Topic 8: Grey Matter Flashcards
Describe the nervous system
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What is the difference between a neuron and a nerve?
The neurone is a single cell, but a nerve is a bundle of the axons of many neurons surrounded by a protective covering.
In a basic neuron, v fine dendrites conduct impulses towards the cell body. A single long axon transmits impulses away from the cell body.
Draw a motor neurone
Draw and describe sensory and relay neurons
Sensory neuron: carry impulses from sensory cells to the CNS.
Relay neurons: found mostly w/in the CNS. Can have many connections w other nerve cells. Relay neurons= aka connector and interneurons
Describe a reflex arc
Rapid, involuntary responses to stimuli w simple nerve pathways.
A stimulus is detected by receptors which generate a nerve impulse. Sensory neurons conduct an impulse to the CNS along a sensory pathway. Sensory neurons enter the spinal cord through the dorsal route.
Sensory neuron forms a synapse w a relay neuron. Relay neuron forms synapse w a motor neuron that leaves the spinal cord via the ventral route.
Motor neuron carries impulses to an effector which produces a response
Describe and explain the pupil reflex.
Light rays passing thru the pupil stimulate receptor cells in the retina. Impulses pass along the optic nerve to the brain.
Radial and circular antagonistic muscles in the iris control pupil size. This allows correct light exposure to avoid eye damage. These muscles are controlled by the autonomic nervous system.
Bright light: parasympathetic nerve impulse. Circular muscles contract, radial muscles relax. The pupil constricts, less light enters.
Dim light: sympathetic nerve impulse. Circular muscles relax, radial muscles contract. Pupil dilates, more light enters the eye.
What is the resting potential?
Neurons have a negative resting potential across the cell membrane. This means inside the neuron is more - than outside as hay mas + charge outside the neuron.
So an anion requires energy to enter the inside of the cell, and a cation would gain energy to enter.
The resting potential is the voltage across the membrane while the neuron is at rest, about -70 mV.
Describe and explain how the resting potential is maintained
Resting potential is maintained by keeping more + ions outside the cell than in via a sodium-potassium pump. This uses ATP to pump 3 Na+ ions out and 2 K+ into the cell. Hydrolysis of ATP provides energy for this. This results in a loss of 1 + charge from the cell each time, setting up the - resting potential.
But, sera high K+ conc inside & high Na+ conc outside the cell. The neurone membrane is permeable to K+ via K channels, not Na! Buildup of K+ inside the neuron causes K+ to leave thru the channel by facilitated dif.
So an electrical AND conc gradient act on the K+ ions. The electrical gradient (Na-K pump) pumps K+ into the cell, but the conc gradient pulls K+ out the cell. At -70mV, the 2 gradients counteract each other y no hay net movement of K+
What are action potentials?
When neurons conduct an impulse, pd across the membrane is briefly reversed, making the inside of the axon + and the outside -. This is depolarisation.
PD becomes +40mV for 3 ms before returning to resting state asap. This is to allow more impulses to be conducted. This return to resting potential is called repolarisation.
The large change in voltage across the membrane= the action potential.
Describe depolarisation in an action potential
A neuron is stimulated, some depolarisation occurs. Change in voltage changes the Na+ gate shape, opening some voltage-dependent Na+ channels. Na+ flow in, increasing depolarisation. This opens even more sodium gates (positive feedback) till ALL Na+ gates open. So action potentials are all or nothing responses.
Hay higher conc of Na+ outside the axon, so Na+ rapidly flow in thru open voltage dependent Na channels. This builds up + charge inside and reverses polarity of the membrane. Potential difference reaches +40 mV.
What is repolarisation?
After 0.5 ms, voltage-dependent Na+ channels close, Na+ permeability of the membrane returns to usual v low level. Voltage-dependent K+ channels open.
K+ thus diffuse down the conc and electrical gradient, leaving the axon. Inside the cell once again becomes more negative, or repolarised. The cell also relies on the Na/K pump, which actively transports 3 Na+ out and 2 K+ in the cell.
