Section 5 Flashcards
What are the four functional zones of neurons that are involved in the transmission of neural impulses? What happens in each?
- The input zone: Contains the dendrites and cell body. This is where the incoming signals are received.
- The trigger zone: Contains the axon hillock. This is the part where axon potentials are initiated.
- Conducting zone: Contains the axon. This is the part where action potentials are conducted to their target locations.
- Output zone: Contains the axon terminals. This is the part that releases chemical messengers to communicate with other cells.
Describe transmission down an axon (conduction). How does this help the signal get transmitted?
When the action potential is initiated, the inside of the membrane becomes more positive. The positive charge influences adjacent areas of membrane and brings them from resting to threshold potential thus triggering another action potential. This repeats down the entire axon, like a wave down the neuron.
There is no signal decay, as the magnitude of the signal is refreshed with each new action potential.
What is one way propogation?
The wave of action potentials travels only in one direction - away from the cell body towards the axon hillock. If they went the opposite way, this would result in breakdown of neuronal communication.
How does the axon ensure one way propagation?
The absolute and relative refractory period for action potentials causes the sodium channels to remain inactive after depolarization. This prevents an action potential from being generated in the opposite direction.
Does a strong signal cause a bigger action potential?
No. All action potentials are considered all-or-none, and they all have the same magnitude.
Because of this, a strong signal does not cause a bigger action potential. It can however trigger an increased frequency of action potentials.
Where do we find oligodendrocytes, and where do we find Schwann cells?
Oligodendrocytes are supporting cells of the CNS
Schwann cells are supporting cells of the PNS
What are the regions of exposed fibre called?
nodes of Ranvier
What is a major difference between myelinated and non-myelinated fibres?
In non-myelinated fibres, the wave of excitation moves down the entire axon.
In myelinated fibres, the wave of excitation jumps from one Node of Ranvier to the next. This is called saltatory conduction and it allows myelinated fibres to conduct impulses up to 50 times faster than unmyelinated axons.
What are neuron to neuron junctions called?
Synapses (synaptic cleft)
Is synaptic transmission electrical or chemical in nature?
Chemical. This is because chemicals called neurotransmitters cross the synapse instead of an electric current.
What are neurotransmitters?
Chemical messengers which transmit signals across a chemical synapse, from one neuron to another neuron, muscle cell, or gland.
Describe the process of synaptic transmission.
- Electrical potential reaches axon terminal, and the change in membrane potential opens voltage-gated Ca2+ channel. Ca2+ flows down its conc gradient into the cell.
- Calcium flows into the axon terminal, triggering exocytosis of the synaptic vesicles containing neurotransmitters.
- The neurotransmitters then diffuse across the synaptic cleft and interact with specific receptors on the postsynaptic (the next) neuron. This binding opens chemically-gated ion channels, through which ions flow into the cell and can modulate the postsynaptic membrane potential. This creates a graded potential, and if strong enough, will travel to the axon hillock. The graded potentials can either stimulate (an excitatory synapse) or inhibit (an inhibitory synapse) an action potential.
What does the interaction of a neurotransmitter with its receptor do in an excitatory synapse?
It opens nonselective cation channels that allow the postsynaptic movement of both Na+ and K+. K+ will most likely exit the cell down its concentration gradient, though it will still cause a slight depolarization but will not result in an action potential firing. This slight depolarization will bring the membrane potential closer to its threshold and therefore called an excitatory postsynaptic potential (EPSP).
What does the interaction of a neurotransmitter with its receptor do in an inhibitory synapse?
Inhibitory synapses generally result when the neurotransmitter interacts with its receptor to activate either Cl- or K+ channels.
Increasing K+ allows K+ to move down its conc gradient and flow out of the cell. Increasing Cl- allows Cl- to move down its conc gradient and enter the cell. Both of these events cause the inside of the cell to become slightly more negative.
These small hyperpolarizations move the membrane potential away from the threshold and are therefore called inhibitory postsynaptic potentials (IPSPs) as it would now take a stronger excitatory input to trigger an action potential.
What is summation?
What is it?: Summation is like a neuron’s decision-making process. It adds up small signals (graded potentials) to decide whether to send a message to other neurons.
How it works: Graded potentials (positive or negative signals) travel to the “axon hillock,” where they’re added together. If the total is strong enough (reaches a threshold), the neuron fires and sends a message; otherwise, it stays quiet.
Summary: Summation is how neurons decide when to communicate with others based on the total strength of incoming signals.
