Nervous System

Chapter Objectives

After studying this chapter, students should be able to . . . 1. describe the structure of a neuron and explain the functional significance of its principal regions. 2. classify neurons on the basis of their structure and function. 3. describe the locations and functions of the different types of supporting cells. 4. explain what is meant by the blood-brain barrier and discuss its significance. 5. describe the sheath of Schwann and explain how it functions in the regeneration of cut peripheral nerve fibers. 6. explain how a myelin sheath is formed. 7. define depolarization, repolarization, and hyperpolarization. 8. explain the actions of voltage-regulated Na+ and K+ channels and describe the events that occur during the production of an action potential. 9. describe the properties of action potentials and explain the significance of the all-or-none law and the refractory periods. 10. explain how action potentials are regenerated along a myelinated and a nonmyelinated axon. 11. describe the events that occur in the interval between the electrical excitation of an axon and the release of neurotransmitter. 12. describe the two general categories of chemically regulated ion channels, and explain how these operate using nicotinic and muscarinic ACh receptors as examples. 13. explain how ACh produces EPSPs and IPSPs, and indicate the significance of these processes. 14. compare the characteristics of EPSPs and action potentials. 15. compare the mechanisms that inactivate ACh with those that inactivate monoamine neurotransmitters. 16. explain the role of cyclic AMP in the action of monoamine neurotransmitters, and some of the actions of monoamines in the nervous system. 17. explain the significance of the inhibitory effects of glycine and GABA in the central nervous system. 18. list some of the polypeptide neurotransmitters, and explain the significance of the endogenous opioids in the nervous system. 19. discuss the significance of nitric oxide as a neurotransmitter. 20. explain how EPSPs and IPSPs can interact and discuss the significance of spatial and temporal summation and of presynaptic and postsynaptic inhibition. 21. describe the nature of long-term potentiation and discuss its significance.

The Nervous System: Neurons and Synapses

The Nervous System is composed of two different parts:
Central Nervous System which is composed of the Brain and the Spinal cord
Peripheral Nervous System is composed of the Cranial nerves and Spinal Nerves

• Parasympathetic - "Rest and Digest"
• Sympathetic - "Fight or Flight"

Homeostasis is maintained by an appropriate balance between sympathetic and parasympathetic activity.

Nerve- Any bundle of nerve fibers running to various organs and tissues of the body
Neuron- A cell specialized to conduct and generate electrical impulses and to carry information from one part of the brain to another.


Neuron is a single nerve cell; a nerve is a bundle of many nerve fibers (axons and dendrites of neurons) Neurons are responsible for integrating information received from the internal and external environment; they are specialized for excitability and conductivity and consist of a cell body, dendrites and an axon. There are three types of neurons in the body. We have sensory neurons, interneurons, and motor neurons. Neurons are a major class of cells in the nervous system and the functional unit of the nervous system (but they are outnumbered by glial cells).
Neurons are sometimes called nerve cells, though this term is technically imprecise, as many neurons do not form nerves. In vertebrates, neurons are found in the brain, the spinal cord and in the nerves and ganglia of the peripheral nervous system. Their main role is to process and transmit information.
Neurons have excitable membranes, which allow them to generate and propagate electrical impulses. Sensory neuron takes nerve impulses or messages right from the sensory receptor and delivers it to the central nervous system. A sensory receptor is a structure that can find any kind of change in it's surroundings or environment.


Each neuron can carry a signal in only one direction (from the cell body down the axon). They are long lived cells, having been in your body since before birth or in the few years after birth.
Neurons of the peripheral nervous system can be very long cells, since the cell body must reside in or near the central nervous system. Motor neurons of the PNS have short dendrites and their cell body resides in the spinal cord. They have a long axon that goes to the muscle or gland they innervate. Sensory neurons of the PNS have long dendrites going from the region they receive sensory input from to the basal ganglia next to the spinal cord (where their cell bodies are located). Sensory neurons have a short axon going into the spinal cord and synapsing with other neurons.


Check out the Mouse Party and other cool demonstrations about how drugs work! Test your response time by playing this game (try it at different times of day or after drinking caffeine!)


