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The Nervous system Flashcards

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1
Q

What is the function of the nervous system?

A

It detects changes, or stimuli, inside the body and in the environment, processes and stores information and initiates responses.

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2
Q

What is a stimulus?

A

A detectable change in the internal or external environment of an organism that produces a response in that organism.

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3
Q

What are sensory receptors?

A

Specialised sensory cells which give an organism its senses. They act as transducers, converting energy in one form to electrical energy. The impulses then travel along neurones and initiate a response in an effector.

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4
Q

What are the two parts comprising the nervous system?

A

The central nervous system (CNS) and the peripheral nervous system (PNS).

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5
Q

Describe the central nervous system

A

This comprises of the brain and the spinal cord and processes information provided by a stimulus. Both are surrounded by protective meninges, and the spinal cord contains white matter and grey matter.

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6
Q

What is white matter?

A

An area which contains myelinated neurones and few cell bodies.

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7
Q

What is grey matter?

A

An area which contains unmyelinated neurones and many cell bodies of relay and motor neurones.

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8
Q

Describe the peripheral nervous system

A

This comprises of the somatic nervous system, containing the fibres of sensory neurons, which carry impulses from the receptor to the CNS, and motor neurones which carry impulses away from the CNS to the effector. It also comprises the automatic nervous system, which provides unconscious control of the functions of internal organs.

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9
Q

How is a the simplest type of nervous response, a reflex arc, described?

A

It is an inborn response to a stimulus that is rapid, involuntary and protective.

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10
Q

State the steps of a reflex arc

A

Stimulus –> Receptor –> Sensory neurones –> CNS –> Motor neurones –> Effector (muscle or gland)

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11
Q

What is a nerve net?

A

The simplest type of nervous system. It is a diffuse network of cells that group into ganglia, but do not form a brain.

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12
Q

What two types of cells make up a nerve net?

A

Ganglion cells, which provide connections in several directions and sensory cells, which detect stimuli.

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13
Q

Why do we use the Hydra to study nerve nets?

A

-It has a simple pattern
-It is easily to manipulate in experiments
-It regenerates rapidly

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14
Q

How does the Hydra’s nerve net work?

A

It allows the Hydra to sense light, physical contact and chemicals. In response, it can contract, perform locomotion, hunt and feed. A large stimulus triggers more cells which triggers a larger response.

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15
Q

Describe the comparable features of a nerve net

A

-Receptors respond to small types of stimuli, so number of effectors is small
-One type of neuron
-Neurones are branched
-Transmits impulses in both directions along neurones
-Lots of synapses involved
-Unmyelinated neurones leads to slow response

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16
Q

Describe the comparable features of vertebrate neurones

A

-Receptors respond to multiple types of stimuli so number of effectors is large
-Three types of neurone
-Neurones are not branched
-Transmits impulse in one direction down a neurone
-Impulses pass in one direction from point of stimulus
-Few synapses involved
-Myelinated neurones leads to rapid response

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17
Q

What is the function of sensory neurones?

A

They carry impulses from the sense receptors to the CNS.

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18
Q

What is the function of motor neurones?

A

They carry impulses from the CNS to the effector organs.

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19
Q

What is the function of relay neurones?

A

They receive impulses from the sensory neurones and transmit them to motor neurones.

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20
Q

What makes up a motor neurone?

A

Dendrites, cell body, nucleus, axon, myelin sheath, Schwann cells, Nodes of Ranvier, axon endings and synaptic endbulbs.

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21
Q

What does the dorsal root ganglion contain?

A

Cell bodies of the sensory neurones.

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22
Q

What is the function of the dendrites in a motor neurone?

A

They receive signals from other neurones at synapses and transmits the impulses to the cell body.

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23
Q

What is the function of the cell body in a motor neurone?

A

It contains a nucleus and granular cytoplasm, and co-ordinates a response.

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24
Q

What is the function of the nucleus in a motor neurone?

A

It controls neurone activity and contains DNA to code for proteins and neurotransmitters.

