Grace
Define resting potential and action potential (depolarization and repolarization).
Resting potential is the electrical potential across the plasma membrane of a cell that is not conducting an impulse.
Action potential is the reversal and restoration of the electrical potential across the plasma membrane of a cell, as an electrical impulse passes along it (depolarization and repolarization).
What two phases do action potentials consist of?
Depolarization of the membrane potential (change from negative to positive)
Repolarization of membrane potential (a change back from positive to negative).
State one advantage and one disadvantage of the Myelin Sheath.
Advantage: it improves the speed of electrical transmission via saltatory conduction
Disadvantage: it takes up a lot of space within an enclosed environment
State the functions of the nervous system
Receiving information from the external environment
Generating a response to the information
Explain the structure and function of a dendrite.
Dendrites are short branched fibers
They receive chemical information from other neurons or receptor cells
They convert the chemical information into electrical signals for transmission
Explain the process of a nerve impulse passing along a non-myelinated neuron.
Resting potential rises above threshold level.
Voltage gated sodium channels open.
Sodium ions flow into the cell, more sodium channels open.
Inside of cell develops a net positive charge compared to the outside and results in deposium channels open.
Potassium ions flow out of the cell.
Cell develops a net negative charge compared to the outside and results in repolarization.
Concentration gradients restored by sodium-potassium pumps.
Resting potential is restored.
Define and outline saltatory conduction.
Saltatory conduction is the “jumping” of electrical impulses from one Node of Ranvier to the next.
It is enable through the mylenation of the axon.
Its purpose is to speed up the nerve transimissions which would be much slower in continuous transimssion.
Define synapse, synaptic cleft and effector.
Synapse: a junction between cells in the nervous system
Synaptic cleft: a fluid-filled gap which separates presynaptic and postsynaptic cells so that electrical impulses cannot pass across
Effector: muscles and glands which carry out a response to a stimulus
What are the 3 major types of neurons?
Sensory neurons
Motor neurons
Relay neurons
Outline the function of a neurotransmitter
Neurotransmitters are released as a response to the depolarization of a presynaptic neuron
The neurotransmitters bind to receptors on postsynaptic neurons
They can trigger either an excitatory response, which increases the likelihood of an action potential being fired, or they can trigger an inhibitory response, which decreases the likelihood of an action potential being fired
Outline the cause and consequence of the refractory period after depolarization.
The refractory period refers to the period of time following a nerve impulse before the neuron is able to fire again. In a normal resting state, sodium ions are concentrated outside the neuron and potassium ions are mainly inside; this is the period of resting potential. Following depolarisation, an influx of sodium and repolarisation an efflux of potassium, the distribution of sodium and potassium is largely reversed. Before a neuron can fire again, the resting potential must be restored via the antiport action of the sodium-potassium pump.
Outline hyperpolarization in the neuron.
Hyperpolarization occurs when the potential difference is less than -70mV
This means that it is a more negative charge than when the neuron is at its resting potential
As the cell repolarizes and the K+ channel allows potassium to move out the cytoplasm to restore resting potential its concentration is not immediately re-established
The concentration gradients take a few milliseconds to be restored thus causing hyperpolarization
Describe that cause of and effect of membrane potential reaching the threshold potential.
Cause: local currents reduce the concentration gradient in the non-polarized section of the neuron
The membrane potential rises from the resting potential of -70mV to about -50mV.
Effect: sodium channels in the axon are voltage-gated and open when a membrane potential of -50mV is reached (the threshold potential)
Polarization is caused by the opening of sodium channels
Explain the function of the myelin sheath
Myelin acts as an insulator, and has nodes of ranvier between them
The myelin sheaths allow the impulse to travel faster because the action potiential must jump from one node to the next
This also allows for less energy to be used as the impulse does not need to travel through the entire axon
Distinguish between the functions of the 3 major types of neurons.
Sensory Neurons: Sensory neurons are responsible for converting external stimuli from the organism’s environment and converting them into electrical impulses.
Relay Neurons: Relay neurons are found between the sensory input neurons and the motor output neurons in the brain and spinal cord. They allow for sensory and motor neurons to communicate.
Motor Neurons: Motor neurons connect to muscles, glands and organs throughout the body. These neurons transmit impulses to the skeletal and smooth muscles in order to control muscle movements.
Outline the secretion, action, reabsorption and formation of acetylcholine.
