Thalamus

Sensory Projection

*The pulvinar is sometimes classified as nonspecific thalamic relay nuclei because of its diffuse projections to the parietal, temporal, and occipital association cortex. It is involved in behavioural orientation toward relevant stimuli. Among the thalamic relay nuclei, projections to the primary sensory and motor areas tend to be the most localized. These specific relay nuclei lie mainly in the lateral thalamus. All sensory modalities, with the exception of olfaction, have specific relays in the lateral thalamus en route to their primary cortical areas. The classifications between specific and association nuclei are not necessarily "set in stone" since some of the association nuclei also have fairly specific functional projections (e.g. pulvinar) There are specific relays that go to the cortex VPL takes our trunk and appendages somatosensory information and project to the cortex VPM takes information from head and neck to primary somatosensory cortex LGN & MGN take visual and auditory information to our visual and auditory cortices Pulvinar is a "mixed bag." Some argue that it is a specific relay nuclei while others say that it is non-specific. It receives both auditory and visual information and projects to both visual and auditory cortices

Autonomic, Limbic related nuclei

- Our autonomic/limbic related nuclei are the anterior, LD & MD - MD is a "mixed bag"

Non-specific nucleus

-Intralaminar= CM, Parafasicular and Medial VA -Pulvinar -MD The intralaminar nuclei lie within the internal medullary lamina. Like the relay nuclei, they receive inputs from numerous pathways and have reciprocal connections with the cortex. They are sometimes classified along with other "nonspecific" relay nuclei, but it should be noted that unlike relay nuclei (pulvinar and MD), their main inputs and outputs are from the basal ganglia. Intralaminar nuclei can be divided into two functional regions: The caudal intralaminar nuclei include the large centromedian nucleus and are involved mainly in basal ganglia circuitry; the rostral intralaminar nuclei also have input and output connections with the basal ganglia. In addition, the rostral intralaminar nuclei appear to have an important role in relaying inputs from the ascending reticular activating systems (ARAS) to the cortex, maintaining the alert, conscious state. These non-specific thalamic nuclei send reduced projections to widespread regions The CM and Parafascicular nuclei also receive motor information from the basal ganglion

Pulvinar

-the largest thalamic nucleus -has reciprocal connections with the association cortex of the occipital, parietal, and posterior temporal lobe -from the lateral LGB, MGB & Superior colliculus -visual, auditory, and other -lesions of dominant side may result in sensory aphasia The Pulivinar-LP complex incorporates visual pathway of considerable size It is unlikely that any single function can be attributed to the pulvinar Role in visual plasticity Lesions of the pulvinar show evidence of visual neglect and difficulty in discriminating salient stimulus from among distracting stimuli Role in attention

Burst Mode

BURST FIRING Lower noise (background) so the signal coming in is going to maximize the original EPSP Therefore, you're going to get a huge signal detection compared to background noise Link between what comes in and what goes out is much lower We have t-type Ca++ channels on our thalamic neurons that responds to the burst of firing. The intrinsic properties of this is that when the noise level is low, these Ca++ channels are inactivated. These channels must be activated in order to get burst firing. In order to do that, we have to be partially hyperpolarized. With the partial hyperpolarization, our threshold is lower. This is called our low-threshold stimulus. Once the Ca++ channel is activated, you get that burst firing. With burst, its not linear. Remember the neurons had to be partially hyperpolarized in order to get the burst firing so you are going to get a large signal with a little bit of noise. Burst is during our sleep; it helps regulate sleep-wake cycles Example: sleeping in the middle of the night and the fire alarm goes off and you wake up a bit disoriented Burst firing is though of as getting our attention Burst = low input, high output

Sensory Loss

Damage to VPL or VPM Cause numbness on the contralateral body and face Deficit may be more noticeable in the face, hand, and foot than in the trunk or proximal extremities Larger lesions may be accompanied by hemiparesis or hemiaopia which would suggest the involvement of the internal capsule, LGN or optic radiations

