The pattern of sensory discharge can determine the motor response in young Xenopus tadpoles

Young Xenopus tadpoles were used to test whether the pattern of discharge in specific sensory neurons can determine the motor response of a whole animal. Young Xenopus tadpoles show two main rhythmic behaviours: swimming and struggling. Touch-sensitive skin sensory neurons in the spinal cord of immobilised tadpoles were penetrated singly or in pairs using microelectrodes to allow precise control of their firing patterns. A single impulse in one Rohon-Beard neuron (= light touch) could sometimes trigger “fictive” swimming. Two to six impulses at 30–50 Hz (= a light stroke) reliably triggered fictive swimming. Neither stimulus evoked fictive struggling. Twenty-five or more impulses at 30–50 Hz (= pressure) could evoke a pattern of rhythmic bursts, distinct from swimming and suitable to drive slower, stronger movements. This pattern showed some or all the characteristics of “fictive” struggling. These results demonstrate clearly that sensory neurons can determine the pattern of motor output simply by their pattern of discharge. This provides a simple form of behavioural selection according to stimulus. Accepted: 28 November 1996     

The inferior colliculus of the mustached bat has the frequency-vs-latency coordinates

In the mustached bat, the central auditory system contains FM–FM (delay-tuned) neurons which are specialized for processing target-distance information carried by echo delays. Mechanisms for creating the FM–FM neurons involve delay lines, coincidence detection and amplification. A neural basis for delay lines can be a map representing response latencies. The aim of the present study is to explore whether the central nucleus of the inferior colliculus has a latency axis incorporated into iso-best frequency slabs. Responses of single or multiple neurons were recorded from the inferior colliculus of unanesthetized mustached bats with tungsten-wire electrodes, and their response latencies were measured with tone bursts at their best frequencies and best amplitudes or 65 dB SPL. In the dorsoventral electrode penetrations across the inferior colliculus, response latency systematically shortens from ˜12 to ˜4␣ms. Tonotopic representation in the inferior colliculus is somewhat complex. Iso-best frequency slabs are tilted and/or curved, but they orient more or less ventrodorsally. Nevertheless, the latency axis is evident in each iso-best frequency slab, regardless of best frequency. The inferior colliculus has the frequency-vs-latency coordinates. Accepted: 2 October 1996     

Neural correlates of habituation of the proleg withdrawal reflex in larvae of the hawk moth, Manduca sexta

The larval proleg withdrawal reflex of the hawk moth, Manduca sexta, exhibits robust habituation. This reflex is evoked by deflecting one or more mechanosensory planta hairs on a proleg tip. We examined neural correlates of habituation in an isolated proleg preparation consisting of one proleg and its segmental ganglion. Repeated deflection of a single planta hair caused a significant decrease in the number of action potentials evoked in the proleg motor nerve (which carries the axons of proleg retractor motor neurons). Significant response decrement was seen for interstimulus intervals of 10 s, 60 s and 5 min. Response decrement failed to occur in the absence of repetitive stimulation, the decremented response recovered spontaneously following a rest, and electrical stimulation of a body wall nerve facilitated the decremented response (a neural correlate of dishabituation). Adaptation of sensory neuron responses occurred during repeated hair deflections. However, when adaptation was eliminated by direct electrical stimulation of sensory neurons, the response in the proleg motor nerve still decreased significantly. Muscle recordings indicated that the response of an identified proleg retractor motor neuron decreased significantly during habituation training. Thus, habituation of the proleg withdrawal reflex includes a central component that is apparent at the level of a single motor neuron. Accepted: 20 December 1996     

Multisegmental motor activity in the segmentally restricted gin trap behavior in Manduca sexta pupae

Stimulation of sensory neurons innervating hairs in the gin traps on the abdomen of Manduca sexta pupae evokes a rapid bending of the abdomen that is restricted to one or more of the three articulating posterior segments. However, electrical stimulation of the gin trap sensory nerve in an isolated abdominal nerve cord evokes characteristic motor neuron activity in every abdominal segment. To determine if the segmentally distributed motor activity also occurred in intact animals and how it contributed to the segmentally restricted reflex movement, mechanical stimulation of the sensory hairs in intact animals was used to evoke reflex responses that were recorded as electromyograms synchronized with video recordings of the behavior. Motor activity was monitored during movements to determine if there was activity in many segments when the movement was restricted to one segment. Coordinated muscle activity was evoked throughout the abdomen in response to stimulation of any of the three gin traps, even when movement was restricted to one segment. Differences in the timing of ipsilateral and contralateral motor activity among segments allowed the closing of gin traps to be segmentally restricted. These findings suggest that the neural circuit underlying the gin trap reflex is distributed throughout the abdominal nerve cord. This network generates a complex, yet coordinated, motor pattern with muscular activity in many abdominal segments that produces a localized bending reflex. Accepted: 10 January 1997     

