Identification of two populations of corticothalamic neurons in cat primary somatosensory cortex

Abstract

Extracellular and intracellular recordings of corticothalamic (CT) cells were performed in the primary somatosensory cortex of the cat. CT neurons were antidromically activated by electrically stimulating the ventroposterior lateral (VPL) nucleus of the thalamus and were classified into two types according to their physiological properties. Type 1 had no spontaneous activity and no identifiable somatic receptive field. Type 2 fired action potentials spontaneously and responded to mechanical stimulation of the skin or underlying tissues. Axonal conduction velocities were slower for type 1 cells and their cell bodies were located slightly deeper in the cortex than those of type 2 cells. Both types of CT neurons exhibited inhibitory postsynaptic potentials in response to VPL stimulation but an early synaptic excitation and rebound discharge was observed almost exclusively in type 2 cells. These results suggest that only type 2 CT cells can modify the activity of thalamic neurons through a corticothalamic feedback loop.

Axonal conduction properties of antidromically identified neurons in rat barrel cortex

“…Slower conducting axons tend to display greater activitydependent increases in conduction velocity, perhaps rendering slower arriving signals more efficacious by means of temporal summation (see Waxman and Swadlow, 1977). Together, the findings suggest that, as proposed by others (Landry and Dykes, 1985), corticothalamic systems may exert a range of effects.…”

“…Thus, without the benefit of antidromic activation, many extracellularly studied efferent neurons would be undetectable using standard electrophysiologic techniques. Similarly, Landry and Dykes (1985) reported that 63% of antidromically activated CT neurons in SI of anesthetized cats were unresponsive to tactile stimulation, and Swadlow reported, in awake, restrained rabbits, that 45 and 49% of CT neurons in the vibrissa and forelimb cortex, respectively, were not peripherally driven (Swadlow, 1989(Swadlow, , 1990. In a more recent study, 89% of CT neurons were found to be unresponsive (Swadlow and Hicks, 1996).…”

A Corticothalamic Switch: Controlling the Thalamus with Dynamic Synapses

“…VPm cells were injected with positive current to produce sustained discharge (~12 Hz) comparable to tonic spiking during awake states (Hirsch et al, 1983; Steriade et al, 1993), and then the effects of optical CT stimuli on that spiking were measured. L6 neurons, including CT cells, in vivo are often reported to have very low spiking rates (Kwegyir-Afful and Simons, 2009; Landry and Dykes, 1985; Lee et al, 2008; O’Connor et al, 2010; Swadlow, 1989), therefore we elected to stimulate the CT pathway at 0.1 Hz initially. Such CT stimulation caused robust biphasic modulation of VPm spiking (Figure 1D).…”

“…While CT activity is reported to be sparse under many conditions (Kwegyir-Afful and Simons, 2009; Landry and Dykes, 1985; Lee et al, 2008; Swadlow, 1989), studies have shown that L6 neurons, including CT cells, can fire for sustained periods and at moderately high rates (~10 Hz or more) when alert animals are presented with appropriate sensory stimuli or during spontaneous gamma-band oscillations (Bereshpolova et al, 2013, Soc. Neurosci., abstract, 259.03; Briggs and Usrey, 2009a, b; O’Connor et al, 2010; Swadlow and Weyand, 1987; Zhou et al, 2010).…”

Intrinsic properties of and thalamocortical inputs onto identified corticothalamic-VPM neurons

“…These neurons are unlikely to project to the claustrum, another target of layer VI, as cortico-claustal neurons have apical dendrites that typically reach the pial surface (Katz 1987), a morphological feature not observed in our data set. Previously, it has been shown that multiple classes of CT cells can be identified physiologically based on axonal conduction velocities (Harvey 1980;Tsumoto and Suda 1980;Weyand 1981, 1987;Kwegyir-Afful and Simons 2009) and these have been correlated with distinct morphological phenotypes (Landry and Dykes 1985). Furthermore, in the Tree Shrew visual cortex there are distinct CT systems that All traces from the same cell, +1.0 mA current pulses of 5 ms duration were utilized which evoked an initial EPSP 41 mV.…”