Types of Neurons

Structure of a neuron

Neurons have three different parts to them. They all have an axon, a cell body and dendrites. The axon is the part of the neuron that conducts nerve impulses. Axons can get to be quite long. When an axon is present in nerves, it is called a nerve fiber. A cell body has a nucleus and it also has other organelles. The dendrites are the short pieces that come off of the cell body that receive the signals from sensory receptors and other neurons.

Synapse:

Chemical synapses are specialized junctions through which the cells of the nervous system signal to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow the neurons of the central nervous system to form interconnected neural circuits.
Transmissions across the majority of synapses in the nervous system is one-way and occurs through the release of chemical neurotransmitters from the presynaptic axon endings called terminal boutons.




Myelin Sheath
Schwann cells contain a lipid substance called myelin in their plasma membranes. When schwann cells wrap around axons, a myelin sheath forms. There are gaps that have no myelin sheath around them; these gaps are called nodes of Ranvier. Myelin sheathes make excellent insulators. Axons that are longer have a myelin sheath, while shorter axons do not. The disease multiple sclerosis is an autoimmune disease where the body attacks the myelin sheath of the central nervous system.

Neuroglia- Support cells, also known as glial cells. They are more abundant than neurons. They do not transport messages.


Astrocytes, Oligodendrocytes, and Schwann cells
-Oligodendrocytes have extensions that form the myelin sheath around axons in the Central Nervous System. Nerve impulses travel faster along myelinated axons than unmyelinated axons (saltatorial conduction is the term to describe jumping from one node to another).

-Schwann cells perform the same function as oligodendrocytes, but are found in the peripheral nervous system. Instead of having extensions that wrap around axons, the entire cell wraps itself around and around.


-Astrocytes are most common in the brain. They have lots of small feet that go to capillaries and neurons.This is the cell in which all things must go through to get to the brain. They must perform this duty because the capillaries have tight junctions in which messages cannot be passed. This is what is called the blood brain barrier. Many drugs cannot enter the astrocytes. They must instead use a precursor drug. Astrocytes are found in the CNS.

In class we used the example of Dopamine which cannot pass the blood brain barrier, so instead you must use the precursor of L-dopa which can pass the "BBB" (blood brain barrier). The L-dopa then tells the brain to make dopamine.




Electrical Activity of Axons
The permeability of the axon membrane of sodium to potassium is regulated by gated channels.
-Resting membrane potential is -70 mV (voltage). This is sightly permeable to potassium and impermeable to sodium.
-An excess positive charge on the outside of the cell and the excess of the negative charge inside the cell collect close to the plasma membrane These exceeds charges are only a very small fraction of the total number of ions in and outside the cell.
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A more in depth look at Electrical Activity of Axons

The Nerve Impulse

When a nerve is stimulated the resting potential changes. Examples of such stimuli are pressure, electricity. chemicals., etc. Different neurons are sensitive to different stimuli(although most can register pain). The stimulus causes sodium ion channels to open. The rapid change in polarity that moves along the nerve fiber is called the "ACTION POTENTIAL." This moving change in polarity has several stages: Depolarization The upswing is caused when positively charged sodium ions(Na+) suddenly rush through open sodium gates into a nerve cell.The membrane potential of the stimulated cell undergoes a localized change from-65 millivolts to 0 in a limited area. As additional sodium rushes in, the membrane potential actually reverses its polarity so that the outside of the membrane is negative relative to the inside. During this change of polarity the membrane actually develops a positive value for a moment(+40 millivolts). The change in voltage stimulates the opening of additional sodium channels (they are voltage-gated). This is an example of a positive feedback loop.
Repolarization (The down-swing) is caused by the closing of sodium ion channels and the opening of potassium ion channels. Release of positively charged potassium ions (K+) from the nerve cell when potassium gates open. Again, these are opened in response to the positive voltage--they are voltage gated. This expulsion acts to restore the localized negative membrane potential of the cell (about -65 or -70 mV is typical for nerves).
Refractory phase is a short period of time after the depolarization stage. Shortly after the sodium gates open they close and go into an inactive conformation. The sodium gates cannot be opened again until the membrane is repolarized to its normal resting potential. The sodium-potassium pump returns sodium ions to the outside and potassuim ions to the inside. During the refractory phase this particular area of the nerve cell membrane cannot be depolarized. This refractory area explains why action potentials can only move forward from the point of stimulation. Increased permeability of the sodium channel occurs when there is a deficit of calcium ions. when there is a deficit of calcium ions (Ca+2) in the interstitial fluid the sodium channels are activated (opened) by very little increase of the membrane potential above the normal resting level. The nerve fiber can therefore fire off action potentials spontaneously, resulting in tetany. Could be caused by the lack of hormone from parathyroid glands. Could be caused by hyperventilation, which leads to a higher pH, which causes calcium to bind and become unavailable. Speed of conduction. This area of depolarization/repolarization/recovery moves along a nerve fiber like a very fast wave. In nonmyelinated fibers, conduction is hundreds of times faster because the action potential only occurs at the nodes of Ranvier by jumping from node to node. This is called "saltatory" conduction. Damage to the myelin sheath by the disease can cause severe impairment of nerve cell function. Some poisons and drugs interfere with nerve impulses by blocking sodium channels in nerves. See discussion on drug at the end of this outline.

