The Internal Capsule: Some Pricey Brain Real Estate

The internal capsule is one pricey piece of brain real estate! It contains all of the pathways that allow information to be transferred between the cerebral cortex and the spinal cord, brainstem, and subcortical structures (ie: thalamus, basal ganglia). It is divided into an anterior limb, posterior limb, and genu (ie: the area where the anterior and posterior limbs meet).

The anterior limb contains axons that send information between the thalamus and the cingulate gyrus and pre-frontal cortex. It also contains axons in the frontopontine pathway (ie: axons going from the frontal cortex to a portion of the brainstem known as the pons).

The genu contains the corticobulbar tract, which originate in the motor areas of the frontal lobes and extend to the cranial nerve nuclei in the brainstem. It also contains axons that connect the motor section of the thalamus (ie: VA and VL nuclei) with the motor areas of the frontal cortex.

The posterior limb contains the corticospinal tract, which are axons that come from the motor area of the frontal cortex and travel all the way to the anterior horns of the spinal cord where α-motor neurons are located. The posterior limb also contains sensory information coming from the body via the medial lemniscus and the anterolateral (aka: spinothalamic tract) systems.

Internal Capsule MRI

The blood supply to most of the internal capsule comes from the lenticulostriate arteries. These small arteries originate from the first portion of the middle cerebral artery. Two other important arteries also supply portions of the internal capsule: the anterior choroidal artery and the recurrent artery of Heubner. The anterior choroidal artery is a branch of the internal carotid. It supplies the inferior portion of the posterior limb. The recurrent artery of Heubner is a branch of the anterior cerebral artery. It supplies the inferior portions of the anterior limb and the genu.

Anatomy of the Internal Capsule
Division Major Communication Tracts Blood Supply
Anterior limb

– Tracts between the frontal lobe and pons (brainstem)

– Tracts between the thalamus and prefrontal cortex

– Tracts between the thalamus and cingulate gyrus

– Lenticulostriate arteries (branches of the middle cerebral artery)

– Recurrent artery of Heubner (branch of the anterior cerebral artery)

Genu – Tracts between the motor cortex in the frontal lobe and the cranial nerve nuclei in the brainstem (aka: corticobulbar tract)

– Lenticulostriate arteries (branches of the middle cerebral artery)

– Recurrent artery of Heubner (branch of the anterior cerebral artery)
Posterior limb

– Tracts between the motor cortex of frontal lobe and anterior horn of spinal cord (aka: corticospinal tract)

– Medial lemniscus tract (a continuation of the dorsal columns), which carries information about light touch, vibration, and pressure sensation from the body and spinal cord.

– Anterolateral (aka: spinothalamic) tract, which carries pain and temperature information

– Lenticulostriate arteries (branches of the middle cerebral artery)

– Anterior choroidal artery (branch of the internal carotid)

Importance in Disease

Thalamic Hemorrhage
Thalamic intracerebral hematoma
compressing the posterior limb
of the internal capsule

Damage to the internal capsule can be devastating neurologically because it contains so many vital tracts.

For example, a stroke of the anterior choroidal artery can lead to posterior limb damage. This can cause paralysis of the contralateral (ie: opposite) arm and leg secondary to interruption of the corticospinal tract.

Posterior limb disruption can also cause co-existent sensory deficits including an inability to feel light touch, pain, and temperature due to damage of the spinothalamic and medial lemniscal pathways.

Hypertensive hemorrhages in the thalamus or basal ganglia can compress the adjacent fibers of the internal capsule leading to similar clinical findings. The head CT to the right shows a thalamic hemorrhage secondary to severely elevated blood pressure. The patient had compression of the posterior limb of the internal capsule. As a result she was unable to move her left arm and leg, and could not feel pain or light touch on the left side of her body.


The internal capsule is a vital structure. It contains many communication pathways between the brain’s cortex, brainstem, spinal cord, and subcortical nuclei (ie: thalamus, basal ganglia). Its blood supply comes from branches of the middle cerebral artery (ie: lenticulostriates), anterior cerebral artery (ie: recurrent artery of Heubner), and the internal carotid (ie: anterior choroidal artery). Lesions in this area caused by strokes or hypertensive hemorrhages can have devastating clinical consequences.

Other Pertinent Articles…

References and Resources

  • Greenberg MS. Handbook of Neurosurgery. 9th Edition. New York: Thieme, 2006. Chapter 25.
  • Chowdhury F, Haque M, Sarkar M, et al. White fiber dissection of brain; the internal capsule: a cadaveric study. Turk Neurosurg. 2010 Jul;20(3):314-22. doi: 10.5137/1019-5149.JTN.3052-10.2.
  • Simon RP, Aminoff MJ, Greenberg DA. Clinical Neurology, Seventh Edition (LANGE Clinical Medicine). Seventh Edition. New York: McGraw Hill, 2009.
  • Nolte J. The Human Brain: An Introduction to its Functional Anatomy. Sixth Edition. Philadelphia: Mosby, 2008.
  • Bickley LS, Szilagyi PG. Bates’ Guide to Physical Examination and History Taking. Ninth Edition. New York: Lippincott Williams and Wilkins, 2007.

Cerebellum, Purkinje, Mossy, and All That Jazz!

Cellular Anatomy

The cerebellum is a large outgrowth on the backside of the brainstem that looks like a piece of cauliflower. It is the great "modulator" of movement. It compares the movement that the brain wants to do with what the body is actually doing. It then makes fine adjustments to ensure that the intended movement is smooth and fluid. It works in conjunction with the basal ganglia and motor cortex to modulate movement.