Non-spiking local interneurones mediate abdominal extension related descending control of uropod motor neurones in the crayfish terminal abdominal ganglion

The role of non-spiking local interneurones in the synaptic interactions between abdominal extension-evoking descending interneurones and uropod motor neurones in the terminal abdominal ganglion of the crayfish Procambarus clarkii (Girard) was investigated electrophysiologically. Continuous electrical stimulation of the lateral region of the 3rd-4th abdominal connective that included abdominal extension evoking interneurones excited the opener motor neurones and inhibited the closer, reductor motor neurone. Spikes from a single descending interneurone evoked consistent and short latency (0.8–0.9 ms) excitatory postsynaptic potentials (e.p.s.ps) in the opener motor neurones, and evoked rather long-latency (1.5–2.7 ms) inhibitory postsynaptic potentials (i.p.s.ps) in the reductor motor neurone. Many non-spiking interneurones also received depolarizing p.s.ps (0.8–2.5 ms in latency) that were usually faster than i.p.s.ps of the reductor motor neurone if both neurones were recorded sequentially in the same preparation. Non-spiking interneurones received convergent inputs from several descending interneurones and made inverting connection with the reductor motor neurone. Elimination of descending inputs to a particular non-spiking interneurone could reduce the inhibitory response of the reductor motor neurone. These observations strongly suggested that descending inhibitory inputs to the closer, reductor motor neurone were mediated by non-spiking interneurones. Furthermore, some non-spiking interneurones made output connections with the opener motor neurones. The disynaptic pathway through non-spiking interneurones is significant to control and modulate the opening pattern of the uropod during abdominal extension. Accepted: 27 December 1996     

Directionality of auditory nerve fiber responses to pure tone stimuli in the grassfrog, Rana temporaria. II. Spike timing

We studied the directionality of spike timing in the responses of single auditory nerve fibers of the grass frog, Rana temporaria, to tone burst stimulation. Both the latency of the first spike after stimulus onset and the preferred firing phase during the stimulus were studied. In addition, the directionality of the phase of eardrum vibrations was measured. The response latency showed systematic and statistically significant changes with sound direction at both low and high frequencies. The latency changes were correlated with response strength (spike rate) changes and were probably the result of directional changes in effective stimulus intensity. Systematic changes in the preferred firing phase were seen in all fibers that showed phaselocking (i.e., at frequencies below 500–700 Hz). The mean phase lead for stimulation from the contralateral side was approximately 140° at 200 Hz and decreased to approximately 100° at 700 Hz. These phaseshifts correspond to differences in spike timing of approximately 2 ms and 0.4 ms respectively. The phaseshifts were nearly independent of stimulus intensity. The phase directionality of eardrum vibrations was smaller than that of the nerve fibers. Hence, the strong directional phaseshifts shown by the nerve fibers probably reflect the directional characteristics of extratympanic pathways. Accepted: 23 November 1996     

Dynamic responses of series force receptors innervating the opener muscle apodeme in the blue crab, Callinectes sapidus

Receptors monitoring muscle force innervate the opener muscle apodeme in the walking legs of the blue crab, Callinectes sapidus. Biocytin backfills reveal 9–15 bipolar neurons with somata as large as 60 μm positioned at the distal end of the apodeme. Sensory endings insert into the apodeme and are in series with the opener muscle. The axons of these neurons form the opener apodeme sensory nerve that merges with the most distal branch of the opener motor nerve. Recordings reveal that the receptors are not spontaneously active nor do they respond to passive muscle stretch. Isometric muscle contraction evoked by stimulating the opener excitor motor neuron is the adequate stimulus for receptor firing. Most significant is the finding that during contraction, over a wide range of forces, the firing rate of individual receptors closely parallels the rate of change of isometric force. The peak instantaneous frequency typically occurs at the force derivative maximum, but not at maximum force development. Thus, receptors of the opener apodeme sensory nerve more closely monitor changes in isometric force rather than the total force achieved. Accepted: 20 September 1996     

