Topic 8: Grey Matter Flashcards

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1
Q

Resting Membrane Potential of a neuron

A
  • The outside of the membrane is positively charged compared to the inside because there are more sodium ions outside the cell than there are on the inside.
  • This means the membrane is polarised (has a potential difference/voltage). The voltage of the membrane in about -70.
  • The resting membrane potential is primarily established by a sodium-potassium pump which works by active transport (uses ATP) to move three sodium ions (3Na+) outside of the neuron for every 2 potassium ions (2K+) which move into the neurons.

This process sets up 2 concentration gradients:
1. There is an inward concentration gradient for sodium - high conc. outside, low conc. inside. Sodium ion channels are primarily shut at this stage, which maintains this gradient.
2. There is an outward concentration gradient for potassium - high con. on inside, low conc. on outside. Potassium ion channels are mainly shut a this stage, which maintains this gradient.

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2
Q

Describe what happens when an action potential reaches a neuron

A
  1. A stimulus to the neuron triggers the opening of sodium ion channels. If the stimulus is above the threshold for that neuron then it will trigger more sodium ion channels to open (positive feedback).

Depolarisation:
When the sodium ion channels open, sodium ions diffuse into the cell down their electrochemical gradient (+ve to -ve). and concentration (high to low) gradients. This makes the inside of the neuron less negative.

Repolarisation:
At a potential difference of around +40mV*, the voltage-gated sodium ion channels close and voltage-gated potassium channels open. Potassium will now diffuse out of the neuron down the electrochemical ion concentration gradient. The inside of the cell becomes more negative again.

Hyperpolarisation
Potassium ion channels are slow to close so there’s a slight ‘overshoot’, in which the neuron becomes more negative than the resting potential because too many K+ ions leave the cell.

Refractory Stage
During repolaristaion and hyperpolarisaton the neuron cannot receive another stimulus (sodium ion channels cannot open again until the resting potential has been re-established)

The resting membrane potential & concentration gradients of Na+ & K+ are re-established using the sodium-potassium pump.

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3
Q

What is an action potential

A

An action potential is a wave of positive charge which moves along the axon. It is an all-or-nothing response because it is only one size (+40mV). If the body wants to communicate a stronger message then it needs to either:
- Stimulate more neurons
- Send the message more frequently

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4
Q

Explain how an action potential moves along an axon

A

The action potential travels along the neuron as a wave of depolarisation - some sodium ions that enter the neuron diffuse sideways, which causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part changing the potential differnece, thus creating local currents - thus stimulating another action potenial.

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5
Q

Why is the refractory stage important

A
  • Allows time for the neurone to reset.
  • Ensures that action potentials don’t overlap.
  • Ensures that action potentials are unidirectional (can pass in one direction only).
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6
Q

Motor Neurones

A

Structure:
- Many short dendrites. Cell body at one end. One long axon carries nerve impulses from the cell body to effector cells.

Function
- Involved in transmitting electrical impulses from the CNS to muscles & glands in the body

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7
Q

Sensory Neurones

A

Structure:
- One long dendron carries nerve impulses from receptor cells to the cell body, which is in the middle of the neurone. One short axon carries nerve impulses for the cell body to the CNS.

Function:
- Transmits impulses from receptors to the CNS

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8
Q

Relay neurones

A

Structure:
- Many short dendrites carry impulses from sesnory neurones to the cell body. An axon carries nerve impulses from the cell body to motor neurones.
No myelin sheath

Function:
- Located within the CNS, involved in transmitting the electrical impulses from sensory neurones to motor neurons.

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9
Q

General structure & function of neurones

A

Neurones help co-ordinate communication within the nervous system
They all have a cell body composed of a nucleus as well as organelles such as mitochondria in the cytoplasm. These provide energy (in the form of ATP) needed for active transport of ions into & out of the cell in an impulse. They also contain dendrites (extensions involved into conducting imulses towards the cell body), and axons (which conduct impulsess away from the cell body).

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10
Q

Detail the role of myelination?

