Chapter 5 - Nerve Cells And Nerve Impulses Flashcards Preview

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0

What is the Nervous System ?

•It is the communication network and control centre of the body.

•It is also involved in maintaining homeostasis, a task it shares with the endocrine system.

1

What does the nervous system consist of?

The nervous system can be divided into two main parts:

•the central nervous system (CNS)
•the peripheral nervous system.

Both apart of ONE nervous system.

2

Explain the role of the Central nervous system (CNS).

•It consist of the brain and spinal cord. It is the control centre for the whole nervous system.

•The nerves that connect the central nervous system with the receptors, muscles and glands make up the peripheral nervous system (PNS).

3

What are the basic structural and functional units of the nervous system?

Nerve Cells or Neurons.

8

Describe the structure of Neurons.

Neurons vary in size and shape, but they all consist of a cell body and two different types of extension from the cell - the dendrites and the axon.

9

What is the term Nerve Fibre used for?

It is used for any long extension of a nerve cell, but usually refers to an axon.

10

What does the cell body of the Neuron contain?

The nucleus and other organelles.

11

Describe the structure of Dendrites.

•Usually fairly short extensions of the cytoplasm of the cell body

•Often highly branched

•They carry messages, or nerve impulses, into the cell body.

12

Describe the structure of the Axon.

•often a single, long extension of the cytoplasm.

•It usually carries nerve impulses away from the cell body.

•Most axons are covered with a later of fatty material called the myelin sheath.

•Those that have a myelin sheath are called myelinated fibres and those that don't are said to be unmyelinated.

13

How long is an Axon?

•It is usually longer than the dendrites, but the length of the axon can vary enormously.

•Those in the brain may be only a few mm long, while the axons that run from the spinal cord to the foot may be a
m+ long.

14

When is the term nerve fibre used?

It is used for any long extension of a nerve cell, but usually refers to an axon.

15

Describe interneurons.

•AKA Connector neurons or association neurons

•They have many branches that are able to send or receive messages to or from adjacent neurons.

16

What is a Grey Matter?

Grey areas that consist of nerve cell bodies and unmyelinated fibres.

17

What is a White Matter?

White areas which are composed of myelinated fibres (the lipid myelin that covers these fibres is white in colour).

18

What wraps around the axon?

Outside the brain and spinal cord the myelin sheath is formed by special cells called Schwann cells, which wrap around the axon.

19

What are Nodes of Ranvier?

They are gaps in the myelin sheath that are located at intervals along the axon.

20

What are the 3 important functions of the Sheath?

• Acts as an insulator

• Protects the axon from damage

• Speeds up the movement of nerve impulses along the axon.

21

What is the function of the Neurilemma?

•It is a structure formed by the outermost coil of the Schwann cell which is located around the myelin sheath.

•It helps on the repair of injured fibres.

22

What are the 3 functional type of neuron?

•Sensory (or receptor) neurons
•Motor (or effector) neurons
•Relay neurons

23

What is the function of Sensory Neurons?

They carry messages from receptors in the sense organs, or in the skin, to the central nervous system (brain ad spinal cord).

24

What is the function of Motor Neurons?

They carry messages from the central nervous system to the muscles and glands - the effectors.

25

What is the function of Relay Neurons?

Also known as Interneurons, Association neurons, or connector neurons; they are located in the central nervous system and are the link between the sensory and motor neurons.

26

List the Structural types of neurons.

•Multipolar Neurons
•Bipolar Neurons
•Unipolar Neurons

27

Describe Multipolar Neurons.

•Have one axon and multiple dendrites extending from the cell body.

•Most common

•They include most of the interneurons in the brain and spinal cord and also the motor neurons that carry messages to the skeletal muscles.

28

Describe Bipolar Neurons.

•Have one axon and one dendrite.

•Both axon and dendrite may have many branches at their ends.

•Bipolar neurons occur in the eye, ear and nose where they take impulses from the receptor cells to other neurons.

29

Describe Unipolar Neurons.

•They have just one extension, an axon.

•The cell body is to one side of the axon.

•Most sensory neurons that carry messages to the spinal cord are of this type.

30

What are nerves?

•Outside the central nervous system, nerve fibres are arranged into bundles called nerves.

•In a nerve the fibres are held together by sheaths of connective tissue.