If you don't understand this a great illustrated site to look into... http://outreach.mcb.harvard.edu/animations/actionpotential.swf

All or none law

Muscle cells and neurons function on an all-or-none principle. Once the stimulus exceeds a certain threshold, the contraction or nerve impulse in that single cell will happen, and it will happen all the way. It's like when you play dominoes and you line them up and you tilt the first one only so far and not have it fall over, but once you pass the point of no return, they are going down, and all the others will fall down in a quick motion. This principle is fully dependent on those gated ion channels opening, letting sodium ions flood into the cell.

Acetylcholine as a neurotransmitter

Acetylcholine(ACh) is used as an excitatory neurotransmitter by some neurons in the CNS and by somatic motor neurons at the neuromuscular junction. At autonomic nerve endings, ACh may be either excitatory or inhibitory, depending on the organ involved.

The stimulatory effects on ACh on skeletal muscle cells is produced by the binding of ACh to nicotinic ACh receptors. Effects of ACh on other cells occur when ACh binds to muscarinic ACh receptors.

Chemically regulated Channels

The binding of a neurotransmitter to its receptor protein can cause the opening of ion channels through two different mechanisms. These two mechanisms can be illustrated by the actions of ACh on the nicotonic and muscarinic subtypes of the ACh receptors.

Ligand-Gated Channels

A neurotransmitter molecule is the ligand that binds to its specific receptor protein. Part of the protein has extracellular sites that bind to the neurotransmitter ligands, while part of the protein spans the plasma membrane and has a central ion channel.

The nicotinic ACh receptor can serve as an example of ligand-gated channels. Two of its five-polypeptide subunits contain ACh binding sites, and the channel opens when both sites bind to ACh. The opening of this channel permits the simultaneous diffusion of Na+ into and K+ out of the postsynaptic cell. The effects of the inward flow of Na+ predominate, however, because of its steeper electrochemical gradient. This produces the depolarization of an excitatory postsynaptic potential (EPSP)

Although the inward diffusion of Na+ predominates in an EPSP, the simultaneous outward diffusion of K+ prevents the depolarization from overshooting 0mV.

G-Protein-Coupled Channels

The muscarinic ACh receptors are formed from only a single subunit which can bind to one ACh molecule. Unlike the nicotinic receptors, these receptors do not contain ion channels. Binding of ACh (the ligand) to the muscarinic receptor causes it to activate a complex of proteins in the cell membrane known as G-Proteins. They are named because their activity is influenced by guanosine nucleotides.

There are 3 G-protein subunits
1-Alpha
2-Beta
3Gamma

In response to the binding of ACh to its receptor, the alpha subunit dissociates from the other two subunits, which stick together to form a beta-gamma complex. On the specific case, either the alpha subunit or the beta-gamma complex then diffuses through the membrane until it binds to an ion channel, causing the channel to open or close.

The binding of ACh to its muscarinic receptors indirectly affects permeability of K+ channels. This can produce hyperpolarization in some organs and depolarization in other organs.

Scientists have learned that it is the beta-gamma complex that binds to the K+ channels in the heart muscle cells and causes these channels to open. Which leads to the diffusion of K+ out of the postsynaptic cell. The cell the becomes hyperpolarized production an inhibitory postsynaptic potential (IPSP). Such an effect is produced in the heart for example, when autonomic nerve fibers synapse with pacemaker cells and slow the rate of beat.

