Can someone PLEASE help me in how neurones help a pupil dilate ? - a level biology

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JacobBob
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https://imgur.com/a/uuVP10q

I want to understand the sequence. In Mp1, what depolarisation are they talking about ?

Should I say that when light falls on the rods in the retina, the rods cause depolarisation in the bipolar neurones ... ?

Is the sensory neurone a different neurone than bipolar neurones ? If so, do nerve impulses pass through the bipolar neurones first and then the sensory neurone ?
The nerve impulses are transmitted via the optic nerve to the midbrain which then sends impulses via the symapthetic nervous system and the motor neurone to the radial muscles causing them to contract ?
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macpatgh-Sheldon
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Hi,
As you see from the mark scheme, you only need to say the obvious, basic things to get your marks at A level.

To explain to you what happens in a little more detail, SO THAT WHAT YOU NEED TO KNOW BECOMES A TRIFLE (yummy - [my teaching style btw]!):

1. The dimness of the light is first detected by the photoreceptors (rods/cones) (they are sensory neurones)
2. These send impulses via the bipolar neurones then ganglion cells (whose axons make up the optic nerve).
3. These impulses go to a part of the midbrain (oculomotor nucleus if you really want to know - nucleus of the third cranial nerve).
4. This nucleus sends impulses via the sympathetic system (a part of the autonomic [=like automatic] nervous system) (in the ciliary nerve) to the radial muscle of the iris (the part that gives the eye its colour).
5. Radial muscle contracts, pulling the rim of the pupil outwards to dilate the pupil (called mydriasis if you decide to take up optometry or medicine in Sept).

Hope this helps!
M
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JacobBob
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(Original post by macpatgh-Sheldon)
Hi,
As you see from the mark scheme, you only need to say the obvious, basic things to get your marks at A level.

To explain to you what happens in a little more detail, SO THAT WHAT YOU NEED TO KNOW BECOMES A TRIFLE (yummy - [my teaching style btw]!):

1. The dimness of the light is first detected by the photoreceptors (rods/cones) (they are sensory neurones)
2. These send impulses via the bipolar neurones then ganglion cells (whose axons make up the optic nerve).
3. These impulses go to a part of the midbrain (oculomotor nucleus if you really want to know - nucleus of the third cranial nerve).
4. This nucleus sends impulses via the sympathetic system (a part of the autonomic [=like automatic] nervous system) (in the ciliary nerve) to the radial muscle of the iris (the part that gives the eye its colour).
5. Radial muscle contracts, pulling the rim of the pupil outwards to dilate the pupil (called mydriasis if you decide to take up optometry or medicine in Sept).

Hope this helps!
M
Thanks but what should I say about mp1 ?
And what is the sensory neurone here ? macpatgh-Sheldon Jpw1097
Last edited by JacobBob; 1 year ago
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Jpw1097
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(Original post by JacobBob)
Thanks but what should I say about mp1 ?
And what is the sensory neurone here ? macpatgh-Sheldon Jpw1097
I'm assuming this question is asking about the pupil response in the dark. When light hits the photoreceptor (i.e. rod/cone), it actually causes the photoreceptor cell to become hyperpolarised. Therefore in the dark, the photoreceptor is depolarised. The photoreceptor cell synapses with a bipolar cell, and the bipolar cell then synapses with the ganglion cell (this is the sensory neurone in this case). Therefore, in the dark the ganglion cell is not depolarised and does not fire an action potential. As macpatgh-Sheldon has already said, the ganglion cells make up the optic nerve. When light is shone in the eye, the impulse then travels along the optic nerve to the midbrain (part of the brainstem) and synapses with motor neurones which innervate the radial pupillary muscles (via the oculomotor nerve). This then causes the pupillary sphincter muscle to contract, causing the pupil to constrict. In the dark however, I don't believe the dilation of the pupil is due to an increase in sympathetic activity. Yes, sympathetic fibres innervate the radial muscles which, when contracted, dilate the pupil. However, in the absence of light, there will be less parasympathetic activity, and the size of the pupil is determined by the balance between sympathetic and parasympathetic activity. Therefore, a decrease in parasympathetic activity will cause the pupil to dilate, even though the sympathetic activity remains unchanged.
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