Differences In Firing Patterns Research Articles

Subacromial Impingment Syndrome does not alter muscle onset activation patterns during shoulder cardinal movement at different speed and load

This study examines neuromuscular firing patterns in healthy and subjects diagnosed with SIS of the periscapular, prime-moving, and rotator cuff muscles during “clinical” cardinal plane physiological movements at different speeds and loads. EMG recordings were taken in 34 healthy and 34 subjects diagnosed with Subacromial Impingement Syndrome (SIS) of the prime movers, periscapular, and rotator cuff muscles during flexion, scaption, and abduction performed at fast, medium, and slow speeds with a loaded (3 kg) and unloaded arm. Differences in firing patterns between groups were analyzed by fitting mixed linear models with random intercepts per subject, and fixed factors for group, muscle, movement type, speed, and load. No difference in timing of activation was seen between the healthy and SIS. Onset timing of prime movers, periscapular, and rotator cuff muscles were prior to movement in all scenarios studied, with rotator cuff muscles firing last. Speed and load appear to independently vary muscle activation timing in a non-intuitive manner in both healthy and SIS. The lack of different firing neuromuscular patterns in subjects diagnosed with SIS and healthy subjects raises the need to consider individual assessment of motor patterns rather than generalized patterns.

Shoulder muscle onset timing during clinical assessment movements is the same in elite handball players as non-athletes: Implications for clinical assessment

ObjectivesThis study examines neuromuscular firing patterns in overhead athletes and non-athletes of the periscapular, prime-moving, and rotator cuff muscles during “clinical” cardinal plane physiological movements. DesignCohort prospective study. SettingEMG recordings were taken of the periscapular, prime-moving, and rotator cuff muscles during flexion, scaption, and abduction performed at fast, medium, and slow speeds with a loaded (3 kg) and unloaded arm. Participants14 Handball players and 20 non-athletes. Differences in firing patterns between groups were analyzed by fitting mixed linear models with random intercepts per subject, and fixed factors for group, muscle, movement type, speed, and load. Main outcome measuresNo difference in timing of activation was seen between the professional athletes and non-athletes. ResultsSpeed and load appear to independently vary muscle activation timing in a non-intuitive manner in both athletes and non-athletes. Onset timing of periscapular, prime movers and rotator cuff muscles are prior to movement in all scenarios studied, with rotator cuff muscles firing last. ConclusionsOnset activation patterns in overhead athletes are not different to non-athletes during cardinal plane movements.

Characterization of the axon initial segment of mice substantia nigra dopaminergic neurons.

The axon initial segment (AIS) is the site of initiation of action potentials and influences action potential waveform, firing pattern, and rate. In view of the fundamental aspects of motor function and behavior that depend on the firing of substantia nigra pars compacta (SNc) dopaminergic neurons, we identified and characterized their AIS in the mouse. Immunostaining for tyrosine hydroxylase (TH), sodium channels (Nav ) and ankyrin-G (Ank-G) was used to visualize the AIS of dopaminergic neurons. Reconstructions of sampled AIS of dopaminergic neurons revealed variable lengths (12-60 μm) and diameters (0.2-0.8 μm), and an average of 50% reduction in diameter between their widest and thinnest parts. Ultrastructural analysis revealed submembranous localization of Ank-G at nodes of Ranvier and AIS. Serial ultrathin section analysis and 3D reconstructions revealed that Ank-G colocalized with TH only at the AIS. Few cases of synaptic innervation of the AIS of dopaminergic neurons were observed. mRNA in situ hybridization of brain-specific Nav subunits revealed the expression of Nav 1.2 by most SNc neurons and a small proportion expressing Nav 1.6. The presence of sodium channels, along with the submembranous location of Ank-G is consistent with the role of AIS in action potential generation. Differences in the size of the AIS likely underlie differences in firing pattern, while the tapering diameter of AIS may define a trigger zone for action potentials. Finally, the conspicuous expression of Nav 1.2 by the majority of dopaminergic neurons may explain their high threshold for firing and their low discharge rate.

Firing Pattern Characteristics of Tonically Active Neurons in Rat Striatum: Context Dependent or Species Divergent?

