Topic 8 Flashcards
8.1 Know the structure and function of sensory, relay and motor neurones including the role of Schwann cells and myelination.
Spicy, Rice, Spicy, Rice, More, Egg, Rice
Stimulus, receptor, sensory, relay, motor, effector, response
Sensory Neurone
- one long dendron (impulses from receptor cells to cell body)
- cell body is in the middle
- one short axon (impulses from cell body to CNS)
- always myelinated
Relay neurone
Motor neurone
Describe the differences in the structure of a myelinated sensory neurone and a myelinated motor neurone.
- cell body in the middle of the sensory neurone whereas the cell body is at the end of the motor neurone
- sensory neurone has one long dendrone whereas motor neurone has many small dendrites
- sensory neurone has a shorter axon than a motor neurone
The role of Schwann cells
Myelination
- fatty, white substance that coats the axon of some neurones, forms an electrically insulating layer
- main function is it increase the speed impulses that travel along the axon
- Myelin sheath speeds up the transmission of action potentials using Saltatory conduction, as the action potential “jumps” to the nodes of Ranvier.
8.2 Understand how the nervous systems of organisms can cause effectors to respond to a stimulus
Understand how a pupil dilates and contracts
Dim light —> photo receptors in eye —> processes info —> radial muscles in iris are stimulated —>Radial muscles contract = pupil dilation
Bright lights —> photo receptors in the eye —> processes info —> Circular muscles in the iris are stimulated —> Circular muscles contract = pupil contraction
Nervous system
Endocrine system
The peripheral nervous system (PNS)
The autonomic nervous system
Dendrites/Dendrons
- extensions of neurones that receive signals from other cells and conduct action potentials towards the cell body (in the sensory neurone, the dendron is myelinated)
Axons
- extensions of neurones that conduct action potentials away from the cell body to other cells
Cell body of a neuron
- also called the soma
- is the spherical part of the neurone that contains the nucleus
Difference between a nerve cell and a neuron
A neuron is the smallest unit of nervous tissue that is also known as a nerve cell, contains dendrites, cell body, axon and axon terminal.
A nerve is a bundle of axons of several neurons that helps in transmitting signals among various body parts.
The sodium potassium pump
- some intrinsic proteins actively transport K+ into the axon and Na+ out - the NaK pump.
- uses ATP to simultaneously pump 3Na+ out of the cell and 2K+ in.
- results in an overlay negative charge inside the axon.
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How do nerve impulses start?
- Neurones are stimulated by receptor cells
- These contain special Na+ channels that are not voltage - gated, but are gated by the appropriate stimulus
- e.g. chemical - gated Na+ channels in tongue taste receptor cells open when a certain chemical in food binds to them
Resting potential
- achieved by active transport of the NaK pump ( 3 sodium ions out for 2 potassium ions in )
- membrane is more permeable to potassium, so sodium remains outside the neurone and potassium also leaks back out
- leads to a negative potential difference of -70mV
Action potential
- when an action potential arrives/ the neurone is stimulated
- sodium channels open and sodium rushes into the neurone by facilitated diffusion
- causing depolarisation
- At threshold of -50mV more voltage gated Na channels open
- At +30mV Na channels close and K channels open.
- K leaves the neurone by facilitated diffusion and Na is pumped out by active transport.
- Repolarisation
- Hyperpolarisation occurs as too much potassium leaves the cell
- potassium channels close
- resting potential is restored
Saltatory conduction
- Depolarisation at node of Ranvier
- Electric current between nodes
- Potential difference reduced at next node, initiating another action potential
Factors affecting the speed of an action potential
(Myelin sheath)
- Myelin is an electrical insulator
- no insulation at the Nodes of Ranvier
- The depolarisation/action potential jumps from one Node of Ranvier to another
- SALTATORY CONDUCTION
- this means that the entire axon doesn’t need to depolarise, which speeds up the nerve impulse
E.g myelinated - 90 ms-1
Unmyelinated - 30ms-1
Factors affecting the speed of an action potential
(Diameter of an axon)
- Diameter of an axon - greater diameter = faster conduction
- due to an increased surface area (not sa:vol) for ion movement
Factors affecting the speed of an action potential
(Temperature)
- higher temperature = faster nerve impulse (rate of diffusion is faster, enzyme activity is faster e.g. ATPase
Refractory period
- time after depolarisation, where no new AP can start
- time is needed to restore the ions, gated proteins and resting potential
- Na+ channels cannot be opened, as it can not be depolarised again.
The all - or - nothing principle
Action potential only happens if the stimulus reaches a threshold value
- stimulus is strong enough to cause an action potential
- once it starts it travels to the synapse
- (-55mV)
- AP is always the same size
- An AP is the same size all the way along the axon
- The transmission of the AP along the axon is the nerve impulse
How do we detect the size of a stimulus?
- The number of impulses in a given time - the larger the stimulus, the more impulses generated.
- By having neurones with different threshold values-the brain interprets the number and type of neurones and thereby determines the size.
- The number of neurones stimulated can also demonstrate the size
Stimulus
Any change in the internal or external environment is called a stimulus.
