CHAPTER PREVIEW
In this chapter we will explore what sits above the brainstem and inside the brain—the diencephalon and surrounding structures. More specifically, we will learn about the thalamus, basal ganglia, and brain ventricles. We will also survey a few select disorders associated with these structures.
CHAPTER OUTLINE
IN THIS CHAPTER
In this chapter, we will . . .
■ Learn about the four parts of the diencephalon
■ Survey the form and function of the basal ganglia
■ Describe the brain ventricles
■ Discuss the role of cerebrospinal fluid
■ Survey disorders related to damage to the structures listed here
LEARNING OBJECTIVES
■ The learner will list the four parts of the diencephalon and briefly describe the function of each.
■ The learner will draw the basal ganglia and describe the function of these nuclei.
■ The learner will list the names of the four brain ventricles.
■ The learner will describe cerebrospinal fluid's composition and its function.
■ The learner will list and briefly describe disorders associated with the diencephalon, basal ganglia, and brain ventricles.
■ Introduction
■ The Diencephalon
• Thalamus
• Subthalamus
• Hypothalamus
• Epithalamus
■ The Basal Ganglia
• Structure and Function of the Basal
Ganglia
• Internal Capsule and Corona Radiata
• Basal Ganglia Disorders
■ The Brain Ventricles
• Structure and Function
• Disorders of the Ventricles
■ Conclusion
■ Summary of Learning Objectives
■ Key Terms
■ Draw It to Know It
■ Questions for Deeper Reflection
■ Case Study
■ Suggested Projects
■ References
► Introduction
In this chapter, the diencephalon, basal ganglia, and brain ventricles will be explored. Special attention will be paid to what these structures might contribute to speech, language, and hearing.
► The Diencephalon
The diencephalon is located between the cerebrum and the brainstem, resting above the midbrain of the brainstem. Its location makes it a prime area for connecting the cerebral cortex to the rest of the body. It also connects the nervous system to the endocrine system, our hormone system. The diencephalon consists of four parts: the thalamus, subthalamus, hypothalamus, and epithalamus (FIGURE 6-1).
Thalamus
Thalamus is a Greek term meaning “inner chamber” or “bedroom.” The thalamus sits on top of the midbrain and consists of two halves, or hemispheres, each being about the size of a walnut (FIGURE 6-2). In terms of function, the thalamus has been traditionally viewed as a sensory fiber relay station or switchboard between the cerebral cortex and subcortical areas. Perhaps a more modern-day analogy for the thalamus’s function would be an Internet router, but instead of routing a very general signal, it routes specific information to specific cortical areas. More specifically, the thalamus processes all sensory information (except olfaction), routing it to specialization cerebral cortex locations, which in turn process the particular type of sensory information (e.g., vision). The thalamus is involved in motor function, but only indirectly through directing some extrapyramidal fibers, which control more autonomic functions, to the basal ganglia (Sherman, 2006; Webster, 1999).
FIGURE 6-1 Coronal section of the brain showing the epithalamus, thalamus, subthalamus, and hypothalamus.
FIGURE 6-2 Medial view of the brain illustrating the location of the thalamus.
© Alila Medical Images/Shutterstock.
Structure and Function of the Thalamus
The thalamus is made up of several different nuclei. (See TABLE 6-1 for a list of all thalamic nuclei and FIGURE 6-3 for a picture of thalamic nuclei.) These nuclei are specialized in the sense that they have specific functions. We will now survey the thalamic nuclei, paying close attention to those involved with speech, language, or hearing.
Medial Nuclei
The dorsomedial nuclei (DMs) receive information from the hypothalamus, amygdala, and other thalamic nuclei. After processing this information, they project it to the prefrontal cortex and septal area (i.e., the septum pellucidum—a membrane that separates the right and left ventricles) (Webster, 1999). The DMs function in attention and eye-head control. They also are involved in emotion and autonomic control (Castro, Merchut, Neafsey, & Wurster, 2002).
Lateral Dorsal Nuclei
This group contains three nuclei. Information inputs into the lateral dorsal nucleus (LD) from the hypothalamus and other thalamic nuclei and then projects to the septal area, cingulate gyrus, and parahippocampal gyrus. These structures are involved in the limbic system, where emotional processing takes place (Webster, 1999). The lateral posterior nucleus (LP) receives information from two ventral thalamic nuclei and the superior colliculus and then projects to the parietal association cortex (areas 5 and 7). Its functions include eye control and vision. The pulvinar nucleus (P), the largest nucleus in the thalamus, receives input from the superior colliculus and projects to the secondary visual cortex (areas 18 and 19). Like the LP, the P is involved in vision and eye control.
