Diencephalon
From Medical-Wiki
The diencephalon contains several structures, each with the term "thalamus." Most of these structures derive from the developmental vesicle called the diencephalon. The contents of the diencephalon include the dorsal thalamus (commonly called the thalamus), the subthalamus, epithalamus and the hypothalamus. The pineal gland is also part of the diencephalon
Contents |
Thalamus
The thalamus is a walnut-sized structure buried at the center of the cerebrum. Nearly all sensory information that enters the cerebral cortex goes through the thalamus. The major exception to this rule is olfactory information, which goes from the olfactory epithelium to the olfactory bulb and directly to cortex (piriform cortex is a main olfactory target).
The role of the thalamus has been unclear. It relays information from the periphery to cortex, but it does not seem to modify the information. Since there is no apparent reason to have a relay nucleus, the thalamus had been ignored for decades. In fact, the thalamus seems to play a major role in regulating information flow - suppressing sensory input to cortex during sleep, and allowing input during wakefulness.
The thalamus is composed of many nuclei that come in three types.
Primary relay nuclear groups in the dorsal thalamus
Sensory information enters the cerebrum through the primary relay nuclei of the dorsal thalamus. For example, retinal ganglion cells synapse in the dLGN (dorsal lateral geniculate nucleus). Similarly, inputs from the somatosensory system synapse in VPM and VPL (inputs from the head synapse in the ventral posteriomedial nucleus, while inputs from the body synapse in the ventral posteriolateral nucleus).
The primary relay nuclei of the dorsal thalamus include:
Anterior nucelus - receives input from the mammillothalamic tract and hippocampus
VA - Ventral Anterior nucleus, receives input primarily from basal ganglia
VL - Ventral Lateral nucleus, receives inputs from cerebellum
LD - Lateral Dorsal nucleus, receives inpupt from the hippocampus
VPL - Ventral Posteriolateral nucleus, relay nucleus for somatosensory information from the body
VPM - Ventral Posteriomedial nucleus, relay nucleus for somatosensory information from the head
LGN - Lateral Geniculate nucleus, relay nucleus for visual information
MGNv - ventral Medial Geniculate nucleus, relay nucleus for auditory information
Higher order relay nuclear groups in the dorsal thalamus
The thalamus is not just a relay for primary inputs, it also relays information between cortical areas.
Inputs from a particular system project to specific thalamic association nuclei. For example, visual cortical areas project to the pulvinar nucleus. Recordings in the pulvinar have found individual neurons with receptive fields as complex as any visual cortical area. So while neurons in the LGN do not have substantial orientation selectivity, there are sharply tuned neurons in the pulvinar.
Outputs from the these higher order relay nuclei go to higher cortical areas, presumably relaying inputs from one cortical area to the next higher cortical area. For example, primary visual cortex projects to neurons in the pulvinar, and these pulvinar neurons probably project to secondary visual areas.
The best defined higher order nuclear groups include:
DM - Dorsomedial nucleus, which receives input from the prefontal cortex, the olfactory cortex (eg. piriform cortex), and limbic structures.
LP - Lateral Posterior nucleus, which receives input from the parietal lobe
PO - Posterior nucleus, relays information between somatosensory cortical areas.
Pulvinar - receives input from occipital, parietal and temporal lobes, areas that process visual information.
MGNd - dorsal Medial Geniculate nucleus, relays information between auditory cortical areas.
Ventral thalamus - the thalamic reticular nucleus
The TRN sits like a shield covering the ventral surface the thalamus and projects locally to the other nuclei of the thalamus.
The TRN receives input from both the cortex and brainstem that are thought to control states of consciousness. Because the TRN has exclusively inhibitory outputs (GABA) onto the dorsal thalamic nuclei, it is thought to "gate" information through the thalamus.
When inactive the TRN has no effect on the relay of information through the thalamus - sensory inputs come in from the periphery and are faithfully transmitted to cortex.
When active, however, the TRN inhibits thalamic relay nuclei and sensory information does not get relayed to cortex. This is thought to account for our relative insensitivity to sounds and light when we are asleep. Brainstem and cortical activity during sleep specifically activates the TRN, shutting down relay of information to the cortex.
