Chapter 2 - Nerve Cells And Nerve Impulses

Question 1

What do neurons do?

Answer 1

Receive information and transmit it to other cells. A small gap separates the neurons.

Question 2

What is a membrane?

Answer 2

The surface of a cell. It is composed of 2 layers of fat molecules that are free to flow around one another. Most chemicals can not cross the membrane, but specific protein channels in the membrane permit a controlled flow of water, oxygen, sodium, potassium, calcium, chloride, and other important chemicals

Question 3

What is the nucleus?

Answer 3

A part of the cells that contains the chromosomes

Question 4

What is a mitochondrion?

Answer 4

It is the structure that performs metabolic activities, providing the energy that the cell requires for all other activities. . It requires fuel and oxygen to function.

Question 5

What are ribosomes?

Answer 5

The sites at which the cell synthesises new protein molecules. Proteins provide building materials for the cell and facilitate various chemical reactions. Some ribosomes float freely within the cell. Others attach to the endoplasmic reticulum.

Question 6

What is the endoplasmic reticulum?

Answer 6

A network of thin tubes that transport newly synthesised proteins to other locations

Question 7

Describe a motor neuron

Answer 7

It has its soma in the spinal cord. It receives excitation from other neurons through its dendrites and conducts impulses along its axon to a muscle.

Question 8

Describe a sensory neuron

Answer 8

It is specialised at one end to be highly sensitive to a particular type of stimulation, such as light, sound or touch. The soma is located on a stalk off the Amin trunk. See p31

Question 9

Describe a dendrite

Answer 9

They are branching fibres that get narrower near their ends. The dendrite’s surface is lined with special synaptic receptors, at which the dendrite receives information from other neurons. The greater the surface area of a dendrite, the more info it can receive.

Question 10

What are dendritic spines?

Answer 10

These are the short outgrowths that increase the surface area available for synapses.

Question 11

What is the soma?

Answer 11

The cell body that contains the nucleus, ribosomes, and mitochondria. Most of the metabolic work of the neuron occurs here.. The cell body is covered with synapses on its surface in many neurons.

Question 12

What is the axon?

Answer 12

A thin fibre if constant diameter. It is the neuron’s information sender, conveying an impulse towards other neurons or an organ or muscle.

Question 13

What is the purpose of the myelin sheath?

Answer 13

It is an insulation covering for the axon. It increases the efficiency with which information is sent down the axon.

Question 14

What are nodes of Ranvier?

Answer 14

These are the interruptions in the myelin sheath. These are the points where the action potentials fire and then send information down the axon to the next node.

Question 15

What is a presynaptic terminal?

Answer 15

An axon has many branches, each if which swells at its tip, forming a presynaptic terminal or button. This is the point from which the axon releases chemicals that cross through the junction between one neuron and the next

Question 16

What is an afferent axon?

Answer 16

An afferent axon brings information into a structure (admits). Every sensory neuron is an afferent to the rest of the nervous system.

Question 17

What is an efferent axon?

Answer 17

An efferent axon carries information away from a structure(exit). Every Motor neuron is an efferent from the nervous system. A given neuron can be an effect on one structure and an afferent to another.

Question 18

What is an interneuron?

Answer 18

A cell’s dendrites and axon are entirely contained within a single structure. Eg. An interneuron of the thalamus has its axon and all it’s dendrites within the thalamus. It is also called an intrinsic neuron.

Question 19

What are the different types of glia?

Answer 19

The brain has several types of glia with different functions. These include the Astrocyte, the Schwann cell, the micro glia, the radial glia, and the oligodendrocyte.

Question 20

What are astrocytes?

Answer 20

The star shaped astrocytes wrap around the presynaptic terminals of a group of functionally related axons. By taking up ions released by axons and then releasing them back to
Axons, an Astrocyte helps synchronise the activity of the axons, enabling them to send messages in waves. They also remove waste material created when neurons die and control the amount of blood flow to each brain area. During periods of heightened activity in some brain areas, astrocytes dilate the blood vessels to bring more nutrients to that area.

Question 21

What are microglia?

Answer 21

They are very small cells. They remove waste materials as well as viruses, fungi, and other microorganisms. In effect, they function like part of the immune system.

Question 22

What are oligodendrocytes?

Answer 22

They can be found in the brain and spinal cord. They wrap around axons to create myelin sheaths that surround and insulate certain vertebrate axons.

Question 23

What are Schwann cells?

Answer 23

Found in the periphery of the body, they are the specialised glia that wrap around axons creating a myelin sheath.

Question 24

What are radial glia?

Answer 24

They guide the migrant of neurons and their axons and dendrites during embryonic development. When this finishes, most radial glia become neurons, with some becoming astrocytes, and oligodendrocytes.