|
TABLE 6-1 Thalamic Nuclei |
|||
|
Group |
Thalamic Nucleus |
Afferent Inputs |
Efferent Outputs |
|
Medial |
Dorsomedial (DM) |
Hypothalamus, amygdala, other thalamic nuclei |
Prefrontal cortex, septal area |
|
Lateral dorsal |
Lateral dorsal (LD) |
Hypothalamus; other thalamic nuclei |
Septal area, cingulate gyrus, parahippocampal gyrus |
|
Lateral posterior (LP) |
Ventral posterior medial and lateral nuclei |
Superior and inferior parietal lobe |
|
|
Pulvinar (P) |
Superior colliculus, visual association cortices, frontal eye cortex |
Superior colliculus, visual association cortices, frontal eye cortex |
|
|
Lateral ventral |
Ventral anterior (VA) |
Basal ganglia, cerebellum |
Motor cortices |
|
Ventral lateral (VL) |
Basal ganglia, cerebellum |
Motor cortices |
|
|
Ventral posterior lateral (VPL) |
Medial lemniscus, spinothalamic tract |
Postcentral gyrus |
|
|
Ventral posterior medial (VPM) |
Trigeminothalamic tract |
Postcentral gyrus |
|
|
Medial geniculate body (MGB) |
Inferior colliculus |
Primary auditory cortex |
|
|
Lateral geniculate body (LGB) |
Optic tract |
Primary visual cortex |
|
|
Anterior |
Anterior nucleus (AN) |
Mammillary body |
Cingulate cortex |
|
Intralaminar |
Centromedian Parafascicular |
Reticular formation Other thalamic nuclei |
Cerebral cortex Basal ganglia |
|
Reticular |
Reticular |
Cerebral cortex, basal ganglia, other thalamic nuclei |
Other thalamic nuclei, basal ganglia |
|
Midline |
Midline |
Amygdala, cingulate gyrus, hypothalamus |
Amygdala, cingulate gyrus, hypothalamus |
Data from: Webster, D. B. (1999). Neuroscience of communication (2nd ed.). San Diego, CA: Singular Publishing.
Lateral Ventral Nuclei
This grouping contains six nuclei. The ventral posterior lateral nucleus (VPL) and ventral posterior medial nucleus (VPM) are often grouped together as the ventral posterior (VP) nuclei because they have the same projections and functions. The VPL receives input from the medial lemniscus and spinothalamic tract. The spinothalamic tract transmits pain, temperature, and crude touch information from the body to the thalamus. The VPM receives its inputs from the trigeminothalamic tract, which has the same sensory function as the spinothalamic tract, but from the several cranial nerves associated with the face (V, VII, IX, X) instead of the body in general. The VPL and VPM both project from the thalamus to the sensory cortex (areas 3, 2, 1) and are thus involved in somatosensory function. The VPM is the more crucial nucleus to remember for speech because it relays sensory information from the speech structures as they move and make articulatory contact.
FIGURE 6-3 The thalamic nuclei and their projections.
The ventral lateral nucleus (VL) and ventral anterior nucleus (VA) also are grouped because they have the same inputs and projections. Both receive input from the basal ganglia and the cerebellum and then project to motor cortices, the VL to the primary motor cortex (area 4) and the VA to the premotor cortex (area 6) and supplementary motor area. The VL functions in executing movements, whereas the VA is involved in motor planning; both are involved in speech production.
The final two nuclei in this grouping are sometimes classified under lateral ventral nuclei, as we have done here, or sometimes grouped together under the metathalamus (meta is Greek for “after”). The medial geniculate body (MGB) relays auditory information from subcortical midbrain structures (i.e., inferior colliculus) to the primary auditory cortex of the temporal lobe, and thus is important for our sense of hearing. Visual information is sent via the optic nerve to the lateral geniculate body (LGB) of the thalamus, which then projects to the primary visual cortex of the occipital lobe. The MGB is the auditory center of the thalamus, whereas the LGB is the visual center. Many confuse the functions of the MGB and LGB, but a helpful mnemonic is medial = music (hearing) and lateral = light (vision).
Anterior Nucleus
The anterior nucleus (AN) is involved in emotional processing due to its connection to the limbic system. The mammillary bodies receive input from the amygdala and hippocampus, which are part of the limbic system, and then project to another limbic structure, the cingulate cortex. Because of these connections, the AN is intimately involved in our emotional processing system.
Intralaminar Nuclei
There are two intralaminar nuclei of the thalamus, the centromedian intralaminar nucleus and the para- fascicular intralaminar nucleus. These nuclei receive input from the reticular formation and other thalamic nuclei and project to the basal ganglia and numerous places in the cerebral cortex. Their connections would suggest a role in arousal and motor function.
Reticular Nuclei
The reticular nuclei receive input from the cerebral cortex, basal ganglia, and other thalamic nuclei but do not project information to the cerebral cortex. Rather, they make connections to other thalamic nuclei and the basal ganglia (Webster, 1999). Kandel, Schwartz, Jessell, Siegelbaum, and Hudspeth (2013) report that the reticular nuclei monitor all the information between the cerebral cortex and the thalamus and act as a filter for information ascending to the cortex.
Midline Nuclei
These nuclei are interconnected with the amygdala, cingulate gyrus, and hypothalamus. Their function is not well understood, but they may facilitate emotional processing.
Blood Supply to the Thalamus
The anterior portion of the thalamus (VA, VL, and anterior DM) is supplied by the tuberothalamic arteries that branch off of the internal carotid artery’s (ICA’s) posterior communicating artery (P-com) (FIGURE 6-4). The paramedial thalamus is supplied by the paramedian thalamic artery that arises from the posterior cerebral artery (PCA). The inferior lateral portion of the thalamus is supplied by PCA’s inferolateral arteries, which are also known as the thalamogeniculate arteries. The posterior thalamus’s blood supply is provided by the PCA’s posterior choroidal arteries (Li et al., 2018).
