05-01-07, Nervous system

Communication is vital to the survival of the living organism. To interact with their environment, multicellular organisms have developed a communication system at the cellular level. Neurons allow messages to be carried from one cell to another so that communication among all body parts is smooth and efficient.

Check out this book: Biology - Neuroanatomy - Atlas of Structures, Sections, Systems 6th Ed (Lippincott Williams & Wilkins). There is a surprisingly large lack of information on neuroscience on this hard drive. Hey, there's also the good organic chemistry text: “Organic Chemistry Textbook (390 p.) (U.S.) Excellent.pdf”.

The nervous system controls and coordinates all essential functions of the human body. The nervous system receives and relays information about activities within the body and monitors and responds to internal and external changes.

Four functions of the nervous system

Two major divisions

The function of the spinal cord is to carry messages from the body to the brain, where they are analyzed and interpreted. Response messages are then passed from the brain through the spinal cord and to the rest of the body.

Some peripheral neurons collect information from the body and transmit it toward the central nervous system. These are known as afferent neurons, which take information from the body and transmit to the central nervous system.

Efferent neurons transmit information away from the central nervous system.

It's an enormous network of “one-way streets”. Kind of like New York City.

(my own notes) Sometimes it is easy to get confused with terms in biology and the sciences because things seem to always come in pairs of either opposites or complements, because of our “accounting” approach to the universe: positive/negative, prime/composite, axon/dendrite, input/output, male/female, host/parasite distinctions, effector/affector,

There is one axon per neuron. There are three types of neurons.

May 3rd 02007

Dendrites receive information. Axons transmit information. There is one axon per neuron. Sensory neurons (afferent receptors) carry impulses from the receptor sense organs to the brain and spinal cord. Receptor neurons detect external or internal changes and send the information to the central nervous system in the form of impulses by way of the afferent neurons. The afferent neurons are going to be transmitting the information that the receptor neurons receive.

Motor neurons (efferent neurons) carry impulses from the brain and spinal cord to muscles or glands. Muscles and glands are two types of effectors. In response to signals sent along the efferent motor neurons, muscles contract and other effectors do things (like glands, which secrete).

Interneurons connect sensory and motor neurons and interneurons carry signals between sensory (afferent neurons) and motor neurons (efferent neurons). Interneurons are only found in the central nervous system.

The cell body contains the nucleus and much of the cytoplasm. Most of the metabolic activity of the cell occurs in the cell body (cytoplasm) including the generation of ATP and synthesis of protein.

Dendrites are short extensions extending from the neuron's cell body. The dendrites can connect to the axon terminals. Dendrites receive stimulus (action potentials) and carry impulses from the environment or from other neurons and carry them toward the cell body.

An axon is a long fiber that carries action potentials away from the cell body. Each neuron has only one axon. Axons send stimulus. The axon ends in a series of small swellings called axon terminals.

May 8^th 02007

Yesterday we started on nervous system action potentials and the way that sodium and potassium is regulated through the neuron's axons and dendrites to manage the voltage over the plasma membrane surface of the neuron.

Due to active transport neuron contains more potassium cations and fewer sodium cations than the surrounding medium. Inside the neuron also contains many negative charges, proteins, molecules, and ions.

Resting potential

A nerve cell has electrical potential across its cell membrane because of a difference in the number of positively and negatively charged ions in and out of the cell. Because of active transport, nerve cells have more potassium ions and fewer sodium ions than the surrounding medium outside of the cell wall. Inside the cells, you have negatively charged molecules, proteins and ions.

Potassium ions can leak out across the membrane more easily than sodium ions can leak in. So this means that there is a greater likelihood of potassium ions to leak through the membrane into the outside medium than it is likely for sodium ions to leak in, however this does not include the cases where there is active transport and so on.

Net result of the leakage of positively charged ions out of the cell is a negative charge on the inside of the neuron's cell membrane. So this means that when charge does start to reverse it's flow, it will then change from “inside to outside” to “outside to inside” to stabilize the charge.

Charge difference between within the cell and outside on the cell membrane is known as resting potential of the neuron's cell plasma membrane. There is more positive charge outside in the resting potential state than there is positive charge inside the neuron in this state. At this point, the neuron is said to be polarized. Thinks that are polar have charges at their poles. Water is still an example of polar molecules.

The neuron stays in the polarized state until it is stimulated. This is either through external stimulation or neighbor stimulation (i.e. via the neuron near to it at the dendrites connected to the axon terminals of the last neuron).

The ability of a neuron to respond to a given stimulus and convert that stimulus into a nerve impulse is known as excitability. Nerve cells have excitability, “just like you”.

The moving impulse

An impulse causes a movement of ions across the cell membrane of a neuron. Actually, through the cell membrane, not over the surface necessarily. This will temporarily change the charge at the cell membrane and this essentially “rolls along the axon” of the nerve cell. The cell membrane of a neuron contains thousands of tiny molecules known as gates (sodium and potassium gates). They allow for the transport of sodium and potassium ions. There is also going to be the sodium-potassium pump.

These molecular gates embedded in the plasma membrane allow either sodium or potassium ions to pass through. The pumps are not the gates. The gates allow only one or the other, but the pumps can work with both types of ions.

Generally, more often than not, the gates of the neuron are closed.