A

Myelination is a layer of fatty substance/lipid formed from schwann cells wrapped around the neurone - can increase the speed of impulses by acting as an electrical insulator, and allowing impulses to travel by saltatory conduction.

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11
Q

What is tho role of myelination in saltatory conduction

A

Myelination plays a key role in saltatory conduction byinsulating axons and allowing action potentials to “jump” from one node of Ranvier to the next, which speeds up the transmission of nerve impulses.

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12
Q

Describe the process of synaptic transmission

A
  1. An action potential triggers a calcium influx:
    When an AP arrives at the presynaptic membrane the depolarisation stimulates voltage-gated calcium ion channels to open. Calcium ions enter the neurone
  2. Calcium influx causes Neurotransmitter Release:
    The influx of calcium ions into the synaptic knob causes the synaptic vesicles to move to the presynaptic memberane. They then fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft via exocytosis.
  3. The Neurotransmitters trigger an Action potential in the postsynaptic neurone:
    The neurotransmitters diffuse across the synaptic cleft and binds to specific receptors of the postsynaptic membrane, stimulating the opening of sodium ion channels in the postsynaotic neurone. The influx of sodium ions into the postsynaptic membrane causes depolarisation and an action poetential is genereted on the postsynaptic membrane if the threshold is reach.

Synaptic transmission is contolled with the help of digestive enzymes in the synaptic cleft, which break down excess neurotransmitters to prevent overstimulation on the postsynaptic membrane. These are then taken up by the pre-synaptic membrane and reused. Re-uptake of NTs, the presence of receptors on one side of the synapse only & the refractory period serves to ensure that the AP is unidirectional.

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13
Q

Acetylcholine

A

A neurotransmitter involved in muscle contraction & the control of heart rate

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14
Q

The role of neurotransmitters: excitatory vs inhibitory

A

Excitatory neurones stimulate/the production of an action potential in the postsynaptic membrane by stimulating theopening sodium ion channels.
Inhibitory neurones cause chloride ions to open in the postsynaptic membrane, so negative chloride ions enter, thus causing hyperopolarisation of the postsynaptic membrane. Therefore triggering a new action potential becomes difficult.

Whether or not an action potential is triggered in the post-synaptic membrane depends on the summation of excitatory & inhibitory neurones and if voltage reaches the threshold for that neurone.

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15
Q

What is Synaptic divergence?

A

Impulse diverge in when one neurone connects to many neurones alllowing info to be dispersed to different parts of the body.

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16
Q

What is synaptic convergence?

A

When many neurones connect to one neurone so info is amplified (made stronger).

17
Q

8.5) Describe how light travels through the eye

A

Light enters the eye through the pupil and the amount of light entering is controlled by the muscles of the iris. The lens (the shape of which is controlled by ciliary muscles) of the eye focuses light on the retina where the photoreceptors are located, specifically in the fovea. Subsequently, the nerve impulses recieved by the photoreceptors cells are then carried via the optic nerve to the brain. The point where the optic nerve leaves the eye is known as the blind spot so there are no photoreceptor cells there.

18
Q

Photoreceptors

A

Photoreceptors are light receptors in the eye. The retina contains two types of photoreceptors:

Rod cells:
- Located around the outer retina
- Sensitive to light intensity (detects presence and brightness of light)
- Only produce monochromatic vision (black and white)

Cone cells:
- Grouped together in the fovea
- Sensitive to different wavelengths of visible light
- generates coloured images

19
Q

What are receptors?

A

Receptors are specialised cells that can generate an electrical impulse in a sensory neurone when stimulated by a particular stimulus.

20
Q

Rod and cone cells differ due to the type of vision they provide.
Explain how else to the two photoreceptors differ.

A

They differ by their level of sensitivity - Cones can only work in bright conditions whereas rods are much more sensitive and dim light in sufficient for them to work. This is brought about as a result of convergence - three rods converge onto a single bipolar cell (connecting photoreceptors to the optic nerve) - this acts to amplify the signals triggered by light. Whereas, only one cone cell synapses with each bipolar neurone - so there is no amplification of the signal - making these photoreceptors less sensitive.