Thalamic Disorders
In addition to being a relay station, the thalamus also plays a role in the perception of pain, regulation of cortical arousal, and control of the sleep-wake cycle
(Sherman, 2006). The thalamus receives projections from multiple ascending sensory pathways, including pathways for pain (Ab Aziz & Ahmad, 2006). Damage to the thalamus can result in thalamic pain syndrome, which is also known as Dejerine-Roussy syndrome. This condition involves burning or tingling sensations and possibly hypersensitivity to stimuli that would not normally be painful, such as light touch or temperature change. The condition can be both severe and debilitating.
The thalamus relays important fibers related to cortical arousal from the brainstem’s reticular formation to the cerebral cortex. Damage to specific thalamic regions associated with these fibers can result in disorders of consciousness, such as coma, excessive daytime sleepiness (i.e., hypersomnia), and patients who are passive and do not move or talk (i.e., akinetic mutism) (Castaigne et al., 1981).
What role does the thalamus play in speech and language? Johnson and Ojemann (2000) have proposed that the dominant ventrolateral thalamus (the left ventrolateral thalamus in most people) plays an important role in language and coordinating the cognitive and motor aspects of language. Using electrical stimulation to this area, they were able to induce misnaming and perseverations as well as articulation errors.
Crosson (1984) described what is now called thalamic aphasia, a type of aphasia noted to have three main characteristics. The first is fluent verbal output with semantic paraphasias that often results in jargon. The second characteristic is auditory comprehension that is less severe than one would expect for the severity of verbal output. The third characteristic is minimally impaired or even intact repetition. These symptoms are consistent in cases of damage to the dominant thalamic hemisphere (Crosson, 1992), which is consistent with Johnson and Ojemann’s (2000) findings.
FIGURE 6-4 Blood supply to the thalamus showing four main arteries and the thalamic nuclei they supply. A. Lateral view of the thalamus and its blood supply. B. Dorsal view of the thalamus and its blood supply. Abbreviations: DM, dorsomedial; ICA, internal carotid artery; IL, intralaminar nucleus; LGB, lateral geniculate body; P, pulvinar nucleus; PCA, poster cerebral artery; VA, ventral anterior; VL, ventral lateral; VP, ventral posterior.
TABLE 6-2 Fluent Aphasic Conditions Compared
|
Wernicke |
Transcortical Sensory |
Conduction |
Thalamic |
Anomic |
|
|
Fluency |
+ |
+ |
+ |
+ |
+ |
|
Auditory comprehension |
- |
- |
+ |
= |
+ |
|
Repetition |
- |
+ |
- |
= |
+ |
+, preserved; -, significantly impaired; =, minimally impaired.
Thalamic aphasia defies classical definitions of fluent aphasia (TABLE 6-2). It is obviously different from Wernicke aphasia, in which patients are fluent with paraphasias but have significantly impaired auditory comprehension and repetition. It is also different from transcortical sensory aphasia, which involves significantly impaired auditory comprehension with fluent, neologism-filled speech and preserved but echolalic repetition. Thalamic aphasia is also different from conduction aphasia due to thalamic aphasia’s relatively intact repetition (Papathanasiou, Coppens, & Potagas, 2013). The occurrence of thalamic aphasia suggests that subcortical structures, such as the thalamus, along with the cerebral cortex play an important role in language.
Subthalamus
Given its name, the subthalamus obviously lies below the thalamus; it contains a set of specialized cells called the subthalamic nucleus. Functionally, it has more in common with the basal ganglia than with the thalamus. Specifically, it may play a role in the selection of actions and impulse control. Damage to the subthalamus can result in motor problems like hemiballismus, which is a one-sided involuntary flinging of the limbs sometimes seen in Parkinson disease or other neurological diseases (Das, Romero, & Mandel, 2005). Subthalamic damage may also play a role in obsessive-compulsive disorder and general impulsivity (Carter, 2009; Frank, Samanta, Moustafa, & Sherman, 2007; Mallet et al., 2008). Deep brain stimulation of the subthalamus has been shown to relieve some types of tremors and other involuntary movements (Kitagawa et al., 2000), which demonstrates the subthalamus’s close connection to the basal ganglia (BOX 6-1).
Hypothalamus
The term hypothalamus means “under chamber.” It is about the size of an almond and lies just under the anterior ventral surface of the thalamus (FIGURE 6-6). In terms of function, it can be thought of as a linker and a regulator. It is a linker because it connects the nervous system to the endocrine system (i.e., hormonal system) via the hypothalamus’s pituitary gland. In addition, the hypothalamus is a regulator because it controls aspects of metabolism, body temperature, food intake, circadian rhythms, emotion, and secondary sex characteristics, among other functions (FIGURE 6-7). These functions could all be said to revolve around the idea of homeostasis or maintaining the body’s status quo (Fauci, 2008). The hypothalamus does not appear to play any direct role in speech, language, or hearing, but it may play an indirect role in regulating some substances that be involved in neurotransmitter function, which in turn may affect disorders such as dyslexia, aphasia, and developmental speech delay (Kurup & Kurup, 2003).
