Biology Unit 3.8 - The Nervous System

Question 1

What are the three main functions of the nervous system?

Answer 1

• Detects changes inside the body and in the environment
• Processes and stores information
• Initiates responses

Question 2

Describe the how the nervous system initiates a response…

Answer 2

Stimulus produces a response, which are converted into electrical energy by transducers, which then travel along nerve cells or neurons as electrical or nervous impulses, which then initiate a resonse in an effector

Question 3

What are the two main parts of the nervous system?

Answer 3

• Central Nervous System (CNS) - consists of the brain and spinal cord that process information provided by a stimulus
• Peripheral Nervous System (PNS) - consists of the somatic nervous system and the autonomic nervous system

Question 4

What are neurons?

Answer 4

Highly specialised cells that generate and transmit nerve impulses

Question 5

What are the three types of neurons?

Answer 5

• Sensory neurons - carry impulses from the sensory organ or receptor to the central nervous system
• Motor neurons - carry impulses from the central nervous system to the effector organs
• Relay neurons - receive impulses from the sensory neurons and pass them to the motor neurons

Question 6

What is the function of the cell body?

Answer 6

Has a nucleus, granular cytoplasm, and many ribosomes

Question 7

What is the function of the ribosomes?

Answer 7

Form clusters call Nissil granules and synthesis proteins including neuro-transmitter substances

Question 8

What is the function of dendrites?

Answer 8

Branch off the cell body and receive impulses from other nerve cells

Question 9

What is the function of the axon?

Answer 9

Transmit impulses from the cell body, while the ends form synapses with other neurons

Cytoplasm is called axoplasm which contains many mitochondria

Question 10

What is the function of synapses?

Answer 10

Pass action potentials from neurone to neurone

Question 11

What is the function of Schwann cells?

Answer 11

Wrap themselves around the axon as they grow forming a myelin sheath

Only present in vertebrate nervouse systems

Question 12

What is the function of the fatty myelin sheath?

Answer 12

Acts as an insulator and speeds up the transmission of impulses

Broken by nodes of Ranvier at intervals, which greatly speed up impulse transmission

Question 13

What is the function of nodes of Ranvier?

Answer 13

1 micrometer gaps in the myelin sheath, which allow for rapid transmission

Question 14

What is the spinal cord?

Answer 14

Flattened cylinder of nervous tissue running from the base of the brain to the lumbar region, protected by vertebrae

Question 15

What is the gray matter?

Answer 15

H-shaped central core consisting mainly of nerve cell bodies, dendrites and synapses

Question 16

What is white matter?

Answer 16

Matter that surrounds the grey matter consisting of nerve fibres, while mylein sheaths give it its white colour

Question 17

What is the central canal?

Answer 17

Fluid filled canal containing serebrospinal fluid

Question 18

What is a reflex arc?

Answer 18

Rapid, involuntary response resulting from nervous impulses initiated by a stimulus

Question 19

What is a stimulus?

Answer 19

Detectable change in the environment

Question 20

What is a receptor?

Answer 20

Something that detects energy in one form and convert it into electrical energy

Question 21

What is an effector?

Answer 21

Muscle or gland responsible for the response

Question 22

What is a response?

Answer 22

Action taken by the effector e.g., pulling your arm away from a flame

Question 23

How are nerve impulses transmitted in simple organisms?

Answer 23

• Nervous system known as a nerve net, which is found in the ectoderm
• Nerve net consists of simple nerve cells with short extensions joined to each other
• Transmission of the nerve impulse is slow

Question 24

Compare a nerve net and vertebrate nervous system…

Answer 24

Nerve net:
* Shorter neurons
* Branched
* Neurons can transmit in both directions
* Impulse spreads out in all directions from point of stimulus
* Non-myelinated
* One type of nerve cell
* Slower transmission

Nervous system:
* Long neurons
* Unbranched
* Neurons can transmit in one direction
* Impulse follows a defined route
* Myelinated
* Three types of nerve cell
* Fast transmission

Question 25

What is a nerve impulse?

Answer 25

Impulse transmitted is electrical in nature, measured by a cathode ray oscilloscope, which measures the size and speed

Question 26

What is resting potential?

