Thalamus and Cerebral Cortex
which neuron type transmits intracortical processing info to other parts of the brain?
pyramidal (projection neurons)
2 major types of cortical cells
pyramidal and non-pyramidal (stellate/interneurons)
cells of layer V, where do they project to?
pyramidal, send axons mainly to layer III and V. Large pyramidal neurons in this layer project to basal ganglia, brainstem, or spinal cord
distortion of receptotopic maps
reflect behavioral importance of a certain part of the receptor sheet (somatosensory cortex - large representation of the index finger)
matrix thalamocortical neurons characteristics
small, receive comparatively weak and less well-organized ascending inputs. Project axons widely to layers II/III. Inputs to matrix neurons arise from collaterals of layer V pyramidal cells on their way to brainstem and spinal cord centers.
receptotopic organization
within sensory areas, there is a topographic representation of the receptor periphery in which adjacent areas of a receptor surface are represented by adjacent cell groups
2 types of thalamocortical neurons
1. core 2. matrix
characteristics of basic functional plan of cortex
1. motor areas in frontal, somatosensory in parietal, auditory in temporal, visual in occipital 2. topographic organization, receptotopic 3. receptotopic maps are distorted to reflect behavioral importance of a certain body part 4. map boundaries coincide with cytoarchitectonic boundaries 5. primary sensory areas receive major afferent input from a specific relay nuclei 6. each hemisphere contains multiple, separate, and more-or-less complete motor, somatosensory, auditory, or visual representations
3 functions of thalamus
1. process and relay info to cerebral cortex 2. regulate overall excitability of cerebral cortex 3. serve as integrative center for cortical function
thalamic nuclei corresponding to parietal cortex
pulvinar
core cells of thalamus project to layer ___. Layer ___ neurons project back to core cells (synapse weakly)
IV; VI
principle of localization of function
a given region of cerebral cortex is more involved with one kind of function than with others. Ex. right and left hemispheres are functionally asymmetrical in motor dominance and speech
thalamic nuclei projecting to cingulate gyrus
anterior
cytoarchitecture and Brodmann's areas
arrangement of cells bodies; areas are defined by cytoarchitecture and have very specific thalamic inputs. compositions of different areas reveal homology among mammalian brains. gyro pattern may be different, but cytoarchitectonically defined areas remain constant in relative size and location
cells in layer I
cell-sparse layer, cells are inhibitory
functional columnar organization
cells within a vertically oriented column form an elementary processing unit of interconnected neurons
parallel processing
different cortical regions perform different but related tasks at the same time
thalamic nuclei projecting to frontal lobe
medial
core thalamocortical neurons characteristics
large, receive strong, highly ordered ascending inputs and project their axons primarily to layer IV of cerebral cortex. Receive corticothalamic synapses from layer VI neurons
neocortex
largest expanse of cortical tissue, occupying approx. 90% of cortex in humans
from where do cells in layer V and VI receive input?
layer III, thalamocortical inputs
motor monitors
layer V corticothalamic inputs send motor commands and synapse with thalamic nuclei along the way, to inform them of the motor command
where do inputs to matrix cells come from?
layer V pyramidal cells, on their way to brainstem and spinal cord
cells in layer IV
mainly interneurons, receives most of cortex's afferent input from the thalamus. Project to layers II and III predominantly
type of cells in higher order thalamic nuclei
majority are matrix cells. project to association cortex (everything except primary sensory and motor). these nuclei receive inputs from and project to a number of different cortical areas, providing a means for integrating info via a cortico-thalamic-cortical pathway
Type of cells in first order thalamic nuclei
mostly core-type neurons. nuclei involved in primary sensation, transmit afferent information
areas 6 and 8
motor association cortex in frontal lobe
areas 5 and 7
parietal association cortex
receptive field of cell
part of receptor periphery which when stimulated causes a change in the electrical activity of neuron
how does thalamus regulate overall excitability?
neurons in intralaminar thalamic nuclei receive inputs from widespread regions of the brainstem reticular formation and spinal cord, then project their axons to widespread regions of cortex. these are thought to modulate the activity of large populations of cortical neurons Also...state-dependent thalamic firing modes
state-dependent thalamic firing modes
neurons in thalamus have a voltage- and time-dependent ionic conductance (T-current/calcium current) that enables them to switch between repetitive firing (relay mode) and bursting states. When thalamic neurons are hyper polarized for long period of time, T-current channels become primes, permitting the neuron to fire calcium-mediated bursts of APs. Synaptic inputs from reticular formation contribute to this switching with norepinephrine and ACh. Depolarization leads to switch to relay mode. In relay mode, cell's firing is determined, tightly coupled to sensory inputs.
areas 41 and 42
primary auditory cortex (Gyrus of Heschl)
area 4
primary motor cortex
areas 1,2,3
primary somatic sensory cortex
cells in layers II and III
projection cells, provide input to other II/III cells and to V/VI cells. Many layer III cells are corticocortical (project to other cortical regions in same hemisphere) or callosal (project to opposite hemisphere)
how does thalamus serve as integrative center for cortical function? examples
some thalamic nuclei receive inputs from one or more regions of cerebral cortex and send outputs to other cortical areas, notably association cortex in frontal and limbic regions. Ex. Medial dorsal thalamic nucleus receives inputs from the hypothalamus and amygdala. Also interconnected with prefrontal and temporal cortex. Ex. pulvinar receives visual inputs from LGN and is reciprocally connected with visual cortical areas in parietal, occipital, and temporal lobes
thalamic nuclei corresponding to somatosensory cortex
ventral posterolateral, arcuate
thalamic nuclei corresponding to motor cortex and motor association cortex
ventrolateral, ventroanterior
areas 17,18,19
visual cortex 17=primary visual or striate cortex (lingual and cuneus gyri)