Question 25

Do myelinated axons exist in invertebrates?

Answer 25

No. But many if the axons in vertebrates are myelinated.

Question 26

What happens when cells get a virus?

Answer 26

The cell evicts some of the virus so that the immune system can see it. The immune system then kills the virus and the cell the contains the virus.

Question 27

Why do we need a blood-brain barrier?

Answer 27

Because a cell with a virus is killed by the immune system. This works well where the cells are then replaced but the brain does not replace damaged neurons so a virus in neurons could cause brain damage.

Question 28

What is the blood - brain barrier?

Answer 28

The body builds a wall along the sides of the brain’s blood vessels. This wall keeps most viruses, bacteria, and harmful chemicals out of the brain.

Question 29

What happens to viruses that get into the brain?

Answer 29

Rabies can lead to death. Microglia and others attack viruses or slow their reproduction. Viruses that enter your nervous system will probably stay with you for life. Eg. Shingles

Question 30

What are endothelial cells?

Answer 30

They form the walls of the capillaries. Outside the brain, they are separated by small gaps but in the brain they are joined so tightly that virtually nothing passes between them.

Question 31

What is a problem with the blood barrier?

Answer 31

The barrier keeps out useful chemicals as well as harmful ones. Useful chemicals like amino acids ( the building blocks for proteins) . The brain needs special mechanisms to get these chemicals across the blood-brain barrier.

Question 32

How does the brain get useful chemicals?

Answer 32

1. Small uncharged molecules cross freely (oxygen, carbon dioxide). Water crosses through special protein channels in the wall of the endothelial cells.
2. Molecules that dissolve in the fats of the membrane also cross passively (vitamins A and D and all the drugs that affect the brain (antidepressants, and other psychiatric drugs to illegal drugs such as heroin)
3. Active transport - a protein mediated process that expends energy to pump chemicals from the blood into the brain. Chemicals that are actively transported include glucose, amino acids, purines, choline, some vitamins, iron and certain hormones

Question 33

Explain blood- brain barrier and Alzheimer’s and brain cancer

Answer 33

In people with Alzheimer’s the endothelial cells shrink and harmful chemicals enter the brain. In brain cancer patients, the barrier prevent medicines from getting in, making it harder to treat the cancer.

Question 34

What do neurons require for nutrition?

Answer 34

Glucose. The metabolic pathway that uses glucose requires oxygen (the brain uses up 20% do the body’s oxygen. The brain uses glucose because it is practically the only nutrient that cross the blood-brain barrier.

Question 35

Describe a neuron’s membrane.

Answer 35

1. 8 nanometers thick
2. It has an inner and outer layer of phospholipid molecules
3. Embedded in the phospholipid molecules are cylindrical protein molecules through which the various chemicals pass

Question 36

what is the electrical gradient?

Answer 36

Also known as polarisation, it is the difference in electrical charge between the inside and the outside of the cell.

Question 37

What is the resting potential?

Answer 37

The difference in voltage in a resting neuron

Question 38

What is selective permeability?

Answer 38

Some chemicals pass through the membrane more freely than others. Oxygen, carbon dioxide, urea, and water cross freely through channels that are always open. Most large or electronically charged ions and molecules do not cross the membrane at all. Sodium, potassium, allium, and chloride cross through channels (gates) that are sometimes open

Question 39

Describe the membrane when the neuron is at rest.

Answer 39

1. The sodium channel is closed

2. Potassium channels are nearly but not entirely closed so there is a slow flow of potassium

Question 40

Describe the sodium- potassium pump

Answer 40

1. Transports 3 sodium ions out of the cell whilst drawing 2 potassium ions into the cell.
2. The pump is an active transport that requires energy
3. Sodium ions are10times more concentrated outside the cell
4. The sodium stays out but the potassium slowly leaks back out through the potassium channels carrying a positive charge with them. This leakage increases the electrical gradient across the membrane

Question 41

What is electromotive force?

Answer 41

This electrical gradient has positive and negative ions attracted to each other. Eg. When the sodium channels open the sodium rushes into the neuron because the inside of the cell is negative but the sodium is positive.

Question 42

What is the concentration gradient (diffusion)?

Answer 42

The difference in distribution of ions across the membrane. The ions move from areas of high concentration to areas of low concentration. Eg sodium has high concentration outside the cell so when the channels open, they move away from this area.

Question 43

Describe the forces influencing potassium.

Answer 43

1. Potassium is positively charged and the inside of the cell is negatively charged so the electrical gradient tends to pull the potassium in
2. However, potassium is more concentrated inside the neuron so the concentration gradient tends to push it out.

Question 44

The resting potential takes a lot of energy to maintain. Why is it so important?