21
Q

The action of rod cells in the dark

A

In the dark, the rods aren’t stimulated: Sodium ions diffuse into the cell through open sodium ion channels whilst also being actively pumped out of the cell. As a result the inside of the cell is only slightly more negative compared to the outside, thus causing the membrane to be slighlty depolarised. Consequently calcium ion channels open in the presynaptic membrane and Ca2+ ions diffuse into the rod cell causing the release of a neurotransmitter called glutamate. Glutamate serves to inhibit the bipolar neurone by preventing sodium ion channels from opening in the post synaptic memebrane, so depolarisation doesn’t occur and an action potential isn’t triggered. It creates an IPSP (a temporary hyperpolarisation in the post-synaptic membrane) and as a result no information is transmitted to the brain.

22
Q

The action of rod cells in the light

A

Rods contain a light-sensitive pigment called rhodopsin which absorbs light energy. In the presence of enough light intensity, rhodopsin splits into retinal & opsin. Retinal changes structure from cis retinal to trans retinal, which activates opsin. Opsin binds to the membrane of the rod cell thus causing sodium ion channels to close without affecting their active removal. Because sodium ions cannot diffuse back in, the membrane become hyperpolarised. As a result, Calcium ion channels don’t open in the pre-synaptic celft and no neurotransmitter in released into the synaptic cleft. Consequently sodium ion channels remain open in the post-synaptic cleft and sodium ions can diffuse in. Once a threshold is reached more sodium ion channels open and an action potential is generated in the bipolar neurone.
Opsin is reconverted to rhodopsin afterwards using enzymes, energy and vitamin A. Trans retinal&raquo_space; cis retinal.

23
Q

Why do rod cells contain lots of mitochondria?

A

To provide ATP for resynthesis of rhodopsin and active transport of Na+ ions.

24
Q

8.8) Cerebrum

A

Largest part of the brain composed of two halves - cerebral hemispheres. Involved in controlling vision, thinking, learning & emotion as well as voluntary control of the body. Different parts of the cerebrum have different functions:
- Frontal Lobe: Thinking & decision making
- parietal lobe: controls sensory info (touch, spatial awareness)
- Occiptial Lobe: processes visual info
- Temporal Lobe: processes auditory info

25
Q

8.8) Cerebellum

A

Located underneath the cerebrum & plays and important role in co-ordinating muscle movements as well as balance.

26
Q

8.8. Corpus callosum

A

A band which connects the two cerebral hemispheres

27
Q

Hypothalamus

A

Found just beneath the middle part of the brain is involved in thermoregulation as well as production of hormones involved in the control of the pituitary gland.

28
Q

Medulla Oblangata

A

Located at the base of the brain controls many vital body processes such as breathing, heart rate & blood pressure.

29
Q

CT scans

A

Use X-rays to produce cross-section images of the brain. Denser structures absorb more radiation than less dense structures and therefore they show up lighter on the scan. Produces 2D & 3D images.

Strengths:
- Can be used for medical diagnosis which show up damaged/abnormal areas.
- Generally cheaper than MRI scanners.

Limitations:
- Static images - shows structure but not function, which have to be worked out through behavioural changes.
- Lower quality images & weaker resolution than MRI scanners.

30
Q

MRI

A

Magnetic Resonance Imaging scanners use magnetic field and radio waves for imaging soft tissue.

Strengths:
- Can be used for diagnosis as diseased tissue can be seen.
- Can determine the exact size and location of tumours as they show up a lighter colour.
- Gives 2D/3D images
- Higher quality images for soft tissue than CT. Better resolution.

Limitations:
- Gives static image - only shows structure, function has to be worked out.
- MRI scanners cost a lot more than a CT scanner.
- Not good for people with metallic implants or pace makers.

31
Q

FMRIs

A

Uses magnetic fields & monitors oxygen uptake to show brain activity.

Strengths:
- Shows abnormal activity in the brain, which can be used for medical diagnosis.
- Shows live activity - can investigate structure and function.
- Detailed, high resolution images

Weakness:
- Expensive
- Safe, painless and non-invasive
- May not be appropriate for people with metal or electronic implants.