In a smooth muscle cells of the stomach the binding of ACh to its muscarinic receptors causes a different type of G-protein alpha subunit to dissociate and bind to gated K+ channels. The binding of the G-protein subunit to the gated K+ channels causes them to close rather than open. The diffusion of K+ which occurs at an outgoing rate in the resting cell is reduced to below resting levels. The resting membrane potiential is maintained by a balance between cations flowing into the cell and cations following out a reduction in the outward flow of K+ produces a depolarization. This depolarization produced in these smooth muscle cells results in contraction of the stomach

Acetylcholinesterase (AChE)

The inactivation of ACh is achieved by means of an enzyme called acetylcholinesterase or AChE which is present on the postsynaptic membrane or immediately outside the membrane with its active site facing synaptic cleft.

Acetylcholine in the PNS

Somatic motor neurons form synapses with skeletal muscle cells (muscle fibers). At these synapes, or neuromuscular junctions, the postsynaptic membrane of the muscle fiber is known as a motor end plate. The EPSPs produced by ACh in the skeletal muscle fibers are often called end-plate potentials. Voltage-regulated channels produce action potentials in the muscle fiber, and these are reduced by other voltage-regulated channels along the muscle plasma membrane

If in any stage in the process of neuromuscular transmission is blocked, muscle weakness sometimes leading to paralysis and death may result

Autonomic motor neurons innervated cardiac muscle, smooth muscles in blood vessels and visceral organs, and glands. There are two classifications of autonomic nerves, sympathetic and parasympathetic. Most of the parasympathetic axons that innervate the effector organs use ACh as their neurotransmitter.

QUESTIONS

1. Which support cell creates the blood brain barrier?
a. Glial Cell
b. Neurons Cell
c. Schwann Cell
d. Satellite Cell
e. Astrocyte

2. When I was a kid my mother would yell at my brother for sniffing glue. She would tell him he would lose all his brain cells if he kept doing this. Was my mother correct?
a. no, cells re-grow
b. due to mitosis we do not need to worry about the amount of cells we have
c. we have an endless supply, so sniff away
d. brain cells are limited, she was correct
e. this was found true by brain cell studies.

3. When the child touched the hot stove the nerve impulse shot back up to the cell body through what?
a. Axon
b. Dendrite
c. Neuron
d. A and B the impulse was going back and forth at the same time

4. My aunt was diagnosed with multiple sclerosis which is a chronic, degenerating, remitting and relapsing disease that progressively destroys _______ in the Central Nervous System?
a. Schwann cells
b. microglia
c. oligodendrocyte cells
d. axons
e. astrocytes

5. Which of the following neurons are pseudounipolar?
a. autonomic motor neurons
b. somatic motor neurons
c. neurons in the retina
d. sensory neurons
e. all of the above

6. Which of the following may be produced by the action of nitric oxide?
a. dilation of blood vessels
b. constriction of blood vessels
c. erection of the penis
d. a and c only
e. all of the above

7. The supporting cells that form myelin sheaths in the peripheral nervous system are:
a. oligodendrocytes
b. satellite cells
c. Schwann cells
d. astrocytes
e. microglia

8. Where do dendrites transmit electrical impulses?
a. Away from the cell body
b. Towards the cell body
c. Around the cell body
d. Inside the cell body
e. All of the above

9. What is the nervous system divided into?
a. The central nervous system
b. The peripheral nervous system
c. Neurons
d. Cell body
e. a and b only

10. The absolute refractory period of a neuron:
a. occurs only during the repolarization phase
b. occurs only during the depolarization phase
c. is due to the high negative polarity of the neuron
d. occurs during depolarization and the first phase of repolarization

11. Which ion is responsible for the release of neurotransmitters during a synapse?
a. potassium ion
b. calcium ion
c. sodium ion
d. chlorine ion
e. negative ion

12. Which of the following is not true of the neuron?
a. composed of cell body, dendrites, and axon
b. long living
c. need individual neuron for every single muscle fiber
d. very interconnected, particularly in the brain
e. more inhibitory neurons than excitatory neurons

13. The most common cell in the brain is the:
a. astrocyte
b. schwann cell
c. microglia
d. oligodendrocyte