Tonically active neurons (TANs)--presumably, striatal cholinergic interneurons--exert a strong influence on striatal information processing. Primate studies often describe a characteristic TAN response comprising suppressed activity followed by rebound firing that occasionally is preceded by a brief activation. By contrast, studies in behaving rats report pronounced excitation during movement. These differences in firing patterns may be due to variations in behavioral conditions or could stem from the fact that TANs in rodents use different neuronal mechanisms. If similar/different task conditions yield similar/different activity patterns, then the two species may share neuronal mechanisms; however, if similar task conditions yield different activity patterns, then the two species use different neuronal mechanisms. To evaluate these possibilities, we recorded TAN activity in the ventral and dorsolateral striatal regions in rats performing a simple instrumental task similar in concept to one used in primate studies. We demonstrate that TAN activity is substantially influenced by event context; yet, under identical task conditions, primate and rat TANs display similar activity patterns, whereas under different conditions they do not. Our results suggest that the observed differences in firing patterns likely reflect dissimilarities in task attributes rather than species-dependent neuronal mechanisms and call for re-evaluation of the excitatory response in primate research.

Lobule‐specific membrane excitability of cerebellar Purkinje cells

Cerebellar Purkinje cells (PCs) are the sole output of the cerebellar cortex and function as key to a variety of learning-related behaviours by integrating multimodal afferent inputs. Intrinsic membrane excitability of neurons determines the input-output relationship, and therefore governs the functions of neural circuits. Cerebellar vermis consists of ten lobules (lobules I-X), and each lobule receives different sensory information. However, lobule-specific differences of electrophysiological properties of PC are incompletely understood. To address this question, we performed a systematic comparison of membrane properties of PCs from different lobules (lobules III-V vs. X). Two types of firing patterns (tonic firing and complex bursting) were identified in response to depolarizing current injections in lobule III-V PCs, whereas four distinct firing patterns (tonic firing, complex bursting, initial bursting and gap firing) were observed in lobule X. A-type K(+) current and early inactivation of fast Na(+) conductance with activation of 4-aminopyridine-sensitive conductances were shown to be responsible for the formation of gap firing and initial bursting patterns, respectively, which were observed only in lobule X. In response to current injection, PCs in lobule X spiked with wider dynamic range. These differences in firing pattern and membrane properties probably contribute to signal processing of afferent inputs in lobule-specific fashion, and particularly diversity of discharge patterns in lobule X, as a part of the vestibulocerebellum, might be involved in strict coordination of a precise temporal response to a wide range of head movements.

Sleep-waking discharge profiles of dorsal raphe nucleus neurons in mice

We have recorded, for the first time, in non-anesthetized, head-restrained mice, a total of 407 single units throughout the dorsal raphe nucleus (DR), which contains serotonin (5-hydroxytryptamine, 5-HT) neurons, during the complete wake-sleep cycle. The mouse DR was found to contain a large proportion (52.0%) of waking (W)-active neurons, together with many sleep-active (24.8%) and W/paradoxical sleep (PS)-active (18.4%) neurons and a few state-unrelated neurons (4.7%). The W-active, W/PS-active, and sleep-active neurons displayed a biphasic narrow or triphasic broad action potential. Of the 212 W-active neurons, 194 were judged serotonergic (5-HT W-active neurons) because of their triphasic long-duration action potential and low rate of spontaneous discharge, while the remaining 18 were judged non-serotonergic (non-5-HT W-active neurons) because of their biphasic narrow action potential and higher rate of spontaneous discharge. The 5-HT W-active neurons were subdivided into four groups, types I, II, III, and IV, on the basis of differences in firing pattern during wake-sleep states, their waking selectivity of discharge being in the order type I>type II>type III>type IV. During the transition from sleep to waking, the vast majority of waking-specific or waking-selective type I and II neurons discharged after onset of waking, as seen with non-5-HT W-specific neurons. Triphasic DR W/PS-active neurons were characterized by a low rate of spontaneous discharge and a similar distribution to that of tyrosine hydroxylase-immunoreactive, dopaminergic neurons. Triphasic DR slow-wave sleep (SWS)-active and SWS/PS neurons were also characterized by slow firing. At the transition from sleep to waking, sleep-selective neurons with no discharge activity during waking ceased firing before onset of waking, while, at the transition from waking to sleep, they fired after onset of sleep. The present study shows a marked heterogeneity and functional topographic organization of both serotonergic and non-serotonergic mouse DR neurons and suggests that they play different roles in behavioral state control and the sleep/waking switch.