Epithalamus
The epithalamus lies superior and posterior to the thalamus. It consists of the pineal gland, habenula, and stria medullaris. The pineal (“pine cone”) gland is an endocrine gland that gets its name from its pinecone shape (Figure 6-6). It is about the size and shape of a grain of rice, being about 5 to 8 millimeters in size. It produces a hormone known as melatonin, which is involved in regulating the sleep-wake cycle, as well as our circadian rhythms, and in gonad development. After puberty, this gland hardens due to a buildup of calcium (i.e., calcification) and becomes a useful landmark in neuroimaging because of its dense structure. The famous philosopher Rend Descartes believed the pineal gland was the seat of the soul.
BOX 6-1 Deep Brain Stimulation
Deep brain stimulation is like a pacemaker for the brain. It has been shown to be effective for treating chronic pain as well as tremors and other involuntary movements. Deep brain stimulation is enabled through a neurosurgical procedure to open the skull and place a device that provides a continuous electrical signal to specific areas of the brain (FIGURE 6-5). For treating chronic pain, the target would be specific thalamic nuclei. For treating tremor and other involuntary movements, the internal part of the globus pallidus (GPi) and the subthalamic nucleus could be targets. The procedure has brought relief to patients with many different diagnoses, including Parkinson disease and Huntington disease. Some advantages of the procedure are (1) it can be done on both sides of the brain to control symptoms affecting both sides of the body, (2) its effects are reversible, and (3) it can control symptoms on a continuous 24-hour basis. There are risks for the procedure as there are for any medical procedure. Some risks include cerebral hemorrhage, cerebrospinal fluid leaking, or an infection at the surgical site.
FIGURE 6-5 Illustration of electrode placement in deep brain stimulation.
National Institute of Mental Health (NIMH), National Institutes of Health (NIH,). Brain stimulation therapies. Retrieved from https://www.nimh.nih.gov/health/topics/brain-stimulation-therapies/brain-stimulation-therapies.shtml
FIGURE 6-6 The hypothalamus shown in relation to the thalamus.
The habenula (“rein”) is a group of nuclei that lies anterior to the pineal gland. It is involved in olfactory reflexes, such as when we salivate at the smell of food (i.e., parotid salivary reflex) or gag in response to a noxious odor. The habenula is also involved in stress responses due to connections to the limbic system as well as our reward processing system (Andres, During, & Veh, 1999; Matsumoto & Hikosaka, 2008). The stria medullaris, a white matter tract, connects the habenular nuclei to the limbic system (Swenson, 2006).
► The Basal Ganglia
Structure and Function of the Basal Ganglia
The basal ganglia are a group of structures that make up most of the remaining subcortical gray matter regions of the brain. They consist of three large nuclei, the caudate nucleus, globus pallidus, and putamen (FIGURE 6-8). The globus pallidus (Latin for “pale globe”) and putamen (Latin for “shell”) reside together but are separate from the caudate nucleus. Along with the midbrain’s substantia nigra and subthalamic nuclei, these structures are key components of the basal ganglia and the extrapyramidal motor system. Damage to this motor system results in dyskinesias or movement disorders.
FIGURE 6-7 The pituitary gland and its functions.
The caudate nucleus (Latin for “having a tail”) is in the shape of an arch (FIGURE 6-9). It has a bulbous head anteriorly and a thin tail that leads into a second bulge, the amygdala (part of the limbic system). The caudate is separated from the globus pallidus and putamen by the internal capsule. Functionally, the caudate and putamen are one nucleus called the striatum, a term that means “striped.” Anatomically, the putamen and the globus pallidus are lumped together under the name lenticular (Latin for “lens”) nucleus. The globus pallidus has two nuclei, an external (GPe) and an internal (GPi).
FIGURE 6-8 Coronal view of main structures of the basal ganglia.
FIGURE 6-9 Lateral view of the basal ganglia in isolation.
Functionally, the basal ganglia have two major pathways that run through them (FIGURE 6-10). There is a direct pathway from the striatum to the medial globus pallidus to the VA and VL thalamic nuclei, which facilitates movement. In addition, an indirect pathway runs from the striatum to the lateral globus pallidus to subthalamic nuclei back to the medial globus palli- dus and finally back to the VA and VL thalamic nuclei. This indirect pathway functions to inhibit movement (Castro et al., 2002).
The basal ganglia also have many connections to the cerebral cortex, but their connections to cortical motor areas are the most significant. Using the neurotransmitter dopamine, which is produced in the midbrain’s substantia nigra, the basal ganglia regulate important extrapyramidal motor functions such as posture, balance, arm swinging, and other body movements (e.g., walking). This regulation includes activating, sustaining, and inhibiting motor movements.