During this repolarisation phase the cell is in its absolute refractory period- Na+ channels are inactivated & wont respond to any stimuli.
How is the resting potential restored?
The membrane is v permeable to K+ and lots more ions move out, making p.d more negative than -70 mV. This is hyperpolarisation.
Resting potential is re-established by closing voltage-dependent potassium channels. Potassium ions diffuse back into the axon to recreate resting potential.
Why is the refractory period so important?
The refractory period lasts til all voltage-dependent Na and K channels close, and resting potential is restored.
The refractory period ensures impulses travel in 1 direction and limits the number of action potentials- ensures action potentials are separated from one another.
Therefore hay limited number which can pass along a neurone in a given time
How are action potentials sped up?
In reality the wave of depolarisation across the membrane is quite slow. This is where the myelin comes in! No hay Na+ channels in the mylein, but nodes of Ranvier between the myelin have Na channels.
When + charge reaches a Schwann cell from the node, the Na ions rush in and bump other Na+ really quickly across the myelin to reach the next node. This is saltatory conduction, and the myelin sheath insulates the axon bc its hydrophobic.
A wider axon diameter also means faster conduction because it means less resistance to flow.
When would a stimulus generate an action potential?
A stimulus must be above a threshold level of -55 mV to generate an action potential.
The all or nothing affects of the a.potential means the size of the stimulus has no affect on the size of the a.potential.
But, the stimulus size affects the Hz of impulses and the number of neurons in a nerve that conduct impulses.
What are synapses?
A synapse links 2 or more neurons juntos. It allows action potentials from one neuron to be communicated to the next. Info travels from the presynaptic neuron, across the synaptic cleft (gap), to the postsynaptic neuron.
The synapse is filled w synaptic vesicles containing neurotransmitters like Acetylcholine. These are chemicals that stimulate the post synaptic neuron.
Synapses control nerve pathways to allow a flexible response. Synapses also allow a coordinated response by integration of info from diff neurons.
Describe synapse transmission
Action potential arrives. The membrane depolarises. Calcium ion channels open. Ca2+ ions enter the neuron, causing synaptic vesicles w neurotransmitter like ACh to fuse w the presynaptic membrane.
ACh is released across the synaptic cleft, into the post synaptic membrane and binds w specific, complimentary receptor proteins.
ACh binds to the comp receptor, changing the protein shape, opening cation channels. Na+ flow thru the channels. The membrane depolarises and initiates an action potential.
When released from the receptor the neurotransmitter will be taken up across the presynaptic membrane or it can diffuse away and be broken down.
What does the extent of depolarisation depend on?
The extent of depolarisation depends on the amount of Aceylcholine reaching the postsynaptic membrane.
This will depend on the number and Hz of impulses, and the number of functioning receptors in the postsynaptic membrane.
What are excitory synapses?
Excitory synapses make the postsynaptic membrane more permeable to Na+. Usually MANY impulses arriving al mismo tiempo produce sufficient depolarisation to generate an action potential. The fact that each impulse adds to the effect of others is called summation. Hay 2 types:
Spatial summation: impulses from different neurones generate an action potential in the postsynaptic neuron.
Temporal: several impulses along one neuron produce an action potential in the postsynaptic neuron.
Draw and label the structure of the eye
Describe the structure of rods and cones w/in the retina
3 layers of cells make up the retina.
The rods and cones synapse w bipolar neurone cells which synapse w ganglion neurons, whose axons make up the optic nerve.
Light hitting the retina has to pass through the layers of neurons BEFORE reaching the rods and cones.
Describe rods
Contains rhodopsin, made of opsin (lipoprotein) and retinal
Rhodopsin breaks down into its 2 parts in the presence of light. This is called bleaching
Rhodopsin takes time (and ATP!) to reform, while it is reforming it cannot be activated again
Rhodopsin is v sensitive and can detect a single photon
How do axons compete for target cells?
Axons compete for target cells in the visual cortex. Every time a neuron fires onto a target cell, the synapses of another neuron sharing the SAME target cell are weakened and release less neurotransmitter. If this repeatedly happens the synapses that are not firing will be cut back.
