The human nervous system is incredibly complex and controls all aspects of how we experience our lives (behaviour, development, emotions, etc.). Despite the multitude of research that has been and is being performed, there is so much that is unknown. The information we do have constantly changes and evolves as research unveils new theories. The human nervous system includes the central nervous system (CNS) as well as the somatic and autonomic nervous systems. I will review the CNS only in this article, which consists of the brain and spinal cord. The brain is studied via two main methods: 1) experimental ablation, which involves damaging specific areas of the brain and observing effects on behaviour; and 2) electrical stimulation, which involves stimulating parts of the brain with electrodes and similarly observing the effects.
There are two main types of nerve cells in the CNS: neurons and glia. Neurons send electrical signals to convey messages to other neurons, muscles, or glands. Glia mainly provide structural support, but other types of glia also perform different functions. For example, some remove waste and debris, some build myelin sheath, and others guide migration and growth of axons and dendrites during embryonic development. Recent studies have revealed that glia are also involved in conducting electrical impulses along the neurons. The structure of a neuron includes then soma, dendrites and axon (reference image below). The soma is the cell body that contains the nucleus and other cellular structures. The dendrites are narrowing tree-like branches off the soma that receive messages from other neurons. The greater their surface area, the more information they receive. The axon is a thin fibre of constant diameter and is usually longer than the dendrites. It conveys impulses to other neurons. A neuron has multiple dendrites, but only one axon. All three of these structures are lined with synaptic receptors. The axon is covered with a fatty myelin sheath which increases the speed of conduction of the electrical impulses. This sheath is segmented – it has gaps, which are called Nodes of Ranvier. The signal jumps from node to node, resulting in increased speed – this is called saltatory conduction. There are two types of axons: efferent axons carry information away from a structure, while afferent axons carry information into a structure.
The blood-brain barrier protects the CNS from infection. It keeps the majority of chemicals out of the brain. Most substances that enter the bloodstream cannot enter the brain. This is a safety system that prevents viruses from accessing the brain. If a virus does successfully enter, it will most likely stay with that individual for the rest of their life. Only small, uncharged molecules (eg. oxygen and carbon dioxide) and molecules that dissolve in the fats of the membrane (eg. vitamins A and D, heroin, marijana, and antidepressants) cross the barrier.
The membrane of a neuron maintains an electrical gradient, which is a difference in electrical charge between the inside and outside of the cell. There is a slightly negative charge inside the membrane in comparison with the outside. There is a high concentration of sodium ions outside the cell and a high concentration of potassium ions inside the cell. Stimulation of a neuron causes an action potential to occur, in which the negative proteins from inside rush out and the positive proteins from outside rush in, in order to keep the balance of the charge. This is how messages are sent throughout our nervous system. The voltage of a resting cell is approximately -70mV. An action potential occurs when the neuron is stimulated to -55mV, called the threshold of excitation. If this threshold is not reached, an action potential will not occur. Once the threshold is reached, a rapid depolarization up to +40Mv occurs – known as an action potential. The neuron then goes back down to resting state, but overshoots. This is known as the refractory period, which lasts about one millisecond. During this period, there is an absence of action potentials. The cell must return to resting stated (-70mV) before another action potential can occur. The graph below illustrates this process.
If the threshold of excitation is reached and an action potential occurs, this is known generally as excitation. On the other hand, if an action potential does not occur, this is known as inhibition, as the message will not be passed on. Action potentials are based on the all-or-none law, meaning that the intensity of the stimulus does not affect the intensity of the action potential. Rather, if a stimulus is intense, the number of action potentials will be greater.
Synapses are gaps between neurons, the location at which electrical signals are passed on to the next cell. There are three types of axons: axo-axonic, axo-dendritic, and axo-somatic. The neuron that delivers the message is called the pre-synaptic neuron, while the neuron that receives the message is called the post-synaptic neuron. Inhibition is necessary and crucial because if neurons are constantly excited, a seizure results. The chemicals that transmit the specific message to another neuron are called neurotransmitters. Over 100 types of neurotransmitters have been identified to date, and each one conveys a different message. Neurotransmitters are created and transported in vesicles in the pre-synaptic cell. An action potential causes the membrane of the vesicle to fuse with the membrane of the cell, dumping the neurotransmitter into the synaptic cleft (see image below). The neurotransmitter attaches onto receptor sites on the post-synaptic cell. Different neurotransmitters fit into corresponding receptor sites like a lock and key.