Answer 44

The sodium-potassium pump costs a lot of energy but when the next excitation occurs, the neuron can respond rapidly

Question 45

What is an action potential?

Answer 45

A message sent by axons.

Question 46

What happens if the negative change within the neuron increases?

Answer 46

Hyperpolarisation - this means an increase in polarisation.

Question 47

What happens if the charge within the neuron becomes increasingly positive?

Answer 47

Depolarisation - this reduces polarisation. If the stimulation reaches the threshold of excitation then action potential will occur.

Question 48

What is the threshold of excitation?

Answer 48

A point of depolarisation where an action potential will occur. This means that the sodium and potassium channels open. The peak of the action potential varies from one axon to another but remains the same for an axon

Question 49

Explain what happens with an action potential?

Answer 49

1. A the start sodium is mostly outside, potassium is mostly inside
2. When the membrane is depolarised the sodium and potassium channels open. These are voltage gated channels
3. At first the opening of the potassium channel produces little effect.
4. The opening of the sodium channels makes a big difference as the concentration gradient and the electrical gradient both encourage the sodium to enter the neuron.
5. When the depolarisation reaches the threshold, the sodium channels open fully
6. At the peak of the action potential the sodium channels close( even at this point, there is still more sodium outside the cell)
7. At the peak, the inside of the cell has a positive charge. The concentration gradient and the electrical gradients push potassium out of the cell.
8. The cell reaches a temporary hyperpolarisation.
9. The cell reaches resting potential, the channels are closed and the sodium- potassium pump works hard to get the extra sodium out of the cell.

Question 50

What are voltage gated channels?

Answer 50

Their permeability depends on the voltage difference across the membrane. As the membrane becomes depolarised, the channels open.

Question 51

What do local anaesthetic drugs do?

Answer 51

They attach to the sodium channels of the membrane, preventing sodium from entering, and thereby stopping action potential. The receptors still feel the pain but the axons can’t transmit the message.

Question 52

What does back propagate mean?

Answer 52

Sometimes the message goes back into the cell body and the dendrites. At his point, the dendrite becomes more susceptible to the structural changes responsible for learning.

Question 53

Explain all or none law.

Answer 53

The amplitude and velocity of an action potential are independent of the intensity of the stimulation that initiated it. If the stimulation does not reach the threshold, then there will be no action potential. It doesn’t matter by how much the stimulation exceeds the the hold, the action potential will be the same size

Question 54

What happens during the refractory period?

Answer 54

This occurs immediately after an action potential. It resists the production of further action potentials. There are 2 sub periods within this period.

Question 55

What happens during the absolute refractory period?

Answer 55

This is the first of the sub periods. The membrane cannot produce an action potential, regardless of the stimulation.

Question 56

What happens during the relative refractory period?

Answer 56

This is the second sub period during which a stronger than usual stimulus is necessary to initiate an action potential.

Question 57

What is the axon hillock?

Answer 57

It is part of the motor neuron, a swelling where the axon exits the soma

Question 58

Explain the propagation of the action potential.

Answer 58

The sodium enters a point on the axon. The positive ions travel. Down the axon to neighbouring regions. The positive charges slightly depolarise the next area of the membrane, causing it to reach its threshold and open its voltage gated sodium channels. The charge now moves to the next point along the axon. The positive charge will not cause action potentials backwards because they are still in a period of refractory.

Question 59

How can the speed of the message down an axon be increased?

Answer 59

1. Increase the diameter of the axon

2. Myelin sheath (found only in vertebrates)

Question 60

What are nodes of ranvier?

Answer 60

Gaps in the myelin sheath along the axon

Question 61

Explain how an action potential travels down a myelinated axon. (Saltatory conduction)

Answer 61

The first action potential occurs at the axon hillock or the first node of ranvier. The action potential can not regenerate between the nodes because sodium channels are virtually absent. After the action potential occurs at a node, sodium ions push a chain of positive ions along the axon to the next node where they regenerate the action potential.

Question 62

How does multiple sclerosis impact on the axon?

Answer 62

The immune system attacks the myeline sheaths. After the axon was initially myelinated, it lost its sodium channels under the myelin sheath. Therefore, the message dies out before it reaches the next node.

Question 63

What are local neurons?

Answer 63

Some small neurons have no axon. These neurons can only exchange info with their closest neighbours. The are called local neurons. They do not follow the all or none law. Instead it has a graded potential.

Question 64

What is a graded potential?

Answer 64

These are used by local neurons. A membrane potential that varies in magnitude in proportion to the intensity of the stimulus. The change in membrane potential is conducted to adjacent areas of the cell, in all directions gradually decaying as it travels. Those various areas of the cell contact other neurons, which they excite or inhibit through synapses. They are difficult to study because they are so small.