Control of heart rate and role of receptors

Autonomic nervous system - controls involuntary activities of internal muscles and glands.

Sympathetic nervous system - stimulates effectors, so speeds up any activity. Helps us cope with stressful situations by heightening our awareness and preparing us for fight or flight.

Parasympathetic nervous system - inhibits effectors so slows down any activity - concerned with conserving energy and replenishing the body's reserves. (Acts in normal resting conditions)

Changes to the heart rate are controlled by the medulla oblongata:

> A centre that increases heart rate which is linked to the sinoatrial node via the sympathetic nervous system

> A centre that decreases heart rate which is linked to th sinoatrial node via the parasympathetic nervous system.

Receptors respond to:  > Chemical changes in the blood   > Pressure changes in the blood

> Chemoreceptors = found walls of the carotid arteries = sensitive to changes in blood pH.

> In solution carbon dioxide forms an acid > lowers pH

1. Increased metabolic / muscular activity

2. More carbon dioxide produced by increased respiration of the tissues

3. Blood pH is lowered

4. Chemical receptors in carotid arteries increase frequency of impulses to the medulla oblongata.

5. Centre in medulla oblongata thats speeds up heart rate - increases frequency of impulses sent to SA node via the sympathetic nervous system.

6. SA node increases heart rate

7. Increased blood flow removes carbon dioxide faster > returns to normal

When blood pressure is higher than normal...

Transmit nervous impulse to the centre in the medulla oblongata that decreases heart rate. Impulses then sent via the parasympathetic nervous system to the SA node > decreases heart rate

When blood pressure is lower than normal...

Transmit nervous impulse to centre in medulla oblongata that increases heart rate. This centre sends impulses via the sympathetic nervous system to the SA node > increases heart rate.

Sensory reception - Pacinian corpuscle

Specific to a single type of stimulus > Respond to changes in mechanical pressure

> Produces a generator potential by acting as a transducer - converts the energy of a stimulus into a nervous impulse = generator potential

Structure and function of Pacinian corpuscle...

> Single sensory neurone in middle - a stretch-mediated sodium channel in its plasma membrane

> Permeability to sodium ions changes as it changes shape

Resting state = stretch-mediated sodium channels to narrow to let any sodium ions pass along them. Neurone of the Pacinian corpuscle has a resting potential.

Pressure applied = Pacinian corpuscle changes shape and membrane around neurone stretches. This stretching widens the sodium channels > sodium ions diffuse into neurone.

Influx of sodium ions > changes potential of membrane (depolarised) producing a generator potential. > Creates an action potential > passes along neurone and others to the CNS.

Rceptors in the eyes...

Light receptor cells found on the retina = rod cells and cone cells = transducers

Rod cells - light intensity

Cone cells - colour of light

> Cannot distinguish between wavelengths of light

> More numerous than cone cells

> Many rod cells share single sensory neurone (retinal convergence) = can respond to low intensity light

> Threshold value has to be exceeded before a generator potential is created in the bipolar cells

> More chance threshold value will be exceeded due to several rod cells attached to each bipolar cell

> Rhodopsin breaks down > generator potential (low intensity light is sufficient)

> Only a singe impulse generated regardless of how many neurones are stimulaed.

> Therefore cannot distinguish between separate sources of light = low visual accuity

> Found at the peripheries of the retina - light intensity lowest

> Only respond to high light intensity

> Contain iodopsin - requires high light intensity to break down - cannot detect colour in low light

> Each cone cell has a connection to a single bipolar cell. 

> The brain can distinguish between separate sources of light

> Good visual acuity

> Found at the fovea (part of retina opposite the pupil)