M5 S1 : Communication and homeostasis

Question 1

What is a stimulus?

Answer 1

Any change in the internal or external environment (like temp, light intensity or pressure).

Question 2

What are receptors? What are the different types?

Answer 2

They detect stimuli. They are specific and only detect one particular stimulus (pressure, light or glucose). Some receptors are cells (photoreceptors) and some receptors are proteins on cell surface membrane (glucose receptors are proteins found on pancreatic cell membranes ).

Question 3

What are photoreceptors?

Answer 3

Receptor cells that connect to the nervous system.

Question 4

What are effectors?

Answer 4

Cells that bring about a response to a stimulus, to produce an effect.

Question 5

What are examples of of effectors?

Answer 5

1. Muscle cells
2. Cells found in glands (like the pancreas).

Question 6

Why do receptors and effectors work together?

Answer 6

They play an important role in communicating information from one particular of an organism to another. This is to make sure that activities of different organs are coordinated to keep the organism working properly.

Question 7

How can cell signalling occur? What are the examples?

Answer 7

1. Between adjacent cells : In the nervous system, cells communicate by secreting chemicals (neurotransmitters) which send signals to adjacent cells (like other nerve cells or muscle cells).
2. Between distant cells : Hormonal works by cells releasing chemicals in blood and act as signals to distant cells. Cell-surface receptors allow cells to recognise the chemicals involved in cell-signalling.

Question 8

Why is it important that organisms respond to stimuli?

Answer 8

To increase their chances of survival.

Question 9

What are the 3 main types of neurone?

Answer 9

1. Sensory neurone
2. Motor neurone
3. Relay neurone

Question 10

What is the function of the sensory neurone?

Answer 10

Transmit nerve impulses from receptors to the CNS (the brain and spinal chord).

Question 11

What is the function of the motor neurone?

Answer 11

Transmit nerve impulses from CNS to effectors.

Question 12

What is the function of the relay neurone?

Answer 12

Transmit nerve impulses between sensory neurones and motor neurones.

Question 13

What do all neurones contain?

Answer 13

They all have a cell body with a nucleus (plus cytoplasm and all the other organelles that is usually in a cell). The cell body has extensions that connect to other neurones. Dendrons and dendrites (smaller branches of dendrons) carry nervous impulses towards the cell body. Axons carry nervous impulses Away from the cell body.

Question 14

What is the structure of a sensory neurone?

Answer 14

They have short dendrites and one long dendron to carry nervous impulses from receptor cells to the cell body, and one short axon that carries nerve impulses from the cell body to the CNS.

Question 15

What is the structure of a motor neurone?

Answer 15

They have many short dendrites that carry nerve impulses from the CNS to the cell body, and one long axon that carries nerve impulses from the cell body to effector cells.

Question 16

What is the structure of a relay neurone?

Answer 16

It contains many short dendrites that carry nerve impulses from sensory neurones to the cell body, and one axon that carries nerve impulses from the cell body to motor neurones.

Question 17

What are nerve impulses?

Answer 17

Nerve impulses are electrical impulses. They are also called action potentials.

Question 18

Why are sensory receptors said to act as transducers.

Answer 18

Different stimuli have different forms of energy (e.g. light or chemical energy). But your nervous system only sends information in the form of nervous impulses. Sensory receptors convert the energy of a stimulus into electrical energy. They act as transducers (something that converts one form of energy into another).

Question 19

What is the resting potential?

Answer 19

When a nervous system receptor is in resting state (not being stimulated), there is a difference between the inside and outside of the cell (the inside in negatively charged relative to the outside). This means there is a voltage across the membrane (or a potential difference). This potential difference is called the resting potential. This resting potential is generated by ion pumps and ion channels.

Question 20

What is the generator potential?

Answer 20

When a stimulus is detected, the cell membrane is excited and becomes more permeable, allowing more ions to move in and out of the cell. This alters the potential difference. The change in potential difference due to a stimulus is called the generator potential.

A bigger stimulus excites the membrane more, causing a bigger movement of ions and a bigger change in potential difference and therefore a bigger generator potential is produced.

Question 21

How is an action potential produced?

Answer 21

If the generator potential is big enough, it will trigger an action potential (nerve impulse) along a neurone. An action potential is only triggered if the generator potential reaches a certain level, called the threshold level. If the stimulus is too weak, the generator potential won’t reach the threshold, so there is no action potential.

Question 22

What is the Pacinian corpuscle and how does it produce an action potential?

Answer 22

Pacinian corpuscle are mechanoreceptors. They detect mechanical stimuli (pressure and vibrations). They are found in your skin. They contain the end of a sensory nerve ending wrapped in lots of layers of connective tissue (lamellae).

When a Pacinian corpuscle is stimulated (like when you get tapped on the shoulder) the lamellae deforms and press on the sensory nerve endings. This causes deformation of the stretch-mediated sodium channels in the sensory neurones cell membrane. The sodium ion channels open and sodium ions diffuse into the cell, creating a generator potential. If the generator potential reaches the threshold, it triggers an action potential.

Question 23

What is the resting potential of a membrane?

Answer 23

In a neurones resting state (when it is not being stimulated) the outside of the membrane is positively charged compared to the inside (as more positive ions outside than inside). So the membrane is polarised (has a charge). The voltage across a membrane at rest is called the resting potential. It is about 70 mV.

Question 24

What things control the resting potential being created and maintained?

Answer 24

By the use of sodium-potassium pumps and potassium ion channels in a neurone’s membrane.

1. Sodium-potassium pumps use active transport to move 3 sodium ions (Na+) out of the neurone for every 2 potassium ions (K+) moved in. This requires ATP.
2. Potassium ion channels allow facilitated diffusion of potassium ions (K+) out of the neurone down their concentration gradient.

Question 25

How are sodium-potassium pumps and potassium ion channels used to create and control the resting potential?

Answer 25

1. The sodium-potassium pumps move sodium ions out of the neurone, but the membrane isn't permeable to sodium ions so they can't diffuse back in. This creates a sodium electrochemical gradient ( concentration gradient of ions) because there are more positive sodium ions outside than inside. 2. The sodium-potassium pumps also move potassium ions into the neurone. 3. When the cell is at rest, most potassium ion channels ae open. This means that the membrane is permeable to potassium ions, so some diffuse back out through. Even though the ions are moving in and out of the cell, in total more positive ions move out of the cell than enter. This makes the outside of the cell positively charged compared to the inside.

Answer 26

When a neurone is stimulated, sodium ion channels in the cell membrane open. If the stimulus is big enough, it causes a rapid change in potential difference. This causes the cell membrane to become depolarised.

Question 27

What are voltage-gated ion channels?

Answer 27

Channels that only open at a certain voltage.

Answer 28

1. Stimulus : This excites the neurone cell membrane, causing sodium ion channels to open. The membrane gets more permeable to sodium, so sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative. 2. Depolarisation : If potential difference reaches the threshold, voltage gated sodium ion channels open and more sodium ions diffuse into the neurone (positive feedback). 3 : Repolarisation : Eventually the sodium ion channels close and the voltage-gated potassium ion channels open. The membrane is more permeable to potassium so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. This starts to get the membrane back to its resting potential.