Dale G. Ferrington

Responses of spinothalamic tract cells in the superficial dorsal horn of the primate lumbar spinal cord.

1. The responses of thirty‐five spinothalamic tract (s.t.t.) cells in or near lamina I of the dorsal horn were examined in chloralose‐ and barbiturate‐anaesthetized monkeys (Macaca fascicularis). Many of the cells could be classified on the basis of receptive field properties as either wide dynamic range (w.d.r.) cells or as high‐threshold (h.t.) cells. 2. Thalamic stimulation sites for antidromic activation of the s.t.t. cells were in or around the ventral posterior lateral nucleus. Axons of the s.t.t. cells had a mean conduction velocity of 17 m/s (33 and 14 m/s for w.d.r. and h.t. cells, respectively). Mean minimum afferent conduction velocity averaged 37 m/s (52 and 23 m/s for w.d.r. and h.t. cells, respectively). Background activity was low (mean of 2.3 impulses/s). 3. An alternative classification of the cells was based on a kappa means cluster analysis of the responses to a series of mechanical stimuli. The response profiles for a given cell were normalized, and those of the s.t.t. cells in or near lamina I were analysed along with the responses of a population of s.t.t. cells, largely in laminae IV‐VI, that had been described previously. S.t.t. cells in or near lamina I were distributed amongst three of the four groups of cells determined by the cluster analysis (types 2‐4). 4. Vibratory stimuli excited most of the w.d.r. but none of the h.t. cells tested. Best frequencies were 5‐10 Hz (at 100 and 500 microns indentations). 5. Most w.d.r. but few h.t. cells responded to cutaneous cooling. All of the cells responded to noxious heating, but w.d.r. cells had steeper stimulus‐response curves. 6. After a series of noxious heat stimuli, the thresholds for noxious heat were lowered and responses to lower‐intensity noxious heat stimuli were enhanced (sensitization). However, responses to more intense stimuli were reduced (inactivation). Similar changes were seen in the responses to graded mechanical stimuli. 7. It is concluded that s.t.t. cells in or near lamina I can signal noxious cutaneous stimuli but have poor coding abilities for innocuous mechanical stimuli. Some of these cells respond to innocuous thermal stimuli, but their role in thermoreception is unclear. The small receptive fields suggest that these cells could contribute to stimulus localization.

Human tactile detection thresholds: modification by inputs from specific tactile receptor classes.

1. Human detection thresholds for a vibratory stimulus applied to the volar surface of the index finger were examined under conditions where afferents from specific tactile receptor classes were simultaneously activated from the thenar eminence. The experiments were designed to test whether stimuli which have been shown previously to induce afferent inhibition of ‘tactile’ neurones in the cuneate nucleus of the cat could modify human subjective performance in a tactile detection task. Conditioning stimuli to the thenar eminence were usually of three forms; steady indentation to engage slowly adapting tactile receptors; 300 Hz vibration to engage Pacinian corpuscles; and 30 Hz vibration to engage the intradermal, rapidly adapting tactile receptors which are thought to be Meissners corpuscles.

Functional capacities of tactile afferent fibres in neonatal kittens

1. Responses were recorded from individual tactile afferent fibres isolated by microdissection from the median nerve of pentobarbitone‐anaesthetized neonatal kittens (1‐5 days post‐natal age). Experiments were also conducted on adult cats to permit precise comparisons between neonatal and adult fibres.

Actions of single sensory fibres on cat dorsal column nuclei neurones: vibratory signalling in a one-to-one linkage.