Of course, loud sounds will startle us out of sleep so there must be a mechanism to let strong stimuli get through to cortex and "wake us up." This is thought to happen through the inactivation and de-inactivation of a specific ion channel in thalamic relay cells. When the TRN is silent, thalamic relay cells are regular activated (depolarized) by input. This near-constant depolarization inactivates a T-type calcium channel. When the TRN is active, the relay cells are more hyperpolarized, so the T-type calcium channel is de-inactivated. When a big enough depolarization comes into a relay cell (that is, when a big stimulus like a loud sound comes in) the relay cell depolarizes and activates the T-type calcium channels. Once the T-type channels activate, they cause the relay cell to fire a large burst of action potentials - sending a powerful signal on to sensory cortex.
So the activity level of the TRN determines whether the thalamus is in "tonic mode" in which sensory stimuli are normally transmitted to the cortex ("wakefulness") or in "burst mode" in which sensory stimuli don't get to the cortex ("sleep") unless the stimulus is very strong and a burst of action potentials "wakes up" the cortex.
Connections
Sensory input comes to relay nuclei through a variety of pathways. For example, visual inputs to the LGN arrive via the optic tract. Inputs to the MGNd arrive through the brachium of the inferior colliculus. Inputs to VPM and VPL arrive from the medial lemniscus. The output of relay nuclei project to cerebral cortex. These glutametergic (excitatory) outputs project through the TRN, and give off collaterals that synapse within the TRN.
The bulk of the axons reach cortex through the internal capsule. The internal capsule flows into the corona radiata, the posterior portion of which projects to the visual cortex and is called the optic radiations.
Connections from cortex back to the thalamus (either to the TRN or directly to the relay nuclei) also travel through the internal capsule.
Blood supply to the thalamus
Most of the thalamus is supplied by branches of the posterior cerebral artery.
The LGN is supplied by at least 2 arteries: "The lateral geniculate body has a dual blood supply from the anterior choroidal artery (branch from internal carotid artery) and from the lateral choroidal artery (branch from the posterior cerebral artery)." (Journal of Neurology, Neurosurgery and Psychiatry)
Hypothalamus
The hypothalamus is dealt with in detail in its own section, so we only briefly review its anatomy and location here for completeness. Diagram of the anatomy of the thalamus.
The hypothalamus sits below (ventral) to the thalamus. It is responsible for many homeostatic functions, including, among others, temperature control, satiety, fluid balance, and circadian rhythms.
Given its diverse homeostatic functions, it is understandable that the hypothalamus receives inputs from a wide diversity of sites. Most of its cortical inputs come from the limbic system. Equally important, it detects blood-borne hormonal and chemical signals. Its major outputs include projections to the neural portion of the pituitary gland, reciprocal connections to the limbic system, and connections with brainstem and spinal cord nuclei.
Blood supply to the hypothalamus comes from perforating branches directly off the Circle of Willis.
Subthalamus
Most of the subthalamus is just a rostral extension of the midbrain, with parts of the substantia nigra and the red nucleus, and the midbrain reticular formation, which is called the zona incerta in the subthalamus. In addition, it contains one nucleus, conveniently called the subthalamic nucleus. The subthalamic nucleus plays a role in motor control and is interconnected with the basal ganglia.
Epithalamus
The epithalamus consists of the pineal gland and habenular nuclei.
Pineal gland
The pineal gland controls endocrine functions that depend on the length of the day. In lower vertebrates the pineal gland contains photoreceptors and directly measures the length of a day by the amount and duration of light it receives. In mammals (and birds) there are no photoreceptors in the pineal gland, but it does receive input from the visual system.
In the dark the pineal gland secretes melatonin. As days get longer in the spring melatonin secretion decreases, which, in many mammals, induces fertility in both males and females. Recent evidence also suggests that melatonin levels affect immune responses.
Habenula
The nuclei of the habenula receive input from the stria meduallaris of the thalamus and project out through the habenulointerpeduncular tract. This is a rarely discussed pathway that seems to connect the limbic system and the reticular formation of the brainstem. This connection suggests a role in mediating the influence of the limbic system on arousal states.
Structures to identify on sections
To have a good handle on the thalamus, you should be able to identify all the structures shown in Nolte pp. 393-396.
To have a good handle on the anatomy of the hypothalamus, be able to identify the structures shown in Nolte pp. 562-563.
References
The most authoritative online reference of thalamus is written by S. Murray Sherman, of Neurobiology at the University of Chicago.
J. Nolte (2002) The Human Brain: An introduction to its functional anatomy. 5th edition. Mosby, Inc. St. Louis. Ch. 16.
C Luco, A Hoppe, M Schweitzer, X Vicuna and A Fantin (1992) Visual field defects in vascular lesions of the lateral geniculate body. Journal of Neurology, Neurosurgery, and Psychiatry. 55:12-15.
