Neuroanatomy: Gray and White Matter

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Questions and Answers

What is the primary function of the thalamus?

Regulating hormonal balance

Controlling voluntary muscle movements

Relaying sensory information to the cerebral cortex (correct)

Managing autonomic functions

Which sensory modality does NOT pass through the thalamus?

Somatic sensation

Hearing

Vision

Olfaction (correct)

Which structure is part of the diencephalon along with the thalamus?

Hypothalamus (correct)

Corpus callosum

Medulla oblongata

Cerebellum

What forms the lateral boundary of the internal capsule?

Lentiform nucleus Signup and view all the answers

What is a key feature of thalamic circuits?

Reciprocal cortical-thalamic connections Signup and view all the answers

Which part of the internal capsule lies between the caudate nucleus and the anterior part of the lentiform nucleus?

Anterior limb Signup and view all the answers

Which function is primarily associated with the hypothalamus?

Controlling autonomic and neuro-endocrine functions Signup and view all the answers

What is the shape of the thalamus described as?

Egg-shaped Signup and view all the answers

What type of sensory information does the posterior column pathway primarily convey?

Vibration sense and proprioception Signup and view all the answers

Where do primary sensory neurons carrying fine touch information first make their synapses?

Dorsal column nuclei in the medulla Signup and view all the answers

What is the main pathway for pain and temperature sense?

Anterolateral pathway Signup and view all the answers

Which of the following correctly describes the pathway taken by axons carrying proprioception information after entering the spinal cord?

They ascend ipsilaterally to the medulla and then cross over. Signup and view all the answers

What happens to the sensory pathways at the level of the thalamus?

They cross over to the opposite side before synapsing. Signup and view all the answers

What is the role of the thalamus in the sensory pathways?

To serve as a relay center for sensory signals Signup and view all the answers

Which statement is true regarding the primary sensory neurons?

Their cell bodies are located in the dorsal root ganglia. Signup and view all the answers

What is conveyed by both the posterior column and anterolateral pathways?

Some aspects of touch sensation Signup and view all the answers

What is the primary location of gray matter in the spinal cord?

In the center of the spinal cord Signup and view all the answers

Which structures are considered gray matter nuclei within the brain?

Basal ganglia and cranial nerve nuclei Signup and view all the answers

What term is used for white matter pathways that connect structures on opposite sides of the CNS?

Commissure Signup and view all the answers

Which statement correctly describes the arrangement of nerve roots in the spinal cord?

Dorsal roots convey mainly afferent sensory information Signup and view all the answers

In which part of the human nervous system do clusters of cell bodies become known as ganglia?

PNS Signup and view all the answers

What explains the length discrepancy between the bony vertebral canal and spinal cord during development?

The canal grows faster than the spinal cord Signup and view all the answers

What is the primary function of afferent pathways in the nervous system?

To carry signals toward a structure Signup and view all the answers

What is the correct organization of the nervous system segments as viewed from the dorsal and ventral sides?

Motor systems are more anterior; sensory systems are posterior Signup and view all the answers

What role does the fornix play in the brain's limbic system?

It connects the hippocampal formation to the hypothalamus and septal nuclei. Signup and view all the answers

Which area is primarily responsible for language comprehension?

Wernicke's area Signup and view all the answers

What type of cortex is involved in integrating multiple sensory modalities?

Heteromodal association cortex Signup and view all the answers

What may occur as a result of lesions in the limbic system?

Deficits in consolidating immediate recall into long-term memories. Signup and view all the answers

Which structure is involved in the formation of new memories?

Hippocampal formation Signup and view all the answers

What type of aphasia results from damage to Wernicke's area?

Receptive or sensory aphasia Signup and view all the answers

Where is the premotor cortex located in relation to the primary motor cortex?

It is adjacent to the primary motor cortex. Signup and view all the answers

What kind of hallucinations may be associated with limbic dysfunction?

Olfactory hallucinations Signup and view all the answers

What is a common behavior exhibited by patients with frontal lobe lesions when performing tasks?

They tend to perseverate on a single action. Signup and view all the answers

Which of the following is NOT a personality change associated with frontal lobe lesions?

Increased empathy Signup and view all the answers

What characterizes the gait of patients with frontal lobe lesions?

A unsteady magnetic gait with shuffling feet Signup and view all the answers

Which condition is associated with lesions in the visual association cortex?

Achromatopsia Signup and view all the answers

What is the primary source of blood supply to the anterior cerebral hemisphere?

Internal carotid arteries Signup and view all the answers

Which arteries contribute to the blood supply of the posterior circulation of the brain?

Posterior cerebral arteries Signup and view all the answers

What is the main function of the internal jugular veins?

Drain blood from the brain Signup and view all the answers

Which phenomenon may occur due to lesions in the visual association cortex?