1. The synaptic linkage between single, identified sensory fibres associated with Pacinian corpuscle (P.c.) receptors and central neurones of the dorsal column nuclei was examined in decerebrate or anaesthetized cats. Paired recordings were made from individual neurones in the gracile division of the dorsal column nuclei and from the hind‐limb interosseous nerve in which it is possible to identify and monitor the activity of each P.c. fibre activated when recording from the intact nerve with a platinum hook electrode. Individual P.c. fibres were activated by vibration delivered with an 0.2 mm diameter probe to the interosseous P.c. receptors. 2. Thirty‐five P.c. fibre‐gracile neurone pairs were isolated in which activity in the single, identified P.c. fibre evoked suprathreshold responses (mean latency +/‐ S.D., 10.3 +/‐ 1.5 ms) in the gracile neurone. A single impulse arriving over one P.c. fibre could generate pairs or triplets of output spikes from several target neurones thus revealing a potent synaptic organization within the dorsal column nuclei for the transmission and amplification of weak sensory signals. 3. The potency of the linkage for some pairs resulted in post‐synaptic response levels of up to 400 impulses s‐1 when a single input fibre was discharging one impulse on each vibration cycle at 200‐400 Hz. 4. Gracile neurones driven by single P.c. fibres had phase‐locked responses to vibration at frequencies of up to 400‐500 Hz. However, the responses displayed much greater phase dispersion than those of P.c. fibres, indicating that a major component of phase dispersion in the vibration‐induced responses of dorsal column nuclei neurones is attributable to the properties of the synaptic linkage between an individual fibre and the target neurone. 5. The potent actions of single, identified P.c. fibres on their target neurones are consistent with the hypothesis that phase‐locked responses in dorsal column nuclei neurones to vibration at 100‐400 Hz may reflect the functional domination of the target neurones output by one or a few of its converging fibres.

Integrative processing of vibratory information in cat dorsal column nuclei neurones driven by identified sensory fibres.

1. In decerebrate or anaesthetized cats, the vibration‐induced responses of dorsal column nuclei neurones were examined, first, when their input came from simultaneously recorded pairs or other combinations of identified Pacinian corpuscle (P.c.) afferent fibres of the interosseous nerve, and secondly, when different convergent sets of P.c. fibres were engaged by footpad vibration. 2. Suprathreshold actions were observed on individual dorsal column nuclei neurones from two or more identified P.c. fibres. Recruitment of these convergent fibres usually led to summation in the dorsal column nuclei neurone as reflected in higher response levels compared with those evoked by single‐fibre inputs. 3. When the input was increased from one to two or more identified P.c. fibres the dorsal column nuclei neurones could retain a single, dominant phase of response to high‐frequency (greater than 100 Hz) vibration even though these fibres, in isolation, evoked responses in the target neurone at substantially different latencies. However, on average, phase locking was significantly tighter in response to single‐fibre input than to multiple P.c.‐fibre input. 4. Dorsal column nuclei neurones were also able to retain phase‐locked responses to high‐frequency vibration when phase differences between different convergent inputs were systematically introduced to alter the degree of synchrony in the activity arriving over convergent, identified P.c. fibres. 5. When the input to dorsal column nuclei neurones came from the skin it was found that with the recruitment of two converging sets of P.c. fibres the dorsal column nuclei neurones were able to retain phase‐locked responses to high‐frequency vibration even when phase shifts were introduced between the two sets of P.c. inputs. 6. In conclusion, the observed integrative processing by dorsal column nuclei neurones of vibration‐induced inputs arriving over identified, convergent P.c. fibres, or sets of P.c. fibres, is consistent with our hypothesis that the retention of phase‐locked responses to vibration at frequencies greater than or equal to 100 Hz may reflect the functional domination of the target neurone by just one or a few of its convergent input fibres.

High gain transmission of single impulses through dorsal column nuclei of the cat

Paired recordings were made in the cat from neurones of dorsal column nuclei and from intact pacinian sensory fibres of the hindlimb interosseous nerve. Direct evidence is presented for central neurones being driven by single impulses arriving over just one sensory nerve fibre. Transmission through this sensory relay appears to be optimized for the detection of minimal sensory inputs. Two mechanisms operate for the amplification of such inputs. First, individual sensory fibres can exert divergent, suprathreshold actions on multiple target neurones, and second, a single impulse coming over one input fibre can induce pairs or bursts of output spikes from its target neurones.