Damage to the basal ganglia can be classified in different ways. LaPointe and Murdoch (2014) suggest two categories: dyskinesias, which are involuntary movements, and akinesias, which are involuntary postures. The category of dyskinesias include tremors (rhythmic shaking), athetosis (slow, writhing movements of the head and hands), chorea (quick, abrupt fidgeting of the hands and/or feet), ballismus (quick flinging of a limb), and tics (quick, stereotyped motor or vocal behaviors). In contrast, akinesias include rigidity (limb resistance to passive movement), dystonia (simultaneous agonist and antagonist muscle contraction resulting in distorted movements and postures), and bradykinesia (slow movements).
Patients with basal ganglia damage can have both dyskinesias and akinesias at the same time. For example, the three hallmark characteristics of Parkinson disease include two akinesias (bradykinesia and rigidity) and one dyskinesia (tremor). Another example is Huntington disease, which typically has one dyskinesia (chorea) and one akinesia (dystonia). There are also basal ganglia disorders that have just one category, like Tourette syndrome and its motor and vocal tics (BOX 6-2).
FIGURE 6-10 The basal ganglia circuit illustrating the direct and indirect pathways.
BOX 6-2 Tourette Syndrome
Tourette syndrome (TS) is a neurological disorder characterized by involuntary motor and/or vocal tics. A tic is a repetitive, involuntary behavior. TS is named after a French neurologist named Georges Gilles de la Tourette (1857-1904), who first described the condition in 1885. The exact etiology of the condition is unknown, but there is evidence through twin studies that it may be inherited. The symptoms of TS emerge in early childhood (age 3 to 9 years), and the condition affects males three to four times more often than it does females. About 200,000 people in the United States suffer from TS, and another 1 in 100 may have a mild FIGURE 6-11, undiagnosed form of the condition. The symptoms of the disease are sometimes the worst in adolescence and improve as the person ages. As mentioned earlier, the primary symptom is tics. Motor tics can involve behaviors like eye blinking, facial grimacing, or sudden jerks of the head. Vocal tics can include grunting or barking sounds, throat clearing, or sniffing. TS is diagnosed through clinical presentation and ruling out other neurological disorders. There is no cure for the condition, but neuroleptic drugs (i.e., antipsychotic drugs like aripiprazole [Ability]) do bring some relief to some people with TS, although patients may complain that the drugs' side effects are worse than the symptoms of TS.
FIGURE 6-11 Georges Gilles de la Tourette.
Internal Capsule and Corona Radiata
Almost all sensory information is relayed through the thalamus with the exception of olfaction, which has its own pathway via the olfactory bulbs. Fibers between the cortical surface and the thalamus create a fan-shaped sheet of axons called the corona radiata (“radiating crown”), which carries nearly all neuron traffic to and from the cerebral cortex (FIGURE 6-12). Much cerebral activity takes place in this dense white matter area because disorders involving it result in significant deficits in cognitive, social, and emotional abilities. Multiple sclerosis is a disorder in which the myelin-producing oligodendroglia are lost, resulting in the white myelin sheath that surrounds axons being either thinned or lost altogether. This leads to multiple scars in places like the corona radiata, and thus the name multiple sclerosis or “multiple scarring” (Reich et al., 2007). The neurons involved can no longer effectively transfer signals, and patients suffer motor, cognitive, and sometimes even psychiatric problems (Daroff, Fenichel, Jankovic, & Mazziotta, 2012).
As the fibers of the corona radiata course down, they taper as they enter the internal capsule, a narrow space between the caudate nucleus and the lenticular nucleus (Catani & de Schotten, 2012). The internal capsule is illustrated in Figure 6-11. The internal capsule bends as it passes between the thalamus and the basal ganglia; the bend is called the genu (Latin for “knee”) of the internal capsule. Lesions to the genu can affect the corticobulbar tract, an important motor pathway for voluntary motor function in the head and neck (e.g., speech), and lead to either hemiplegia or hemiparesis.
Basal Ganglia Disorders
Parkinson Disease
What Is It?
Parkinson disease (PD) is a progressive extrapyrami- dal movement disorder involving degeneration of the substantia nigra and thus the loss of dopaminergic innervation of the striatum. Dopamine has a dampening effect on motor movement; thus, with a loss of dopamine, the dampening effect is lost and muscles become too rigid (FIGURE 6-13). As a result, the direct pathway no longer functions correctly and the indirect pathway dominates function, resulting in overinhibition of movement.
FIGURE 6-12 The internal capsule and corona radiata.
Cause
The cause of PD is unknown, though both environmental toxins and genetics have been suggested etiologies. The disease typically begins around 60 years of age, usually with increasing tremors being the first symptom. In about 10% to 15% of cases, disease onset occurs before age 50 years, as in the case of Michael J. Fox (BOX 6-3).
FIGURE 6-13 The brain in a healthy state versus in a Parkinsonian state. Note the loss of the dampening effect in Parkinson disease.
BOX 6-3 Michael J. Fox and Parkinson Disease
Michael J. Fox is a Canadian American actor best known for his roles in the television show Family Ties and the Back to the Future movies. In 1991 he was diagnosed with early-onset PD, but he kept his condition private until 1998 when his symptoms became more pronounced. As his symptoms worsened, FIGURE 6-14 he reduced his on-screen activities and began to do voice-over work on films like Stuart Little. In 2000, Fox established the Michael J. Fox Foundation, an institution dedicated to finding a cure for PD. He has managed his condition through the use of the drug carbidopa- levodopa (Sinemet), a drug that reduces the symptoms of PD, and through a surgery called thalamotomy, during which select portions of the thalamus are destroyed. Fox continues to appear in television and film, often playing characters who display PD characteristics.