31

What is the difference between Neuron, Nerve Fibre and Nerve.

A Neuron is a single nerve cell, a Nerve Fibre is any long extension of cytoplasm of a nerve cell body, although the term usually refers to an axon and a Nerve is a bundle of nerve fibres held together by connective tissue.

32

What is a Synapse?

•It is a small gap between one neuron and the next.

•Most synapses occur between the end of branches of an axon of one neuron and a dendrite or the cell body of another neuron.

•Messages have to be carried across the synapse.

33

What is a Neuromuscular junction?

It is similar to a synapse; it is a tiny gap which occurs where an axon meets a skeletal muscle cell.

34

What is a nerve impulse?

•It is the message that travels along a nerve fibre.

•Nerve impulses are transmitted very quickly, making it possible for the cost to respond rapidly to any change in the internal or external environment.

35

What is an electrochemical change?

•It is a nerve impulse that travels along a nerve fibre.

•It is described as electrochemical because it involves a change in electrical voltage that is brought about by changes in the concentration of ions inside and outside the cell membrane of the neuron.

36

What does the speed of the nerve impulses depend on?

It depends on whether or the nerve fibre is myelinated or unmyelinated and also on the diameter of the fibre.

37

What's the difference between nerve impulses through myelinated and unmyelinated fibres?

•In unmyelinated fibres the impulse travels steadily along the fibre. (max 2m/s or 7km/h)

•In myelinated fibres the myelin sheath is not continuous.

•It is punctuated by the gaps called the nodes of Ranvier.

•The nerve impulses jump from one node to the next.

38

What is a Saltatory Conduction?

•It is a jumping conduction which allows the nerve impulse to travel much faster.

•Depending on the diameter of the fibre, impulses can travel at speeds from 18m/s (65km/h) up to 140m/a (500km/h).

39

What is the quickest way the body has of mounting homeostasis?

Sending messages in the form of nerve impulses.

40

What do these electrical charges have to do with nerve impulses and nerve cells?

•When some chemical substances are dissolved in water they break up into electrically charged particles called ions.

•This happens to some of the substances dissolved in the fluid around and inside cells.

41

What positive and negative ions do the cells consist of ?

Extracellular fluid - contains a high concentration of sodium chloride. (Most of the Charged particles are Na+ and Cl-)

Intracellular Fluid - contains a high concentration of potassium (K+) and the negative ions come from a variety of organic substances made by the cell.

42

Describe Membrane Potential.

It is particularly large in nerve and muscle cells.

43

What is the membrane potential for unstimulated nerve cells?

•It is known as the resting membrane potential, it can be measured and is about -70mV.

•This means that the potential of the inside of the membrane is 70mV less than that of the outside.

44

What causes a resting membrane potential of neurons?

It is mainly due to the differences in the distribution of potassium ions (K+) and sodium ions (Na+) on either side of the cell membrane.

45

What is the concentration of sodium and potassium ions?

The concentration of sodium ions is about 10 times higher outside the neurons than within, and that of potassium ions is about 30 times greater inside the neuron than outside.

46

Explain the permeability of the cell membrane.

The cell membrane is highly permeable to potassium and chloride ions, only slightly permeable to sodium ions and impermeable to the large negatively charges organic ions.

47

What makes the inside of the cell become electrically negative?

There is a tendency for potassium ions to diffuse out of the cell, which results in inside of the cell becoming electrically negative.

48

How does the cell membrane maintain this potential difference?

1. It actively moves ions across the membrane. This activity is describe as a sodium-potassium pump and it transports sodium ions out of the cell and potassium ions in.

2. The cell membrane is not equally permeable to all ions and there are large numbers of negative charged ions trapped inside the cell.

49

When is a membrane said to be Polarised?

•Although Na+ and K+ are both + charged, there are not enough potassium ions inside the cell to counteract the effect of the large number of negative ions.

•Thus, the membrane is maintained so that the inside is negative in relation to the outside.

•In this stage the membrane is said to be polarised.

50

How does Depolarization occur?

•If a sufficiently strong stimulus is applied to a nerve fibre, the membrane becomes more permeable to sodium ions so that sodium ions move across the membrane and into the cell.

•This inward movement is too great to be balanced by an outward movement of potassium ions and the membrane becomes depolarised.

