The substantia nigra is a layer of pigmented nuclei composed of gray matter and located between the reticular formation of the midbrain and the cerebral crura.
This midbrain basal ganglia structure is further divided into a reticulata (a network-like construction) and a compacta (a close-together construction) part based on the position of neurons within each part.
The substantia nigra is responsible for the production of dopamine in the brain, as well as for the control of the reward-seeking processes.
In addition, it’s also involved in some of the cognitive processes of the brain, including learning, as well as in the regulation and control of motor planning, eye movement, behaviors, emotions, and addictions to drugs.
What Is the Structure and Function of the Substantia Nigra?
Even though it’s often referred to as one single structure of the brain, there are – in fact – two distinct parts of the substantia nigra:
- Substantia nigra pars reticulata
- Substantia nigra pars compacta
Substantia Nigra Pars Reticulata
Substantia nigra pars reticulata is a midbrain structure whose neuronal cells project their axons to the thalamus, midbrain, and the brain stem. Its main function is to direct the information received from the sensory stimuli back to the striatum, or to the thalamus and the brain stem.
These neuron fibers receive signals from the striatum via two neuronal pathways: the striato-nigral direct pathway, and the indirect striato-pallido-subthalamic-nigral pathway.
The pars reticulata also processes the GABA signals, i.e. the signals from the neurotransmitters that control many of the cortical functions, vision, as well as anxiety.
Substantia Nigra Pars Compacta
Substantia nigra pars compacta mainly consists of dopaminergic cells that produce the neurotransmitter dopamine.
Being a part of the basal ganglia structure, the substantia nigra pars compacta regulates movement and the rewarding system of the brain.
What Is a Dopaminergic System?
The substantia nigra is part of the dopaminergic system of the brain. The dopaminergic system comprises bundles of dopaminergic neurons, i.e. tracts that produce the neurotransmitter dopamine, which makes it one of the biggest dopamine producers in the brain.
These sets of neuronal fibers project toward the other brain structures in order to release the dopamine which is synthesized in the neuron cells of the tracts.
The dopaminergic pathways these neuron tracts create along this process support various functions of the brain, such as reward, motivation, control of the neuroendocrine system, etc.
What Is Dopamine and Why Is It Important?
Dopamine is a neurotransmitter that is produced in the brain. It serves as a chemical agent that supports the transmission of sensory information between neurons.
Its production is triggered when the brain is expecting a reward, as a result of the encoded information gained from the sensory stimuli, which constitutes the cycle of motivation, reward, and reinforcement.
For instance, when we associate a certain type of activity with some kind of pleasure as a reward, the anticipation of that particular reward signals the brain to start producing dopamine.
Any enjoyable activity or sensation can trigger it – the smell of our favorite food, the lyrics of our feel-good song, the expectation of a happy event, etc.
This neurotransmitter is not only responsible for our positive feeling, but also for various vital functions of the body. The list includes:
- heart and kidney function
- memory and focus
- mood and emotions
- executive functioning
- pain processing
- pancreatic function and insulin regulation
- motor control
- blood flow
- stress response
- digestion
- pleasure-seeking behavior
- reward-seeking behavior
- sleep
Together with other neurotransmitters, such as adrenaline and serotonin, dopamine supports and stimulates the cognitive processes, planning, and productivity.
What Type of Neurons Produces Dopamine?
Dopaminergic neurons are part of the structure of substantia nigra pars compacta. They are not many in number, yet they play a significant role in the processes that require dopamine, such as the control of voluntary movement, mood, behavior, stress, and addiction.
These neurons contain neuromelanin and are related to the most prominent neurological condition – Parkinson’s disease. Namely, the loss of dopaminergic neurons results in the progressive dysfunction of the central nervous system.
What Are the Dopamine Pathways of the Brain?
There are 4 major dopamine pathways of the brain:
- Mesolimbic Dopamine Pathways
- Mesocortical Dopamine Pathways
- Nigrostriatal Dopamine Pathways
- Tuberoinfundibular Dopamine Pathways
Mesolimbic Dopamine Pathways
This dopamine pathway supports the primary function of the substantia nigra – pleasure and reward. Stimulation of the brain’s nucleus accumbens with pleasurable sensations (such as food, drugs, etc.) leads to an increase of the dopaminergic activity within the mesolimbic dopamine pathways, that is, stimulates the release of dopamine.
This neurotransmitter sends the information to the nucleus accumbens which, in turn, creates positive feelings.
Mesocortical Dopamine Pathways
The dopaminergic projections within the mesocortical dopamine pathway transmit the action potentials to the prefrontal cortex via dopaminergic neuron fibers.
Since the prefrontal cortex plays a vital role in the cognitive processes of the brain, as well as in decision making, and the consolidation and storage of working memory, when damage is inflicted to this structure, the affected individual could lose the ability to make decisions and to focus on a task.
Nigrostriatal Dopamine Pathways
The nigrostriatal dopamine pathways support motor planning. The dopamine neurons project their fibers from the substantia nigra to the parts of the basal ganglia – the caudate and the putamen.