An impulse starts when pressure or other sensory inputs disturb the plasma membrane, causing sodium gates to open. Once sodium gates are opened, sodium is going to flow inwards. The sodium gates are going to allow sodium ions to rush into the cell. Allow positively charged sodium ions to flow inside the cell. The inner membrane becomes temporarily more positive than the outside membrane and this temporary state is called depolarized.

Page 358 of your textbook or in the packet that was given, Figure 1218 will demonstrate what is going on here pretty well as a supplement to what we are talking about.

As the impulse passes, the potassium gates open, allowing the positively charged potassium ions to flow out of the neuron. Sodium is going to flow in, the potassium is going to flow inwards. So this is the point of repolarization. Now the inner membrane of the neuron is negative, and the outside is more positive, etc. As this negative charge comes into that area, it's going to disturb the plasma membrane right next to it, and that's going to trigger the same thing, the same steps of the positive charges going in, and this is going to run the whole length. It is like a wave.

So, when repolarization is established, once again negatively charged on the inside and it's positively charged on the outside.

Depolarization and repolarization of a neuron membrane is called action potential. Action potential is another name for nerve impulse. After the nerve impulse, there is a period where the neuron has to wait in order to conduct another nerve impulse, and this is called the refractory period.

The refractory period is a very shot period during which the sodium-potassium pump continues to return sodium ions to the outside and potassium ions to the inside of the axon. This returns the neuron to the resting potential. The refractory period returns the neuron to the resting potential. The refractory period has a higher charge, then a lower charge, then back up to the resting potential of the neuron.

An impulse is not an electric current, it is a wave of depolarization and repolarization. The change in charge disturbs the plasma membrane next, and this continues down the membrane until it reaches the end.

Know how anesthetics work relative to the neural plasma membrane. Might have something to do with reducing the ability of a nerve to signal, or something like that.

A nerve impulse is actually the movement of an action potential along a neuron as a series of voltage-gated ion channels open and close. This nerve impulse is much slower than an electric current.

“Unlike an electric current”, the strength off an impulse is always the same- the “strength” is apparently the voltage of these impulses. So, kind of like a muscle contraction, there is either an impulse to a stimulus or there is not—it's an all or nothing response. The impulse will either trigger all the way down or it will not trigger at all.

Central nervous system

May 10^th, 02007

Missed yesterday due to the AP Calculus AB exam. Also missed earlier this week on Monday. Notes were taken on Monday. Wednesday might have been something other than notes.

Neuroglia - nonexcitable cells of neural tissue that support, protect, and insulate the neurons.

Dendrite input, axon output. The Myelin sheath is the fatty insulating sheath that surrounds all but the smallest nerve fibers. Sensory receptors are those dendritic end organs that are specialized to respond to stimulus. A graded potential is a local change in membrane potential that declines with distance and is not conducted along the nerve fiber. An action potential is a large transient depolarization event, including polarity reversal, that is conducted along the nerve fiber known as nerve impulse.

Saltatory conduction is the transmission of an action potential along a myelinated fiber in which the nerve impulse appears to leap from node to node. This is probably because of the nodes of Ranvier and the Myelin sheaths that insulate the nerve fibers. Sensory transduction is the conversion of stimulus energy into a nerve impulse.

Nervous system

Histology of the nervous tissue. The nervous system is densely packed full of neurons and supporting cells that leaves only 20% extracellular space. The supporting cells wrap around the neurons and help the transmittance of information from one area of the nervous system to another over long ranges. Supporting cells are nine times more numerous than neurons and retain their ability to reproduce throughout their entire lives, and for this reason, many brain tumors are gliomas (tumors formed by uncontrolled proliferation of glial cells).

Supporting cells:

The nervous system is more than just simple neurons, but instead is condensed ecology of many supporting cells that facilitate the operations of the neurons. Satellite cells effectively regulate the chemical environment, the Schwann cells and oligodenodrocytes provide insulation, the ependymal cells help out with the satellite cells, the microglia are like warriors except not technically lymphocytes, and astrocytes help integrate the capillaries and neurons. Also see the neuron maps for Purkinje cells, stellate cells, granule cells, and basket cells.

The spinal cord is continuous with the brain and emerges from an opening at the base of the skull, stretches down for approximately 42 to 45 centimeters from the vertebral column. There are 31 pairs of spinal nerves that are a part of the peripheral nervous system and that are directly connected to the spinal cord.

Nerves are bundled axons. Each spinal nerve, or bundle of axons associated with the peripheral nervous system, consists of a dorsal root and a ventral root. The dorsal root contains neurons that carry signals to the central nervous system. You have lots of neurons coming from a generalized area, and they all sort of lead into one of the many dorsal roots, and then that goes directly to the central nervous system and potentially to the upper brain, or maybe in reflex arcs stop at the spinal cord and so on.

Within the spinal cord and elsewhere in the body are interneurons. In addition to carrying impulses to and from the brain, the spinal cord regulates reflexes. Reflexes are the simplest responses to a stimulus (sneeze, blink). A reflex produces a rapid motor response to a stimulus because the sensory neuron synapses directly with a motor neuron in the spinal cord. As soon as it hits the spinal cord, it goes right back and towards motor neurons. Reflexes do not go to the brain. Most reflexes do not reach the brain.