Parkinson Disease (PD)

Parkinson Disease (PD)

In his famous piece, “Essay on the shaking palsy,” James Parkinson described a condition he called “paralysis agitans” and believed the medulla, part of the brainstem, was involved. Based on his observations that individuals with the condition showed little to no cognitive impairment, Parkinson hypothesized that the cerebral cortex, the outer crust of the brain, was not involved.
 
We now know, thanks to Jean Martin Charcot and others who were studying the condition, that Parkinson Disease does involve pathological changes within the cerebral cortex.  55-60 million people are affected with Parkinson Disease worldwide, and this number will continue to climb as life expectancy increases and the world population ages. Idiopathic Parkinson’s Disease, Familial Early Onset PD, and Juvenile Onset PD make up the majority of cases. Women are less likely to develop PD because estrogen may be protective.

The classic motor symptoms of PD

(1) Pill Rolling Tremor. Tremors occur due to rhythmic contractions of opposing muscle groups. In PD, the tremor typically occurs at a frequency of 3-5 muscle contraction cycles per second (Hz). The tremor occurs at rest, usually starts unilaterally in the hands and fingers, and diminishes during voluntary movement and sleep. As the disease progresses, the tremor becomes more prominent, bilateral, and eventually rigidity develops. 

(2) Bradykinesia or hypokinesia (slowed movement). Patients with PD are slow in their movements, have difficulty with fine motor control, and experience problems writing and/or using their hands to play instruments or type on a computer keyboard. Spontaneous movements and associated movements are diminished. When we talk to others, we use facial gestures and hand movements to express our emotions; this requires spontaneous movement. When we walk, we swing our arms and move our legs in a specific way to keep our balance; this requires associated movements. When we feel thirsty while reading this lesson, we may reach for our cup of coffee (or water, or wine); this requires initiation of movement. All of these are affected in patient’s with PD which leads to the classic findings of “masked-like” facial expressions, minimal arm swing/shuffling gait, and delayed initiation of voluntary movement. 

(3) Rigidity (Cogwheel Rigidity). When apposing muscle groups are contracting at the same time or are always in a contracted state, this leads to muscle rigidity. When you try to move the arms of an individual with PD, you will find it difficult. There is a passive resistance to movement similar to the negativism or waxy flexibility seen in catatonia. 

(4) Posture problems. Standing or sitting up straight requires the axial/postural muscles. Patient’s with PD have difficulty maintaining appropriate posture and are seen stooped or hunched over. 

The classic “non-motor” symptoms of PD

(1) Cognitive deficits. About 30% of patients with PD may have an accompanying dementia or other forms of cognitive loss. The cognitive deficits seen in patients with PD are thought to be a result of damage to the cholinergic system (especially the nucleus basalis of Meynert) and buildup of insoluble protein aggregates within cortical and sub-cortical structures. 

(2) Constipation. Likely related to cholinergic dysfunction. 

(3) Olfactory deficits. Diminished sense of smell may occur before motor symptoms.

(4) Autonomic Dysfunction. Disrupted sympathetic innervation to the heart and other important organs may lead to bradycardia and hypotension, urinary retention, postural hypotension, sweating, and drooling. 

(5) Rapid Eye Movement Sleep Behavior Disorder (RBD) and other sleep disturbances. During sleep, patients with PD may “act out” their dreams. REM sleep is normally characterized by muscle paralysis. RBD occurs when paralysis of muscles does not occur, and patients move around while dreaming. Insomnia and other sleep disturbances are common in patients with PD.

(6) Mood and personality changes. Depression, apathy, anxiety, irritability, agitation, and psychosis often occur in patients with PD–likely related to destruction of serotonergic projections from the raphe nucleus, noradrenergic projections from the locus coeruleus, cholinergic neurons, dopaminergic projections from the ventral tegmental area, and glutamatergic/GABAergic neurons. 

The Motor System: A Brief Review

The two main motor tracts that control skeletal muscles for movement originate in the motor cortex of the brain and are called the corticospinal tract (or pyramidal tract) and the corticobulbar tract. The corticospinal tract starts in the motor cortex and many, but not all, of the neuronal fibers cross over (or decussate) at the medullary pyramids to eventually synapse on lower motor neurons in the contralateral anterior horn of the spinal cord. Therefore, the right motor cortex primarily controls the left side of the body. Similarly, the corticobulbar tract originates in the motor cortex and many, but not all, of the neuronal fibers cross over and synapses on lower motor neurons in the brain stem. Therefore, the right motor cortex primarily controls movement of the muscles on the left side of the body.