FIGURE 6-14 Michael J. Fox. Kemp, Burns, and Brown (2008) report that about 20% of patients with PD will also develop dementia.
Signs and Symptoms
The three main signs of PD are bradykinesia, tremor, and rigidity (FIGURE 6-15). Bradykinesia (Greek for “slow movements”) is the most significant of the three, robbing the patient of the ability to make timely, smooth movements. Bradykinesia is seen most easily in the Parkinsonian gait, which is characterized by short, shuffling steps and pedestal turning (a slow turning of direction through many small steps). The PD tremor is visible when the hands are at rest (i.e., a resting tremor) and disappears when the person intentionally uses his or her hand. The tremor vibrates at about 3 to 5 Hz. Rigidity in those with PD is most obvious in the face. They display what is called masked facies, which is impairment in the facial muscles that leaves the face looking expressionless. Rigidity is also present in the respiratory muscles, leaving the person with less-than-ideal breath support for speech. Patients often complain of hypophonia, or weak voice.
Diagnosis and Treatment
Diagnosis of PD is made through a neurological exam. At this time, there is no test that clearly identifies the disease. In addition, there is no cure for PD. Dopamine-based drugs can alleviate the symptoms for a time but do nothing to treat the underlying disease process. Pallidotomy, a surgical procedure in which lesions are made on the medial globus pallidus, has had mixed success in reducing akinesia and increasing movement. In another surgical method, deep brain stimulation, an electrode is implanted in the brain and electrically stimulates the medial globus pallidus, which disrupts its inhibiting effect.
FIGURE 6-15 Clinical presentation of Parkinson disease.
Huntington Disease
What Is It?
Huntington disease (HD) is a progressive hereditary neurological disorder (FIGURE 6-16). It affects 12 out of every 100,000 people and commonly presents between the ages of 35 and 42 years.
Cause?
HD is caused by a mutation on chromosome 4. This mutation is passed to offspring through an autosomal dominant inheritance pattern, meaning that parents with HD have a 50% chance of having children who eventually develop the disease. When HD appears, it degenerates the basal ganglia and enlarges the brain’s ventricles (FIGURE 6-17).
FIGURE 6-16 The autosomal dominant pattern of inheritance. Note the 50% chance of parents passing the Huntington gene onto their offspring.
FIGURE 6-17 A coronal section of a normal brain versus a brain with Huntington disease.
© Blamb/Shutterstock.
BOX 6-4 A Case of Huntington Disease
Joe was diagnosed with HD when he was 36 years old. His first symptoms were mood problems and a decrease in cognitive abilities. These symptoms worsened over time and were joined by involuntary writhing movements called chorea. In addition, Joe developed obsessive-compulsive behaviors. One of these was an obsession with collecting train paraphernalia, which quickly cluttered his house and frustrated his wife Marilyn. As Joe continued to deteriorate, he became a safety risk for wandering and falls. Eventually Marilyn had to put Joe in assistive care. Joe grew increasingly unstable and, during a home visit, he took a gun from his gun safe and shot himself Joe's two children have both been tested for HD and, fortunately, neither child has the gene.
Signs and Symptoms
The first signs are being fidgety and clumsy. In many ways, HD is the opposite of PD. In HD, the loss of neurons in the striatum results in impairment to the indirect, inhibiting pathway, resulting in increased movement (hyperkinesia) in the form of involuntary writhing movements (chorea). Worsening depression and dementia also occur in this population. Some patients, because of their diagnosis and prognosis, opt to commit suicide (BOX 6-4).
Diagnosis and Treatment
HD is hypothesized through a person’s clinical presentation and then confirmed through a genetic test. Computed tomography and magnetic resonance imaging scans may be ordered, but sometimes changes in the basal ganglia are not apparent. Like PD, there is no cure for HD. Medications are available to reduce symptoms such as chorea. Psychiatric medications can be prescribed to treat depression, psychosis, and behavior problems. There also are medications that improve the memory issues characteristic of dementia. As the person’s condition worsens, it usually becomes more and more difficult for the family to care for him or her, and care in a long-term nursing home may eventually be needed.
Basal Ganglia and Aphasia
It is unclear whether damage to the basal ganglia causes aphasia. Two aphasic syndromes have been suggested. Damasio, Damasio, Rizzo, Varney, and Gersh (1982) reported Wernicke-type aphasia; Wallesch and colleagues (Brunner, Kornhuber, Seemuller, Suger, & Wallesch, 1982; Wallesch, 1985; Wallesch et al., 1983; Wallesch & Wyke, 1985) described transcortical motor aphasia (TABLE 6-3) . More recently, Radanovic and Scaff (2003) reported three patients with motor-articulatory issues (not aphasia) and one patient with transcortical motor aphasia. It appears that damage to the basal ganglia can lead to a variety of deficits, including dysarthria, dysphonia, and comprehension, naming, and repetition problems (Radanovic & Scaff, 2003).