Encoding of sound duration by neurons in the auditory cortex of the little brown bat, Myotis lucifugus

Responses of 117 single- or multi-units in the auditory cortex (AC) of bats (Myotis lucifugus) to tone bursts of different stimulus durations (1– 400 ms) were studied over a wide range of stimulus intensities to determine how stimulus duration is represented in the AC. 36% of AC neurons responded more strongly to short stimulus durations showing short-pass duration response functions, 31% responded equally to all pulse durations (i.e., all-pass), 18% responded preferentially to stimuli having longer durations (i.e., long-pass), and 15% responded to a narrow range of stimulus durations (i.e., band-pass). Neurons showing long-pass and short-pass duration response functions were narrowly distributed within two horizontal slabs of the cortex, over the rostrocaudal extent of the AC. The effects of stimulus level on duration selectivity were evaluated for 17 AC neurons. For 65% of these units, an increase in stimulus intensity resulted in a progressive decrease in the best duration. In light of the unusual intensity-dependent duration responses of AC neurons, we hypothesized that the response selectivities of AC neurons is different from that in the brainstem. This hypothesis was validated by results of study of the duration response characteristics of single neurons in the inferior colliculus. Accepted: 8 November 1996     

Central patterning and reflex control of antennular flicking in the hermit crab Pagurus alaskensis (Benedict).

1. The effects of altering sensory input on the motoneuronal activity underlying antennular flicking have been tested. 2. Removal of the short segments of the outer flagellum results in a reduction of the number of spikes/burst in the fast flexor motoneurones A31F and A32F. 3. During a flick the delay between the burst in motoneurone A31F and the burst in motoneurone A32F is insensitive to alteration of sensory input. 4. Sensory feedback from the flexion phase of a flick is necessary for the activation of either extensor motoneurone. Evidence is presented to suggest that this feedback is primarily from joint-movement receptors at the MS-DS and DS-OF joints. 5. The results are incorporated into a model in which the patterns of flexor activity result from some specified properties of three components: a trigger system, a follower system, and the spike initiating zone of the flexor motoneurones. The trigger system determines when a flick will occur. The follower system determines the number of flexor spikes during a flick. Properties of the spike initiating zone determine the spike frequency and the timing between bursts in the flexor motoneurones. Extensor activity in the model is reflexively elicited by feedback from phasic, unidirectional receptors sensitive to joint flexion. 6. The functional significance of reflex control of extensor activity is discussed in relation to the form and proposed function of antennular flicking. It is suggested that this form of control is adapted to the function of antennular flicking because flexion at the MS-DS joint is not always necessary for the fulfilment of the fuction of a flick.     

Feature-detecting auditory neurons in the brain of a sound-producing fish

The mormyrid fish Pollimyrus adspersus has auditory specializations for sound pressure detection and uses acoustic displays in its natural social behavior. In this paper it is shown that auditory neurons in the mesencephalon (torus semicircularis) are activated selectively by temporal features of complex sounds. Single neurons were recorded while presenting sounds to fish underwater. The stimuli were acoustic click trains, 400 ms in duration, and were synthesized with differing inter-click-intervals (ICIs). The natural sounds of this species are composed similarly and the range of ICIs synthesized overlapped with the natural range (5–40 ms). One-third of the neurons studied were strongly selective for a narrow range of ICIs, increasing spike rate by ten fold or more at the best ICI compared to the minimum response observed. The best ICI for interval selective neurons remained stable when the sound pressure of the stimulus was changed. Neurons that were selective gave phasic responses to tone bursts, and most had non-monotonic rate level functions. The origin of interval selectivity is discussed and a time-based computational mechanism is proposed. Accepted: 20 December 1996     

Indices of skeletal muscle damage and connective tissue breakdown following eccentric muscle contractions