Networking nerve cells

GABAergic cells of the thalamic reticular nucleus (R) are innervated by collateral branches of thalamocortical relay cells (TCR) and corticothalmic fibers as they transverse The reticular nucleus receives bi-directional input The reticular nucleus also projects to domain-specific areas of the thalamus Inputs from the periphery (Aff) or intrinsic brain structures (IN) excite relay neurons (TCR) which as it transverses to the cortex excites reticular neurons These reticular neurons then project back to the same TCR neuron to form an inhibitory feedback connection to the relay cells Fibers returning to the thalamus from the cortex (to which the particular dorsal thalamus just excited) excite reticular neurons in the same sector and the reticular projection to the dorsal thalamus is part of an inhibitory feedforward connection This bi-directional circuitry allows the cortex to induce and/or maintain thalamocortical synchrony

Synpatic properties

Ionotropic receptor: The binding causes a conformational change that opens the ion channel which forms the central core of the receptor complex. Metabotropic receptor: The conformation change in the receptor ends in activation of G-protein,which intern leads to a cascade of biochemical reactions in the membrane and cytoplasm of relay cells. Neurotransmitters in the thalamus Glutamate (AMPA receptor) GABA (y-aminobutyric acid) GABA (A or B receptor) Acetylcholine (nicotine or muscarinic receptor) Noradreneline Serotonin histamine

Thalamus

Is part of the diencephalon The epithalamus is limbic related. Habenular nucleus is involved in sleep and drugs, hoarding behaviors, stress recognition, etc. The pineal gland is related to cycles of the system Sub-thalamic nucleus is part of the basal ganglia nucleus Hypothalamus is the regulator of the endocrine system For example, It regulates the pituitary gland

LGN

Magnocellular-motion and spatial analysis (layers 1 & 2) Dorsal pathway Parvocellular-detailed form and color (layers 3-6) Ventral pathway

Motor related

Our motor-related nuclei are the VL, VI and VA These nuclei get a lot of information from the basal ganglia and the cerebellum and then send projections up to the cortex

Medial Geniculate

Processes auditory input related to sound intensity (loudness) and frequency (pitch) Receives input from inferior colliculus, lateral lemniscus and primary auditory cortex

Nuclear Divisions

Right and left thalamus are connected by the interthalamic adhesion/massa intermedia Transveres the 3rd ventricle Not considered a commisure since there is a lack of decussating fibers Function is currently unknown The thalamus is divided into a medial nuclear group, lateral nuclear group, and anterior nuclear group by a Y-shaped white matter structure called the internal medullary lamina. Nuclei located within the internal medullary lamina itself are called the intralaminar nuclei. The midline thalamic nuclei are an additional thin collection of nuclei lying adjacent to the third ventricle, several of which are continuous with and functionally very similar to the intralaminar nuclei. Finally, the thalamic reticular nucleus (to be distinguished from the reticular nuclei of the brainstem) forms an extensive but thin sheet enveloping the lateral aspect of the thalamus.

Basic Function

Sensory integration and relay Motor integration and relay Awareness of nociceptive stimuli Emotional and subjective response to sensation Memory and instinctive behaviour Activation and arousal Lesions of the thalamus can produce disturbances in sensation, motor function, cognitive function, memory, emotional behavior, and levels of arousal. Damage to the thalamus can lead to permanent coma.

Synchronization

Temporal coincidence of specific and non-specific thalamic activity generates the functional states that characterize human cognition Oscillations in the 40Hz range are synchronized between a cortical area and its thalamic relay by corticothalamic projections The question remains how to synchronize specific thalamic relay with widespread cortical activation? Thalamus sits in the perfect position in order to bind the cortical processes toghether

Extreme networking

Thalamic relay neurons may receive around 5000-8000 synapses 44% derived from corticothalamic fibers Around 40% are GABAergic mostly derived from the reticular nucleus The cortex has the potential to exert disynaptic, inhibitory effect on the thalamus

Arterial supply of the thalamus

The Posterior Cerebral artery supplies the thalamus: Polar artery Paramedian artery Inferolateral (thalamogeniculate) Posterior (medial and lateral) choroidal artery