TABLE 6-3 Nonfluent Aphasic Conditions Compared
|
Broca |
Transcortical Motor |
Mixed Transcortical |
Global |
|
|
Fluency |
- |
- |
- |
- |
|
Auditory comprehension |
+ |
+ |
- |
- |
|
Repetition |
- |
+ |
+ |
- |
+, preserved; -, significantly impaired.
► The Brain Ventricles
Structure and Function
In addition to white and gray matter, the brain contains spaces called ventricles. There are four ventricles in the brain: a right and left lateral ventricle (also called first and second ventricles), a third ventricle, and a fourth ventricle (FIGURES 6-18 and 6-19). The right and left ventricles look like horseshoes, the third ventricle like a misshapen donut, and the fourth ventricle like a diamond. The left and right ventricles each have three horns (i.e., projections). The anterior horn is located in the cerebral hemisphere’s frontal lobes, and the posterior horn is in the parietal lobe. The inferior ventricle horn is located in the temporal lobe. The body of the lateral ventricle connects the anterior and posterior horns. The third ventricle is located at midline in the diencephalon. It articulates with the two lateral ventricles via the intraventricular foramen. The third ventricle narrows near the midbrain to form the long cerebral aqueduct, which leads to the fourth ventricle located posterior to the pons and anterior to the cerebellum. A pair of lateral recesses course under the cerebellum and connect with the subarachnoid space of the meninges at the cerebellomedullary cistern. This connection means that the ventricular system and the subarachnoid space are continuous.
All the brain’s ventricles are filled with cerebrospinal fluid (CSF), a clear and colorless fluid that looks like plasma. CSF is also found in the subarachnoid space of the meninges (FIGURE 6-20). The choroid plexus (Latin for “the delicate knot”) is a structure located in each ventricle that produces CSF at a rate of 400 to 500 milliliters (mL) each day. CSF moves between the ventricles via the interventricular foramen and the cerebral aqueduct. Overall, there is approximately 125 mL of CSF in the nervous system, which is replenished every 7 hours. Old CSF is absorbed into the venous system through the arachnoid villi, which are small bumps that protrude into the brain’s venous system. This allows CSF to exit the subarachnoid space and enter the bloodstream.
FIGURE 6-18 Coronal view of the ventricular system. (Fourth ventricle not pictured.)
FIGURE 6-19 Lateral view of the ventricular system.
FIGURE 6-20 Cerebrospinal fluid (CSF) shown residing in the meninges.
CSF has four basic functions. First, it protects brain tissue by acting as a water cushion. Second, it lightens the weight of the brain from 1,400 grams to about 25 grams through buoyancy (think of how much lighter you seem in a pool). Third, it reduces waste by removing metabolic waste from the nervous system. Fourth, CSF helps to transport nutrients and hormones to the brain.
Disorders of the Ventricles
The most well-known condition involving the ventricles and CSF is hydrocephalus (Latin for “water brain”), in which CSF accumulates in the brain ventricles causing brain tissue to be compressed against the skull (FIGURE 6-21). There are two forms of hydrocephalus, obstructive and nonobstructive. In obstructive hydrocephalus, a narrowing (stenosis) of the passageways that connect the ventricles can lead to CSF buildup because CSF cannot freely move through the system. Nonobstructive hydrocephalus involves problems in the absorption of CSF (and in rare cases, the production of CSF). For example, a person may not reabsorb old CSF, resulting in swollen brain ventricles.
Hydrocephalus can be congenital or acquired through brain injury, meningitis, or tumor. It is a very dangerous condition in that, if not treated, it can result in increased intracranial pressure leading to severe brain damage and even death. It is probably a more serious condition for adults than for infants because adult skulls are fused. Infant skulls do not completely fuse until about 18 months of age, and thus, less pressure is put on the brain.
Most cases of hydrocephalus are treated with surgically inserted shunts, which act as drainage pipes for excess CSF (FIGURE 6-22). Typically, a small incision is made in the scalp and skull, and the shunt is placed through the brain into one of the ventricles. The line is then run down the skin and into the peritoneal cavity, the tissues of which can absorb the incoming CSF. Potential problems can arise with a hydrocephalic shunt. When the scalp is cut and a hole is made in the skull, a hemorrhage between the skull and brain can occur. In addition, a hemorrhage can occur in the brain itself, resulting in damaged brain tissue and further neurological problems. Patients may also experience bleeding in the ventricles. There is also a risk of introducing a bacterial infection into the CSF. Lastly, sometimes shunts become plugged and need to be redone (Shipley, 2012).
FIGURE 6-21 A. Skull of child with hydrocephalus. B. Photo of a child with hydrocephalus.
(A) Courtesy of Vimont, Engelmann/National Library of Medicine; (B) © Donal Husni/NurPhoto/Getty Images.
FIGURE 6-22 Illustration of a ventricular-peritoneal shunt.
► Conclusion
When we think of the brain, we usually think of a bumpy, spherical mass. This chapter has attempted to open the brain up and see the interesting structures inside. Who knew that we had not one, but four holes in our head! Much attention has been given to the cerebral hemispheres, but as this chapter has attempted to demonstrate, there is much going on below the surface.