 Indirect indices of exercise-induced human skeletal muscle damage and connective tissue breakdown were studied following a single bout of voluntary eccentric muscle contractions. Subjects (six female, two male), mean (SD) age 22 (2) years performed a bout of 50 maximum voluntary eccentric contractions of the knee extensors of a single leg. The eccentric exercise protocol induced muscle soreness (P < 0.05 Wilcoxon test), chronic force loss, and a decline in the 20:100 Hz percutaneous electrical myostimulation force ratio [P < 0.01, repeated measures analysis of variance (ANOVA)]. Serum creatine kinase (CK) and lactate dehydrogenase (LDH) activities were elevated (P < 0.01, repeated measures ANOVA) following the bout. The mean (SD) CK and LDH levels recorded 3 days post-exercise were 2815 (4144) IU · l^–1 and 375 (198) IU · l^–1, respectively. Serum alkaline phosphatase activity showed no changes throughout the study, and a non-significant increase (P = 0.058, repeated measures ANOVA) in pyridinoline was recorded following the bout. Urinary hydroxyproline (HP) and hydroxylysine (HL) excretion, expressed in terms of creatinine (Cr) concentration, increased after exercise (P < 0.05 and P < 0.01, respectively, repeated measures ANOVA). An increased HP:Cr was recorded 2 days post-exercise and HL:Cr was increased above baseline on days 2, 5, and 9 post-exercise. This indirect evidence of exercise-induced muscle damage suggests that myofibre disruption was caused by the eccentric muscle contractions. Elevated urine concentrations of indirect indices of collagen breakdown following eccentric muscle contractions suggests an increased breakdown of connective tissue, possibly due to a localised inflammatory response. Accepted: 9 October 1996     

Segmentally distributed metamorphic changes in neural circuits controlling abdominal bending in the hawkmoth Manduca sexta

During the metamorphosis of Manduca sexta the larval nervous system is reorganized to allow the generation of behaviors that are specific to the pupal and adult stages. In some instances, metamorphic changes in neurons that persist from the larval stage are segment-specific and lead to expression of segment-specific behavior in later stages. At the larval-pupal transition, the larval abdominal bending behavior, which is distributed throughout the abdomen, changes to the pupal gin trap behavior which is restricted to three abdominal segments. This study suggests that the neural circuit that underlies larval bending undergoes segment specific modifications to produce the segmentally restricted gin trap behavior. We show, however, that non-gin trap segments go through a developmental change similar to that seen in gin trap segments. Pupal-specific motor patterns are produced by stimulation of sensory neurons in abdominal segments that do not have gin traps and cannot produce the gin trap behavior. In particular, sensory stimulation in non-gin trap pupal segments evokes a motor response that is faster than the larval response and that displays the triphasic contralateral-ipsilateral-contralateral activity pattern that is typical of the pupal gin trap behavior. Despite the alteration of reflex activity in all segments, developmental changes in sensory neuron morphology are restricted to those segments that form gin traps. In non-gin trap segments, persistent sensory neurons do not expand their terminal arbors, as do sensory neurons in gin trap segments, yet are capable of eliciting gin trap-like motor responses. Accepted: 10 January 1997     

The tarso-pretarsal chordotonal organ as an element in cockroach walking

Many types of sense organs have been demonstrated to show repetitive discharges during walking that could provide informational cues about leg movements and other parameters of locomotion. We have recorded activities of receptors of the distal (tarsal) segments of the cockroach hindleg in restrained and freely moving animals while they were videotaped. These recordings show peaks of activities at the onset and termination of the stance phase. We have morphologically and physiologically identified a joint angle receptor, the tarso-pretarsal chordotonal organ, that contributes to the discharges seen late in stance, prior to the onset of leg flexion in swing. This sense organ encodes the angle and rate of change of the most distal leg joint and specifically discharges when the claws are disengaged from the substrate. Applied displacements of the claws in restrained preparations elicit reflex activation of the tibial flexor muscle and a crossed extensor reflex in the opposite hindleg. These reflexes could function to insure that leg flexion in swing does not occur until the claws are disengaged and to enhance support by the opposite hindleg. Thus, the regular discharges of the chordotonal organ could assure efficient and coordinated muscle contractions and movements during normal, unperturbed walking. Accepted: 2 January 1997     

Patterns of activity in the antennular motoneurones of the hermit crab Pagurus alaskensis (Benedict).