One example of a neurotransmitter used by both the central nervous system and peripheral nervous system is acetylcholine which is commonly released at neuromuscular junctions and binds to receptors on muscle fibres to trigger muscle contraction. It is also commonly released within the autonomic nervous system to promote parasympathetic responses (‘rest and digest’). Acetylcholine is created in the axon terminal by combining choline with an acetyl group (derived from mitochondrial Acetyl CoA). Acetylcholine activates a post-synaptic cell by binding to one of two classes of specific receptors (nicotinic or muscarinic). Acetylcholine must be continually removed from the synapse, as overstimulation can lead to fatal convulsions and paralysis. Acetylcholine is broken down into its two component parts by the synaptic enzyme acetylcholinesterase (AChE). AChE is either released into the synapse from the presynaptic neuron or embedded on the membrane of the post-synaptic cell. The liberated choline is returned to the presynaptic neuron where it is coupled with another acetate to reform acetylcholine
Discuss why neonicotinoids can be used as pesticides and state one benefits.
Neonicotinoids bind to acetylcholine receptors in insects’s central nervous system
Acetylchonlinesterase, the enzyme which typically breaks down acetylcholine, cannot breakdown neonicotinoids.
This makes the binding irreversible and blocks the receptors from binding with acetylcholine
This prevents synaptic transmission and results in the paralysis and death of the insects.
Benefits include it being an effective insecticide and/or it not being highly toxic to humans and other mammals.
Outline the changes that lead to the depolarization of an axon as an action potential travels along a neuron.
Local currents diffuse from adjacent depolarised section of axon
Resting potential becomes reduced
Voltage-gated ion channels affected and the sodium channels open, which causes sodium to diffuse in
There are less positive charges outside and more inside, so the membrane polarity reversed
The entry of sodium causes more sodium gates to open, and so membrane potential rises from -70mV to +40 mV ( -+ 10 mV)
What is a synapse?
The connections between neurons, neurons and glands, neurons and receptors, and neurons and muscles
The synapse is a gap between the presynaptic neuron and the post synaptic neuron
The space is about 20nm
The gap is there to prevent an unwanted nerve signal between the 2 neurons
What is the function of the soma?
The soma contains the nucleus and organelles
It is where essential metabolic processes occur
Explain the steps in the process of neurotransmission across a synapse.
Action potential reaches the end of a presynaptic neuron.
Voltage gated calcium channels open.
Calcium ions flow into the presynaptic neuron.
Vesicles with neurotransmitters inside the presynaptic neuron fuse with the plasma membrane.
Neurotransmitters diffuse in the synaptic cleft and bind to receptors on the postsynaptic neuron.
The receptors are channels which open and let sodium ions into the postsynaptic neuron.
The sodium ions cause the postsynaptic membrane to depolarize.
This causes an action potential which passes down the postsynaptic neuron.
Neurotransmitters in the synaptic cleft are degraded and the calcium ions are pumped back into the synaptic cleft.
State the name and function of three different parts of the brain.
Answers may include any three of the following:
Medulla Oblongata
Automonic control of gut muscles, breathing, blood vessels and heart muscle.
Cerebellum
Coordinates unconscious functions and movement
Hypothalamus
Interface between brain and pituitary gland (synthesizing hormones)
Pituitary Gland
Stores and releases hormones
Cerebral hemispheres
Integrates high complex functions such as memory and emotions.
What happens when AP reaches terminal end of neuron.
The depolarization of the terminal end of the presynaptic neuron will cause voltage gated calcium channels to open
Calcium ions diffuse into the presynaptic neuron
Synaptic vesicles merge with the membrane
Exocytosis occurs and the neurotransmitters spill into synapse and diffuse across
Neurotransmitters bind to receptors on the postsynaptic cell
Sodium channels open on postsynaptic cell
The presynaptic cell pumps calcium ions back to synapse
The neurotransmitters are absorbed into the postsynaptic nerve
Explain the process of exocytosis
The plasma membrane engulfs the molecules
Fluidity allows for the movement of molecules across the membrane
The membrane sinks out and engulfs the molecule
The phospholipids pinch off and form a vesicle around the molecule
An example of this is neurotransmitters leaving the presynaptic neuron
Draw a Diagram of a neuron
Must Include:
- dendrite
-nucleus
-axon
- cell body
- myelin sheath
- schwann cell
- node of ranvier
- axon terminal