Drugs produce effects by altering these neurotransmitters. There are two types of drugs: agonists, which increase the effects of neurotransmitters; and antagonists, which block the effects. Most drugs affect the neurotransmitter Dopamine, which makes one feel pleasure. Methamphetamine increases the release of Dopamine into the synapse, cocaine blocks the reuptake of Dopamine to leave it in the synapse longer, and hallucinogens act by stimulating certain types of Serotonin receptors.
There are three sections in the human brain: the hindbrain, midbrain and forebrain. The hindbrain is the posterior part and consists of the medulla, pons and cerebellum. The medulla is an extension of the spinal cord and controls biological processes such as breathing, heart rate and salivation. The pons is the location where the cranial nerves cross to the opposite side of the brain (this is why the left side of the brain controls the right side of the body, and vice versa). The cerebellum controls movement and balance. The midbrain, obviously located in the middle of the brain, contains the superior and inferior colliculi and the substantia nigra. The superior colliculus is involved in vision and the inferior colliculus is involved in audition. The substantia nigra gives rise to Dopamine-containing pathways. The forebrain is the largest part of the brain and has two hemispheres and four lobes.
The occipital lobe is known as the primary visual cortex as it receives visual input. The parietal lobe is the primary somatosensory cortex, as it is the primary target for touch sensations and information from muscle-stretch receptors and joint receptors. It also processes spatial and numerical information. The temporal lobe is the primary target for auditory information. It also processes more complex aspects of vision, such as the perception of movement and face recognition. The left temporal lobe is essential for understanding spoken language. The frontal lobe contains the primary motor cortex and the prefrontal cortex, which is responsible for higher-order processes such as planning for the future and organization. This is the area that is affected by schizophrenia – the area develops in the late teens to early twenties, the same time as the onset of schizophrenia. Many sub-cortical structures play an equally large role in human life. The thalamus is the main relay station – virtually every message passing through the brain goes to the thalamus first. The basal ganglia plays a large role in movement. The limbic system is responsible for emotion. The hypothalamus conveys messages to the pituitary gland, which alters the release of hormones. The brain has four ventricles that are filled with cerebrospinal fluid. The fluid supports and cushions the brain and provides buoyancy, nutrition and hormones. The corpus callosum is a bundle of axons that separates the two hemispheres. Lastly, the convolutions in the brain allow for a large amount of tissue to fit into the skull. Generally, the more convoluted the brain, the more intelligent a species is. Humans have the most highly convoluted brain (as you may have guessed). The hill of a convolution is called a gyrus and the valley is call a sulcus.
Virtually all psychological disorders have been linked to an abnormality in brain functioning. Addiction causes changes in the nucleus accumbens. This area releases Dopamine and controls our feeling of “wanting.” Drugs affect this area by increasing sensitivity which, in turn, increases one’s desire for a certain drug. Depression has a strong family link, especially for relatives of women with early-onset depression. It is associated with decreased activity in the left hemisphere. All anti-depressants increase Serotonin activity in some way (eg., increasing release or decreasing reuptake). Schizophrenia has a strong genetic predisposition. Studies suggest that it is caused either by genes or difficulties early in life that impair brain development in ways that lead to behavioural abnormalities beginning in adulthood. Another possible cause is a childhood infection in which a parasite invades the brain. Schizophrenics show mild brain abnormalities, especially in the temporal and frontal lobes. The prefrontal cortex is very slow to mature and the symptoms of the disorder are caused by excess Dopamine. As a result, patients are prescribed antipsychotic medications, which block Dopamine.
It is clear that a tremendous amount of research has been devoted to the CNS; specifically, the various parts of the brain and their functions. I believe that further research should focus on how these specific individual parts influence one another to produce a combined perception of one’s world (this is known as the “binding problem”). Another area of focus should be on the role of glial cells in transmission of electrical impulses and how central they are to this process relative to neurons.
The more the CNS is studied, the more complex it is recognized to be. We are only at the tip of the iceberg in understanding these complexities. It may take decades before a full or, at least, more comprehensive understanding has been attained. It is clear that the CNS is more than just the sum of the individual cells and, hopefully, we will soon be able to comprehend the details of its interactive functions.