Electrophysiological Characteristics of Globus Pallidus Neurons

Extracellular recordings in primates have identified two types of neurons in the external segment of the globus pallidus (GPe): high frequency pausers (HFP) and low frequency bursters (LFB). The aim of the current study was to test whether the properties of HFP and LFB neurons recorded extracellularly in the primate GPe are linked to cellular mechanisms underlying the generation of action potential (AP) firing. Thus, we recorded from primate and rat globus pallidus neurons. Extracellular recordings in primates revealed that in addition to differences in firing patterns the APs of neurons in these two groups have different widths (APex). To quantitatively investigate this difference and to explore the heterogeneity of pallidal neurons we carried out cell-attached and whole-cell recordings from acute slices of the rat globus pallidus (GP, the rodent homolog of the primate GPe), examining both spontaneous and evoked activity. Several parameters related to the extracellular activity were extracted in order to subdivide the population of recorded GP neurons into groups. Statistical analysis showed that the GP neurons in the rodents may be differentiated along six cellular parameters into three subgroups. Combining two of these groups allowed a better separation of the population along nine parameters. Four of these parameters (Fmax, APamp, APhw, and AHPs amplitude) form a subset, suggesting that one group of neurons may generate APs at significantly higher frequencies than the other group. This may suggest that the differences between the HFP and LFB neurons in the primate are related to fundamental underlying differences in their cellular properties.

Physiological and Morphological Characterization of Local Interneurons in the Drosophila Antennal Lobe

The Drosophila antennal lobe (AL) has become an excellent model for studying early olfactory processing mechanisms. Local interneurons (LNs) connect a large number of glomeruli and are ideally positioned to increase computational capabilities of odor information processing in the AL. Although the neural circuit of the Drosophila AL has been intensively studied at both the input and the output level, the internal circuit is not yet well understood. An unambiguous characterization of LNs is essential to remedy this lack of knowledge. We used whole cell patch-clamp recordings and characterized four classes of LNs in detail using electrophysiological and morphological properties at the single neuron level. Each class of LN displayed unique characteristics in intrinsic electrophysiological properties, showing differences in firing patterns, degree of spike adaptation, and amplitude of spike afterhyperpolarization. Notably, one class of LNs had characteristic burst firing properties, whereas the others were tonically active. Morphologically, neurons from three classes innervated almost all glomeruli, while LNs from one class innervated a specific subpopulation of glomeruli. Three-dimensional reconstruction analyses revealed general characteristics of LN morphology and further differences in dendritic density and distribution within specific glomeruli between the different classes of LNs. Additionally, we found that LNs labeled by a specific enhancer trap line (GAL4-Krasavietz), which had previously been reported as cholinergic LNs, were mostly GABAergic. The current study provides a systematic characterization of olfactory LNs in Drosophila and demonstrates that a variety of inhibitory LNs, characterized by class-specific electrophysiological and morphological properties, construct the neural circuit of the AL.

Differences in firing pattern of first order and higher order thalamocortical relays in anesthetized rats - burst properties and phase relationship to cortical EEG

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The NMDA-to-AMPA ratio at synapses onto layer 2/3 pyramidal neurons is conserved across prefrontal and visual cortices.

To better understand regulation of N-methyl-d-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor complements across the cortex, and to investigate NMDA receptor (NMDAR)-based models of persistent activity, we compared NMDA/AMPA ratios in prefrontal (PFC) and visual cortex (VC) in rat. Whole cell voltage-clamp responses were recorded in brain slices from layer 2/3 pyramidal cells of the medial PFC and VC of rats aged p16-p21. Mixed miniature excitatory postsynaptic currents (mEPSCs) having AMPA receptor (AMPAR)- and NMDAR-mediated components were isolated in nominally 0 Mg2+ ACSF. Averaged mEPSCs were well-fit by double exponentials. No significant differences in the NMDA/AMPA ratio (PFC: 27 +/- 1%; VC: 28 +/- 3%), peak mEPSC amplitude (PFC: 19.1 +/- 1 pA; VC: 17.5 +/- 0.7 pA), NMDAR decay kinetics (PFC: 69 +/- 8 ms; VC: 67 +/- 6 ms), or degree of correlation between NMDAR- and AMPAR-mediated mEPSC components were found between the areas (PFC: n = 27; VC: n = 28). Recordings from older rats (p26-29) also showed no differences. EPSCs were evoked extracellularly in 2 mM Mg2+ at depolarized potentials; although the average NMDA/AMPA ratio was larger than that observed for mEPSCs, the ratio was similar in the two regions. In nominally 0 Mg2+ and in the presence of CNQX, spontaneous activation of NMDAR increased recording noise and produced a small tonic depolarization which was similar in both areas. We conclude that this basic property of excitatory transmission is conserved across PFC and VC synapses and is therefore unlikely to contribute to differences in firing patterns observed in vivo in the two regions.

