Two weeks ago, I stated the body is made of trillions of cells communicating with each other through two major pathways.
To recap, the electrical network is the realm of nerves. Electrical impulses move along neurons from one end to the other. The myriad of connection pathways possible from one neuron to the next determines to a large degree how information is interpreted.
The junction between neurons is a region called the synapse, containing a synaptic cleft - a small gap approximately 20 to 40 nanometers across.
There are two different types of synapses. The first is slightly narrower than typical and the electrical signal can leap across much like the spark between two wires. These are high specialized synapses and relatively rare.
The vast majority of synapses rely on chemicals called neurotransmitters to carry information from one neuron to the next - from a sending neuron at the tip of the axon to the dendrites of the receiving neuron.
Neurotransmitters are constructed within the sending neuron and packaged in small vesicles. When an action potential is received, the vesicles meld with the cell membrane and release the chemicals into the cleft. They diffuse across the cleft driven by the concentration gradient - imagine a drop of food colouring dispersing in a glass of water.
It is important to note that the more vesicles which fuse with the membrane, the more neurotransmitter in the cleft.
This has two effects. The first is to ensure a stronger stimulation of the receiving neuron. Each molecule of a neurotransmitter fits into a receptor on the receiving neuron's surface much like a key turning a lock. Each receptor fires off a signal. The more signals sent, the faster the neuron pulses, and the stronger the signal sent to the axon.
The second effect is to make the signal last longer. More neurotransmitters in a cleft means it takes longer for the concentration to decrease. Imagine a crowd trying to leave the CN Centre by the main doors. The larger the crowd, the longer it is going to take for everyone to get out.
As an added layer, there are dozens of known neurotransmitters which can complicate communication, particularly as they can be mimicked by other molecules. This is the basis of many of the drugs which alter our state of mind.
But not all communication between cells is managed by neurons. Endocrinology is the study of hormones and these are chemical compounds with a very different communication system from our nerves.
Hormones are chemical messengers released from specific secretory cells found in various glands. They are secreted in response to specific stimuli which includes signals from neurons. Once they are released into the blood stream, they influence any and every cells which has an appropriate receptor site.
There are key differences between hormonal and neuronal communication.
The first is the extent of communication. Across a single synapse, only two neurons can communicate. Depending upon the number of dendrites a neuron has, it might be able to affect 10,000 other neurons.
Hormones, on the other hand, can easily affect every cell in the body - several trillion. For example, the change in hormones brought on by puberty have a profound effect on all sorts of different systems in the body.
A second major difference is their time signature. A neuron fires off signals which might last as long as a millisecond. The effects of hormones can take hours or days to emerge and can last a lifetime. After all, how often do the effects of puberty disappear after a while?
A third major difference is the extent of their actions. Neurons can only stimulate or inhibit the next neuron in line. Hormones can change the activity of a protein, turn on and off biochemical pathways, alter the overall metabolism of a cell, cause cells to grow or to atrophy or divided or even initiate cell death.
Consider testosterone which increases muscle mass or progesterone which causes the proliferation of cells in the uterine lining during the luteal phase.
Both increase the total number of cells in the body. On the other hand, the thyroid hormone is responsible for the death of the cells in a tadpole's tail as it metamorphosizes into a frog.
Together neurons and hormones make up the neuroendocrine axis which begins in the hypothalamus and is responsible for regulating both our autonomic nervous system and hormonal systems.
Hormones secrete in the hypothalamus can induce the pituitary gland to release hormonal signals which enter general circulation and can release further hormones. For example, being frightened is not simple a state of mind.
It is a state of body as this cascade of hormones results in the release of hormones from the adrenal glands.
The human body is a complex mixture of chemical and electrical interactions which regulate all of the cells which make us, us.
