Dissector Answers - Neuroanatomy 3

Learning Objectives:

Upon completion of this session, the student will be able to:

  1. List brain structures that are part of the limbic system; describe the role of limbic structures in regulating functions relevant to survival, and specify the role of individual limbic structures in controlling emotions.
  2. Identify anatomical location of amygdala, explain the role of the prefrontal cortex-amygdala connection in initiating fear and rage, in adults and in children; describe symptoms of diseases associated with damaged or atrophied amygdala.
  3. Identify the following surface features of the brain: five lobes (frontal, parietal, occipital, temporal and insula); primary motor area; primary sensory areas (somatic, visual and auditory); motor association area; sensory association areas; prefrontal cortex (lateral, medial and orbitofrontal regions); and uncus.
  4. Identify the following internal features of the brain: cerebral cortex; cerebral white matter (association, commissural and projection fibers); thalamus and hypothalamus; basal nuclei, including nucleus accumbens, the striatum consisting of caudate nucleus and lentiform nucleus (putamen and globus pallidus).
  5. Identify the following components of the limbic system: hippocampus, mammillary bodies, amygdala, parahippocampal gyrus, and cingulate gyri.

Learning Objectives and Explanations:

1. List brain structures that are part of the limbic system; describe the role of limbic structures in regulating functions relevant to survival, and specify the role of individual limbic structures in controlling emotions. (N98, N99A, N99B, N101, N102, TG7-30, NI28, NI31A, NI31B, NI31C)
The limbic system consists of: The limbic system:
  1. Regulates activities essential for survival:
    • Feeding
    • Fighting
    • Fearing
    • Reproduction (Fornication - just so you get 4 F's)
  2. Generates emotions and affects memories that enhance individual and species survival in the context of social interaction.
  3. The limbic system achieves many of these objectives by regulation of the hypothalamus.
Cingulate cortex helps people to understand how to alter behavior in a variety of situations. It helps people recognize alternatives, such as selecting a menu item or the pros and cons of taking a new job. Parahippocampal cortex is important for memory storage, encoding and retrieval.

Nucleus accumbens (also called ventral striatum) is involved in anticipation of reward and reward approach. Anhedonia occurs when this area is lesioned. Approach reward is eliminated. Firing of N. accumbens neurons increases when anticipating rewards. The striatum is the major input to the basal nuclei which chooses which action/thought/emotion will occur.

Hippocampus: key structure in memory.

Summary of limbic system:
  1. Main roles of the limbic system: (1) Limbic system function developed in small mammals/ hypothalamus/ survival; (2) Limbic system functions developed in humans/emotion and cognitive control/survival in social context
  2. Limbic system: cortex (cingulate gyrus and parahippocampal gyrus) and additional structures: roles in regulating emotions, memory or saliency
  3. Amygdala and intense emotions; fear; amygdala and PFC; development; atrophy of amygdala in 2 diseases (below)
2. Identify anatomical location of amygdala, explain the role of the prefrontal cortex-amygdala connection in initiating fear and rage, in adults and in children; describe symptoms of diseases associated with damaged or atrophied amygdala. (N98A, N99B, N101, N102, N103, N104A, N104B, N137, N138, N139, TG7-46, TG7-48, TG7-49, TG7-56A, TG7-56B, TG7-72, TG7-73)

The amygdala modulates our reactions to events that are important for our survival. Events that warn us of imminent danger or events that signal the presence of food, sexual partners, rivals, children in distress, and so on. The amygdala lets us react almost instantaneously to the presence of a danger. So rapidly that often we startle first, and realize only afterward what it was that frightened us.

Stimulation of amygdala elicits aggression, which can be inhibited by stimulation of the ipsilateral frontal cortex. Bilateral ablation of the amygdala has been carried out in humans for treatment of rage attacks, characterized by irritability that builds up over several hours or days to a state of dangerous aggressiveness. This controversial operation has been successful in eliminating such attacks.

The orbitofrontal cortex (OFC), part of the prefrontal cortex or PFC, is important in signaling expected rewards/ punishments of an action. This makes the brain capable of comparing the expected reward/punishment with the actual delivery of reward/punishment, making the OFC critical for adaptive learning.

Patients with ventromedial PFC lesions show defects in emotional response and regulation. Reduced social emotions such as compassion, shame and guilt (associated with moral values). Poorly regulated anger and frustration, can't process gender specific social cues.

Emotions:

Children have less control over their emotions, because axons that send information from the Cortex to the limbic system are not yet developed. In addition, the neurons of the prefrontal cortex that provide much of our rational control over our emotions do not mature until early adulthood. In contrast, the amygdala is mature at birth and thus exerts a heavy influence on children.

Amygdala: Bilateral lesion, Kluver-Bucy syndrome - Symptoms:

  1. Docility, absence of emotional responses (fear, rage, and aggression cease to exist)
  2. Compulsion to be overly attentive to all sensory stimuli and to examine all objects orally, visually or tactilely; hyperorality and Hyperphagia
  3. Hypersexuality
  4. Visual agnosia

Urbach-Wiethe disease: Calcifications in the temporal lobe - The symptoms vary, but may include a hoarse voice, lesions on and scarring of the skin, easily damaged skin with poor wound healing, dry wrinkly skin, and beading of papules around the eyelids. These result from a generalized thickening of the skin and mucous membranes. Many patients with Urbach-Wiethe disease also have neurological symptoms. About 50-75% of diagnosed cases also demonstrate bilateral symmetrical calcifications of the medial temporal lobes. These calcifications are the result of a buildup of calcium deposits in the blood vessels of the region. Over time the vessels harden and the tissue supplied by them dies, producing lesions. Neuropsychiatric presentation can be similar to Kluver-Bucy syndrome.