Differentiation of presumed serotonergic dorsal raphe neurons in relation to behavior and wake-sleep states.

Using extracellular single unit recording, either alone or in combination with microdialysis application of drugs, we examined the characteristics of presumed serotonergic dorsal raphe neurons during wake-sleep states in the freely moving cat. Recordings were made from a total of 272 neurons in the dorsal raphe nucleus. Of these, 240 (88%) were classified as serotonergic on the basis of their typical long-duration action potential, slow discharge activity, and reduced spontaneous discharge rate during paradoxical sleep compared to during slow-wave sleep. An inhibitory response to serotonergic agonists and a slow conduction velocity were seen in all neurons of this type tested or identified by stimulation of the main ascending serotonergic pathway. These presumed serotonergic dorsal raphe neurons could be subdivided into two typical previously identified groups (types I-A and I-B) and four atypical new groups (types I-C, II-A, II-B, and II-C) according to differences in firing patterns during wake-sleep states. The typical neurons were evenly distributed in the dorsal raphe nucleus and their activity was related to the level of behavioral arousal, since they discharged regularly at a high rate during waking and at progressively slower rates during slow-wave sleep, and ceased firing either during slow-wave sleep with ponto-geniculo-occipital waves and paradoxical sleep (type I-A) or only during paradoxical sleep (type I-B). In contrast, the atypical subgroups were unevenly distributed in the dorsal raphe nucleus and exhibited firing patterns distinct from those of the typical neurons, such as sustained tonic activity during paradoxical sleep (types I-C and II-C) or showing their highest rate of tonic discharge during slow-wave sleep, with suppression of discharge during both waking and paradoxical sleep (type II-B). From these data we suggest that presumed serotonergic dorsal raphe neurons play different roles in behavioral state control and that there is functional topographic organization in the dorsal raphe nucleus.

Morphological and electrophysiological properties of principal neurons in the rat lateral amygdala in vitro.

In this study, we characterize the electrophysiological and morphological properties of spiny principal neurons in the rat lateral amygdala using whole cell recordings in acute brain slices. These neurons exhibited a range of firing properties in response to prolonged current injection. Responses varied from cells that showed full spike frequency adaptation, spiking three to five times, to those that showed no adaptation. The differences in firing patterns were largely explained by the amplitude of the afterhyperpolarization (AHP) that followed spike trains. Cells that showed full spike frequency adaptation had large amplitude slow AHPs, whereas cells that discharged tonically had slow AHPs of much smaller amplitude. During spike trains, all cells showed a similar broadening of their action potentials. Biocytin-filled neurons showed a range of pyramidal-like morphologies, differed in dendritic complexity, had spiny dendrites, and differed in the degree to which they clearly exhibited apical versus basal dendrites. Quantitative analysis revealed no association between cell morphology and firing properties. We conclude that the discharge properties of neurons in the lateral nucleus, in response to somatic current injections, are determined by the differential distribution of ionic conductances rather than through mechanisms that rely on cell morphology.

Dependence of GABAergic Synaptic Areas on the Interneuron Type and Target Size

In the neostriatum, several types of interneuron with distinct firing patterns and expression of neuroactive substances are known to exist. We found two types of neostriatal interneurons, parvalbumin-containing fast-spiking (FS) cells and somatostatin-containing low-threshold spike (LTS) cells to both be immunoreactive for GABA at their axon terminals in immersion-fixed brain slices from rat. To reveal the differences in synaptic connections between these two types of GABAergic interneurons, the postsynaptic target and their synaptic structure were compared by three-dimensional reconstructions from electron microscopic images of intracellularly stained axon terminals. FS cells made a greater proportion of synaptic contacts onto somata than LTS cells. Although terminal boutons of FS and LTS cells were similar in volume, their synaptic junctional areas differed in size distribution and relation to the dimensions of postsynaptic dendritic shafts or spines. Whereas the synaptic junctional areas of FS cells (0.024-0.435 microm(2); n = 28) sharply and linearly increased with the circumference of the postsynaptic dendrites or spines (0.939-5.146 microm), the slope for the junctional area of LTS cells (0.02-0.103 microm(2); n = 29) against circumference (0.844-4.252 microm) was less steep, and a much weaker correlation was seen. In addition to the differences in firing patterns, expressed molecules, axonal arborizations, and postsynaptic targets, this variation in dependency of the synaptic area on the target size suggests functional differentiation of GABAergic interneurons.

