The internal capsule is an area of white matter

White matter

White matter is one of the two components of the central nervous system and consists mostly of myelinated axons. White matter tissue of the freshly cut brain appears pinkish white to the naked eye because myelin is composed largely of lipid tissue veined with capillaries. Its white color is due to...

 in the brain

Brain

The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

 that separates the caudate nucleus

Caudate nucleus

The caudate nucleus is a nucleus located within the basal ganglia of the brains of many animal species. The caudate nucleus is an important part of the brain's learning and memory system.-Anatomy:...

 and the thalamus

Thalamus

The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...

 from the lenticular nucleus. The internal capsule contains both ascending and descending axons.

It consists of axon

Axon

An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

al fibres that run between the cerebral cortex

Cerebral cortex

The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

 and the pyramids of the medulla.

Components

The internal capsule is V-shaped when cut both coronally (on the same plane as the face) and horizontally/transversely (the same plane as the brim of a top hat

Top hat

A top hat, beaver hat, high hat silk hat, cylinder hat, chimney pot hat or stove pipe hat is a tall, flat-crowned, broad-brimmed hat, predominantly worn from the latter part of the 18th to the middle of the 20th century...

).

When cut horizontally:

• the bend in the V is called the genu
• the anterior limb
  Anterior limb of internal capsule
  The anterior limb of internal capsule contains:* fibers running from the thalamus to the frontal lobe;* fibers connecting the lentiform and caudate nuclei;* fibers connecting the cortex with the corpus striatum; and...
  
  or crus anterius is the part in front of the genu, between the head of the caudate nucleus
  Caudate nucleus
  The caudate nucleus is a nucleus located within the basal ganglia of the brains of many animal species. The caudate nucleus is an important part of the brain's learning and memory system.-Anatomy:...
  
   and the lenticular nucleus
• the posterior limb
  Posterior limb of internal capsule
  The posterior limb of internal capsule is the portion of the internal capsule posterior to the genu.-Anterior:The anterior two-thirds of the occipital part of the internal capsule contains fibers of the corticospinal tract, which arise in the motor area of the cerebral cortex and, passing downward...
  
  or crus posterius is the part behind the genu, between the thalamus and lenticular nucleus
• the retrolenticular portion is caudal to the lenticular nucleus and carries optic tracts including the geniculocalcarine radiations.
• the sublenticular portion is beneath the lenticular nucleus and are tracts involved in the auditory pathway from medial geniculate nucleus to the primary auditory cortex (Brodmann Area 41)

Fibers

Working anterior to posterior:

• The anterior limb of the internal capsule contains:

1) Frontopontine (corticofugal) fibers project from frontal cortex to pons

Pons

The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...

;

2) Thalamocortical fibers (part of the thalamocortical radiations) connect the medial

Medial dorsal nucleus

The medial dorsal nucleus is a large nucleus in the thalamus.It is believed to play a role in memory.-Anatomy:...

 and anterior nuclei of the thalamus to the frontal lobe

Frontal lobe

The frontal lobe is an area in the brain of humans and other mammals, located at the front of each cerebral hemisphere and positioned anterior to the parietal lobe and superior and anterior to the temporal lobes...

s (these are severed during a prefrontal lobotomy).

• The genu contains corticobulbar fibers, which run between the cortex
  Cerebral cortex
  The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
  
   and the brainstem.

• The posterior limb of the internal capsule contains corticospinal fibers, sensory fibers (including the medial lemniscus and the anterolateral system) from the body and a few corticobulbar fibers.

Other fibers within the internal capsule

• The retrolenticular part contains fibers from the optic system, coming from the lateral geniculate nucleus
  Lateral geniculate nucleus
  The lateral geniculate nucleus is the primary relay center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain....
  
   of the thalamus
  Thalamus
  The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
  
  . More posteriorly, this becomes the optic radiation
  Optic radiation
  The optic radiation is a collection of axons from relay neurons in the lateral geniculate nucleus of the thalamus carrying visual information to the visual cortex along the calcarine fissure.There is one such tract on each side of the brain.-Parts:A distinctive...
  
  . Some fibers from the medial geniculate nucleus
  Medial geniculate nucleus
  The Medial Geniculate Nucleus or Medial Geniculate Body is part of the auditory thalamus and represents the thalamic relay between the inferior colliculus and the auditory cortex...
  