1. Using electromyogram recordings from the antennular muscles of intact animals and recordings from the antennular nerves of partially dissected preparations, the patterns of activity in specific antennular motoneurones have been described during antennular flicking and antennular withdrawal. 2. The slow extensor motoneurone A30S is active during flicking in addition to the phasic component of the antennular motor system (A30F, A31F and A32F). 3. The flexion phase of a flick is the result of a burst of variable duration and number of spikes within flexor motoneurones A31F and A32F. 4. The extension phase of a flick is the result of a burst of variable duration and number of spikes in extensor motoneurones A30F and A30S. 5. Extension-withdrawal and slow flexion-withdrawal reflexes, tonic flexion withdrawal and maintained flexion at the MS-DS joint usually result from activity in part of the tonic component of the antennular motor system:moto-neurones A30S, A31S and A32S. 6. Fast flexion-withdrawal reflexes result from a burst of spikes in motoneurone A31F-S which constitutes the phaso-tonic component of the antennular motor system. 7. During high-frequency activity (15-60/sec), reciprocity exists between the slow flexor motoneurones A31S and A32S and slow extensor motoneurone A30S.     

Temporal selectivity for complex signals by single neurons in the torus semicircularis of Pleurodema thaul (Amphibia: Leptodactylidae)

Responses of auditory neurons in the torus semicircularis (TS) of Pleurodema thaul, a leptodactylid from Chile, to synthetic stimuli having diverse temporal patterns and to digitized advertisement calls of P. thaul and three sympatric species, were recorded to investigate their temporal response selectivities. The advertisement call of this species consists of a long sequence of sound pulses (a pulse-amplitude-modulated, or PAM, signal) having a dominant frequency of about 2000 Hz. Each of the sound pulses contains intra-pulse sinusoidal-amplitude-modulations (SAMs). Synthetic stimuli consisted of six series in which the following acoustic parameters were systematically modified, one at a time: PAM rate, pulse duration, number of pulses, and intra-pulse SAM rate. The carrier frequency of these stimuli was set at the characteristic frequency (CF) of the isolated units (n = 47). Response patterns of TS units to synthetic call variants reveal different degrees of selectivities for each of the temporal variables, with populations of neurons responding maximally to specific values found in the advertisement call of this species. These selectivities are mainly shaped by neuronal responsiveness to the overall sound energy of the stimulus and by the inability of neurons to discharge to short inter-pulse gaps. Accepted: 30 October 1996     

Coupling between muscle activities and muscle torques during horizontal-planar arm movements with direction reversal.

In this study we investigated the hypothesis that the simple set of rules used to explain the modulation of muscle activities during single-joint movements could also be applied for reversal movements of the shoulder and elbow joints. The muscle torques of both joints were characterized by a triphasic impulse. The first impulse of each joint accelerated the limb to the target and was generated by an initial burst of the muscles activated first (primary mover). The second impulse decelerated the limb to the target, reversed movement direction and accelerated the limb back to the initial position, and was generated by an initial burst of the muscles activated second (secondary movers). A third impulse, in each joint, decelerated the limb to the initial position due to the generation of a second burst of the primary movers. The first burst of the primary mover decreased abruptly, and the latency between the activation of the primary and secondary movers varied in proportion with target distances for the elbow, but not for the shoulder muscles. All impulses and bursts increased with target distances and were well coupled. Therefore, as predicted, the bursts of muscle activities were modulated to generate the appropriate level of muscle torque.     

Chemical communication in heliothine moths. VII. Correlation between diminished responses to point-source plumes and single filaments similarly tainted with a behavioral antagonist

Addition of (Z)-11-hexadecenyl acetate (Z11-16:Ac) into a normally attractive binary blend of Heliothis virescens pheromone components resulted in a suppression of upwind flight and source location by males. Male response was reduced even at the lowest dosages of Z11-16:Ac tested but upwind flight and source location were most clearly reduced when the loading of Z11-16:Ac reached 10% or more of the (Z)-11-hexadecenal (Z11-16:Ald) loading (the major component present in the binary blend). Similar patterns of suppression in response were noted when Z11-16:Ac was added to binary blends of pheromone components at both 10 and 100 μg loadings of Z11-16:Ald. Males in casting flight following upwind flight in a mechanically generated pulsed plume, responded to the interception of a subsequent, single binary-blend filament by making a toward-source upwind surge. Responses of males to a single filament that was tainted by a level of Z11-16:Ac that had allowed some reduced level of upwind flight and source location to occur in the previous plume experiments were diminished compared with their control counterparts. Analysis of the flight tracks revealed that the surges in response to single tainted filaments were stunted because males made fewer significant changes in course angles steered, airspeeds generated, and in the tempo of counterturns executed. Accepted: 28 December 1996     

Electrical potentials from the eye and optic nerve of Strombus: effects of electrical stimulation of the optic nerve.

