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Does the depolarization needed to generate a second action potential increase as the interval betwee

Touch Light touch is detected by receptors in the skin.

  1. Each type of channel protein has a specific function in the electrical activity of neurons.
  2. Proposed transmembrane structures of four types of gated ion-channel proteins. Threshold stimulus strength required to elicit an action potential during the relative reftractory period.
  3. During the rising phase of an action potential the current flow is dominated by a.

These are often found close to a hair follicle so even if the skin is not touched directly, movement of the hair is detected. In the mouse, light movement of hair triggers a generator potential in mechanically-gated sodium channels in a neuron located next to the hair follicle. This potential opens voltage-gated sodium channels and if it reaches threshold, triggers an action potential in the neuron.

Touch receptors are not distributed evenly over the body. The fingertips and tongue may have as many as 100 per cm2; the back of the hand fewer than 10 per cm2. This can be demonstrated with the two-point threshold test. With a pair of dividers like those used in mechanical drawing, determine in a blindfolded subject the minimum separation of the points that produces two separate touch sensations.

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The ability to discriminate the two points is far better on the fingertips than on, say, the small of the back. The density of touch receptors is also reflected in the amount of somatosensory cortex in the brain assigned to that region of the body. Proprioception Proprioception is our "body sense". It enables us to unconsciously monitor the position of our body. It depends on receptors in the muscles, tendons, and joints. If you have ever tried to walk after one of your legs has "gone to sleep", you will have some appreciation of how difficult coordinated muscular activity would be without proprioception.

The Pacinian Corpuscle Pacinian corpuscles are pressure receptors. They are located in the skin and also in various internal organs. Each is connected to a sensory neuron. Pacinian corpuscles are fast-conducting, bulb-shaped receptors located deep in the dermis.

They consist of the ending of a single neurone surrounded by lamellae. They are the largest of the skin's receptors and are believed to provide instant information about how and where we move. They are also sensitive to vibration. Pacinian corpuscles are also located in joints and tendons and in tissue that lines organs and blood vessels.

Pressure on the skin changed the shape of the Pacinian corpuscle. This changes the shape of the pressure sensitive sodium channels in the membrane, making them open. Sodium ions diffuse in through the channels leading to depolarisation called a generator potential. The greater the pressure the more sodium channels open and the larger the generator potential. If a threshold value is reached, an action potential occurs and nerve impulses travel along the sensory neurone.

The frequency of the impulse is related to the intensity of the stimulus. Adaptation When pressure is first applied to the corpuscle, it initiates a volley of impulses in its sensory neuron.

However, with continuous pressure, the frequency of action potentials decreases quickly and soon stops. This is the phenomenon of adaptation. Adaptation occurs in most sense receptors.

It is useful because it prevents the nervous system from being bombarded with information about insignificant matters like the touch and pressure of our clothing. Stimuli represent changes in the environment. If there is no change, the sense receptors soon adapt.

But note that if we quickly remove the pressure from an adapted Pacinian corpuscle, a fresh volley of impulses will be generated. The speed of adaptation varies among different kinds of receptors. Receptors involved in proprioception - such as spindle fibres - adapt slowly if at all.

This is a graded response: If the generator potential reaches threshold, a volley of action potentials also called nerve impulses are triggered at the first node of Ranvier of the sensory neuron. In living cells nerve impulses are started by receptor cells. These all contain special sodium channels that are not voltage-gated, but instead are gated by the appropriate stimulus directly or indirectly.

This is the all or nothing law. How are Nerve Impulses Propagated?

Once an action potential has started it is moved propagated along an axon automatically. The local reversal of the membrane potential is detected by the surrounding voltage-gated ion channels, which open when the potential changes enough.

The ion channels have two other features that help the nerve impulse work effectively: For an action potential to begin, then the depo larisation of the neurone must reach the threshold value, i. This means that, although the action potential affects all other ion channels nearby, the upstream ion channels cannot open again since they are in their refractory period, so only the downstream channels open, causing the action potential to move one-way along the axon.

The refractory period is necessary as it allows the proteins of voltage sensitive ion channels to restore to their original polarity. Check Point Nerve Impulses travel in one direction This is due to the refractory period i.

Action potentials are more difficult to generate during this period relative to resting potential -70mV [back to top] Action potentials can travel along axons at speeds of 0. This means that nerve impulses can get from one part of a body to another in a few milliseconds, which allows for fast responses to stimuli. The speed is affected by 3 factors: Temperature - The higher the temperature, the faster the speed.

So homoeothermic warm-blooded animals have faster responses than poikilothermic cold-blooded ones. Axon diameter - The larger the diameter, the faster the speed. This explains why squid have their giant axons. Myelin sheath - Only vertebrates have a myelin sheath surrounding their neurones. The voltage-gated ion channels are found only at the nodes of Ranvier, and between the nodes the myelin sheath acts as a good electrical insulator.

The action potential can therefore jump large distances from node to node 1mma process that is called saltatory propagation.