Posterior Column-medial Leminiscal pathway

The medial lemniscus axons terminate in the ventral posterior lateral nucleus (VPL) of the thalamus. The neurons of the VPL then project through the posterior limb of the internal capsule in the thalamicsomatosensory radiations to reach the primary somatosensorycortex (areas 3, 1, and 2) in the postcentral gyrus. Synaptic inputs to the primary somatosensory cortex from both the face and body occur mainly in cortical layer IV and the deep portions of layer III, with some inputs also reaching layer VI

Association related nuclei

The pulvinar and LP are the associated-related nuclei Some claim its specific and some claim its non-specific

Basic Anatomy

The thalamus is composed of several nuclei: Sensory nuclei (VPL, VPM, LGN/LGB, MGB, pulvinar) Motor-related (VI, VA, and VL) autonomic and limbic related (anterior, LD, MD) and nuclei related to association areas (pulvinar, and LP). Nonspecific thalamic nuclei (intralaminar nuclei such as CM parafascicular and medial VA) send diffuse connections to widespread regions of the cerebral cortex and to other thalamic nuclei. The reticular nucleus of the thalamus helps to regulate the excitability of the thalamic projection nuclei.

Importance of Thalamus

The thalamus is the gateway to the cerebral cortex Conveys sensory, motor, and autonomic information from the brainstem and spinal cord All sensory projections are first processed through the thalamus Except the olfaction Thalamic nuclei are reciprocally interconnected with regions of the cortex Estimated that for every 1 thalamocortical projection there are 40 corticothalamic projections

2 physioligical states

There are 2 different physiological state in the thalamus: the tonic mode and the burst mode They are two different response modes that reflect the status of these neurons Relaying this information to the cortex The tonic relay is really a real representation of the world The burst mode is going to be a distorted view of our world Tonic allows a linear translation to the brain, while the burst is kind of like a heads up In our tonic, you have a lot of background; these neurons are always firing but when a signal does come in, its linear (the input level is the same as the output level) for every EPSP that comes in, that same output that is generated, and then it goes back down to baseline In our burst, you still have your background but its not as big so you end up with these huge spikes. Its not linear. You have a low background and then all of a sudden, you get this huge firing

Trigemminal Sensory Pathway

There is an analogous pathway called the trigeminal lemniscus conveys touch sensation for the face via the ventral posterior medial nucleus of the thalamus (VPM) to the somatosensory cortex. Figure Legend: Trigeminal Sensory System Nuclei and Pathways The mesencephalic, chief, and spinal trigeminal nuclei convey different sensory modalities. Sensory information for the face then travels via VPM at the thalamus to the somatosensory cortex. 3 major components of the Trigeminal pathway: 1) Mesencephalic: proprioception from muscles of mastication and tongue and extra ocular muscles. 2) Jaw-jerk reflex 3) Chief: fine touch and dental pressure (when you clench your teeth) - There is also the Spinal trigeminal: crude touch, pain and temperature. Trigeminal pathway that goes to the VPM has a somatotopic humunculus too In people who do not have upper motor neuron regions, you can't elicit the jaw jerk reflex

THE REGULATOR:RETICULAR NUCLEUS

Thin sheet located just lateral to the rest of the thalamus and medial to the internal capsule The only thalamic nucleus that does not project to the cortex Receives inputs from other thalamic nuclei and the cortex then projects back to the thalamus Almost a pure population of GABAergic neurons Regulates the thalamic activity

Tonic

This is the truest form of relay, little processing involved TONIC FIRING Larger EPSP = larger response 4 things he wants us to remember about Tonic firing: Tonic is going to reflect linear summation (EPSP in = EPSP out) Spontaneous activity is higher in tonic This means you are going to have a larger background, which helps maintain linearity Lower signal to noise ratio It's not going to have huge spikes (helps maintain linearity) You have more noise (background) compared to the signal Background is high and the signal comes in and is right above it Minimizes non-linearity distortions This is how we get real reconstruction of the world (no distortions)

Anterolateral-Spinothalamic Tract

VPL relays somatosensory spinal inputs to the cortex, considered a specific relay nuclei. VPL receives contralateral limbs and trunk representations while VPM contains the head, face and intraoral structures. Each large body part such as finger or toe tends to be represented in VPL The spinothalamic and posterior column-medial lemniscal pathway reach separate neurons within the VPL.