Unit 11: The nervous system Flashcards
(39 cards)
What are the two divisions of the nervous system
- Central nervous system (CNS): consists of brain and spinal cord
- Peripheral nervous system (PNS): consists of nerves that carry messages to and from the CNS to the rest of the body.
What are the two types of cells in the nervous system
- Neurons: Cells that transmit nerve impulses between parts of the nervous system. Many neurons grouped together = nerve
- Neuroglia: Cells that support and nourish neurons.
Dendrite
Neuron structure
extensions that receives signals from other neurons and sends them towards a cell body
Cell body
Neuron structure
Contains the nucleus and organelles to keep the cell alive. Relays an impulse from dendrite to axon
Axon
Neuron structure
Conducts an impulse away from the cell body towards other neurons or effectors. A membranous tube containing axoplasm (cytoplasm of the axon) and axomembrane (membrane of axon).
Myelin sheath
Neuron structure
Protective coating of a type of neuroglial cells called Schwann cells, around larger axons and dendrites.
Nodes of Ranvier
Neuron structure
Gaps in the myelin sheath; speeds up transmission of an impulse.
Motor neurons function and structure
Relays messages from an interneuron in the CNS to an effector (organ, muscle fiber, or gland).
Structure: Short dendrites and long axons
Sensory neurons function and structure
Relays messages to an interneuron in the CNS. May be equipped with special endings called sensory receptors that detect environmental changes and alerts the sensory neuron.
Structure: Long dendrites and short axons
Interneurons function and structure
Lies entirely within the CNS; relays messages between neurons (i.e., from a sensory neuron to a motor neuron or between other interneurons).
Structure: Length of dendrites and axons vary depending on their location/function within the CNS.
Oscilloscope
Nervous system uses nerve impulse to convey information. Nerve impulse can be analyzed using a voltmeter called oscilloscope, conducted on two points (inside and outside on an axon), as voltage is a measure of the electrical potential differences between two points.
Resting potential (Voltage, and the cause of such voltage)
When the axon is not conducting an impulse
Voltage: -65 millivolts; The inside of the axon is negative compared to the outside.
The polarity is caused by two things:
1. Sodium-potassium pump: Membrane protein that actively transports Na+ out of and K+ into the axon.
2. Large negatively charged organic molecules inside the axon that are too large to cross the membrane.
When do nerve impulses occur? Threshold?
Nerve impulse occur as a response to a stimulus; stimulus strong enough to reach a certain threshold. Nerve impulse is a “all-or-nothing” response to a stimulus – there are no partial action potential.
Active potential (Voltage, and two stages)
Rapid change in polarity across the axomembrane as a nerve impulse occurs.
1. Depolarization: Charge inside axon changes from – to +
Voltage: +40 millivolts; The inside becomes more positive than the outside.
-Sodium gates open
-Na+ diffuse along its concentration gradient, into the axon.
2. Repolarization: Charge inside axon changes from + back to -
Voltage: -65 millivolts
-Potassium gates open
-K+ diffuse along its concentration gradient, out of the axon
Voltage is restored at repolarization, but the locations of the Na/K ions are switched
Refractory period
Returning the ion distribution back to normal
- Sodium gate closes; nerve impulse cannot move backwards
- Over when the sodium-potassium pump restored the ion distribution
Nonmyelinated vs Myelinated nerve fibers
nonmyelinated axons/dendrites: action potential travels down one section at a time.
myelinated axons/dendrites: the sodium and potassium gated channels are concentrated at the nodes of Ranvier. Ion exchanges are happening only at the nodes, so transmission of a nerve impulse is much faster. Called saltatory transmission as the action potential “jumps” from node to node.
Synaptic cleft
The gap between the axon terminal and the dendrite
Presynaptic vs Postsynaptic membrane
- Presynaptic membrane: Membrane of the first neuron that is sending the impulse
- Postsynaptic membrane: Membrane of the next neuron that is receiving the impulse
Neurotransmitters
Molecules that carry the electrical messages from one neuron to another
Two classes:
1. Excitatory neurotransmitters: Cause an action potential to be generated
2. Inhibitory neurotransmitters: Stop an action potential from generating
Axon terminal & Synaptic vesicles
Axon terminal: The end of an axon
Synaptic vesicles: Storage of neurotransmitters in the axon terminal
Impulse transmission: Synaptic transmission
First stage
- Nerve impulse reaches an axon terminal
- Calcium gates open, and Ca2+ ions move into the axon terminal.
- Sudden rise in Ca2+ stimulates synaptic vesicles to merge with the presynaptic membrane
- Neurotransmitters are released into the synaptic cleft
- Neurotransmitters diffuse across the synaptic cleft and bind to specific receptors on the postsynaptic membrane of the next neuron
Impulse transmission: Synaptic integration
Second stage
- Neurotransmitters may be transmitting excitatory or inhibitory signals
- Neurons integrate all these signals
- The combination of excitatory and inhibitory messages must reach the threshold for a new action potential to be generated in the second neuron
Impulse transmission: Neurotransmitter breakdown
Once the neurotransmitter has initiated a response, they have to be inactivated, in order to prevent continuous stimulation/inhibition of the postsynaptic membrane
- Some neurons contain enzymes that can rapidly inactivate the neurotransmitter. (i.e., monoamine oxidase breaks down norepinephrine (NE), acetylcholinesterase (AChE) breaks down acetylcholine (ACh))
- the postsynaptic membrane reabsorbs the neurotransmitter, for either repackaging into synaptic vesicles or molecular breakdown.
Effect of drugs at a synapse
- Cause a neurotransmitter to leak out of the synaptic vesicle into the synaptic cleft; increases number of neurotransmitters so its action is enhanced
- Prevent the release of neurotransmitter; decreases number of neurotransmitter so its actions are inhibited.
- Promote the release of neurotransmitter; same effect as 1
- Prevent reuptake of neurotransmitter by the presynaptic membrane; the neurotransmitter is around for longer, so its effect is prolonged
- Block the enzyme that breaks down a neurotransmitter; same effect as 4
- Bind to a receptor mimicking a neurotransmitter; body responds as if the neurotransmitter was present
