Oh look at that...we're getting to the Nervous System. Another very interesting body system. When we talked last time about the endocrine system, we touched on fact that it influences vital body functions such as metabolism (thyroid), cellular glucose uptake (insulin), or growth for example. It does that in a slow and sustainable way by having glands secrete chemicals into the blood stream, and then having them attach to receptors on target cells. Those cells in turn change their behavior and ... grow, let glucose in, or modify their glucose metabolism (making energy).
The nervous system functions similarly, in that it can quickly send signals through an electrical wire network (A huge one!) to every area of your body. At the end of those wires (which are cellular extensions called axons), they release a chemical (called a neurotransmitter), which attaches to receptors on the receiving cell. Those cells then react...a muscle for example contracts, or a pixel of a picture that you look at becomes a nerve impulse that is sent to your brain to make you 'see' the image.
So, today we first discuss the basic physiology the nervous system is based on; mainly how we can send this electrical impulse through the body.
Secondly, we will go through some basic anatomy.
As always, please print the lecture notes in your preferred format and watch the videos first.
Then, answer the review questions and email them to me at [email protected] to get credit.
17.1. Together with the endocrine system, the nervous system controls & coordinates what goes on within all of our body’s cells. As we learned, endocrine glands secrete hormones into the blood stream, from where they find their way to target cells with specific receptors on them, which then prompt the cell to act in a specific way. The nervous system works fundamentally in the same way, except that nerve cells don’t secrete their substance (now called neurotransmitter) into the blood stream, but rather send electrical signals along cellular extensions (cable-like structures commonly called nerves) directly to target cells, where neurotransmitters are secreted and then picked up by specific receptors, which prompt the cell into a specific action. The nervous system fundamentally consists of ‘cables’ called nerves that send electrical impulses through the body, and a brain, which decides what signals to send to which cells. Can you name those two parts?
17.2. In order for a brain (or spinal cord for low level decisions) to control our cells functioning, it has to be informed to what is going on both internally, as well as outside of our bodies. Some nerves therefore will carry informative messages into the brain, while others send commands to body cells. We touched on this concept before discussing the negative feedback loop, remember? Can you name the parts of this basic neural functioning?
17.3. The endocrine system controls body functions that are ongoing and take time, such as growth or the cell’s uptake of nutrients, for example. Sending hormones through the blood stream is perfect for that. On the other hand, the nervous system controls body cell functions that require a quick response such as moving the arms to turn the steering wheel in the car in order to avoid an accident. To send these signals with maximum speed, the body makes use of the concepts of electricity, which is generated when negative charges (electrons) travel through wires. What molecules in our body can generate electricity?
17.4. Magnets either have a positive or a negative side. The positive side is attracted to the negative of another magnet; two positive sides repel each other. Charges work the same way. If allowed to flow freely, would you expect positively charged molecules to move towards a field with a more positive charge or one with a more negative one?
17.5. In the body, charges are carried on chemical molecules that have weight and mass. If they are separated from one another by boundaries and cannot move around freely, they become a great tool to create nerve impulses, or electrical surges within the body. What cellular structure can establish such a boundary?
17.6. Since the molecules cannot cross this boundary, the body can use ion pumps to actively move specific ions to one side of the boundary and others to the other. This way, the body sets up steep chemical gradients for specific ions - in nerves, sodium ions are pumped to the outside of the cell and potassium to the inside. When a nerve impulse is elicited, specific channels in the plasma membrane open allowing a specific type of ion to cross the membrane freely (passively). Will that ion move towards or against its concentrated field (up or down the concentration gradient)?
17.7. Another concept is the one of voltage. It is the difference in charge between 2 fields; in our body’s discussion, that means the area just inside and just outside the cell/plasma membrane. If specific ion channels open and these little charged molecules cross the membrane, voltage changes. These detectable changes is the electricity! The body uses specific amounts of changes in voltage as triggers to create nerve impulses that are sent along nerves through the body. Wild, right? To do this, the body’s excitable cells need to have an established voltage while at rest; it’s called the Resting Membrane Potential. What structures in the body make sure this RMP is kept up?
