lecture 14 - animal nervous systems 1 Flashcards
What are the functions of the nervous system?
allows animals to:
sense and respond to the environment
coordinate movement
regulate internal functions of the body
Describe a neuron
Neurons are electrically excitable cells that receive and transmit information. Dendrites are extensions that receive information, axons transmit information.
Neurons receive signals from other nerve cells or from specialized sensory endings. Neurons transmit signals to other neurons or other cell types, such muscles cells or secretory cells in glands.
What is membrane potential?
charge difference between the inside and the outside of the cell
Describe the resting membrane potential
The Sodium Potassium pump moves sodium ions out of the cell and potassium ions into the cell. (3 sodium ions out, 2 potassium ions in.) Potassium leak channels allow some potassium ions to move back out of cytoplasm and into extracellular space (down their concentration gradient), making the outside more positive than the inside.
The resting membrane potential is polarized (=more negative inside the cell than outside the cell). The resting potential is around -70mV (but varies for different types of nerve cells).
In mammals:
Electrochemical equilibrium for Na+: ~ +60mV
Electrochemical equilibrium for K+: ~ -90mV
What is electrochemical equilibrium?
condition in which no net ionic flux occurs across a membrane because ion concentration gradients and opposing transmembrane potentials are in exact balance. The resting membrane potential is not at equilibrium for sodium or potassium, but closer to the equilibrium for potassium.
What is an action potential?
Action potentials are generated at the axon initial segment (not the axon hillock) in response to membrane depolarization
Nerve cells (and also muscle cells) respond to changes in their membrane potential; they are electrically excitable.
- When a neuron is excited, its membrane potential becomes less negative (=depolarized). At the axon initial segment, a depolarization above a threshold potential leads to the formation of an action potential. If threshold is not reached, no action potential is generated (all-or-nothing response). An action potential is a rapid, short-lasting rise and fall in membrane potential.
Describe how action potentials are generated
Depolarization of the membrane potential at the axon initial segment leads to the opening of voltage-gated sodium channels
sodium enters the cell
the membrane gets more depolarized
more sodium channels open (positive feedback). Sodium channels close shortly after opening (independent of change in voltage). Voltage-gated potassium channels and sodium channels open at a similar threshold, but potassium channels have a slower response (so open later).
Opening of potassium channels leads to movement of potassium out of the cell
membrane becomes more polarized (more negative). Because of the increased membrane permeability to potassium, the membrane potential briefly falls below the resting potential (hyperpolarization), close to the equilibrium potential for potassium.
As voltage-gated potassium channels close, and sodium potassium pumps restore ion concentrations on either side of the membrane, the resting membrane potential is restored.
Describe action potential propagation
Action potentials are propagated along the axon
Action potentials are self-propagating: membrane depolarization by an action potential leads to the opening of neighbouring voltage-gated sodium channels - action potential - opening of neighbouring voltage-gated sodium channels - action potential…
What is the refractory period?
Refractory period (due to inactive sodium channels and hyperpolarization) prevents backwards travelling of the action potential. During the absolute refractory period, an action potential cannot be generated, no matter how strong the stimulus. During the relative refractory period, only a very strong stimulus can elicit an action potential.
What factors affect nerve conduction velocity?
The conduction velocity of action potentials depends on the axon diameter (axons with a bigger diameter have lower resistance, allowing the charge to spread faster)
The conduction velocity also depends on the myelination of the axon.
(conduction velocity also depends on other factors, such as voltage change created by action potential, ‘leakiness’ of the membrane for charge)
- Conduction velocity is greatly increased by axon myelination
Describe axon myelination
In vertebrates, glial cells (Schwann cells -peripheral nervous system - and oligodendrocytes - central nervous system) form insulating myelin sheaths around axons
- Many vertebrate axons are myelinated. Glial cells form lipid-rich layers called myelin that wrap around axons.
- Schwann cells insulate axons of the peripheral nervous system, oligodendrocytes insulate axons in the central nervous system
Explain saltatory conduction
After a membrane depolarization occurs, the charge difference between the inside and outside of the membrane decays with distance from the depolarization event. The decay is increased by leakage of current across the cell membrane.
In a non-myelinated axon, voltage-gated sodium channels need to be in close proximity, so that a new action potential can be generated at a distance where the depolarization is above threshold for eliciting a new action potential.
The insulation by myelin enables the spreading of a charge from an action potential over a greater distance along the axon because less charge is lost across the membrane. Therefore, the charge from an action potential in a myelinated axon can elicit and action potential further away compared to an action potential in a non-myelinated axon. Because the spreading of charge along the axon is much faster compared to the generation of new action potentials, action potential propagation is much faster in myelinated axons.
Describe signal transmission at a chemical synapse
When action potential reaches axon terminal, the depolarization leads to the opening of voltage-gated Ca2+ channels. Ca2+ concentration is higher outside the cell, therefore opening of the channels leads to diffusion of calcium ions into the cell. Rise of calcium ions in the cell leads to fusion of vesicles containing neurotransmitters with the membrane. Neurotransmitters are released into the synaptic cleft.
- Neurotransmitters bind to receptors on postsynaptic membrane
- opening of ion channels (e.g. sodium channels)
- change in postsynaptic membrane potential
The action of the neurotransmitters terminate when they are broken down, taken up into cells, or diffuse away from the synapse.
The properties of the chemical synapse ensure that signal transmission only occurs in one direction.
Describe excitatory post synaptic potential (EPSP)
Depolarization of membrane potential as result of neurotransmitter binding (opening of Na+ channels)
The depolarization decreases with distance from the stimulus
- If the depolarization is strong enough (if threshold depolarization for opening of sodium channels at axon initial segment is reached), it can lead to the formation of an action potential at the axon initial segment
- If EPSPs are generated in quick succession (high frequency of stimulation), they are summed. Summed subthreshold EPSPs might lead to depolarization of the membrane above threshold at the axon initial segment and set off an action potential. (temporal summation)
EPSPs can also be summed over space; subthreshold EPSPs received from several axon terminals at the same time might lead to depolarization above threshold at the axon initial segment and set off an action potential. (spatial summation)
Describe inhibitory postsynaptic potentials (IPSPs)
Hyperpolarization of membrane potential as a result of neurotransmitter binding (opening of K+ or Cl- channels)
Neurotransmitter binding to receptors on the postsynaptic membrane can also lead to a membrane hyperpolarization, by opening of potassium channels (potassium moves out of the cell), or by opening of chloride channels (chloride moves into the cell). Hyperpolarization inhibits generation of an axon potential.
