Corpus striatum, thalamus and amygdala

Overview of the Internal Anatomy of the Cerebral Hemispheres

• Topics Covered:

  • Internal anatomy of the cerebral hemispheres

  • 3D arrangements of the hemispheric grey and white matter

  • Relationships of grey and white matter to the ventricular system

  • 3D relationships between:

  • Corpus striatum (Basal Ganglia or BG)

  • Thalamus

  • Internal capsule

  • Ventricles

  • MRI imaging of the BG: coronal, axial, and sagittal views

  • Ventral striatum

Ventricular System

• Lateral Ventricle:

  • Filled with cerebrospinal fluid (CSF)

  • Consists of different parts known as horns:

  • Frontal Horns: Occupy the frontal lobes

  • Occipital Horn: Extends backwards resembling thin fingers into the occipital region

  • Temporal Horn: Passes downwards and forwards into the temporal lobe

  • Body: The main part of the lateral ventricle, often referred to as the middle portion

  • Trigone or Atrium: Triangular region at the posterior aspect where three horns meet

  • 3rd Ventricle:

  • Occupies the midline region of the forebrain (diencephalon)

  • Name derived from Greek term meaning "in between brain"

  • Contains thalamus and hypothalamus

  • Interventricular Foramen of Monro:

  • Allows CSF to drain from the lateral ventricles to the 3rd ventricle

  • Orientation: Lateral ventricles are tilted like the axis of a planet in contrast to aligned sagittal plane

Corpus Striatum (Basal Ganglia)

• Structure:

  • Caudate Nucleus: C-shaped structure with:

  • Head, body, and tail

  • Head and body are located along the sidewall of the lateral ventricle, while the tail is situated in the roof of the temporal horn

  • Lentiform Nucleus:

  • Lens-shaped structure divided into:

    • Putamen: Shell-like outer portion

    • Globus Pallidus: Medial, rounded part, paler due to myelinated axons

  • The thalamus is closer to the midline and lies posterior to the corpus striatum

Internal Capsule

• Definition:

  • A V-shaped white matter structure consisting of projection fibers with myelinated axons that connect the cerebral cortex with the brainstem (BS) and vice versa

• Structure and Function:

  • Anterior Portion: Anterior limb, covers anterior face of lentiform nucleus

  • Posterior Portion: Posterior limb, covers posterior face of lentiform nucleus

  • Genu: The knee bend at the center of the internal capsule

  • Retrolentiform Part: Sweeps behind the lentiform nucleus

• Coronal Section Overview:

  • Anterior Limb: Seen in coronal sections where thalamus is visible

  • Posterior Limb: Visible in coronal sections without a visible thalamus

  • Representation:

  • 5 white lines in the internal capsule resembling an open book

  • Anterior and posterior limbs situated between caudate and lentiform nuclei

Coronal Section Examination

• Anterior Limb Configuration:

  • Close to midline, next to the frontal horn of the lateral ventricle and the head of the caudate nucleus

  • Cone-shaped lentiform nucleus appears laterally with its base oriented laterally and apex towards the midline

• Posterior Limb Configuration:

  • Body of lateral ventricle seen above thalamus, in contact with the body of the caudate nucleus

  • Appearance similar to a snowman:

  • Large body (thalamus) and small head (caudate nucleus)

  • The lentiform nucleus is situated laterally with the cone structure positioned outside

Input and Output Connections of the Basal Ganglia

• Definition of Striatum:

  • Comprised of caudate nucleus and putamen, referred to as input regions of the basal ganglia

  • Characterized by a darker color due to the presence of myelinated axons from cortical regions

• Output Region (Pallidum):

  • Closer to midline, projects to the thalamus

• Function of Connections:

  • Connections are arranged in closed loops that arise from the cortex and return there, contributing to voluntary movement and various non-motor functions, such as:

  • Emotional regulation

  • Cognitive processes

• Dysfunction Implications:

  • Associated with movement disorders (e.g., Parkinson’s disease) and neuropsychiatric conditions (e.g., OCD, Tourette's syndrome)

Parallel Basal Ganglia Loops

• Functionality:

  • Three sets of parallel loops involved in:

  • Cognition

  • Voluntary movement

  • Reward-based learning

• Ventral Striatum:

  • Located in anterior part of BG, where caudate nucleus and putamen fuse under the anterior limb of the internal capsule

Septum Pellucidum and Septal Area

• Septum Pellucidum:

  • Thin membrane separating frontal horns of lateral ventricles

  • Septal Area: A small region below the septum, sometimes referred to as the pleasure center of the brain

  • Nucleus Accumbens Septi:

  • Located next to septal area, representing the ventral striatum where caudate nucleus and putamen are fused

• Behavioral Functionality:

  • Ventral striatum is strongly connected with the limbic lobe, involving emotional responses and reward-based learning

  • Each part of the striatum serves different input channels:

  • Putamen: Voluntary movement

  • Caudate Nucleus: Cognition

  • Ventral Striatum: Emotional regulation, behavior, and reward-based learning

Biochemical Characteristics of Ventral Striatum

• Receptor Population:

  • Rich in dopamine, opiate, nicotinic, and cannabinoid receptors

  • Key role in reward-based learning and various forms of addiction.

Overview of the Internal Anatomy of the Cerebral Hemispheres

• Topics Covered:

  • Internal anatomy of the cerebral hemispheres, focusing on subcortical structures.

  • $3D$ arrangements of the hemispheric grey matter (nuclei) and white matter (projection, association, and commissural fibers).

