in the patients for forward body perturbations, indicating that poorer control of trunk position could have contributed to their poorer head stabilization. These results indicate that the vestibular system plays an important role in head and trunk stabilization for both head and body perturbations. However, the more normal head accelerations of the patients with infant vestibular loss also indicate that other mechanisms, possibly involving neck reflexes, can at least partially substitute for the vestibular system to provide head and trunk stabilization.

Adolescent↗
PubMed2002

Radiosurgery for residual or recurrent nonfunctioning pituitary adenoma.

OBJECT: Nonfunctioning pituitary adenomas comprise approximately 30% of all pituitary tumors. The purpose of this retrospective study is to evaluate the efficacy and role of gamma knife radiosurgery (GKS) in the management of residual or recurrent nonfunctioning pituitary adenomas. METHODS: A review was conducted of the data obtained in 42 patients who underwent adjuvant GKS at the University of Pittsburgh between 1987 and 2001. Prior treatments included transsphenoidal resection, craniotomy and resection, or conventional radiotherapy. Endocrinological, ophthalmological, and radiological responses were evaluated. The duration of follow-up review varied from 6 to 102 months (mean 31.2 months). Fifteen patients were observed for more than 40 months. The mean radiation dose to the tumor margin was 16 Gy. Conformal radiosurgery planning was used to restrict the dose to the optic nerve and chiasm. Tumor control after GKS was achieved in 100% of patients with microadenomas and 97% of patients with macroadenomas. Gamma knife radiosurgery was equally effective in controlling adenomas with cavernous sinus invasion and suprasellar extension. No patient developed a new endocrinological deficiency following GKS. One patient's tumor enlarged with an associated decline in visual function. Another patient experienced a deterioration of visual fields despite a decrease in tumor size. CONCLUSIONS: Gamma knife radiosurgery can achieve tumor control in virtually all residual or recurrent nonfunctioning pituitary adenomas. Dose sparing facilitates tumor management even when the adenoma is close to the optic apparatus or invades the cavernous sinus.

Adenoma↗
PubMed1981

Motions and interactions of phospholipid head groups at the membrane surface. 2. Simple alkyl head groups.

As a reference point for comparison with more complex head groups, a set of phospholipids with simple alkyl head groups has been studied. These analogues resemble the naturally occurring phospholipids, except they have phosphomethanol, -ethanol, -1-propanol, and -1-butanol as head groups. The gel-to-liquid-crystalline phase transition temperatures were measured with differential scanning calorimetry, and the phase properties of multilamellar dispersions were examined with phosphorus-31 NMR. The effect of the head-group size was found to be rather small. These lipids were synthesized with deuterium labels incorporated into the alcohol portion at all positions in the head groups except butanol, which was labeled only in the C-1 position. Determination of the 2H residual quadrupole splittings led to an analysis of the head-group ordering properties. Specifically, these data showed that increasing the length of the head group leads to a more perpendicular orientation of the head group relative to the bilayer surface. Phosphorus-31 chemical shift anisotropy data were also compatible with this result. Measurement of surface pressure-area diagrams of monolayers of these compounds revealed that at high pressures (30 dyn/cm) all four lipids occupied similar areas (40-44 A2/molecule), yet at lower pressures, the larger the head group, the larger the occupied surface area. This result suggests that in a bilayer the fatty acyl chains occupy similar areas independent of the alkyl head-group size, and the larger head groups cannot pack properly without some conformational adjustment. The addition of phosphatidylcholine with its relatively bulky head group decreases the area available to the alkyl head groups, pushing the alkyl head groups out of the plane of the bilayer surface. Cholesterol, on the other hand, acts as a "spacer", increasing the area available to the head group, and leads to the opposite effect; i.e., the head groups can relax into a conformation more parallel to the bilayer surface. These data illustrate the types of steric effects which can be expected at the membrane surface. Dynamic properties were investigated by measurement of the 2H spin-lattice (T1) NMR relaxation times. These relaxation times could be compared with those from other parts of a phospholipid molecule, namely, the glycerol backbone and the fatty acyl chains. The rates of segmental motion in these head groups were similar to the first C-2 to C-8 segments of the fatty acyl chains, indicating considerable head-group flexibility.

Alkylation↗
PubMed1997

Eye-head coordination during head-unrestrained gaze shifts in rhesus monkeys.

We analyzed gaze shifts made by trained rhesus monkeys with completely unrestrained heads during performance of a delayed gaze shift task. Subjects made horizontal, vertical, and oblique gaze shifts to visual targets. We found that coordinated eye-head movements are characterized by a set of lawful relationships, and that the initial position of the eyes in the orbits and the direction of the gaze shift are two factors that influence these relationships. Head movements did not contribute to the change in gaze position during small gaze shifts (<20 degrees) directed along the horizontal meridian, when the eyes were initially centered in the orbits. For larger gaze shifts (25-90 degrees), the head contribution to the gaze shift increased linearly with increasing gaze shift amplitude, and eye movement amplitude saturated at an asymptotic amplitude of approximately 35 degrees. When the eyes began deviated in the orbits contralateral to the direction of the ensuing gaze shift, the head contributed less and the eyes more to amplitude-matched gaze shifts. The relative timing of eye and head movements was altered by initial eye position; head latency relative to gaze onset increased as the eyes began in more contralateral initial positions. The direction of the gaze shift also affected the relative amplitudes of eye and head movements; as gaze shifts were made in progressively more vertical directions, eye amplitude increased and head contribution declined systematically. Eye velocity was a saturating function of gaze amplitude for movements without a head contribution (gaze amplitude <20 degrees). As head contribution increased with increasing gaze amplitude (20-60 degrees), peak eye velocity declined by >200 degrees/s and head velocity increased by 100 degrees/s. For constant-amplitude eye movements (approximately 30 degrees), eye velocity declined as the velocity of the concurrent head movement increased. On the basis of these relationships, it is possible to accurately predict gaze amplitude, the amplitudes of the eye and head components of the gaze shift, and gaze, eye, and head velocities, durations and latencies if the two-dimensional displacement of the target and the initial position of the eyes in the orbits are known. These data indicate that signals related to the initial positions of the eyes in the orbits and the direction of the gaze shift influence separate eye and head movement commands. The hypothesis that this divergence of eye and head commands occurs downstream from the superior colliculus is supported by recent electrical stimulation and single-unit recording data.

Animals↗