1. Photic stimulation of the mature eye of Strombus can evoke in the optic nerve 'on' activity in numerous small afferent fibres and repetitive 'off' bursts of afferent impulses in a smaller number of larger fibres. 2. Synchronous invasion of the eye by electrically evoked impulses in small optic nerve fibres (apparently the 'on' afferents, antidromically activated) can evoke a burst of impulses in the larger 'off' fibres which propagate away from the eye. Invasion of the eye via one branch of optic nerve can evoke an answering burst in another branch. 3. Such electrically evoked bursts are similar to light-evoked 'off' bursts with respect to their impulse composition, their ability to be inhibited by illumination of the eye, and their susceptibility to MgCl2 anaesthesia. 4. Invasion of the eye by a train of repetitive electrically evoked impulses in the absence of photic stimulation can give rise to repetitive 'off' bursts as well as concomitant oscillatory potentials in the eye which are similar to those normally evoked by cessation of a photic stimulus. 5. The electrically evoked 'off' bursts appear to be caused by an excitatory rebound following the cessation of inhibitory synaptic input from photoreceptors which can be antidromically activated by electrical stimulation of the optic nerve. 6. The experimental results suggest that the rhythmic discharge of the 'off' fibres evoked by the cessation of a photic stimulus is mediated by the abrupt decrease of inhibitory synaptic input from the receptors.     

Reaction to the perturbation of the vertical posture under different conditions of standing and in the case of additional support

The reaction of equilibrium restoration in response to the perturbation of the vertical posture of a subject standing on a stable or unstable support was studied. Perturbation was induced by a sudden forward or backward shift of the support surface. In some of the experiments, the subject was holding onto a handrail suspended on a long soft belt with a fixed upper end. The results of the study showed that the reaction to support movement depended on the direction of the perturbation. The soleus muscle was activated first upon a backward movement, and the anterior tibial muscle was activated first upon a forward movement, with a latency of about 50 ms. Within 30–70 ms, bursts of activity were also detected in the respective antagonist muscles. Sudden movement of the platform caused bursts of impulses in the arm muscles even in the absence of contact with the handrail. These impulse bursts had a longer latency (80–130 ms) and probably played an auxiliary role in the process of the restoration of balance. In the case of standing on an unstable support, the latency of leg muscle activation increased. When a subject was holding onto a handrail, the intensity of impulse bursts decreased in the leg muscles and increased in the arm muscles, while the latent periods of the bursts in the arm muscles decreased. This effect proved to be still more pronounced in the case of balance maintenance on an unstable support. Thus, the change in the response to external perturbations during maintenance of the vertical posture on an unstable support demonstrates that an additional contact of the hand changes the adjustment of the posture control system.     

The effect of exercise and restraint on pectoral muscle metabolism in pigeons

The effect of various activity regimes on metabolism of pigeon pectoralis was examined by measurement of blood lactate following exercise, total lactate dehydrogenase activity of pectoral muscle, and proportions of specific isoenzymes of pectoral muscle lactate dehydrogenase. Sprint-trained birds had the highest pectoral muscle lactate dehydrogenase activity (1409 IU · g⁻¹ wet tissue), while endurance-trained birds had the highest peak lactate levels (287 mg · dl⁻¹, extrapolated from decay curves) and fastest half-time of the lactate response (4.8 min) following exercise, but the lowest lactate dehydrogenase activity (115 IU · g⁻¹ wet tissue). Immobilization of one wing for 3 weeks following endurance training produced a marked increase in lactate dehydrogenase activity of the immobilized muscle, compared to that in the contralateral pectoralis and endurance-trained muscle. Aerobic forms of the lactate dehydrogenase enzyme (that favor conversion of lactate to pyruvate) predominated in pectoral muscle of endurance-trained birds, while cage-confined birds exhibited primarily the anaerobic isoenzymes. These results demonstrate that conversion of pectoral muscle lactate dehydrogenase isoenzymes, total lactate dehydrogenase activity, and half-time of lactate response after exercise is dependent on activity regime in pigeons. In this respect, pigeon pectoral muscle responds to training and disuse in a manner similar to that of mammalian skeletal muscle. Accepted: 10 September 1996