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25
What is the function of the axon in a motor neurone?
It transmits impulses away from the cell body towards the synaptic endbulbs.
26
What is the function of the myelin sheath in a motor neurone?
It protects and insulates the axon and is formed by Schwann cells.
27
What is the function of the Schwann cells in a motor neurone?
They wrap around the axon and secrete myelin which protects and insulates the axon.
28
What is the function of the Nodes of Ranvier in a motor neurone?
They are gaps in the axon, 1mm apart which increase transmission of the impulse 100x times, with the impulse travelling 100m/s.
29
What is the function of the axon endings in a motor neurone?
They are where a neurone forms a synapse with another, resulting in an excitatory or an inhibitory response.
30
What is the function of the synaptic endbulbs in a motor neurone?
They convert electrical impulses to chemical signals and contain synaptic vesicles which release neurotransmitter into the synaptic cleft between connecting neurones.
31
What is the resting potential inside the neurone?
-70mV.
32
Why is a neurone an excitable cell?
As it can change its resting potential.
33
How is the resting potential maintained in the neurone?
-Na+ and K+ pumps, working all the time to pump 3Na+ out and 2K+ into the cytoplasm of the axon, building an electrochemical gradient. -Leaky K+ ion channels allow K+ to diffuse out, increasing the positive charge outside the exon. -Negatively charged proteins inside the cell membrane, which increase the negative charge inside the axon (PO3^-4).
34
What is a nervous impuse?
The transmission of a change in potential along a nerve fibre, associated with the movement of Na+ ions.
35
How can the voltage change of a neurone be picked up?
By a pair of microelectrodes which are fed into an oscilloscope, which is used to measure the magnitude and speed of transmission of the impulse.
36
Describes the steps concluded by an oscilloscope when a neurone receives a stimulus
1. The energy of the stimulus causes some of the voltage-gated sodium channels in the axon membrane to open. The sudden increase in permeability of the membrane to Na+ ions allows them to rapidly diffuse into the axon, down their concentration gradient and the charge in the axon increases from -70mV to +40mv. The membrane is depolarised and Na+ channels close. 2. The potassium channels open and K+ ions diffuse out down their concentration gradient. The cell becomes less positive and more diffuse out and the membrane is repolarised. 3. More K+ ions diffuse out than Na+ ions diffuse in, so the charge becomes even more negative at -90mV, making the membrane hyperpolarised. 4. The Na+/K+ pumps pump K+ back in and Na+ ions back out the cell, restoring the balance of the resting potential.
37
What is the potential difference of the neurone when its membrane is depolarised?
+40mV.
38
What is the potential difference of the neurone when its membrane is hyperpolarised?
-90mV.
39
Order the steps of an action potential
Resting potential --> Depolarisation (Na+ gates open) --> Repolarisation (K+ gates open) --> Hyperpolarisation.
40
How is a wave of depolarisation generated when an action potential is established?
Once the action potential is established, Na+ ions diffuse along the axon and depolarise adjacent sections of the membrane, opening more voltage-gated Na+ channels.
41
What is the absolute refractory period and its significance?
Where Na+ channels are inactivated at the initial action potential site and cannot open until the resting potential is re-established. This lasts 1ms and ensures that the action potential is not propagated back in the direction from which it came, and the nerve impulse travels in one direction only.
42
What is the relative refractory period?
When during the hyperpolarisation period, if an impulse is strong enough, a new action potential may pass. This lasts for 3-4ms after the absolute refractory period and occurs while the Na+/K+ pumps are restoring the resting potential.
43
What is the threshold value for a stimulus to initiate an impulse?
-55mV.
44
Does the size of the nervous impulse initiated in the neurone change?
No, the action potential initiated is always the same size (+40mV) and remains the same size as it is propagated along the axon.
45
What is the 'all or nothing principle' and its significance?
Where an impulse is only initiated over a threshold value, and is always the same size. It allows the action potential to act as a filter, preventing minor stimuli from setting up nervous impulses, so the brain is not overloaded with information.
46
What are the three major factors determining the speed of conduction of nerve impulses?
1. Temperature 2. Diameter of the axon 3. Myelination
47
How does temperature affect the speed of conduction of nervous impulses?
Ions move faster at higher temperatures as they have more kinetic energy, so birds and mammals with warm blood transmit nerve impulses quicker and have faster responses that other animals.
48