Prosopagnosia Signup and view all the answers

Study Notes

Gray Matter

Found in clusters of cells called nuclei, located deep within the cerebral hemispheres and brainstem.
Examples include basal ganglia, thalamus, and cranial nerve nuclei.
In the cerebral hemispheres, the gray matter cortex is outside, while the white matter is inside.
In the spinal cord, the opposite is true: white matter pathways lie on the outside, while the gray matter is in the center.
In the brainstem, gray matter and white matter regions are found both on the inside and on the outside, although most of the outside surface is white matter.

White Matter Pathways

Several different names with similar meaning are used for white matter pathways in the CNS, including tract, fascicle, lemniscus, and bundle.
Pathways carrying signals toward a structure are called afferent, while those carrying signals away from a structure are called efferent.
A white matter pathway that connects structures on the right and left sides of the CNS is called a commissure.
Axons in the PNS form bundles called peripheral nerves, or simply nerves.
Clusters of cell bodies in the PNS are referred to as ganglia.

Spinal Cord and Peripheral Nervous System

The human nervous system develops in segments similar to those of simpler animals, such as segmented worms.
The segments in the head expand and fuse together, forming the cerebral hemispheres and brainstem.
Twelve pairs of cranial nerves exit these segments.
The spinal nerves arise from the segments of the spinal cord.
Each segment gives rise to both sensory and motor nerve roots on each side of the body.

Spinal Cord Organization

Throughout the nervous system, motor systems tend to be more ventral, or anterior, and sensory systems more dorsal, or posterior.
The same holds true for the spinal cord: dorsal nerve roots convey mainly afferent sensory information into the dorsal spinal cord, while ventral nerve roots carry mainly efferent motor signals from the ventral spinal cord to the periphery.
The segments and nerve roots of the spinal cord are named according to the level at which they exit the bony vertebral canal.
There are cervical, thoracic, lumbar, and sacral nerve roots.

Spinal Cord Development

During development, the bony vertebral canal increases in length faster than the spinal cord.
Therefore, the spinal cord ends at the level of the first or second lumbar vertebral bones (L1 or L2).

Main Somatosensory Pathways

Somatic sensation refers to the conscious perceptions of touch, pain, temperature, vibration, and proprioception (limb or joint position sense).
There are two main pathways in the spinal cord for somatic sensation:
- Posterior column pathways: convey proprioception, vibration sense, and fine, discriminative touch
- Anterolateral pathways: convey pain, temperature sense, and crude touch.
Some aspects of touch sensation are carried by both pathways, so touch sensation is not eliminated in isolated lesions of either pathway.
The primary sensory neuron cell bodies are located outside the CNS in the dorsal root ganglia and have bifurcating axons, with one long process extending to the periphery and one into the spinal cord.

Posterior Column Pathway

Primary sensory neuron axons carrying information about proprioception, vibration sense, and fine touch enter first the spinal cord via the dorsal roots and then the ipsilateral white matter dorsal (posterior) columns to ascend all the way to the dorsal column nuclei in the medulla.
Here they make synapses onto the secondary sensory neurons, which send out axons that cross over to the other side of the medulla.
These axons continue to ascend, now on the contralateral side, and synapse in the thalamus, and from there neurons project to the primary somatosensory cortex in the postcentral gyrus.

Anterolateral Pathway

Primary sensory neurons carrying information about pain, temperature sense, and crude touch also enter the spinal cord via the dorsal roots.
These axons make their first synapses immediately in the gray matter of the spinal cord.
Axons from the secondary sensory neurons cross over to the other side of the spinal cord and ascend in the anterolateral white matter, forming the spinothalamic tract.
After synapsing in the thalamus, the pathway again continues to the primary somatosensory cortex.

Thalamus

The thalamus is an important relay center.
Nearly all pathways that project to the cerebral cortex do so after synapsing in the thalamus.
The thalami are gray matter structures located deep within the cerebral white matter just above the brainstem and behind the basal ganglia.
They are shaped somewhat like eggs, with their posterior ends angled outward, together forming an inverted V in horizontal sections.
The thalamus consists of multiple nuclei.
Each sensory modality, including vision, hearing, taste, and somatic sensation, has a different nuclear area where synapses occur before the information is relayed to the cortex (olfactory inputs are an exception and do not pass directly through the thalamus).
Non-sensory pathways also relay in the thalamus.
For example, there are thalamic nuclei that process information coming from the basal ganglia, cerebellum, limbic pathways, and brainstem reticular formation on the way to the cortex.
An important feature of thalamic circuits is the reciprocal nature of cortical–thalamic connections.
Thus, virtually all cortical regions project strongly via layer VI back to the thalamic areas from which their major inputs arise.

Diencephalon

The thalamus, together with the hypothalamus and epithalamus, form the diencephalon.
The hypothalamus is an important region for control of autonomic, neuro-endocrine, limbic, and other circuits.
The epithalamus encompasses several small nuclei, including the pineal body, habenula, and parts of the pretectum.