Neural firing in ventrolateral thalamic nucleus during conditioned vocal behavior in cats.

The ventrolateral (VL) thalamus in mammals is a site well-situated to show vocalization-related neural activity if there is general or classical motor system involvement in vocal production. It receives input from both the basal ganglia and cerebellum, and forms reciprocal connections with motor cortical areas. The current study examined the activity in cat VL thalamus neurons during instrumentally conditioned vocalization. Units in our sample showed irregular spontaneous firing which could be modulated by slowly occurring fluctuations in intensity of vocalization task performance. Two main types of behavioral events were associated with changes in neural firing rate. The first of these was the ingestion of food reward. More than half of all recordings showed phasic bursting patterns during licking; a similar number had increases in firing preparatory to this phasic activity. The second behavioral event modulating unit responses was vocalization. Approximately 60% of recordings showed activity changes time-locked to vocalization. These responses were almost always excitatory, and often involved changes in firing that preceded vocalization onset. No spatial organization of differences in firing pattern between neurons could be distinguished. Our results suggest that VL thalamus may well be involved in mediating vocal behavior, although its functional role remains an object of speculation. Results are compared with previous studies of vocalization-related activity and of VL thalamus activity.

Opposing effects of 5-hydroxytryptamine on two types of medullary inspiratory neurons with distinct firing patterns

1. Activity of inspiratory neurons was recorded extracellularly from the caudal portion of the nucleus ambiguous (0-3.5 mm rostral to the obex) in decerebrated, spontaneously breathing cats. Using a micropressure ejection method, we tested the responsiveness of the inspiratory neurons to direct applications of serotonin (5-HT) and noradrenaline (NA) in comparison with applications of glutamate and control artificial cerebrospinal fluid (ACSF) by means of a multibarreled micropipette. 2. We made detailed examinations of 52 inspiratory neurons that were excited by glutamate but did not react to control ACSF. Those inspiratory neurons were further classified into two subgroups based on the differences in firing patterns: inspiratory neurons with an augmenting firing pattern ["augmenting I units" (22/52)] and inspiratory neurons with a decrementing firing pattern ["decrementing I units" (30/52)]. 3. Although application of NA produced predominantly inhibitory effects on both the decrementing (22/30) and augmenting I units (20/22), application of 5-HT resulted in distinct or opposing effects on these two types of inspiratory neurons: the decrementing I units (25/30, 83%) were excited by 5-HT, whereas the augmenting I units (17/22, 77%) were inhibited by 5-HT. 4. The excitation of the decrementing I units with 5-HT was characterized by a long onset-latency of response and a prolonged recovery process. The increase in firing rate occurred not only during the inspiratory active phase but also during the expiratory phase.(ABSTRACT TRUNCATED AT 250 WORDS)

A Comparison of Motor Unit Behaviour in Young and Aged Subjects

We evaluated motor unit behaviour in young and aged subjects during ramped tension development and release to 10% maximum voluntary contraction requiring 10 seconds to complete. Six young and six elderly adults performed isometric abductor digiti minimi muscle contractions while motor units were recorded with implanted fine wire electrodes. Computer interactive programs yielded interspike interval (ISI) data for 16 units from the young and for 15 units from the aged subjects. Mean ISIs for young were 88.4 ms for shortening and 96.5 ms for lengthening contractions. Units from the aged had ISIs of 110.5 ms and 117.1 ms for shortening and lengthening contractions. Some evidence for between-group differences in firing pattern during ascending and descending ramps was also found.

Unilateral lesion of the nigrostriatal pathway decreases the firing rate and alters the firing pattern of globus pallidus neurons in the rat.

