Unit 1: The Nervous System Flashcards
Overview (The function of the nervous system):
•The nervous system works in conjunction with the endocrine system to respond to both internal and external environmental change, thereby maintaining homeostasis within the body.
-the nervous system responds via electrochemical messages relayed from the brain.
-the endocrine system responds via chemical messengers relayed through the bloodstream (hormones).
Nerves:
•two main types of nerve cells found in the nervous system:
1. Glial cells
-used for structural and nutritional support.
2. Neurons
-conduct nerve impulses throughout the body.
-supported by glial cells.
Neurons are:
The basic unit of the nervous system.
Neuron structure (dendrites):
-branches which accept nerve impulses from other neurons and carry them towards the cell body.
Neuron structure (axons):
-longer branches which carry nerve impulses away from the cell body.
Neuron structure (myelin sheath):
-a fatty myelin sheath surrounds each axon, insulating the neuron and speeding up the rate of impulse transmission.
Neuron structure (schwann cells):
-a type of glial cell, are responsible for producing the myelin around each axon.
Neuron structure (nodes of ranvier):
-gaps between Schwann cells are referred to as the nodes of ranvier. Electrical impulses “jump” from node to node (saltatory conduction).
Neuron structure (axon terminal):
-once the electrical signal reaches the axon terminal, it is passed on to the dendrites of an adjoining neuron.
Myelination:
-the myelination of neurons is vital for proper signal transduction within the nervous systems.
Saltatory conduction:
-action potentials travelling down the axon “jump” from node to node.
Myelinated neurons:
-make up the white matter of your brain, which is responsible for conducting nerve impulses.
-can regenerate after injury.
Unmyelinated neurons:
-make up the grey matter of your brain, which is responsible for processing information and generating nerve impulses.
-cannot regenerate after injury.
Types of neurons:
-three main types of neurons carry signals to and from the brain (sensory neurons, interneurons, motor neurons).
Types of neurons (sensory neurons):
-afferent- moving towards CNS.
-gather information from sensory receptors (i.e. touch/sight/sound/taste receptors) and transmit these impulses to the brain.
Types of neurons (interneurons):
-process and integrate incoming sensory info from sensory neurons and relay outgoing information to motor neurons.
Types of neurons (motor neurons):
-efferent- moving away from CNS.
-transmit information from the brain to muscles (effectors), glands, and other organs.
Reflex arc:
•a neural circuit that passes through interneurons in the spinal cord for immediate response.
-simplest nerve pathway.
-neural circuit through the spinal cord.
-reflexes are involuntary & unconscious (no brain coordination).
5 essential components for a reflex arc:
- Receptor.
- Sensory neuron.
- Interneuron (in spinal cord).
- Motor neuron.
- Effector.
The speed of an impulse:
-the speed of an impulse along the nerve fiber is dependent on 2 things:
1. Myelin: myelinated axons send impulses faster than non-myelinated axons because the impulse jumps from node to node all along the axon.
2. Diameter of axon: bigger = faster.
Conduction of nerve impulses (electrical event):
-a nerve impulse or action potential has both a chemical and electrical component (hence the term electrochemical impulse).
Four stages of a nerve impulse:
- Polarized/resting state.
- Depolarization.
- Repolarization.
- Refractory period.
Four stages of a nerve impulse
(1. Resting state):
•the difference in charge across the membrane of a resting neuron is called resting membrane potential.
-the inside of a neuron has a slight negative charge at rest, whereas the outside has a slight positive charge.
-this results in a resting potential of -70mV.
Four stages of a nerve impulse
(Resting state & the Sodium-Potassium pump):
•A) at rest Na+ ions are found mostly outside the axon membrane and K+ ions are found mostly within the axon, along with larger negatively charged ions that cannot pass through the membrane.
•B) membrane is impermeable to Na+ ions.
•C) membrane is slightly permeable to K+ ions so some K+ ions leak out (leaky K+ ion channels).
•D) as a result, slightly more positive ions are outside the membrane, making the inside relatively negative.
•E) the sodium-potassium pump along the membrane surface maintains the electrical potential difference by transporting 3 Na+ ions out of the cell and 2 K+ ions inside the cell.
•F) as a result, an excess of positive charge accumulates outside of the cell membrane. A constant membrane potential of -70mV is maintained.
Four stages of a nerve impulse
(2. Depolarization):
-action potentials occur when a neuron is stimulated by an electrical impulse.
•A) an impulse causes sodium gates to fully open, thus allowing Na+ to diffuse freely across the membrane.
