B3 RESPONSE TO INTERNAL STIMULI

What’re receptors?

cells which detect a specific stimulus and covert some sort of energy into a generator potential, which can cause an action potential in the sensory neurone

What is the generator potential?

the change in electrical potential in a receptor when it is stimulated

What is a pacinian corpuscle?

a receptor which is specific to pressure

What does a pacinian corpuscle look like?

How do pacinian corpuscles work?

• pressure distorts the layers on the pacinian corpuscle

• the stretch mediated sodium channels open

• sodium ions diffusee in

• causes a potential difference across the membrane, leading to depolarisation

• this is the generator potential

• the greater the pressure, the larger the generator poteential

• if the generator potential reaches the threshold value an action potential is created

What is the eye?

• a sense organ

• has receptor cells found in the retina

What’re the 2 types of receptor cells found on the retina?

• rod cells

• cone cells

What’re characteristics of rod cells?

• distributed evenly throughout the retina

• do not allow colour vision

• very sensitive to light

• low visual acuity (image is poorly resolved)

What’re characteristics of cone cells?

• found mostly in the fovea

• allow colour vision

• contain pigments sensitive to red, green and blue light

• not as sensitive to light

• high visual acuity giving a sharp image

What’re the light sensitive pigments found in rod cells?

rhodopsin

What’re the light sensitive pigments found in cone cells?

iodopsin

What does light do to the pigment?

causes it to break down, altering its chemical structure leading to the production of a generator potential

How’re rods and cones connected to nerves?

• have synapses which connect them to bipolar neurones

• have synapses with neurones in the optic nerve

Why do rods have a poor visual acuity?

several rods are connected to one bipolar neurone, meaning that the light which falls on several rod cells will only generate one impulse to the brain and the brain cannot distinguish between the separate light sources that generated them

How are rods sensitive to low light intensitities?

the stimulation of several rods results in enough neurotransmitters released to reach the threshold value in the bipolar neurone in low light intensities (retinal convergence)

Why do cones have a high visual acuity?

each cone synapses with its own bipolar neurone, meaning that light falling on 2 cone cells generates 2 impulses to the brain and the brain can therefore distinguish between the light sources that generated them, so the brain can see them as separate

Why are the cone cells not very sensitive to light?

each cone cell has to produce enough neurotransmitter to reach the threshold in the bipolar neurone, and this is only likely in bright light, as in low light intensities there is not enough neurotransmitter to create an action potential in the bipolar neurone

What type of muscle is the heart muscle?

myogenic as it will beat without any nervous stimulation

What acts as the hearts natural pacemaker?

sinoatrial node

What is the sinoatrial node controlled by?

the medulla oblongata in the autonomic nervous system which controls the rate at which it sends out waves of electrical activity

What does the heart muscle look like?

How does ventricular systole occur?

• sinoatrial node generates a wave of electrical activity which spreads across both atria causing atrial systole

• a layer of non-conductive tissue stops the wave of electrical activity from spreading into the ventricles

• the wave of electrical activity spreads to the atrioventricular node

• after a short delay the atrioventricular node conveys a wave of electrical activity down the septum through the bundle of His

• the bundle of His splits in 2 which carry the electrical activity down the apex into the pukinje fibres which extend up the outer walls of the ventricles

• this causes the ventricles to contract simultaneously

What nervous system is heart rate controlled by?

the autonomic

What’re the two antagonistic branches in the autonomic nervous system?

• sympathetic

• parasympathetic

What is the sympathetic nervous system?

• stimulates effectors

• speeds up

• fight or flight

• neurotransmitter is noradrenaline

What is the parasympathetic nervous system?

• inhibits effectors

• controls activity at rest

• neurotransmitter is acetylecholine

What’re the 2 parts of the medulla oblongata’s cardioregulatory centre?

• acceleratory centre (linked to the sinoatrial node by the sympathetic nervous system)

• inhibitory centre (linked to the sinoatrial node by the parasympathetic nervous system)

What’re the 2 types of receptors important in the control of heart rate?

• baroreceptors (respond to changes in blood pressure)

• chemoreceptors (respond to changes in pH)

Where are baroreceptors and chemoreceptors found?

in the aorta and the carotid artery

What happens if blood pressure rises above normal?

• baroreceptors detect this and send impulses to the medulla

• more impulses are sent to the sino atrial node via parasympathetic nerves

• this decreases the frequency of impulses from the sino atrial node across the atria, decreasing the rate at which the artia contract

• this means the atriaventricular node fires less frequently so the ventricles contract less frequently and heart rate decreases

What happens if blood pressure falls below normal?

• baroreceptors detect this and send impulses to the medulla

• more impulses are sent to the sino atrial node via sympathetic nerves

• this increases the frequency of impulses from the sino atrial node across the atria, increasing the rate at which the atria contract

• this means the atrioventricular node fires more frequently so the ventricles contract more frequently and the heart rate increases

How does changes in carbon dioxide levels affect the pH of blood?

the carbon dioxide forms carbonic acid which dissolves in blood plasma to lower the blood pH (increase in pH leads to an increase in respiration)

What happens if carbon dioxide levels rise so the pH falls?

• chemoreceptors detect this and send impulses to the medulla

• more impulses are sent to the sino atrial node via sympathetic nerves

• this increases the frequency of impulses from the sino atrial node across the atria, increasing the rate at which the atria contract

• this means the atrioventricular node fires more frequently so ventricles contract more frequently and the heart rate increases

What happens if carbon dioxide levels decrease and the pH rises?

• chemoreceptors detect this and send impulses to the medulla

• more impulses are sent to the sino atrial node via parasympathetic nerves

• this decreases the frequency of impulses from sino atrial node across the atria, decreasing the rate at which the atria contract

• this means the atriaventricular node fires less frequently so the ventricles contract less frequently and the heart rate decreases