3. Identify the following surface features of the brain: five lobes (frontal, parietal, occipital, temporal and insula); primary motor area; primary sensory areas (somatic, visual and auditory); motor association area; sensory association areas; prefrontal cortex (lateral, medial and orbitofrontal regions); and uncus. (N98A, N99B, N101, N102, N103, N104A, N104B, N137, N138, N139, TG7-46, TG7-48, TG7-49, TG7-53A, TG7-53B, TG7-56A, TG7-56B, TG7-72, TG7-73. NI19A, NI19B, NI19C, NI20)
The lobes of the cerebral hemispheres are named for the bones that overlie them: frontal bone, parietal bone, temporal bone, and occipital bone.

The frontal lobes lie on the anterior cranial fossa, above the orbits. The temporal lobes sit in the middle cranial fossae behind the orbits. The occipital lobes sit above the cerebellum, which occupies the posterior cranial fossa.

There are 5 lobes of each cerebral hemisphere, but only 4 are visible on the surface of the brain. They are the frontal lobe, Parietal lobe, occipital lobe, and temporal lobe.

The brain features many grooves or sulci separating multiple gyri or rounded ridge-like elevations. The major sulci and fissures include the midline longitudinal fissure separating the two cerebral hemispheres, the lateral sulcus that lies above the temporal lobe, the parieto-occipital sulcus and preoccipital notch (both of which can be difficult to locate) and the central sulcus, which separates the precentral gyrus from the postcentral gyrus.

The central sulcus separates the precentral gyrus, the primary motor cortex, and the postcentral gyrus, the primary sonatosensory cortex.

Down in the bottom of the lateral sulcus, overshadowed by parts of the frontal, parietal, and temporal lobes called opercula, is the insula, the hidden 5th lobe.

Some people consider the limbic system as a 6th lobe, with its cingulate gyrus lying above the corpus callosum, the big connection between the two cerebral hemispheres.

4. Identify the following internal features of the brain: cerebral cortex; cerebral white matter (association, commissural and projection fibers); thalamus and hypothalamus; basal nuclei, including nucleus accumbens, the striatum consisting of caudate nucleus and lentiform nucleus (putamen and globus pallidus). (NI27A, NI27B, NI48B, NII.87.3, NII.87.4, NII.87.5, NII.87.6, NII.87.7, NII.87.8, NII.87.9)

The gray matter in the brain has another layer than is found in the spinal cord, where gray matter is found in a butterfly-shape inside the cord, wrapped with white matter making the fiber tracts up and down the spinal cord. In the brain, the cerebral cortex is an additional layer of gray matter on the surface of the brain. White matter in the brain forms fiber tracts connecting the various parts of the brain and communicating up and down with the spinal cord. The corpus callosum is the huge white matter connection between the cerebral hemispheres, above the lateral ventricles, while the septum pellucidum lies between the lateral ventricles.

Collections of subcortical gray matter include the thalamus, hypothalamus, and basal nuclei. The internal capsule is made of white matter tracts that connect cerebral cortex with other parts of the brain and spinal cord and passes between these collections of gray matter. The thalamus lies beside the third ventricle, and the hypothalamus, as the name implies, lies below it.

The basal nuclei (also called the basal ganglia, but since these are collections of nerve cell bodies within the central nervous system, they must be called nuclei - ganglia are collections of nerve cell bodies outside of the central nervous system, i.e. in the peripheral nervous system, and are the sensory and autonomic ganglia we know and love) include the caudate nucleus and putamen (making the striatum or neostriatum), the globus pallidus, subthalamus, and substantia nigra. The caudate nucleus is C-shaped and lies on the lateral wall of the lateral ventricle, while the putamen is disc-shaped and lies within this C-shape, connected to the head of the caudate and also separated from the head of caudate by the anterior limb of the internal capsule and from the thalamus by the globus pallidus and the posterior limb of the internal capsule. Globus pallidus lies medial to the putamen, nestled into the genu of the internal capsule. The substantia nigra lies within the midbrain behind the cerebral peduncles.

5. Identify the following components of the limbic system: hippocampus, mammillary bodies, amygdala, parahippocampal gyrus, and cingulate gyri. (NI28, NI31A, NI31B, NI31C)

The hippocampus is found in the medial temporal lobe. The mammillary bodies project down from the diencephalon posterior to the stalk of the pituitary and just anterior to the midbrain. They lie medial to the uncus of the temporal lobe, the anterior end of the parahippocampal gyrus. The amygdala lies within the anteromedial temporal lobe, lateral to the uncus. The parahippocampal gyrus curves up and back on the medial surface of the temporal lobe, separated by the posterior end of the cingulate gyrus by the calcarine sulcus. The cingulate gyrus lies just above the corpus callosum.