•B) Na+ rushes into the cell (down their concentration gradient), leading to a slight positive charge on the inside relative to the outside.
•C) this reverses the membrane potential from -70mV to +40mV; the membrane is now said to be depolarized.
•D) once Na+ reaches equilibrium, the membrane once again becomes impermeable to them, and the gates close.
Four stages of a nerve impulse
(Depolarization & threshold potential):
•an action potential is considered to be an “all-or-none” event because any stimulus that fails to achieve a membrane potential of at least -55mV will have no effect.
•note that increasing the stimulus strength does not increase the impulse strength; a neuron will either fire or not fire.
•the intensity of a stimulus is instead experienced as an increased frequency (number) (quick vs slow) of nerve impulses (IMPORTANT!!!).
Four stages of a nerve impulse
(Repolarization):
•once an action potential has peaked, Na+ gates close, and K+ gates open so K+ rushes out of the axon.
-this restores the positive charge outside the membrane; however, Na+ and K+ concentrations are briefly reversed compared to at the resting state (Na+ is higher inside the membrane).
-the sodium-potassium pump kicks in and exchanges Na+ for K+, restoring the initial resting potential of -70mV. This process is called repolarization.
-occurs in a wavelike motion down an axon.
Four stages of a nerve impulse
(Refractory period aka hyperpolarization):
•so much K+ rushes out of the cells that the membrane potential overshoots -70mV and the cell becomes hyperpolarized (-90mV).
•the sodium-potassium pump restores the membrane potential to -70mV.
•until the resting potential of the neuron has been properly restored, a second action potential cannot be conducted along the axon; this is referred to as the refractory period.
•during this time, the membrane cannot be made permeable to Na+, so a second wave of depolarization cannot occur.
•the stronger the impulse, the longer it takes for the nerve to recover.
Signal transduction across a synapse (chemical event):
-once the impulse has traveled down the entire length of the axon, it reaches the axon terminal.
•the axon terminal is in close contact with the dendrites of another neuron.
•for the signal to move to the next neuron, it must cross the space between the axon terminal and the dendrites of the subsequent cell; this space is referred to as the synapse or synaptic cleft.
•synapse or synaptic cleft = space between neurons.
Vesicle:
-like a taxi.
-neurotransmitter release:
•neurotransmitters are required to help “carry” the electrical impulse from one neuron to another.
•1. An action potential reaches the axon terminal.
•2. Calcium channel “gates” within the axon terminal open, causing calcium ions to flow into the cell and trigger the movement of neurotransmitter vesicles towards the presynaptic membrane.
•3. Vesicles fuse to the membrane and neurotransmitters are released into the synapse.
•4. Neurotransmitters diffuse across the synaptic cleft (synapse) to the post-synaptic membrane.
•5. Once neurotransmitters have reached the post-synaptic membrane, they bind with receptors embedded in the membrane.
•6. Binding induces or inhibits an action potential in the corresponding neuron.
•7. Neurotransmitters are then released by the receptors and either return to the presynaptic neuron or are broken down by an enzyme on the post-synaptic membrane.
Neurotransmitter types- there are two types of neurotransmitters:
- Excitatory.
- Inhibitory.
Neurotransmitters (excitatory):
•causes the Na+ channels of the post-synaptic membrane to open, resulting in depolarization and continuing the action potential.
•e.g. acetylcholine is an excitatory neurotransmitter found in muscle cells; causes contraction of the muscle fibre.
•cholinesterase is the enzyme required to break down acetylcholine after the action potential has occurred.
•insecticides and nerve gas block the release of cholinesterase, causing muscles to remain in a state of constant contraction.
Neurotransmitters (inhibitory):
•trigger the K+ channels to open, causing K+ to flow out and thus lowering the membrane potential.
•this leads to hyperpolarization, making it more difficult to generate an action potential.
•e.g. GABA (gamma-aminobutyric acid): problems with GABA are often associated with epilepsy and huntington’s disease.
•e.g. norepinephrine: produced by adrenal glands during flight or fight response. Excitatory or inhibitory (i.e. increase blood glucose levels, decrease digestion rate).
Summation:
•a single action potential and, thus neurotransmitters, from one neuron is usually too small to trigger an action potential in the postsynaptic neuron.
•most neurons have many synapses on their dendrites from many neurons.
•summation: the effect produced by the accumulation of neurotransmitters from two or more neurons on the postsynaptic neuron.
•an excitatory transmission can be enhanced with more excitatory transmissions (to produce an action potential).
•but an inhibitory transmission can delete the effects of an excitatory transmission (and prevent an action potential).