Descending Motor Pathways

The execution of motor movements involves more than just these two tracts. Information from the cortex and subcortical areas are integrated and processed and eventually transmitted to the motor cortex to execute movement. We call these motor “coordination/integration” pathways in the brain “extra pyramidal” because they occur outside of the traditional corticospinal/corticobulbar (pyramidal) motor tracts. The structures that make up the Extrapyramidal motor system include the Basal Ganglia, the Substantia Nigra (in the midbrain), and the subthalamic nucleus.

Extrapyramidal Motor System

The structures that make up the Extrapyramidal motor system include the Basal Ganglia, the Substantia Nigra (in the midbrain), and the subthalamic nucleus. Basal Ganglia, as discussed in the Anatomy sections, include the caudate nucleus, the putamen nucleus, and the globus pallidus. The caudate nucleus and the putamen nucleus are called the “striatum” or “neostriatum” whereas the three nuclei together are referred to as the “corpus striatum.” The globus pallidus is divided into two parts, the internal and external parts. The substantia nigra (SN) is located in the midbrain and is named after the black pigmented neuronal cell bodies seen on anatomical sections. The SN is divided into two parts, the pars reticulata (SNr) and the pars compacta (SNc)  Neuronal inputs to the striatum come from the motor cortex, substantia nigra pars compacta, the thalamus, and subcortical structures like the dorsal raphe nucleus. Information is processed in the striatum and eventually sent to the thalamic nuclei and back to the motor cortex. But from the striatum to the thalamic nuclei, neurons first synapse at the the globus pallidus internal (GPi) and substantia nigra pars reticulata (SNr). From the GPi/SNr, neuronal signals are sent to the thalamic nuclei. To make things more complicated, some of the neurons from the striatum don’t directly synapse on the GPi/SNr and instead take a detour at the globus pallidus external (GPe) and subthalamic nucleus (STN).

 

The extrapyramidal system and the motor nuclei of the thalamus form a loop that obtains information from the cortex and other areas of the brain, integrates and processes it, and relays it back to the motor cortex to modulate motor activity in the corticospinal and corticobulbar tracts. The diagrams can give you a headache but the simple concept is that stimulation of the direct pathway leads to activation of the motor cortex. On the other hand, stimulation of the indirect pathway decreases activation of the motor cortex (“indirect pathway inhibits”). This occurs through a series of glutamatergic and gabaergic connections. Please note that the diagrams are overly simplistic and do not show many of the other neurotransmitters involved (substance P, opioid peptides, neurokinin, etc). 

Dopamine (DA) modulates this circuit. Stimulation of D1 receptors in the striatum by dopamine stimulates the direct pathway and increases motor cortex activity. Stimulation of D2 receptors in the striatum inhibits the indirect pathway. Therefore, the overall effect of dopamine is to increase activity at the motor cortex. Loss of dopamine input from the SNc (as seen in Parkinson’s Disease) or blockade of dopamine receptors via medications (like antipsychotics) will decrease activity in the motor cortex and cause slowed movements. 

Acetylcholine (ACh) has the opposite effect of DA. Stimulation of M2 receptors in the indirect pathway stimulates the indirect pathway whereas stimulation of M1 receptors in the direct pathway inhibit the direct pathway. This leads to an overall net effect of decreased activity at the motor cortex. Note that ACh interneurons are activated by the glutamate neurons from the motor cortex but are normally under inhibitory control by DA from the nigrostriatal tract. This is why we give anticholinergic medications like benztropine (Cogentin) to help with extrapyramidal symptoms (EPS) and/or the slowed movements associated with Parkinson Disease. 

 

L-Dopa

Dopamine does not readily cross the blood brain barrier. However, its precursor, L-Dopa, does. The mainstay treatment for Parkinson’s disease is “dopamine replacement” with L-Dopa. Because L-Dopa is vulnerable to enzymatic degradation (via monoamine oxidases/MAOs,  Catechol-O-Methyl transferases/COMTs, and aromatic amino acid decarboxylases/AADCs) within the intestine, L-Dopa is often administered with blockers of these enzymes (e.g. carbidopa) to ensure L-dopa reaches the brain. Despite the presence of inhibitors of these enzymes, therapeutic doses of L-Dopa still have some conversion to dopamine and norepinephrine in the periphery. This may explain the side effects seen with L-Dopa treatment.  

Side effects of L-Dopa are thought to result from the conversion of L-Dopa to dopamine, norepinephrine, and/or epinephrine in the periphery. Dopamine in the medulla stimulates the chemoreceptor trigger zone to cause nausea/vomiting and dopamine alters gastrointestinal smooth muscle activity leading to gastrointestinal side effects. The conversion of L-Dopa to norepinephrine in the periphery can lead to sympathetic activation of the heart (tachycardia) and other cardiac arrhythmias. Neuropsychiatric symptoms such as depression, anxiety, agitation, aggression, impulsivity problems (gambling, spending) and psychosis (hallucinations and paranoia) can occur as side effects of L-Dopa treatment.

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