Internal Capsule

A large number of nerve fibers interconnect the cerebral cortex with centers in the brainstem and spinal cord, and with the thalamus.
Most of these fibers pass through the interval between the thalamus and caudate nucleus medially, and the lentiform nucleus laterally.
This region is called the internal capsule.
Above, the internal capsule is continuous with the corona radiata; and, below, with the crus cerebri (of the midbrain).
The internal capsule may be divided into the following parts:
- The anterior limb: lies between the caudate nucleus medially, and the anterior part of the lentiform nucleus laterally.
- The posterior limb: lies between the thalamus medially, and the posterior part of the lentiform nucleus on the lateral side.

Limbic System

Includes deeper structures such as the hippocampal formation and the amygdale, located within the medial temporal lobes, several nuclei in the medial thalamus, hypothalamus, basal ganglia, septal area, and brainstem.
These areas are interconnected by a variety of pathways, including the fornix—a paired, arch-shaped white matter structure that connects the hippocampal formation to the hypothalamus and septal nuclei.
Lesions in the limbic system can cause deficits in the consolidation of immediate recall into longer-term memories.
Patients with lesions in these areas may have no trouble recalling remote events but have difficulty forming new memories.
Limbic dysfunction can cause behavioral changes and may underlie a number of psychiatric disorders and epileptic seizures that may begin with emotions such as fear, memory distortions such as déjà vu, or olfactory hallucinations.

Association Cortex

The cerebral cortex contains a large amount of association cortex, which carries out higher-order information processing..
Unimodal association cortex: higher-order processing takes place mostly for a single sensory or motor modality.
- Usually located adjacent to a primary motor or sensory area; for example, unimodal visual association cortex is located adjacent to the primary visual cortex, and unimodal motor association cortex (premotor cortex and supplementary motor area) is located adjacent to the primary motor cortex.
Heteromodal association cortex: involved in integrating functions from multiple sensory and/or motor modalities.

Language Processing

Language is usually perceived first by the primary auditory cortex in the superior temporal lobe when we are listening to speech or by the primary visual cortex in the occipital lobes when we are reading.
From here, cortical–cortical association fibers convey information to Wernicke’s area in the dominant (usually left) hemisphere.
Lesions in Wernicke’s area cause deficits in language comprehension, also sometimes called receptive or sensory aphasia, or Wernicke’s aphasia.
Broca’s area is located in the frontal lobe, also in the left hemisphere, adjacent to the areas of primary motor cortex involved in moving the lips, tongue, face, and larynx.

Frontal Lobe Lesions

Patients with frontal lobe lesions may have particular difficulty when asked to perform a sequence of actions repeatedly or to change from one activity to another.
They tend to perseverate, meaning that they repeat a single action over and over without moving on to the next one.
Personality changes with frontal lobe lesions may include impaired judgment, a cheerful lack of concern about one’s illness, inappropriate joking, and other disinhibited behaviors.
Other patients with frontal lobe lesions may be abulic (opposite of ebullient), with a tendency to stare passively and to respond to commands only after a long delay.
Frontal lesions can also cause a characteristic unsteady magnetic gait, in which the feet shuffle close to the floor, and urinary incontinence.

Visual Association Cortex Lesions

Lesions in the visual association cortex in the parieto-occipital and inferior temporal lobes can produce a variety of interesting phenomena, including prosopagnosia (inability to recognize faces), achromatopsia (inability to recognize colors), palinopsia (persistence or reappearance of an object viewed earlier), and other phenomena.
Seizures in the visual association cortex can cause elaborate visual hallucinations.

Blood Supply to the Brain and Spinal Cord

Brain:
- Two pairs of arteries carry all the blood supply to the brain: the internal carotid arteries (anterior blood supply) and the vertebral arteries (posterior blood supply).
- The vertebral arteries join together in a single basilar artery.
- The anterior and posterior blood supplies from the carotid and vertebrobasilar systems join together in an anastomotic ring at the base of the brain called the circle of Willis.
- The main arteries supplying the cerebral hemispheres arise from the circle of Willis.
- Ordinarily, however, the anterior and middle cerebral arteries derive their main blood supply from the internal carotid arteries (anterior circulation), while the posterior cerebral arteries derive their main supply from the vertebrobasilar system (posterior circulation).
- The main arteries supplying the brainstem and cerebellum also arise from the vertebral and basilar arteries. These include the superior, anterior inferior, and posterior inferior cerebellar arteries.
- Venous drainage for the brain is provided almost entirely by the internal jugular veins.

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Test your knowledge of the structure and function of gray and white matter in the central nervous system. This quiz covers key concepts related to their locations, types of pathways, and anatomical arrangements in the brain and spinal cord. Challenge yourself and uncover more about neuroanatomy!