Functional Properties of Slowly Adapting Mechanoreceptors in Cat Footpad Skin

The functional properties of slowly adapting (SA) afferent fibers innervating cat footpad skin were examined. Measurements were taken of receptive field area; spontaneous activity (less than 1 impulse/sec); the slope of the stimulus-response curve for steady indentations up to 2 mm in amplitude; variability of the interimpulse intervals, as measured by the coefficient of variation of time interval histograms; decay of the response to steady indentation; and sensitivity to sinusoidal vibration (most sensitive at 5-10 Hz). Where comparable tests were performed on glabrous and hairy skin SA fibers, the functional properties of those in glabrous skin more closely resembled SAI fibers than SAII fibers. Additional results from glabrous skin SA fibers suggest that it is distortion of the nerve endings rather than steady indentation or compression that leads to a brisk response. On the measures described above, there appeared to be only one functional class of SA fiber innervating the cat footpad skin.

Somatotopic Organization and Response Characteristics of Dorsal Horn Neurons in the Cervical Spinal Cord of the Cat

Somatotopic organization was examined for 203 dorsal horn cells in spinal segments C6 and C7 of chloralose-anesthetized cats. The ventral paw and toe area were represented medial to a smaller area with input from the dorsal paw. Representation of the ventromedial forelimb was rostral to that of the paw, while the shoulder and dorsolateral limb were represented caudal to it. In 13 out of 22 electrode tracks in which three or more cells were found, the location of receptive fields progressively changed for successively recorded cells. Receptive fields on the paw were closer together and overlapped more than those on the proximal limb. Receptive fields that included glabrous skin were found for only 7 of 203 cells; all were located in the medial third of the C7 dorsal horn. It appears that glabrous skin is underrepresented in the dorsal horn; this may be compensated for a higher levels by input from the lemniscal system. The response characteristics of 172 dorsal horn neurons were examined. Of these units, 135 (78%) had cutaneous receptive fields. An additional 37 cells (22%) responded to manipulation of muscle or tendon and were classified as deep (D) cells. The cells with cutaneous receptive fields were classified as low-threshold (LT) cells (38%), high-threshold (HT) cells (20%), and wide-dynamic-range (WDR) cells (20%). Alternatively, using cluster analysis, 57 cells with cutaneous receptive fields were classified as one of five mechanical types. Type 1 cells responded primarily to low-threshold input, while the other four types fired in characteristic patterns in response to a combination of innocuous and noxious stimuli. LT cells were located more superficially in the spinal cord than the other classes; their average depth below the cord surface was 1.9 mm. WDR cells (mean = 2.1 mm) were located below the LT cells and above the HT and D cells (mean = 2.6 mm).

Temporal patterning in the responses of gracile and cuneate neurones in the cat to cutaneous vibration.

1. Recordings were made in decerebrate cats from gracile and cuneate neurones responding to vibration‐induced inputs from Pacinian corpuscle (P.c.) receptors of the hind‐limb and forelimb footpads. The two groups of neurones were compared, in particular for their capacities for responding to cutaneous vibration with phase‐locked impulse patterns. 2. In both nuclei the P.c. neurones were most sensitive to vibration in the range 80 to greater than 600 Hz. Stimulus‐response relations were similar for the two groups, as were measures derived from these relations such as response levels, absolute thresholds and the dynamic range (defined as the vibration amplitude range over which responses were graded). 3. At frequencies up to 300‐400 Hz, responses for some neurones in both nuclei remained well phase locked to the vibration; however, quantitative analysis using a factorial analysis of variance indicated that the phase locking was poorer in gracile than cuneate neurones. 4. In both nuclei there was marked variability from neurone to neurone in measures of phase locking which may reflect variations in the extent of convergence of P.c. fibres upon different target neurones. For neurones in either nucleus that had comparatively tight phase locking of responses to vibration it is proposed that their output is functionally dominated by one or a few of their convergent P.c. input fibres.

A simple, accurate method for reproducting details of histological sections using a microfiche printer

A rapid, inexpensive method is described for achieving accurate reproduction of histological sections. The method uses a microfiche reader-printer (for library use) which produces A4 size prints. Interchangeable lenses on the microfiche printer permit the magnification of the section to be varied over the range 6.6-72 times. Where large numbers of sections are involved the speed and low cost of the method offers considerable advantages over traditional hand tracing or photomicrography.