17.8. Have you ever hit your funny bone (the inner elbow) and felt a shooting pain down to the pinky? That pressure on what is called the ulnar nerve elicited it to fire! That was a nerve impulse, or also known as Action Potential. What happens is that when a threshold is strong enough, sodium channels in the cell wall open and let a gazillion of these positively charged ions into the cell. That makes the inside of the cell’s charge more positive (or less negative, we could also say)…the voltage changes, and as a result we feel a little shock when we hit that funny bone. This portion of the entire nerve impulses is called de-polarization (named b/c the voltage, or the polarity across the cell membrane is decreased). The other portion is called re-polarization. It’s goal is to send all those positive charges back to the outside of the cell, in order to re-establish the original ‘resting membrane potential’. Which ion does the body use to cause re-polarization?
17.9. A nerve impulse is an electrical wave of opening, and then closing sodium channels along the length of a nerve. It moves forward, because when the voltage in one section changes, sodium channels of the next segment open. Even though this process is fast, it can be sped up with a wonderful trick. Since ions are hydrophillic (=charged molecules), they do not cross a fat barrier (hydrophobic). The body wraps phospholipid bi-layer sheaths around the nerves. At those sections, ions cannot exchange and the impulse has to ’jump’ to the next section with no wrapping. What are those phospholipid bilayer sheets called?
17.10. Now that we know the fundamentals of how a nerve’s electrical impulse works, lets move onto the anatomy. We first concern ourselves studying the small single cell’s within the nervous system, and then move on to study the brain and spinal cord. Two main cell types are the main worker bees in this system. Neurons are the one’s that send nerve impulses around in the body (the stuff we just learned). They actually also receive stimuli (messages) from other neurons, and process the information received. The other set of cells help and support the neurons (giving them nutrients, or defend against intruders for example). They also insulate the nerves, which helps speed up nerve impulses (described in 17.9). What are those types of cells called?
17.11. Neurons come in many shapes and forms. Both our body’s smallest and largest cell type is found amongst them. Generically, we can describe 3 distinct areas of a neuron. The middle section is the cell body itself (known as soma or perikaryon). Many branches come out of it creating receiving processes for other neurons to connect. On the other end of the soma, a single, long (up to 3 feet!) cable arises to send electrical impulses to its end, where a chemical messenger (neurotransmitter) is released to influence the function of either another neuron, a muscle cell, or a gland. What are the two different types of extensions described called?
17.12. To a large extent, neural processing depends on counting up and interpreting all of the received stimuli. The chemicals (neurotransmitters) released at the ends of nerves (the cables) are either excitatory or inhibitory to the receiving neuron. If all the excitatory messages combined are stronger than the inhibitory ones, the neuron will fire its own nerve impulse (action potential) along the long ‘cable’. At its end, the impulse stimulates the release of its own chemical (neurotransmitter - NT). The NT actually then floats around in a narrow space to find its way onto corresponding receptors on the recipient of the message (the next neuron, muscle, or gland). What do we call this area where NT transmission takes place?
17.13. Neural connections between neurons can be formed and destroyed, depending on their usage. Neuronal pathways (connecting neurons in sequence) are most significant for nervous system function. The more a pathway is stimulated, the stronger it becomes. That is why I encourage retrieval exercises for learning this stuff; you know, the flashcards or labelling exercises, where you have to search for answers in your head. It also means, that if we always complain how bad the world is for example, we are more likely to experience depressive moods; on the other hand, if we ’stop and smell the flowers’ so to speak, we encourage gratitude pathways to get fired up and stronger, which likely make us happier. The strengthening of specific neuronal pathways happens when receiving neurons grow extensions back to the neurons that stimulated them. Awesome, right? This reverberating circuit helps neurons that _____ together, ____ together (=strengthen the connection and therefore the thought).
17.14. A couple more concepts. When we look at the brain in particular, it is a very sensitive organ that needs to be well protected. It does that by being enclosed within the skull, a hard helmet-like bone. In addition, a thin layer of watery liquid surrounds the brain, protecting it further by absorbing shock, and making it practically weightless. The problem with the brain being enclosed comes when it gets swollen; the skull provides a finite space that doesn’t allow for expansion much. Infection is the biggest threat for that besides shock-trauma that the brain can get when we hit our head too hard. Since infections travel most likely through the blood stream, the brain’s blood capillaries have processes from neuronal support cells cling to them and seal them off, so that the type of substances that can enter into the brain tissue is limited. What is that barrier called?
17.15. When we look at nerves in our body, the ulnar nerve for example, which gets excited when we hit our funny bone, most of them have ‘cables’ in them that send sensory information from the body to the brain, as well as ones that send motor impulses from the brain back out to the body. What do we call a nerve that carries both sensory/afferent and motor/efferent impulses?
Please send me the answers you came up with to those questions before the class meets to [email protected] Thank you.
No lab for this class.