Parkinson's disease is caused by the loss of nerve cells in the substantia nigra, an area of the brain responsible for initiating voluntary movement.
What are nerve cells? How do they work? What goes wrong in Parkinson's?
Imagine a big room crowded with people; everyone has rigid, spikey hair and a thin pencil-like nose. When someone nods, their nose touches a spike of hair on another person and causes that person to nod; in this way the signal cascades around the room.
Your brain is a bit like this. It is made of billions of nerve cells and each nerve cell is made of dendrites (the spikey hair) attached to a cell body with a protruding axon (the pencil-like nose); but instead of nodding, electrical impulses are passed on.
The surface of the axon has many tiny doors (called ion channels). When the nerve cell is at rest the doors are closed and the inside of the axon is negatively charged (i.e. it contains negatively charged particles). When a nerve cell is stimulated a door is opened, allowing positively charged particles to enter; the positive charge causes other doors to open and therefore a wave of door opening and positive charge flows along the axon.
The end of the axon forms a synapse with a dendrite; they don't actually touch so how is the electrical impulse passed from axon to dendrite? The arrival of the impulse stimulates release of neurotransmitters (e.g. dopamine) from the end of the axon and they cross the gap and bind to receptors on the dendrite. This causes doors to open on the next nerve cell, positively charged particles to enter and the electrical impulse to proceed.
Volunteering to move
Nerve cells stimulate other nerve cells or activate muscles. The brain coordinates and directs the activity of nerve cells to navigate our environment and respond to external stimuli. Our response is either voluntary or involuntary.
To initiate voluntary movement the dorsolateral frontal cortex of the brain receives visual and auditory information from the visual and auditory cortices. In parallel, the putamen supplies memories to the posterior parietal cortex. The dorsolateral frontal cortex and posterior parietal cortex formulate conscious intention; they then signal, via the basal ganglia (including the substantia nigra) and thalamus, the premotor and motor cortex. The motor cortex initiates and coordinates movement with help from the cerebellum and basal ganglia.
What happens in Parkinson's disease
Parkinson's disease is caused by gradual loss of dopamine producing nerve cells in the substantia nigra, which is part of the basal ganglia. Therefore, when electrical impulses arrive at the basal ganglia from the dorsolateral frontal and posterior parietal cortices, reduced levels of the neurotransmitter dopamine makes the activation (across the axon/dendrite gap) of nerve cells leading to the motor cortex less efficient. This disrupts the initiation of movement (despite intact conscious intention) and the coordination of movement.
See Roberts, A The complete human body (Dorsley Kindersley)