Answer 26

Potential difference between the inside and outside of the axon when no impulse in being transmitted

Question 27

Explain resting potential…

Answer 27

• Potential difference across the axon membrane is negative, meaning the membrane can be described as polarised
• Sodium-potassium pumps maintain the polarised state by releasing 3 sodium ions outside and 2 potassium ions inside by active transport
• Sodium ions are passed out faster than potassium ions are brought in
• Means that the outside of the axon is positive relative to the inside

Question 28

Explain action potential…

Answer 28

• Energy of a stimulus opens some voltage-gated sodium channels, causing sodium ions to rapidly diffuse into the axon
• The negative charge becomes a positive charge of +40 mV
• Membrane is depolarised

Question 29

Explain repolarisation…

Answer 29

• Fraction of a second after depolarisation, repolarisation begins
• Potential difference across the membrane becomes more negative than the resting potential, making the membrane hyperpolarised
• Called the refractory period
• Time delay ensures a unidirectional impulse and limits the frequency of impulses
• Restores polarity to the axon membrane

Question 30

How does action potential travel along an axon?

Answer 30

• Depolarisation of one part of the membrane triggers depolarisation of the next
• Local electrical currents are generated, allowing the impulse to be transmitted
• Act as a stimulus and open sodium ion channels, causing depolarisation of the next section

Question 31

What is the all or nothing law?

Answer 31

Size of an impulse is independent of the size of the stimulus, where if strength of the stimulus is below the threshold, no action potential is generated

Question 32

What determines the amount of information carried?

Answer 32

Number of frequency of impulses:

• Strong stimulus - greater frequency of action potential
• Weak stimulus - fewer action potentials
• Very weak stimulus - no action potential generate, allowing the nervous system to filter out low level stimuli

Question 33

What factors affect the speed of transmission?

Answer 33

• Myelination and distance between the nodes of Ranvier
• Diameter of the axon
• Temperature

Question 34

Explain myelination…

Answer 34

Speeds up the rate at which impulses are transmitted by insulating the axon when restitance is low

Sodium ions channels only occur at the nodes of Ranvier, meaning the action potential appears to jump from node to node, known as saltatory conduction - The longer the distance between nodes the greater the speed of nerve transmission

Question 35

Explain axon diameter…

Answer 35

Wider the diameter of the axon, the greater the speed of transmission

Larger diameter decreases the resistance to the movement of ions

Question 36

Explain temperature…

Answer 36

Affects the kinetic energy of the ions; ions move faster at higher temperatures

Question 37

What are the two main types of synapses?

Answer 37

• Electrical synapses - gap or junction of 3nm across that is small enough to transmit electrical impulses from one neurone to the next
• Chemical synapses - gap of 20nm where electrical impulses are transmitted by neurotransmitters

Question 38

Explain the role of synapses in nerve transmission…

Answer 38

• Pre-synaptic membrane’s permeability to calcium ions is altered allowing them to enter through calcium channels, causing synaptic vesicles to fuse with the pre-synaptic membrane
• Neurotransmitters are released into the synaptic cleft where they diffuse across and attach to receptor sites
• Sodium channels open, allowing a rapid influx of sodium ions into the post-synaptic neurone
• Depolarisation occurs; if it is sufficient action will be initiated in the axon of the post synaptic neurone

Question 39

What are neurotransmitters?

Answer 39

Proteins that act as chemical signals which trigger an action potential in the next neurone

Question 40

Explain neurotransmitters…

Answer 40

• Acetylcholine is quickly destroyed by cholinesterase, limiting its effect and preventing the merging of impulses
• Resulting choline and ethanoic acid diffuse back across the pre-synaptic neurone to form acetylcholine again, whcih requires ATP
• Acetylcholine and noradrenaline occur together in the autonomic nervous system

Question 41

How do durgs affect nerve transmission?

Answer 41

Can interfere synaptic transmission, where stimulants amplify transmission, while depressents inhibit transmission

Question 42

How do psychoactive drugs affect nerve transmission?

Answer 42

Act primarily on the CNS affecting neurotransmitters and their receptors, altering brain function

Question 43

How do stimulants affect synaptic transmission?

Answer 43

Pre-synaptic:
* Accelerates neurotransmitter production
* Opens calcium channels
* Accelerates the release of neurotransmitters by exocytosis
* Blocks the removal or recycling of neurotransmiter substances

Post-synaptic:
* Receptors are more sensitive
* Opens sodium channels
* Inhibits cholinesterase activity
* Mimicks the neurotransmitter substance

Question 44

How do depressents inhibit syanptic transmission?

Answer 44

Pre-synaptic:
* Inhibit neurotransmitter production
* Closes calcium channels
* Inhibits the release of neurotransmitters by exocytosis

Post-synaptic:
* Receptors are less sensitive
* Sodium channels close
* Increases cholinesterase activity
* Masking effect of the neurotransmitter substances
* Blocking receptors