32
Q

PET scans

A

Positron emission tomography scans used radioactive isotopes with a short half-life to monitor areas of activity in the brain. Follows the flow of blood (and thus radioisotopes) through the brain and can be used to monitor activity whilst performing different activities. it is detected by the emission of positrons as the radioisotopes decay.

Strengths:
- Very detailed
- Investigates structure & function in real-time (live image).

Limitations:
- Invasive

33
Q

Hubel and Wiesel conducted experiments on animals to investigate the structure and development of the visual cortex.
i) Describe Hubel and Wiesel’s experiment on kittens (3 Marks)
ii) Explain what their experiment showed about the development of the human visual system. (1 Mark)
iii) Do their experiments give evidence for a critical period in the development of the human visual system? Explain your answer, with reference to what is meant by a critical period (2 Marks)

A

i) Hubel and Wiesel stitched shut one eye of very young kittens for several months (1 mark). When they unstitched the eyes, they found that the kitten’s eye that had been stitched up was blind (1 mark). They also found the ocular dominance columns that were stimulated by the open eye had become become bigger and taken over the ocular dominance columns that weren’t visually stimulated for the shut eye - switched dominance (1 mark).

ii) Hubel and Wiesel’s experiment showed that the visual cortex only develops properly if both eyes are visually stimulated in the very early stages of life. (1 mark)

iii) Yes, their experiments give evidence for a critical period in the development of the human visual system because our visual cortex is also made up of ocular dominance columns (1 mark). The critical period is the period of time early in life when it’s critical that you’re exposed to the right visual stimuli for the visual system to develop properly (1 mark).

34
Q

Where in the brain are ocular dominance columns found? What are they and how are they arranged?

A

Ocular dominance columns are found in the visual cortex (an area of the cerebral cortex at the back of the brain). They are groups of neurons, which receive visual info from either the left or the right eye. They are arranged in an alternating pattern (left,right,left,right) across the visual cortex.

35
Q

How have scientists investigated visual development in humans?

A

Cataracts

36
Q

Describe the role of a critical period in the development of the visual system in cats and other mammals.

A

Critical period is the period in early life of animals (including humans) when, for the
proper development of visual cortex, exposure to visual stimuli is critical.
● Baby mammals have lots of neurons (synapses) in the visual cortex, which needs to be
properly organised to process visual information.
● Synapses that do not receive the visual signal during the critical period are destroyed
and the rest retained (strengthened) developing visual cortex properly.
This means if eyes are not stimulated with visual info during this critical period of development, the visual cortex will not develop properly as many of the synapses will be destroyed.

37
Q

Discuss the use of animals in medical research

A

Arguments AGAINST using animals:
- Animals are often different from humans, so drugs tested on animals may have different effects on humans.
- Experiments can cause pain & distress to animals
- There are alternatives to using animals in research e.g. using cultures of human cells.
- Some people think that animals have the right to not be experimented on. They cannot consent.

Arguments FOR using animals
- Utilitarianism argument - the benefits (saving many human lives) may outweigh the costs (harm to animals).
- Animals are similar to humans in certain aspects, so research has led to lots of medical breakthroughs.
- Animal experiments are only done when its absolutely necessary & sceintists follow strict rules e.g. animals must be properly looked after, painkiller & anaesthetics must be used to minimise pain.
- Using animals is currently the only way to the study how a drug affects the whole body. e.g. cell cultures can’t be used to sudy behavioural effects.
- Some people think that humans have a greater right to life than animals because we have more complex brain (i.e. compared to rats, fish & fruit flies).

38
Q

Describe the role of visual stimulation to the development of the visual cortex during the critical period

A
  • Oscular dominance columns (develop in visual cortex) (1)
  • Neurones form synapses with these (cells/columns) (1)
  • (Stimuli/action potentials/impulses) along neurones required to strengthen connections (with cells of ocular dominance columns)
  • Stimulation during the critical period is needed to form (effective) connections in the visual cortex.