Activities of spontaneously firing neurons in the globus pallidus of intact rats and rats that survived unilateral lesions of the nigrostriatal pathway for 3 days, 1 week, or 6-11 weeks were compared. No significant differences in neuronal firing rate, firing pattern, and number of cells per pass were observed between chloral hydrate-anesthetized control and lesioned animals. However, in locally anesthetized animals, pallidal cells fired significantly faster than in chloral hydrate-anesthetized animals, and the lesion caused a decrease in the firing rates of pallidal cells 1 week and 6-9 weeks postlesion. In addition, significant differences in the firing pattern of pallidal cells, as determined by the ratio of the mean to median interspike intervals, were seen between locally anesthetized controls and animals surviving 3 days, 1 week, and 6-9 weeks postlesion. This altered firing pattern tended to return to normal with time. The number of cells per pass was not significantly altered by the lesion. Data from this study suggest that, in locally anesthetized animals, the removal of the tonic dopaminergic input to the basal ganglia causes pallidal cells to decrease their firing rates in a time-dependent fashion and causes reversable firing pattern changes. This suggests that tonically active dopamine neurons, probably acting through the striatopallidal pathway, regulate the firing rate and mechanisms controlling the temporal ordering of spontaneous discharges of globus pallidus neurons.

Selective activation of parasternal muscle fibers according to breathing rate

The activity of single parasternal intercostal muscle fibers of rabbits was recorded during control breathing and during heat tachypnea at near-constant tidal volume. In order to identify with high probability the type of fibers on the basis of available knowledge, differences in firing pattern were enhanced by occluding the airways during an inspiration or adding dead space. Under both conditions the percentage of fibers with relatively high values of peak firing rate (fp) and rising rate of firing frequency ( Δf Δt ) was greater during tachypnea. Fibers with relatively low values of fp and Δf Δt during control breathing showed little or no increase of these parameters during tachypnea. Fibers with low fp and Δf Δt should be slow, and others fast. Hence, greater percentage of fibers with high fp and Δf Δt during tachypnea indicates a recruitment of fast fibers. These findings support our previous hypothesis of selective activation of inspiratory muscle fibers according to the speed of movements.

Correlation between tremor, voluntary contraction, and firing pattern of motor units in Parkinson's disease.

PATIENTS WITH TREMOR OF PARKINSONISM SHOW THREE CHARACTERISTICS OF MOTOR UNIT ACTIVITY: rhythmic spontaneous resting discharge, abnormally low firing rates during voluntary contraction, and consistent differences in firing pattern between small and large motor units. Smaller units discharge once per tremor beat at weak contractions but change into bursts of two or three spikes per beat at stronger forces. Large units are later recruited and fire preferentially once per beat. The large tremor amplitudes can be partly explained by synchronization of unfused twitches of low frequency units which summate more powerfully than the partially fused contractions during physiological tremor, which is about twice as rapid. Tremor is strongly influenced by the force of voluntary contraction. It is strongest at rest or during weak muscular effort and with increasing force becomes continuously of higher frequency and smaller amplitude. Both changes are the consequence of increasing discharge rates of motoneurones at stronger contractions.

Simple spike firing patterns of cat cerebellar purkinje cells in sleep and waking

1. 1. Previous studies of neuronal activity during sleep have indicated that the pattern of firing of neurons was altered as a function of behavioral state (waking, synchronized sleep and desynchronized sleep). Although not evaluated quantitatively, this change in pattern has been assumed to be independent of changes in firing rate and has formed the basis for postulates of physiological mechanism (such as disinhibition) acting differentially in sleep. In a population of identifiable units, we have examined quantitatively the relationship between firing pattern (as measured by interspike interval histograms and by auto-correlation techniques) and firing rate to see if alterations in firing rate could explain differences in firing pattern observed during behavioral state changes. 2. 2. Simple spikes of 39 Purkinje cells in the cerebellar vermis were recorded by extracellular micro-electrodes during natural sleep and waking in unrestrained, unanesthetized cats. 3. 3. An analysis of covariance demonstrated that characteristics of the form of interspike interval histograms (variance, third moment, fourth moment, percentage of intervals at the mode) were a function of the firing rate of the unit. All units firing with the same mean interspike interval duration tended to have the same histogram form; behavioral state was not an important determinant of histogram form except insofar as the unit firing rate was altered. 4. 4. Auto-correlation analysis demonstrated a tendency toward rhythmic firing with period duration near the modal interval. A high degree of rhythmicity was associated with a high firing rate, and a low degree of rhythmicity was associated with a low firing rate, suggesting that this aspect of firing pattern, like that of histogram characteristics, was also a function of firing rate. 5. 5. These findings indicate that, for simple spikes of cerebellar Purkinje cells, changes in firing rate account for differences in firing patterns observed with behavioral state changes.