As can be expected, the cerebellum is highly complex with inputs and outputs going to various regions of the brain and spinal cord. Before we delve into the circuitry of the cerebellum, we need to first understand its cellular architecture…

Unlike most of the cerebral cortex, the cerebellar cortex has only three layers. They include – from most superficial to deep – the molecular layer, the Purkinje cell layer, and the granular layer.

The molecular layer is composed of connections between the dendrites (ie: information receiving processes) of Purkinje cells and the axons (ie: information sending processes) of granule cells. The molecular layer also contains stellate and basket cells, which help modulate the connections between Purkinje and granule neurons.

The Purkinje layer is, you guessed it, composed of Purkinje neurons. These cells send dendrites into the molecular layer where they receive information from granule cells. Purkinje cells also directly receive signals from other areas of the brain and spinal cord. Purkinje cells then send information along to the deep cerebellar nuclei, which are discrete collections of neurons within the cerebellum.

Finally, the granular layer is populated with granule and Golgi cells. They send axons into the molecular layer, which serve to pass on information to the dendrites of Purkinje cells in the molecular layer.

Confused yet???

Input Circuitry

In order for the cerebellum to modulate movement it must receive input from multiple sources. At this point, we have to introduce three new terms: climbing fibers, mossy fibers, and aminergic fibers.

Neurons in an area of the brainstem known as the "olive" (a part of the medulla oblongata) send climbing fibers into the cerebellum. Each climbing fiber forms direct and powerful excitatory connections with multiple Purkinje cells (interestingly, each Purkinje cell only receives input from one climbing fiber).

The second input, mossy fibers, originate from several different areas outside the cerebellum (see table below). These axons form connections with the granule cells. The granule cells, if you remember from our discussion above, send axons into the molecular layer where they form connections to Purkinje cell dendrites. Therefore, mossy fibers indirectly influence Purkinje cells through the "intermediary" granule cells.

Input Connections (ie: Afferent Fibers) to the Cerebellum

Source of Input Type of Fiber Target of Input
Olivary nuclei (brainstem) Climbing fibers Contralateral cerebellum
Pontine nuclei (brainstem) Mossy fibers Contralateral cerebellum
Reticular nuclei (brainstem) Mossy fibers Ipsilateral cerebellum
Ventral spinocerebellar tract Mossy fibers Ipsilateral cerebellum
Dorsal spinocerebellar tract Mossy fibers Ipsilateral cerebellum
Vestibular nuclei Mossy fibers Ipsilateral cerebellum
Locus ceruleus Norepinephrine Bilateral projections
Raphe nucleus Serotonin Bilateral projections

Aminergic fibers originate from the locus ceruleus in the pons, and the raphe nuclei of the midbrain, pons, and medulla. The locus ceruleus fibers "spit" norepinephrine and the raphe nuclei "spit" serotonin onto multiple areas within the cerebellum.

All of these inputs (as well as basket, stellate and Golgi cells, which are intrinsic to the cerebellum itself) are trying to influence the output of the Purkinje neurons. The Purkinje cells ultimately synthesize and pass along all of this competing information via an inhibitory message to the deep cerebellar nuclei.

And that folks brings us to our next section: the output circuitry…

Output from the cerebellum passes exclusively from the deep cerebellar nuclei. The deep nuclei are four discrete collections of neurons, which are given specific (and funky) names; they include the fastigial, globose, emboliform, and dentate nuclei.

Remember that the Purkinje cells inhibit the output of the deep cerebellar nuclei. Therefore, the more active the incoming messages (via mossy and climbing fibers) –> the more active the Purkinje cells –> the less active the output of the deep cerebellar nuclei.

The output of the deep nuclei goes to four major structures outside the cerebellum: the red nucleus, the vestibular nucleus, the reticular formation, and the thalamus. From these structures the information is passed along to the cerebral cortex and/or the spinal cord for additional processing.

Output Connections (ie: Efferent Fibers) from the Cerebellar Nuclei

Source of Output Target of Output Function
Globose nucleus Contralateral red nucleus
Contralateral thalamus
Influences tone of flexor
Emboliform nucleus Contralateral red nucleus
Contralateral thalamus
Influences tone of flexor
Dentate nucleus Contralateral thalamus Influences motor cortex
and coordination
Fastigial nucleus Bilateral vestibular nucleus
Bilateral reticular formation
Influences motor neurons in
spinal cord and helps
control tone of extensor

Ultimately, the output of the cerebellum influences not only coordination, but also the tone of flexor and extensor muscles. This allows movement to be smooth and coordinated (unless of course, you are me on the dance floor… in that case all bets are off!).


The cerebellum is an extremely complex part of the brain. It receives information about an intended movement from the cerebral cortex and compares that to sensory information coming back from the spinal cord. If the intended movement doesn’t match the actual movement the output of the cerebellum attempts to restore balance.

References and Resources

  • Ikai Y, Takada M, Shinonaga N, et al. Dopaminergic and non-dopaminergic neurons in the ventral tegmental area of the rat project, respectively, to the cerebellar cortex and deep cerebellar nuclei. Neuroscience, V51:3, p 719-28.
  • Asanuma C, Thach WT, Jones EG. Brainstem and spinal projections of the deep cerebellar nuclei in the monkey, with observations on the brainstem projections of the dorsal column nuclei. Brain Research Reviews. V5:3, May 1983. pp 299-322.
  • Huang CC, Sugino K, Shima Y, et al. Convergence of pontine and proprioceptive streams onto multimodal cerebellar granule cells. Elife. 2013;2:e00400. Epub 2013 Feb 26.
  • Baehr M, Frotscher M. Duus’ Topical Diagnosis in Neurology: Anatomy, Physiology, Signs, Symptoms. Fourth Edition. Stuttgart: Thieme, 2005.
  • Purves D, Augustine GJ, Fitzpatrick D, et al. Neuroscience. Fourth Edition. Sinauer Associates, Inc., 2007.