How does the diameter of the axon affect the speed of conduction of nervous impulses?
The greater the diameter, the greater the volume in relation to the area of the membrane. More Na+ ions can flow through the axon so impulses travel faster.
49
How does myelination affect the speed of conduction of nervous impulses?
Speeds up the rate of transmission by insulating the axon. Na+ ions flow through the axon but a myelinated nerve fibre only depolarises where the resistance is low at the nodes of Ranvier, where voltage-gated ion channels are, to let more Na+ ions in, resulting in saltatory conduction.
50
What is saltatory conduction?
Where the action potential jumps from one node of Ranvier to an adjacent node in a myelinated axon. The nodes are 1mm apart so saltatory transmission in rapid, an must faster than conduction along a non-myelinated axon.
51
What are synapses?
A gap separating two neurones, which send the nervous impulse between neurones in one direction only.
52
What are the two types of synapse?
Electrical synapses and chemical synapses.
53
Describe an electrical synapse
A gap 3nm long, small enough for an electrical impulse to be transmitted directly from one neurone to the next.
54
Describe a chemical synapse
A gap 20nm long, too big for the nervous impulse to jump. Branches of axons lie close to the dendrites of other neurones and the impulse is transmitted by a neurotransmitter, a chemical that diffuses across the synaptic cleft, from the pre-synaptic membrane of one neurone to the post-synaptic membrane of another, where a new impulse is initiated.
55
Describe the steps of transmitting an impulse across a chemical synapse
1. The arrival of the impulse at the synaptic endbulb alters its membrane permeability, opening voltage-gated calcium channels, so Ca2+ ions diffuse into the endbulb, down their concentration gradient. 2. The influx of Ca2+ ions cause the synaptic vesicles to move towards and fuse with the pre-synaptic membrane. This releases the neurotransmitter acetylcholine, by exocytosis, into the synaptic cleft. 3. The neurotransmitter diffuses across the synaptic cleft and binds to a receptor, an intrinsic protein spanning the post-synaptic membrane. The protein has two receptor sites and two acetylcholine molecules show co-operative binding when they attach. 4. The receptor protein then changes shape and opens a channel for Na+ ions to diffuse in, down their concentration gradient, depolarising the post-synaptic neurone.
56
What are the three ways that acetylcholine is removed from the synaptic cleft?
-Direct uptake of acetylcholine into the pre-synaptic membrane neurone, so that none remains in the cleft to bind to the post-synaptic receptor. -Active transport of Ca2+ ions out of the synaptic endbulb, so more exocytosis of acetylcholine occurs. -Hydrolysis of acetylcholine by acetylcholinesterase in the synaptic cleft, producing choline and ethanoic acid. These products refuse back across the synaptic cleft into the pre-synaptic membrane and reform acetylcholine.
57
Why are mitochondria present in the synaptic endbulbs of the neuron?
As energy in the form of ATP is required to reform acetylcholine, package it into vesicles and perform exocytosis.
58
What are the general functions of synapses?
-They transmit information between neurones -Pass impulses in one direction which generates precision in the nervous system -Act as junctions -Protect the response system from overstimulation since the impulse is always the same size -Filter out low level stimuli due to the -55mV threshold value
59
How can the action potential threshold value be produced?
By temporal summation or spatial summation.
60
What is a drug?
A molecule that has a physiological effect on the body when ingested, inhaled, absorbed or injected.
61
Where do most drugs act?
At synapses, disrupting the normal functioning of neurotransmitters and producing abnormal patterns of nervous impulses.
62
What are the two ways drugs can affect the action of neurotransmitters?
-They can be sedatives, inhibiting the nervous system creating fewer action potentials in post-synaptic neurones. -They can be stimulants, stimulating the nervous system, allowing more action potentials in post-synaptic neurones.
63
What are the two different mechanisms of drug action?
-Drugs may mimic the action of neurotransmitters and bind to the post-synaptic neurone in a similar way, increasing the frequency of action potentials, as it is not removed from the synaptic cleft by hydrolysis and continues to initiate impulses. -Drugs may prevent the breakdown of neurotransmitters. Organophosphates inhibit acetylcholinesterase so acetylcholine is not hydrolysed and remains in the synaptic cleft, causing repeated firing of the post-synaptic neurone leading to muscle spasms.
64
How do psychoactive drugs affect the body?
They act primarily on the CNS by affecting different neurotransmitters or their receptors which affects the firing of neurones. This alters brain function and consequently, mood, consciousness and behaviour.

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