3. Control & Communication
What you need to know...
Nervous control in animals, including structure and function of central nervous system (CNS).
Brain structure: cerebrum, cerebellum and medulla.
Rapid reflex action and reflex arc: sensory, relay and motor neurons.
Receptors detect sensory input/stimuli.
Electrical impulses move along neurons.
A synapse occurs between neurons, allowing chemicals to transfer from one neuron to another.
Endocrine glands release hormones into the blood stream.
Hormones are chemical messengers.
Target tissues have cells with receptors for hormones, so only some tissues are affected by specific hormones.
Blood glucose regulation to include insulin, glucagon, glycogen, pancreas and liver.
Now that you have a better understanding of the roles different types of cells play in multicellular organisms, the majority of the remainder of this unit is spent learning about the systems multicellular organisms have evolved to overcome the problems which come with being a multicellular organism.
The first multicellular problem which we'll deal with is communication. Multicellular organisms obviously will have groups of specialised cells as tissues and organs which are physically distant from each other but need to communicate with each other. In this topic we'll learn about two ways that animals overcome this: nervous control and hormonal control.
Animals such as ourselves have a system of neurons (nerve cells) which allows for rapid communication and control of the organism. Our nervous system is divided into two inter-connected systems. The Central Nervous System (CNS), which consists of the brain and spinal cord, and the Peripheral Nervous System which consists of all other neurons in the body.
The Central Nervous System is the control centre of the organism. All impulses from and to tissues via the neurons of the Peripheral Nervous System pass through the CNS. This allows the CNS to exert control over the actions of the organism's tissues and organs. The different parts of the CNS control different aspsects of the organism. We'll look at the role of each of the two parts of the CNS in turn starting with the brain.
We'll return to the spinal cord in more detail later, but for now it's helpful before looking at the brain to mention that the spinal cord is the most primitive part of the CNS. It responds to the most basic of stimuli and exerts very simple control processes, such as the reflex response. As you can see from the diagram below the brain consists of a number of different parts (it's actually much more complex than this, but these are the only sections you need to know about for your course). A general rule which should prove useful for you is that the closer the part of the brain is to the spinal cord, the more basic/primitive the function it controls.
So, we should therefore expect the medulla to control a basic function whereas the cerebrum should control a much more complex function with the cerebellum somewhere in between. The functions of these parts of the brain are as follows:
Medulla: The medulla is located at the top of the spinal cord. It controls various essential involuntary functions such as heart rate and breathing. These functions are described as involuntary as they occur without your conscious thought.
Cerebellum: The cerebellum is located behind the medulla and below the cerebrum and controls functions such as balance and movement coordination.
Cerebrum: The cerebrum is the largest part of our brains and controls higher order functions such as thought, perception and personality.
As you can see, the rule worked. So, if you can remember which part of the brain is where, you will be able to determine what each part of the brain does depending on its distance from the spinal cord. It works for me! If you're fascinated by the brain and would like to know more, check out this fantastic magazine from the Wellcome Trust.
The spinal cord carries nerve impulses between the brain and the peripheral nervous system, but as part of the CNS it also carries out a control function. As already mentioned, the spinal cord controls quite basic and primitive responses such as the reflex response. A reflex response is an automatic and almost instaneous response to a stimulus. A reflex response normally occurs as a result of a stimulus which indicates potential harm to the organism. The response is a result of the spinal cord, not the brain, which makes it both involuntary and quick.
The rapid reflex action occurs as a result of the reflex arc. The reflex arc consists of a series of neurons which pass through the spinal cord. The three neurons in the reflex arc are the sensory neuron, the relay neuron and the motor neuron as shown in the diagram below.