Dopaminergic neurons comprising the nigrostriatal dopamine pathways trigger purposeful movement. If the number of these neurons becomes reduced, our brain’s control of motor movement could become impaired.
The symptoms of a dysfunctional nigrostriatal dopamine pathway include tremor, spasms, and signs of Parkinson’s disease.
Tuberoinfundibular Dopamine Pathways
The dopamine neurons of the tuberoinfundibular dopamine pathways project their fibers from the paraventricular nuclei of the hypothalamus to the infundibular area of the hypothalamus.
The function of dopamine in this pathway is to inhibit the release of the hormone prolactin, secreted by the pituitary gland.
What Happens If the Substantia Nigra Becomes Dysfunctional?
Damage inflicted on the substantia nigra or dysfunctionality of any of its comprising structures can lead to various medical conditions.
Even though dopamine is related to feelings of happiness and good mood, lack of sleep can drastically lower its levels in the brain. This is the reason why we’re grumpy and moody when we are tired and haven’t had enough sleep.
In addition, there are also several medical conditions related to low levels of dopamine in the brain:
- Parkinson’s disease – A neurological disorder that affects the part of the brain that controls the movement of our body. It occurs when the cells of the substantia nigra start to die out. This disease is characterized by tremors, rigid muscles, difficulty making quick and smooth movements, as well as difficulty in standing and walking.
- Depression – A mood disorder that affects the way an individual feels, perceives events and surroundings, and affects their manner of thinking and behaving. This can lead to behavior with negative consequences.
- Dopamine transporter deficiency syndrome – Also known as infantile parkinsonism-dystonia syndrome, is a rare disorder of movement which results from the loss of dopamine transporting neurons.
- Lethargy – A pathological state of tiredness and fatigue, characterized by a severe lack of energy, motivation, apathy, and enthusiasm. Unlike the majority of neural diseases and conditions, lethargy can be resolved with plenty of rest and sleep, exercising, and proper nutrition.
- Huntington’s disease – A condition that leads to the progressive degeneration of brain cells. The patients affected with this disease note difficulty in concentration, clumsiness, memory lapses, as well as difficulty with swallowing, moving, speaking, and breathing.
- Tourette’s syndrome – A neurological disorder caused by structural abnormalities of the substantia nigra and the nuclei surrounding this basal ganglia structure. Typical symptoms include frequent motor and phonic involuntary tics or movements.
On the other hand, overtly high levels of dopamine can also cause or trigger unpleasant conditions. Such are the following:
- Mania – A state of abnormally elevated mood, intense energy, and extreme and exaggerated behaviors. The affected individuals can also experience psychosis, such as hallucinations and delusions (a distorted image of reality).
- Obesity – Obese individuals have fewer dopamine receptors, which aids in the increase of their body weight. Namely, when less dopamine is stimulated, the brain triggers sugar cravings as compensation.
- Schizophrenia – When the dopamine is overstimulated in the mesocortical dopamine pathways, the axons of the dopaminergic neurons trigger the areas of the brain that signal aggressiveness and psychotic symptoms. This leads the affected individual to a state of distorted reality, or delusion and hallucination.
To Sum Up
Even though the substantia nigra is a small structure of the brain, it plays a big role in many vital functions of the body that are controlled and regulated by the brain. The regulation of the motivation – reward – reinforcement cycle is the most characteristic function of the substantia nigra.
As the biggest producer of dopamine in the brain, it’s responsible for both the feelings of pleasure and happiness, but also for the regulation and control of learning, emotions, addictions, etc.
When there’s a disruption of the flow of information between the dopaminergic neurons, it results in the dysfunctionality of the structures comprising the substantia nigra.
Fun Facts
Did you know?
- The term substantia nigra was derived from Latin, and it means ‘black substance’. This is due to the high concentration of neuromelanin in the neuron cells comprising this brain structure.
- Neuromelanin is the pigment of some neuron cells, which contributes to the specific color of some brain areas and tissues.
- Cerebral crura are a bundle of large, neuron fibers that cross each other, and connect both left and the right cerebral hemisphere with the opposite part of the cerebellum.
- Melanin is the pigment that gives us the dark tan of our skin when we expose it to the sun.
- We can naturally increase our dopamine levels by eating a lot of proteins, less saturated fat, frequent exercising, getting sufficient sleep, relaxing with music, as well as enjoying the sunlight. Also, supplements like iron, niacin, folate, and vitamin B6, also contribute to the natural production of dopamine in the brain.
- Nucleus accumbens is a collection of neurons activated by dopamine and serotonin. They play a significant role in the reward process in the brain, as well as aid the regulation of appetite and desire.
- The nigrostriatal dopamine pathways contain more than 80% of the dopamine in the human brain.
- The prolactin hormone is produced by the pituitary gland and is of great importance for both male and female reproductive health due to its role in lactation. This hormone also regulates some behaviors, as well as the immune system and our body’s metabolism.
- An abnormal increase in the hormone prolactin can negatively affect the menstrual cycle, libido, fertility, or bone health.
- The hormone dopamine supports our good feelings and mood. But, when its levels in the brain increase, it can produce a feeling of euphoria – an intense feeling of happiness. Speaking of euphoria, the most euphoric feeling in the world is love.