13.1-13.5 Neural Communication Flashcards
What are the main three types of neurones?
1) Sensory neurones
2) Motor neurones
3) Relay neurones
What do sensory neurones do?
Sensory neurones transmit nerve impulses from the receptors to the CENTRAL NERVOUS SYSTEM- the brain and spinal cord.
What do motor neurones do?
Motor neurones transmit nerve impulses from the CNS to effectors.
What do relay neurones do?
Relay neurones transmit nerve impulses between sensory neurones and motor neurones.
What key features do neurones have?
1) Cell body
2) Dendrons
3) Axons
What are axons?
Axons are singular, elongated nerve fibres that transmit impulses away from the cell body.
What are dendrons?
- Dendrons are short extensions which come from the cell body.
- These dendrons divide into smaller and smaller branches known as dendrites.
What is the cell body in a neurone?
The cell body contains the nucleus surrounded by the cytoplasm.
What is a myelin sheath and what does it do?
- Myelin sheath is a membrane rich in lipid which surrounds the axon of some neurones.
- It acts as an insulating layer and allows these myelinated neurones to conduct the electrical impulses at a much faster speed than unmyelinated neurones.
What cells make up the myelin sheath?
Schwann cells
What are the gaps between the myelin sheath called?
Nodes of Ranvier
What are the two main features of sensory receptors?
1) They are specific to a single type of stimulus.
2) They act as a transducer- they convert a stimulus into a nerve impulse.
What is the stimulus for mechanoreceptors?
Pressure and movement
What type of receptors are pacinian corpsucles?
Mechanoreceptors
What do sensory receptors convert the stimulus into?
A nerve impulse, known as generator potential.
What are pacinian corpuscles?
Pacinian corpsucles are specific sensory receptors that detect mechanical pressure.
Where are pacinian corpuscles located?
- Deep within skin
- Fingers
- Soles of feet
- Within joints
Describe the structure of a pacinian corpsule.
- End of sensory neurone found within centre of corpscule.
- Surrounded by layers of connective tissue
- Each layer of tissue is separated by layer of gel.
How does the Pacinian Corpsucle convert mechanical pressure into a nerve impulse?
1) In a normal state (resting state), the stretch-mediated sodium ion channels in the sensory neurone’s membrane are too narrow to allow sodium ions to pass through them.
2) When pressure is applied to the Pacinian corpsucle, the corpsucle changes shape. This causes the membrane surrounding the neurone to stretch.
3) When the membrane stretches, the sodium ion channels present widen. Sodium ions can now diffuse into the neurone.
4) The influx of positive sodium ions changes the potential of the membrane- it becomes depolarised. This results in a generator potential.
5) In turn, the generator potential creates an action potential that passes along sensory neurone to CNS
What is resting potential?
Resting potential is the potential difference across the membrane when a neurone is not transmitting an impulse.
What is the potential difference across the membrane at resting potential?
-70mV
How is resting potential created?
1) Sodium ions are actively transported out of the axon whereas potassium ions are actively transported into the axon by a sodium-potassium pump. However their movement is not equal because for every 3 sodium ions pumped out, two potassium ions are pumped in.
2) As a result there are more sodium ions outside the membrane than inside the axon cytoplasm.
3) Therefore sodium ions diffuse back into the axon down its electrochemical gradient, and potassium ions diffuse out of the axon.
4) Most of the gated sodium ion channels are closed, preventing movement of sodium ions whereas many potassium ion channels are open, allowing potassium ions to diffuse out of the axon.
5) As there are more positively charged ions outside the axon than inside the axon, a resting potential of -70mV is created across the membrane.
When does an action potential occur?
An action potential occurs when protein channels in the axon membrane change shape as a result of the change in voltage across its membrane.
Describe the events that occur in an action potential.
1) The neurone has a resting potential. Some potassium ion channels are open but sodium voltage-gated ion channels are closed.
2) STIMULUS: The energy of the stimulus triggers some sodium voltage-gated ion channels to open, making the membrane more permeable to sodium ions. Sodium ions therefore diffuse into the axon down their electrochemical gradient. This makes the inside of the axon less negative.
3) DEPOLARISATION: If the potential difference reaches the threshold value of around -55mV, voltage-gated sodium ion channels open and more sodium ions diffuse into the axon. This is POSITIVE feedback.
4) REPOLARISATION: When the potential difference reaches approx +40mV, the voltage-gated sodium ion channels close and voltage-gated potassium ion channels open. The membrane is now permeable to potassium so potassium ions diffuse out of the neurone down the electrochemical gradient. This starts to get the membrane back to its resting potential. This is NEGATIVE feedback.
5) HYPERPOLARISATION: Potassium ion channels are slow to close so there’s a slight ‘overshoot’ where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential (less than -70mV)
6) RESTING POTENTIAL: The ion channels are reset. The sodium-potassium pump returns the membrane to its resting potential by pumping sodium ions out and potassium ions in, and maintains the resting potential until the membrane’s excited by another stimulus.
