#119 Action potentials

Action potentials are rapid changes in potential difference across the membrane.

Myelin: specialized cells called Schwann cells wrapped along the axon.

- Schwann cells are made of lipids and proteins

- many Schwann cells form the myelin sheath à affects the speed of conduction of electrical impulses

Transmission of nerve impulses:

- impulse/signals are brief changes in the distribution of electrical charge across the cell surface membrane à results in action potentials

- caused by the rapid movement of Na⁺ and K⁺ ions into and out of the axon

Resting potential

- inside the axon: slightly negative

- potential difference: -60mV to -70mV (potential inside the axon is less than that outside the axon)

Neurones, like all cells, have sodium-potassium pumps in their cell surface

membranes. However, in neurones these are especially active. By active transport, they pump out 3 Na⁺ ions for every 2 K⁺ ions brought in.

- the resting potential is produced and maintained by Na⁺ and K⁺ ion pumps:

• membrane proteins
• uses energy from the hydrolysis of ATP for active transport of ions

- there are more channels for K⁺ ions; large, negative molecules inside cell attracts K⁺ ions à less K⁺ ions diffuse out à there is an overall excess of negative ions inside the membrane

Action potentials - the rapid change in potential difference across the membrane

- caused by the change in permeability of the membrane to Na⁺ and K⁺ ions

- voltage gated channels for Na⁺ and K⁺ ions : opens or closes depending on the potential difference across the membrane

When a receptor receives a stimulus, this can reduce the potential difference across the membrane, which causes sodium ion channels to open. This allows sodium ions to flood into the cell, down an electrochemical gradient. (The 'electro' gradient refers to the difference in charge across the membrane. The 'chemical' gradient is the difference in concentration of sodium ions.)

• depolarization: Na⁺ channels open à Na⁺ enter à potential difference is less negative on the inside (now at approx. +30mV)
• potential difference reaches the threshold potential à generates an action potential
• repolarization: Na⁺ channels close, K⁺ channels open à outward movement of K⁺ down their electrochemical gradient removes the positive charge inside the axon

This sequence of events is called an action potential.

• refractory period: period of time where the axon is unresponsive, recovering from an action potential (restoring its resting potential); another action potential cannot be generated until this period is over

  

Transmission of action potentials

An action potential generated in one part of a neurone travels rapidly along its axon or dendron. This happens because the temporary depolarisation of one part of the membrane sets up local circuits with the areas on either side of it. These cause depolarisation of these regions as well. The nerve impulse therefore sweeps along the axon.

*Action potentials only take place at the nodes of Ranvier, where there is no myelin present.

How an action potential carries information

- action potentials have same:

• size (same amplitude)
• speed at which the action potential travels by

- action potentials have different:

• frequency
• number of neurones carrying an action potential

 ---> acts as a representation of the strength of the stimulus

- nature of the stimulus: deduced from the position of the sensory neurone

Initiation of an action potential

Receptors are cells or tissues that sense changes in the internal or external environment. Many types of receptors transform energy (transducers) from a stimulus into the energy of an action potential in a sensory neurone.

- Receptors are stimulated: receptor potential rises above threshold potential à action potential initiated à stimulates sensory neurones to send impulses to CNS

- all-or-nothing law: neurones either do or do not transmit electrical impulses

- threshold levels rarely stay constant

Speed of conduction

- Myelin insulates the membrane of the axon à speeds up rate by which the action potential travels

- “local circuits” exist from one node to the next, thus creating “saltatory conduction” where an action potential jumps from one node to the next

- with myelin: speed of conduction is 50 times faster

- diameter of axon increase = less resistance = faster transmission

Syllabus 2016-2018 15.1  Control and co-ordination in mammals The nervous system provides fast communication between receptors and effectors. Transmission between neurones takes place at synapses. a)   compare the nervous and endocrine systems as communication systems that  co-ordinate responses to changes in the internal and external environment  b)   describe the structure of a sensory neurone and a motor neurone c)   outline  the roles of sensory receptor cells in detecting stimuli and stimulating the transmission of nerve  impulses in sensory neurones (a suitable example is the chemoreceptor cell found in human taste buds) d)   describe the functions of sensory, relay and motor  neurones in a reflex arc e)   describe and explain the transmission of an action potential in a myelinated neurone and its initiation from a resting potential (the importance of sodium and potassium ions in impulse transmission should  be emphasised) f) explain the importance of the myelin sheath (saltatory conduction) in determining the speed of nerve  impulses and the refractory period in determining their frequency g)   describe the structure of a cholinergic  synapse and explain how it functions, including the role of calcium  ions h)   outline  the roles of synapses in the nervous system in allowing transmission in one direction  and in allowing connections between one neurone and many  others (summation, facilitation and inhibitory synapses are not required) i) describe the roles of neuromuscular junctions, transverse system tubules and sarcoplasmic reticulum in stimulating contraction in striated muscle j) describe the ultrastructure of striated muscle with particular reference to sarcomere structure k)   explain the sliding filament  model  of muscular contraction including the roles of troponin,  tropomyosin, calcium  ions and ATP. The endocrine system is a slower system that  controls long-term changes. Fertility may be controlled by use  of hormones. l) explain the roles of the hormones FSH, LH, oestrogen and progesterone in controlling changes in the ovary and uterus during the human menstrual cycle m)  outline  the biological basis  of contraceptive pills containing oestrogen and/or progesterone