6.4: Nerve Impulses

What are the differences between the nervous and hormonal systems?

• Hormonal system has slow transmission, nervous system has fast transmission

• Hormones can travel to all parts of the nody, nerve impulses only travel to specific parts of the body

• Hormones have a widespread response, nervous system has a localized response

• Hormones have a long-lasting response, nervous system has a short-lived response

• Hormones have permanent and irreversible effects, nervous systems has temporary and reversible effects 

What is a myelinated motor neurone?

A nerve cell that carries impulses from the CNS to muscles/glands with its axon covered by a myelin sheath for faster nerve transmission

What is the structure of a myelinated motor neurone?

What is a resting potential? 

 The electrical charge difference across a cell membrane that is not actively transmitting nerve impulses

What is the membrane potential at resting potential?

70mV

How is a resting potential maintained?

•  Na+/K+ pump actively transports Na+ out of axon and K+ into axon (co-transport) using energy from ATP

• An electrochemical gradient is established with  both a higher K+ inside the axon and Na+ conc outside the axon

• Differential membrane permeability means membrane is more permeable to K+ (as it is permanently open)  so it moves out by facilitated diffusion, but Na+ channels are closed

What is an action potential?

 A brief change in the electrical charge across a neurone’s membrane, making the inside positive compared to the outside, which passes along the axon as a nerve impulse

How do changes in membrane permeability lead to depolarisation and generation of an action potential?

• Stimulus: Na+ channels open so the membrane permeability to Na+ increases

• So Na+ diffuses into the axon down an electrochemical gradient

• This causes depolarisation

• Depolarisation: If threshold potential is reached, an action potential is generated

• More voltage-gated Na+ channels opens, so more Na+ diffuses in rapidly

• Repolarisation: voltage-gated Na+ channels close

• Voltage-gated K+ channels open

• K+ diffuses out of axon

• Hyperpolarization: K+ channels are slow to close so too many K+ diffuse out

• Resting potential: Restored by Na+/K+ pump

Explain this graph of an action potential

What is the refractory period?

The time taken to restore the axon to resting potential when no further action potential can be generated as Na+ channels are closed 

What is the importance of the refractory period?

• Ensures discrete impulses are produced as action potentials cannot overlap

• Limits frequency of impulse transmission to prevent overreaction

• Ensures action potentials travel in one direction as they cannot be propagated in the refractory region

How is maximum frequency of impulse conduction calculated and what is the unit?

Maximum frequency= time/duration of refractory period 

Units:

• mpulses sec-1

• action potentials sec-1

• Hz (1 Hz is equal to one impulse per second)

What is the all or nothing principle?

• If depolarisation exceeds threshold potential, an action potential is always produced. 

• This means action potentials are always the same size 

• Bigger stimuli instead increase the frequency of action potentials, instead of their magnitude.

How does the passage of an action potential along an unmyelinated axon result in a nerve impulse?

• Action potential passess as a wave of depolarization

• Influx of Na+ in one region increases the permeability of adjoining region to Na+

• Causing voltage-gated Na+ channels to open to adjoining region depolarizes

How does the passage of an action potential along a myelinated axon result in a nerve impulse?

• Myelination provides electrical insulation

• Depolarization of axon only happens at nodes of Ranvier

• Resulting in Saltatory conduction

• So no need for depolarization along whole length of axon

How does damage to the myelin sheath lead to slow responses/jerky movement?

• No saltatory conduction so depolarisation occurs along the whole length of the axon 

• Nerve impulses take longer to reach neuromuscular junction

• Causing a delay in muscle contraction

• Ions may pass to other neurones

• Causing wrong muscle fibres to contract

What is Saltatory conduction?

The jumping of action potentials from one Node of Ranvier to the next

What are the factors that effect speed of conductance and how do they effect it?

• Myelination and saltatory conduction: depolarisation only happens at nodes of ranvier so impulse does not depolarize the whole length of axon

• Axon diameter: bigger diameter means less resistance to the flow of ions in the cytoplasm

• Temperature: INcreased temp increases rate of diffusion of Na+ and K+ as more kinetic energy, but at a certain temperature active sites of enzymes could denature