Session 2: Neurons and Synapses Flashcards
What is the nervous system?
The system that coordinates the actions of complex organisms via the transmission of electrochemical signals. These signals are transmitted by a specialised network of cells called neurons (nerves). Neurons function to transmit electrical impulses within the nervous system that convert sensory information into electrical impulses in order to rapidly detect and respond to stimuli
What are the three main features of neurons?
Dendrites (short-branched fibres that convert chemical information from other neurons or receptor cells into electrical signals), axon (elongated fibre that transmits electrical signals to terminal regions for communication with other neurons or effectors), and soma (A cell body containing the nucleus and organelles, where essential metabolic processes occur to maintain cell survival).
What is the myelin sheath?
The myelin sheath is an insulating layer that can sometimes surround the axon of a neuron. It improves the conduction speed of electrical impulses along the axon, but it requires additional space and energy.
Compare the central nervous system and the peripheral nervous system.
The CNS is composed of the brain and spinal cord and it contains relay neurons. The peripheral nervous system is composed of cranial nerves, spinal nerves, and peripheral nerves, it contains sensory neurons (transmit info from sensory receptors to CNS) and motor neurons (transmit info from CNS to effectors).
Describe nerve impulses.
When a change in charge occurs. Neurons generate and conduct electrical signals by pumping positively charged ions (sodium & potassium) across their membrane. There is an unequal distribution of ions on each side of the membrane which creates a charge difference called a membrane potential
What is resting potential?
When a neuron is at rest, the difference in electrical charge across the membrane is called the resting potential. In a typical resting potential, the inside of the neuron is more negative relative to the outside (approximately –70 mV). This difference is maintained primarily by sodium-potassium pumps which release 3 Na+ for every 2 K+ they take in. This is an active membrane transport process (requires ATP).
What are action potentials?
The rapid localised change in membrane charge that occur in order to generate and propagate an electrical impulse along the neuron. This occurs in three main stages: depolarization, repolarization, and refractory period. An Action potential will only occur if the initial depolarization exceeds a threshold potential of approximately (55mV)
What is depolarization?
As a stimulus reaches the membrane, sodium channels open allowing the Na+ to flow in, depolarizing the membrane and making it less negative.
What is repolarization?
The sodium channels close and potassium channels open, allowing K+ ions to escape which causes the membrane potential to return to a more negative internal differential.
What is the refractory period?
At this stage, the resting ionic distribution is largely reversed. The resting potential must be restored by the sodium-potassium pump. The nerve can’t fire again until the resting potential is restored.
Describe nerve impulses (action potential)
Nerve impulses are action potentials that move along the length of an axon as a wave of depolarisation. The ion channels that occupy the length of the axon are voltage-gated (they open in response to changes in membrane potential near them). Hence, depolarisation at one point of the axon triggers the opening of ion channels in the next segment of the axon. It is an all-or-none situation: An action potential of the same magnitude will always occur provided the threshold potential (–55 mV) is reached. If the threshold potential is not reached, an action potential cannot be generated and hence the neuron will not fire
What is myelination?
The formation of myelin. Some neurons are covered in the insulating substance called myelin which increase the speed of nerves impulse transmission by saltatory conduction. Along unmyelinated neurons, action potentials propagate sequentially along the axon in a continuous wave of depolarisation. In myelinated neurons, the action potentials ‘hop’ between the gaps in the myelin sheath called the nodes of Ranvier. This results in an increase in the speed of electrical conduction by a factor of up to 100-fold
What are synapses?
Junctions between 2 Neurons. Action potentials are transmitted between neurons via the release of signalling molecules called neurotransmitters. When the neurotransmitter diffuses across the synapse, it interacts with the receiving membrane to cause depolarization that results in propagation of the nerve signal to the next neuron.
Describe the 5 steps of synapse transmissions.
Step 1: Arrival of an action potential at the axon terminal, causing an influx of calcium ions and the vesicles stored near the presynaptic membrane to release their neurotransmitter into the synaptic cleft.
Step 2: The neurotransmitter diffuses across the synaptic cleft to receptors on the postsynaptic membrane of a dendrite on another neuron, delaying the impulse transmission by about 0.5 ms.
Step 3: The neurotransmitter binds to receptors on the post synaptic membrane.
Step 4: Ion channels in post synaptic membrane open, causing an influx of sodium. This response may or may not reach the threshold (-55mV) required to generate a new action potential and nerve impulse.
Step 5: The neurotransmitter is deactivated by an enzyme located on the membrane. Components of the neurotransmitter are actively reabsorbed back into the synaptic knob, recycled, and repackaged.
What are neurotransmitters?
Neurotransmitters maintain signals in the nervous system by binding to receptors on post-synaptic neurons and triggering electrical impulses. They also activate responses by effector organs (such as contraction in muscles or hormone release from endocrine glands). Neurotransmitters may be either excitatory or inhibitory in their effect (some may be both depending on the receptor they bind to). Excitatory neurotransmitters trigger depolarisation, increasing the likelihood of a response. Inhibitory neurotransmitters trigger hyperpolarisation, decreasing the likelihood of a response