SUMMARY OF LEARNING OBJECTIVES
The following were the main learning objectives of this chapter. The information that should have been learned is below each learning objective.
1. The learner will list the four parts of the diencephalon and briefly describe the function of each.
• Thalamus: a sensory fiber relay station or 3. switchboard between the cerebral cortex and subcortical areas that processes all sensory (except olfaction) information flowing to and from the cerebral cortex. 4.
• Subthalamus: a part of the diencephalon that may play a role in the selection of actions and impulse control.
• Hypothalamus: a linkage that connects the nervous system to the endocrine system (i.e., hormonal system) via the hypothalamus’s 5. pituitary gland. In addition, the hypothalamus is a regulator because it controls aspects of homeostasis (e.g., metabolism, body temperature, food intake).
• Epithalamus: a part of the diencephalon that has functions related to the sleep-wake cycle, circadian rhythms, olfaction, and stress responses.
2. The learner will draw the basal ganglia and describe the function of these nuclei.
• The three nuclei are the caudate nucleus, globus pallidus, and putamen.
• These nuclei function via direct and indirect pathways to facilitate and inhibit movement.
The learner will list the names of the four brain ventricles.
• We have four ventricles: a right, left, third, and fourth ventricle.
The learner will describe cerebrospinal fluid’s composition and its function.
• It consists of a clear and colorless fluid that looks like plasma.
• It has four functions: protection, buoyancy, waste reduction, and transport.
The learner will list and briefly describe disorders associated with the diencephalon, basal ganglia, and brain ventricles.
• Thalamic pain syndrome (Dejerine-Roussy syndrome): a condition involving burning or tingling sensations and possibly hypersensitivity to stimuli that would not normally be painful, such as a light touch or a temperature change.
• Thalamic aphasia: a type of aphasia characterized by fluent verbal output, semantic paraphasias, jargon, mildly impaired auditory comprehension, and minimally impaired repetition.
• Parkinson disease: a progressive extrapyramidal movement disorder involving degeneration of the substantia nigra and thus the loss of dopaminergic innervation of the striatum.
• Huntington disease: a progressive hereditary neurological disorder that leads to increased movement (hyperkinesia) in the form of involuntary writhing movements (chorea). Worsening depression and dementia also occur in this condition.
• Aphasia associated with the basal ganglia: Findings are unclear. Damage to the basal ganglia can lead to a variety of deficits, including dysarthria, dysphonia, and comprehension, naming, and repetition problems. Transcortical motor aphasia and Wernicke aphasia are possibilities, according to the literature.
• Hydrocephalus: a condition that occurs when cerebrospinal fluid accumulates in the brain ventricles, causing brain tissue to be compressed against the skull.
KEY TERMS
Akinesia Akinetic mutism Amygdala Basal ganglia Caudate nucleus
Cerebrospinal fluid (CSF) Choroid plexus Corona radiata
Dyskinesia
Genu
Globus pallidus
Homeostasis
Hydrocephalus Hypersomnia Internal capsule Lateral geniculate body (LGB)
Lenticular nucleus
Medial geniculate body (MGB)
Pineal gland
Pituitary gland
Putamen
Striatum
Thalamic aphasia
Thalamic pain syndrome tic
DRAW IT TO KNOW IT
1. Draw a coronal section of the brain (see 2. Figures 6-1 and 6-8) and label the following: epithalamus, thalamus, subthalamus, hypothalamus, caudate nucleus, putamen, globus palli- dus, right lateral ventricle, left lateral ventricle, third ventricle, and fourth ventricle.
Draw a sagittal (or medial) section of the brain (see Figure 6-2) and label the following: medulla, pons, midbrain, thalamus, hypothalamus, cerebellum, lateral ventricle, corpus callosum, and cerebral hemispheres.
QUESTIONS FOR DEEPER REFLECTION
1. Compare and contrast thalamic aphasia to other types of aphasia.
2. What role does the basal ganglia play in speech?
3. Predict what the speech of people with Parkinson disease and people with Huntington disease is like.
CASESTUDY
Ellie is a 41-year-old female who has become progressively ill in the last few years. More specifically, her movements have become increasingly erratic with chorea, her speech slurred, and she has developed severe depression. Ellie has fallen several times and her husband is thinking of putting into a longterm care facility because it is becoming more and more difficult to care for his wife. Ellie’s brother Doug (age 45) is developing symptoms similar to Ellie.
1. Which of the following conditions do you think Ellie and Doug have?
a. Parkinson disease
b. Thalamic pain syndrome
c. Huntington disease
d. Myasthenia gravis
2. Why did you pick the answer you did?
3. What brain structure is most likely involved in this condition?
4. What is the long-term outlook for Ellie and Doug?
SUGGESTED PROJECTS
1. Write a two- to three-page paper on the topic of thalamic aphasia.
2. Find a case study about thalamic aphasia in the scholarly literature and present the case to the class.
3. Write a two- to three-page paper on the role of the basal ganglia in speech production.
4. Find someone with either Parkinson disease or Huntington disease and interview the patient and/or his or her family.
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