•Depolarization occurs only if the level of stimulation exceeds a certain threshold.

51

What is an All-or-none-response?

A response of constant size regardless of the strength of the stimulus; with respect to nerve cells, a nerve impulse is transmitted at full strength or not at all.

52

How does the original polarity of the membrane decrease to zero?

Because of the movement of sodium ions which also results in the inside becoming positive relative to the outside.

53

What is an Action Potential?

-Because of the movement of sodium ions, the original polarity of the membrane decreases to zero and then the inside actually becomes positive relative to the outside.

-It is the rapid depolarization and repolarization of the membrane when the membrane quickly restores to its original condition and it involves a change on the membrane voltage lasting only 1 millisecond (ms).

54

What is the movement of the action potential along a nerve fibre called?

A nerve impulse.

55

When does a Refractory period occur?

•During an action potential, and for a very brief time afterwards, that part of the nerve fibre cannot be stimulated to respond again.

•This is called the Refractory period.

•The refractory period if an action potential is important in the transmission of an impulse along a fibre.

56

How does the action potential move along the membrane away from the point of stimulation in an unmyelinated fibre?

•Domino like; A Current flow caused by deoplarisation of one area causes depolarization immediately adjacent to the site of the original stimulus.

•The process repeats itself along the whole length of the membrane so that the action potential moves along the whole length of the membrane away from the point of stimulation.

57

Does the depolarization begin in. The middle of a fibre?

•In the body this would be unusual as stimulation normally occurs at the end of a fibre.

•But if it does occur, impulses will travel in both directions along the fibre, away from the point of stimulation.

58

Why doesn't the action potential travel along the nerve fibre?

•Each action potential generates another action potential just infront of it.

•Thus, it is the message, or nerve impulse, that travels along the fibre; not the action potential itself.

59

What prevents the nerve impulse from going backwards along the nerve fibre?

The refractory period. During the refractory period of an action potential, another action potential cannot be generated at that point on the fibre.

60

Why do nerve impulses travel much faster along myelinated fibres compared to unmyelinated fibres?

This is because the action potential jumped from one node of Ranvier to the next because in a myelinated fibre, the nerve fibres are insulated from the extracellular fluid except at the nodes of Ranvier.

61

Give an example of a nerve impulse not becoming weaker with distance.

•All or none response; If you stub your toe, nerve impulses will be generated that travel along an axon all the way up to your spinal cord.

•The voltage of the impulses arriving at the spinal cord will be the same as the voltage of those generated at the toe.

62

How do we determine the strength of a stimulus?

First, a strong stimulus causes depolarization of more nerve fibres than a week stimulus; second, a strong stimulus produces more nerve impulses in a given time than a weak stimulus.

63

What are neurotransmitters?

•Transmission across a synapse; at the synapse, special chemicals are released front the ends of the axon.

•These diffuse across the gap and attach to receptors on the membrane of the next neuron.

•These substances are called neurotransmitters.

64

How many directions does the transmission if nerve impulses across a synapse occur in?

1D

65

What is Organophosphates?

•They were contained in Nerve agents (aka nerve gases) used during World War II, which causes the build up of acetylcholine at the neuromuscular junction.

•All muscles in the body then try to contract and loss of muscle control prevents breathing.

66

What is not a normal consequence of ageing?

•Dementia (formerly called senility).

•There are many forms of dementia but the most common form, and the one we hear most about, is Alzheimer's disease.

67

Facts about Alzheimer's disease.

• 65+
• no cure
• memory loss, confusion, mood swings, aggression, general withdrawal
• most costly disease in developed countries due to their long life expectancy

68

What plaques and tangles of proteins known in the brain that are associated with the disease?

•The plaques are masses of protein known as amyloid while the tangles are of a protein called tau.

•It is uncertain that this is the cause of Alzheimer's disease.

69

Facts about Parkinson's Disease.

• 1 in 100 Aussies 60+
• shaking, slow movement, muscle stiffness, stooped posture, impaired speech.
• no cure but medication to relieve the symptoms as available

70

What causes Parkinson's disease?

•It is caused by progressive Degeneration of nerve cells in the centre of the brain.

•This results in a deficiency of the neurotransmitter dopamine.

•When about 70% of the cells that produce dopamine are not functioning normally, the symptoms of the disease disappear.