6.5 Flashcards
Neuron structure
The nervous system has two main parts: The central nervous system is made up of the brain and spinal cord. The peripheral nervous system is made up of nerves that branch off from the spinal cord and extend to all parts of the body.
The three main subparts of a single neuron are its dendrites, cell body and axon.
▪ The dendrites are the receiving part of the neuron. Dendrites receive synaptic inputs from axons, with the sum total of dendritic inputs determining whether the neuron will fire an action potential.
▪ The cell body carries genetic information, maintains the neuron’s structure, and provides energy to drive activities
▪ At the end of the axon are synaptic terminal buttons, which release chemicals called neurotransmitters that continue the impulse chemically to the next neuron or possibly a muscle.
▪ An impulse is always carried from the dendrite end of a neuron along the membrane of the cell body down the axon, and results in a release of a neurotransmitter.
Describe saltatory conduction.
Neurons with an advanced nervous system have axons with a myelin sheath; they are said to be myelinated. The myelin sheath is actually a series of cells called schwann cells that have each wrapped themselves around the axon multiple times, creating multiple layers of the same cell membrane.
The Schwann cells are spaced evenly along any one axon, with small gaps between them; these gaps are called nodes of Ranvier.
Saltatory conduction is the term used to describe the phenomenon whereby an action potential of myelinated axons skips from one node of Ranvier to the next as the impulse progresses along the axon towards the synaptic terminals.
Impulse travels faster. Less energy in the form of ATP is expended.
Resting Potential
The time period during which an area of a neuron is ready to send an action potential, but is not actually sending one, is called the resting potential. This area of the neuron is said to be polarized.Created by the active transport of sodium ions (Na+) and potassium ions (K+) in two different directions.The vast majority of the sodium ions are actively transported out of the axon cell into the intercellular fluid, and the majority of the potassium ions are transported into the cytoplasm.This active transport of sodium and potassium in opposite directions is an active transport mechanism called the sodium-potassium pump. Na/K pump works by transporting three sodium ions ‘out’ for every two potassium ions ‘in
Depolarization
An action potential is often described as a self-propagating wave of ion movements in and out of the neuron membrane. The resting potential requires active transport (the Na/K pump) to set up a concentration gradient of both sodium and potassium ions. As sodium ions are actively transported to the outside of the membrane, they diffuse in when a channel opens. This diffusion of sodium ions is the ‘impulse’ or action potential, and results in the inside of the axon becoming temporarily positive in relation to the outside.
Repolarization
Neurons do not just send one action potential; one neuron may send dozens of action potentials in a very short period of time. When one area of an axon has opened a channel to allow sodium ions to diffuse in, that area cannot send another action potential until ions have been restored to the positions characteristic of the resting potential.
the importance of neurotransmitters.
At the distal end of the axons, as part of the synaptic terminals are swollen membranous areas called terminal buttons. Within these terminal buttons are many small vesicles filled with the chemical neurotransmitters.
Neurotransmitters are the body’s chemical messengers. They are the molecules used by the nervous system to transmit messages between neurons, or from neurons to muscles.
events of chemical synapses
When an action potential reaches the area of the terminal buttons, it initiates the following sequence of events.
Action potential results in calcium ions (Ca2+) diffusing into the terminal buttons.
Vesicles containing the neurotransmitter fuse with the plasma membrane and release the neurotransmitter (exocytosis)
The neurotransmitter diffuses across the synaptic gap (or cleft) from the presynaptic neuron to the postsynaptic neuron.
The neurotransmitter binds with a specific receptor protein on the postsynaptic neuron membrane.
This binding results in an ion channel opening and sodium ions diffusing in through this channel.
This initiates the action potential to begin moving down the postsynaptic neuron because it is now depolarized (the action potential is now self-propagating)
The neurotransmitter is degraded (broken into two or more fragments) by a specific enzyme and neurotransmitter is released from the receptor protein.
The Ion channel closes to sodium ions
Neurotransmitter fragments diffuse back across the synaptic gap to be reassembled in the terminal buttons of the presynaptic neuron (reuptake)
Synapsys can also occur where a motor neuron adjoins muscle- the mechanism is almost the same but results in a muscle undergoing a contraction.
minimum threshold
In depolarization, each action potential must reach a minimum threshold in order to be self propagated. This begins at the first receptor neuron that began the chain of events. If the minimum is reached, the action potential is initiated and begins to self propagate.