   (which carry auditory information) also pass in the retrolenticular internal capsule, but most are in the sublenticular part.
• The sublenticular part contains fibers connecting with the temporal lobe. These include the auditory radiations and temporopontine fibers
  Temporopontine fibers
  In the human nervous system the temporopontine fibers, a component of the corticopontine tract, are lateral to the cerebrospinal fibers; they originate in the temporal lobe and end in the nuclei pontis....
  
  .

Blood supply

The superior parts of both the anterior and posterior limbs and the genu of the internal capsule are supplied by the lenticulostriate arteries, which are branches off of the M1 segment of the middle cerebral artery.

The inferior half of the anterior limb is supplied via the recurrent artery of Heubner

Recurrent artery of Heubner

Heubner's artery , named after the German paediatrician Otto Heubner is a branch from the anterior cerebral artery,, typically from the distal A1 segment or proximal A2 segment, or at the level of the optic chiasm...

, which is a branch of the anterior cerebral artery.

The inferior half of the posterior limb is supplied by the anterior choroidal artery

Anterior choroidal artery

The anterior choroidal artery originates from the internal carotid artery, though it also rarely arises from the middle cerebral artery.-Structures supplied:The anterior choroidal artery serves many structures in the cerebrum:...

, which is a branch of the internal carotid artery

Internal carotid artery

In human anatomy, the internal carotid arteries are two major arteries, one on each side of the head and neck. They arise from the common carotid arteries where these bifurcate into the internal and external carotid artery, and they supply the brain....

.

In summary, the blood supply of the internal capsule is

Anterior limb: lenticulostriate branches of middle cerebral artery

Middle cerebral artery

-External links:*...

 (superior half) & recurrent artery of Heubner

Recurrent artery of Heubner

Heubner's artery , named after the German paediatrician Otto Heubner is a branch from the anterior cerebral artery,, typically from the distal A1 segment or proximal A2 segment, or at the level of the optic chiasm...

 off of the anterior cerebral artery (inferior half)

Genu: lenticulostriate branches of middle cerebral artery

Middle cerebral artery

-External links:*...

Posterior limb: lenticulostriate branches of middle cerebral artery

Middle cerebral artery

-External links:*...

 (superior half) & anterior choroidal artery

Anterior choroidal artery

The anterior choroidal artery originates from the internal carotid artery, though it also rarely arises from the middle cerebral artery.-Structures supplied:The anterior choroidal artery serves many structures in the cerebrum:...

 off of the internal carotid artery

Internal carotid artery

In human anatomy, the internal carotid arteries are two major arteries, one on each side of the head and neck. They arise from the common carotid arteries where these bifurcate into the internal and external carotid artery, and they supply the brain....

 (inferior half)

As in many parts of the body, some degree of variation in the blood supply exists. For example, thalamoperforator arteries, which are branches of the basilar artery

Basilar artery

In human anatomy, the basilar artery is one of the arteries that supplies the brain with oxygen-rich blood.The two vertebral arteries and the basilar artery are sometimes together called the vertebrobasilar system, which supplies blood to the posterior part of circle of Willis and anastomoses with...

, occasionally supply the inferior half of the posterior limb.

Pathology

The lenticulostriate arteries supply a substantial amount of the internal capsule. These small vessels are particularly vulnerable to narrowing in the setting of chronic hypertension and can result in small, punctate infarction

Infarction

In medicine, infarction refers to tissue death that is caused by a local lack of oxygen due to obstruction of the tissue's blood supply. The resulting lesion is referred to as an infarct.-Causes:...

s or intraparenchymal haemorrhage due to vessel rupture.

Lesions of the genu of the internal capsule affect fibers of the corticobulbar tract

Corticobulbar tract

The corticobulbar tract is a white matter pathway connecting the cerebral cortex to the brainstem. The 'bulb' is an archaic term for the medulla oblongata; in modern clinical usage, it sometimes includes the pons as well...

.

The primary motor cortex sends its axons through the posterior limb of the internal capsule. Lesions, therefore, result in a contralateral hemiparesis or hemiplegia. While symptoms of weakness due to an isolated lesion of the posterior limb can initially be severe, recovery of motor function is sometimes possible due to spinal projections of premotor cortical regions that are contained more rostrally in the internal capsule.