So, if you touch something very hot this stimulus is detected by sensory receptors in your skin. This triggers an electrical impulse which travels along the sensory neuron to the spinal cord. As an extreme temperature is likely to cause you harm, this stimulus will trigger a reflex response. The impulse therefore passes along the relay neuron to the motor neuron. The motor neuron carries the impulse to an effector which brings about the response. In this example the effector would be the muscles in your arm which would contract to move your arm away from the hot object. All of this occurs in a fraction of a second and without your conscious thought. The reflex arc is demonstrated in the diagram below.
You will have noticed that there is a gap between each of the neurones in the diagrams above. These are called synapses. The electrical impulse which travels along a neuron can not jump this gap. When the electrical impulse reaches the end of one neuron it stimulates the release of a chemical. This chemical diffuses across the gap between the two neurons. When the chemical reaches the second neuron it stimulates a new electrical impulse which then travels along this neuron. The processes of the synapse are shown in the diagram below.
If you have flash installed you can view an interactive animation of the reflex arc and the synapse here.
Not all of the control and communication which occurs between tissues and cells in an animal are by electrical impulses. Some signals are sent using chemical messengers in the blood stream. These chemical messengers are called hormones. Tissues which produce hormones and release them into the blood are called endocrine glands. We have numerous endocrine glands in our bodies.
Image from Wikimedia Commons
Each hormone molecule carries out a particular function in the body. But if they travel in the blood stream, which obviously goes to all tissues, how do they only bring about a response in their target tissues? The answer is that the cells of tissues have receptor molecules in their cell membranes which are specific to the hormones which act on that tissue. So, if hormone molecules are present in the blood stream which a particular tissue doesn't have the receptors for, nothing will happen at that tissue. But when the hormone does appear which it does have the receptors for, it will detect its presence and bring about the response. The diagram below represents this specificity. The cell has one type of receptor in its membrane but is surrounded by three different hormone molecules. Only the hormone molecule which is complimentary to the receptor present can bind and bring about a response. The other two hormones aren't "seen" by the cell at all.
A good example of a hormone system in our bodies is the way that we control the concentration of the sugar glucose in our blood stream. Why does the concentration of glucose in our blood stream matter? Too little glucose and our cells won't be able to respire, too much glucose in our blood stream and our cells will begin to lose water by osmosis. So, our bodies aim to have a relatively constant concentration of glucose in our blood stream by adding more glucose if it goes down and removing some if it goes up...this process is controlled by the hormones insulin and glucagon.
Before we learn how insulin and glucagon control the concentration of glucose in our bloodstream, you first need to know how glucose can be stored. Glucose molecules are single sugars. They consist of a small ring of six carbon atoms surrounded by oxygen and hydrogen atoms. Glucose can be stored by joining these rings together to form long chains of glucose molecules. Plant cells create a type of long chain of glucose molecules called starch whereas animal cells instead produce glycogen. Starch and glycogen are very similar molecules as they both consist of long chains of glucose molecules. When an animal cell needs more glucose it can get it from breaking down glycogen. The diagram below summarises the relationship between glucose and glycogen.
Glucose is stored as glycogen in our bodies in the liver. Changes in blood glucose concentration are detected in the pancreas. You can see from the diagram below that the liver and the pancreas are separate organs. Hormones are therefore requried travel in the bloodstream to communicate between the pancreas and the liver to bring about the appropriate response.
Image from Wikimedia Commons
So, how are insulin and glucagon involved in the control of blood glucose levels? Both are released from the pancreas into the bloodstream and both are detected by the cells of the liver but they bring about different responses.
Insulin: Insulin is released by the pancreas into the bloodstream when the concentration of glucose in the blood rises, for example after eating a meal. Liver cells detect the presence of insulin in the blood which causes the conversion of glucose to glycogen. This lowers the concentration of glucose in the blood returning it to normal.
Glucagon: Glucagon is also a hormone relased by the pancreas. Glucagon is released when the concentration of glucose in the blood falls, for example after exercise. When glucagon is detected by the liver cells they convert stored glycogen to glucose which raises the blood glucose concetration back to normal.
The diagram below summarises the action of the hormones insulin and glucagon: