Topic 8 Flashcards
What 2 sections is the mammalian nervous system divided into?
Peripheral Nervous System
Central Nervous System
What is the central nervous system made up of?
The brain and the spinal cord
What is the peripheral nervous system made up of?
- Somatic (voluntary) nervous system
-
Autonomic nervous system - made up of 2 antagonistic branches:
- The sympathetic nervous system
- The parasympathetic nervous system
What does the somatic nervous system control?
Voluntary activity
(e.g. clicking through these flashcards)
What does the autonomic nervous system control?
Involuntary activities
e.g. heart rate
What do the sympathetic + parasympathetic branches of the autonomic nervous system do?
Anatgonistic branches - act in opposing ways
Allow involuntary responses to be controlled in both direction/ways
e.g. sympathetic increases heart rate, parasympathetic decreases
What type of cells make up the mammalian nervous system?
What are the 3 most common types?
Neurones:
Sensory
Relay
Motor
What role do sensory neurones play in the nervous system?
Connect sensory receptors to the CNS
What role do relay neurones play in the nervous system?
Found in the CNS
What role do motor neurones play in the nervous system?
Communicate/pass impulses from the CNS to effectors
Label the diagram of a motor neurone including the Schwann cell
Draw + label a diagram of a nerve
What function do Schwann cells have on a neurone?
Wrap around axon + prodyct a layer of lipid/myelin to insulate it
Prevents flow of ions across membrane so insulates axon
Describe how a mammal’s nervous system responds to a stimulus that triggers a reflex response
Sensory neurones carry impulses from receptors to the CNS (made up of the brain + spinal cord)
The CNS (containing relay neurones) processes the information from many sources
It then sends out impulses via motor neurones to effector organs (mainly muscles + glands)
Describe the stages of the pupil reflex
The iris contains pairs of antagonisic muscles (radial + circular) that control its size
These are controlled by the autonomic nervous system
In high light intensity, photoreceptors in the retina cause impulses to pass as high freq. along the optic nerve to cells in the brain.
These then send impulses along parasympathetic neurones to the circular muslces of the iris. The muscles contract, reducing the diameter of the pupil + how much light enters
In low light intensity, impules are sent at low freq. to the brain, causing impulses to be send down sympatheic motor neurones to the radial muscles instead. These cause the pupil to dilate.
What are the names of the antagonisitc muscles in the iris?
Which cause the pupil to constrict/dilate?
Radial - dilation
Circular - constriction
What is resting potential?
The state of nerve fibres when they are not conucting an impulse
Is a dynamic equilibrium that results from an imbalance in the conc of Na+ and K+ + differences in permeability of the cell membrane to ions
What is the potential difference of a neruone/membrane?
The difference in charge across a membrane
Always a comparison - in this case between the cytoplasm + outside of the cell
What is the resting potential of a nerve cell usually?
-70mV
How can the potential difference of a neurone be measured?
Using a microelectrode connected to a voltmeter
1 end put into the cytoplasm, the other outside the cell
Describe how the dynamic equilibrium that causes the resting potential of a neurones is established
The Na+/K+ pump creates a chemical gradient across the cell by pumping Na+ out + K+ in
Because there are fewer K+ ions outside the cell then inside, the ions diffuse out down the chemical gradient.
This causes the inside of the cell to become more negative (due to loss of + charge)
The negative charge inside the cell prevents the further diffusion of K+ + some ions re-enter the cell
Hence the movement of K+ in/out of the cell and thus its resting potential is maintained
Na+ channel proteins are closed at rest, preventing the ions entering the cell
What causes/triggers an action potential?
The depolarisation of a nearby membrane changing the PD to the threshold potential
Describe the stages that cause an action potential
- The threshold potential is reached and depolarisation occurs
- Na+ gates open, causing Na+ to diffuse down the electrochemical gradient into the cell, carrying with it a positive charge.
- As Na+ flows into the cell, depolarisation increases, opening more Na+ gates (positive feedback). Hence depolarisation is all-or-nothing.
- The polarity of the membrane reverses and reaches +40mV
- Na+ gates then close + K+ gates open, causing K+ to diffuse out of the cell down the electrochemical gradient. The PD of the membrane becomes negative again
- The membrane is hyperpolairsed + K+ gates close. K+ ions diffuse back into the cell, restoring the resting potential
Is the speed of transmission of an AP greater along myelinated or non-myelinated axons?
Why?
Greater along non-myelinated axons
As a result of saltatory conduction
What is the flow of ions along a nerve fibre called?
A local circuit
What is the refractory period?
Period in which the AP cannot be generated in the same section of membrane once it has been passed on
Due to movement of K+ back into the cell
Lasts about 5 miliseconds
Ensures impulse travels in one direction along nerve fibre
In nerves without a myelin sheath, what does the speed an impulse travel at depend upon?
Its cross-sectional area
Larger = faster
Describe saltatory conduction
The ‘jumping’ of an AP along a myelinated nerve
The only region of myelinated nerves that can be depolarised are the nodes of Ranvier
These means that the local circuits/movement of ions cover a longer distance than without myelin
Depolarisation of one node causes depolarisation of the next
Hence impulse ‘jumps’ from one node to another
Why do mammals have myelinated nerve cells even though impulses travel fastest through non-myelinated nerves with large cross-sectional areas?
Because mammalian nerves only have a small cross-sectional area
A myelin sheath speeds conduction by limiting depolarisation to the nodes of Ranvier
This allows saltatory conduction
Describe how an action potential crosses a synapse
- AP arrives in pre-synaptic membrane. The membrane depolarises, causing Ca2+ channels to open + Ca2+ enter the neurone
- Ca2+ cause synaptic vesicles containing neurotransmitter to fuse with presynatptic membrane
- Neurotransmitter released into synaptic cleft
- Neurotransmitter binds with receptors on post-synaptic membrane, causing Na+ channels to open + Na+ to enter the post-synaptic memrbane
- The membrane depolarises + initiates the AP again
- The neurotransmitter can be taken up by the presynaptic membrane (whole/broken down) or diffuse away to be broken down
What is acetylcholine?
What does it do?
Neurotransmitter
Found in all nerves of the somatic + parasympathetic autonomic system
What is the name of the neurotransmitter that breaks down acetylcholine?
Acetylcholinesterase
What ways/how can the binding of (different) neurotransmitters to (different) receptors on the postsynaptic membrane affect its potential difference?
A receptor with an excitatory synapse sends excitatory postsynaptic potentials
Opens Na+ channels + brings PD closer to threshold potential
A receptor with an inhibitory synapse sends inhibitory postsynaptic potentials
Postsynaptic membrane is hyperpolarised, moving PD away from threshold potential
What are the 2 types of photoreceptor cell in the human eye?
Where are they found?
What do the do/detect?
Found in the retina
Rods - Black/white vision BUT more sensitive to light intensity + can work in dim light
Cones - Colour vision in bright light. Clustered in the centre of the reina at the back of the eye
Describe how a rod cell detects light
(How rod cells function in light conditions)
- Light energy is absorbed by rhodopsin which splits into retinal + opsin
- Without retinal, the opsin binds to the membrane of the outer segment of the cell
- This causes cation channels to close. The inner segment continues to pump Na+ out of the cell + the membrane becomes hyperpolarised
- This means that glutamate isn’t released across the synapse.
- Glutamate usually inhibits the neurones connecting the rod cells to the neurones in the optic nerve
- There is less inhibition so an AP forms + is transmitted to the brain. The information from the optic nerve is processed by the brain in the visual cortex
Label the diagram of structure of rods + cones in the retina
Describe how rods function in dark conditions
- No light energy so rhodopsin doesn’t split into retinal + opsin
- Lack of opsin bound to outer segment of cell keeps cation channels open
- Na+ diffuse through open cation channels + move down conc. gradient down cell into inner segment
- In the inner segment, Na+ actively pumped out
- Membrane only slightly depolaries, triggering release of glutamate
- Glutamate binds to bipolar cells, preventing depolarisation + inhibiting AP forming
- No signal sent to brain via optic nerve
Describe the role of ATP in the hyperpolarisation of rod cells in the retina
ATP supplies energy for active transport needed to sodium/potassium (cation) pump
This pumps Na+ out of the inner segment + maintains more negative charge inside the membrane