  • Relationships of grey and white matter to the ventricular system components.

  • $3D$ spatial relationships between:

    • Corpus striatum (comprising the Basal Ganglia or BG).

    • Thalamus (the gatekeeper to the cortex).

    • Internal capsule (the primary highway for projection fibers).

    • Ventricles (fluid-filled cavities).

  • MRI imaging of the BG: coronal, axial, and sagittal planes.

  • Ventral striatum and its role in the limbic system.

Ventricular System and CSF Flow

• Lateral Ventricle:

  • Large, paired, C-shaped cavities filled with cerebrospinal fluid (CSF) produced by the choroid plexus.

  • Frontal (Anterior) Horns: Located within the frontal lobes, bounded medially by the septum pellucidum and laterally by the head of the caudate nucleus.

  • Occipital (Posterior) Horn: Extends backwards into the occipital region; its size is often asymmetrical between hemispheres.

  • Temporal (Inferior) Horn: Passes downwards and forwards into the temporal lobe, with the hippocampus forming its floor.

  • Body: The central portion situated within the parietal lobe.

  • Trigone (Atrium): The expanded triangular region where the body, temporal horn, and occipital horn converge.

• $3rd$ Ventricle:

  • A narrow, slit-like midline cavity of the diencephalon situated between the two thalami.

  • Connected to the lateral ventricles via the Interventricular Foramen of Monro.

  • Communicates inferiorly with the $4th$ ventricle through the Cerebral Aqueduct (of Sylvius).

• Orientation: The lateral ventricles follow a curved path reflecting the developmental expansion of the telencephalon, whereas the $3rd$ ventricle is strictly sagittal.

Corpus Striatum (Basal Ganglia Core)

• Structure and Components:

  • Caudate Nucleus: A prominent C-shaped mass of grey matter.

    • Head: Large, bulging into the frontal horn of the lateral ventricle.

    • Body: Tapered portion passing posteriorly in the floor of the body of the lateral ventricle.

    • Tail: Arching over the thalamus into the roof of the temporal horn, ending at the amygdaloid nucleus.

  • Lentiform Nucleus: A lens-shaped mass lateral to the internal capsule.

    • Putamen: The larger, darker, lateral portion forming the outer shell.

    • Globus Pallidus: The medial, paler portion (subdivided into internal and external segments). It is pale due to a high density of myelinated fibers.

• Spatial Relationships: The thalamus lies medial and posterior to the lentiform nucleus, separated from it by the posterior limb of the internal capsule.

Internal Capsule: The Projection Fiber Pathway

• Definition:

  • A massive V-shaped layer of white matter containing all the projection fibers (both ascending and descending) that connect the cerebral cortex with the brainstem and spinal cord.

• Structure and Functional Anatomy:

  • Anterior Limb: Situated between the head of the caudate nucleus and the lentiform nucleus. It carries thalami-cortical and frontopontine fibers.

  • Genu: The "knee" or bend where the anterior and posterior limbs meet. It contains corticobulbar fibers (controlling cranial nerve motor nuclei).

  • Posterior Limb: Situated between the thalamus and the lentiform nucleus. It contains vital corticospinal fibers (motor) and sensory radiations.

  • Retrolentiform Part: Fibers located posterior to the lentiform nucleus, primarily carrying optic radiations from the lateral geniculate body to the visual cortex.

  • Sublentiform Part: Fibers passing beneath the lentiform nucleus, carrying auditory radiations to the temporal lobe.

Coronal Sectional Analysis

• Anterior (Rostral) Level:

  • Features the frontal horns of the lateral ventricles and the head of the caudate nucleus.

  • The Anterior Limb of the internal capsule separates the caudate head from the putamen.

  • The Corpus Callosum is visible superiorly as the roof of the ventricles.

• Posterior (Thalamic) Level:

  • Features the thalamus near the midline and the body of the caudate nucleus superiorly.

  • The Posterior Limb of the internal capsule is clearly visible separating the thalamus from the lentiform nucleus.

  • The "Snowman" appearance: The thalamus represents the large body, while the caudate body represents the smaller head.

Functional Circuitry of the Basal Ganglia

• Striatum as Input:

  • Formed by the Caudate and Putamen. They receive massive excitatory input from the entire cerebral cortex and the substantia nigra.

• Pallidum as Output:

  • The Globus Pallidus Internus ($GPi$) and Substantia Nigra Reticulata ($SNr$) serve as the primary output sources, sending inhibitory signals to the thalamus.

• Circuit Dynamics:

  • Movement is regulated via the Direct Pathway (pro-movement/disinhibitory) and the Indirect Pathway (anti-movement/inhibitory).

  • Non-motor functions include executive function (cognition) via the caudate and emotional regulation via the ventral striatum.

Ventral Striatum and Reward Systems

• Septum Pellucidum: The vertical membrane separating the two frontal horns.

• Ventral Striatum: Found at the junction where the head of the caudate and the putamen fuse inferior to the internal capsule.

  • Nucleus Accumbens: The primary component of the ventral striatum.

  • Functional Role: Integral to the "Reward Center." It connects the limbic system (emotions) to motor output.

• Biochemical Profile:

  • Highly sensitive to Dopamine (via the mesolimbic pathway from the Ventral Tegmental Area).

  • High concentrations of opiate and cannabinoid receptors mediate pleasure and reinforcement.

• Clinical Significance: Dysfunction or over-activity in these circuits is central to addiction, impulsive behaviors, and psychiatric disorders like $OCD$ or Tourette's.