biology- receptors, reflexes and responses

What is a stimulus

a change in the environment which can be detected

What is a receptor

Energy transducer

• Converts the energy in the stimulus into the energy in an action potential

• Some receptors exist as individual cells (eg in skin) while others are concentrated into sense organs (eg in the eye)

External and internal stimuli

Definition of a tropism

Different tropisms

Plant growth response

Phototropism- light

Geotropicsm/gravitropism- gravity

Chemotropism- chemicals

Hydrotropism- water

Thigmotropism- touch

How is movement brought about in plants

Cells can divide

Cells can elongate

Movement towards a stimulus is a positive tropism, away is negative. Tropisms are controlled by hormones.

How does auxin work eg (IAA)

The tip makes auxin

By AT the auxin moves to the dark side

Auxin diffuses down the dark side

Auxin stimulates cell elongation

So the tip bends to the light

How does auxin work in shoots/roots

1. Auxin is made at the top

2. It diffuses down the seedling

3. It settles on the lower side (possibly due to starch grains: statoliths)

4. In shoots auxin stimulates cell elongation → grows up

5. In root auxin inhibits cell elongation → grows down

What is a kinesis

• A non directional response to a stimulus: it explores the environment relatively quickly, making wide turns

• When it find favourable conditions it slows down, makes tighter turns and stops

What is a taxis

A directional response, towards or away from a stimulus

Features of a reflex

Unconscious

Rapid

Fixed- one stimulus = one response

As few synapses as possible

3 neurones involved in a reflex arc

• sensory neurone- links the receptor with the spinal cord

• Short relay neurone that connects the incoming and outgoing neurones

• The motor neurone that transmits impulses to the effector

Reflex arc for online colour change reaction timer

Eyes detected colour change → 2 synapses in retina → optic nerve to visual cortex in brain → motor cortex in brain → motor neuron in hand → muscle contracts

Pascinian corpuscle structure

What is a Pacinian corpuscle

• a receptor found deep in skin- detects pressure and vibration

• Each corpuscle consists of a single sensory neurone- often described as a ‘nerve ending’ surrounded by lamellae (layers) of fibrous connective tissue separated by a viscous gel

• A rapid change in pressure on the corpuscle is transmitted through to the sensory nerve, which deforms, causes stretch-mediated sodium ion channels in the axon membrane to open. This allows sodium ions to diffuse in, creating a generator potential. If this potential reaches a threshold, action potential pass down the sensory nerve. This passes to the brain and we feel the sensation. The greater the stimulus, the higher the frequency of the impulses.

  • The gel acts as 'shock absorber' and quickly allows the sensory nerve to assume its normal shape, this is called adaptation. From this it can be seen that Pacinian corpuscles will detect rapid changes of pressure, but not prolonged pressure. No more impulses will be generated until the pressure changes again. This is an important point about receptors - they will generally respond to changes in environment, not constant stimuli.

How does pascinian corpuscle work (exam answer)

What is retina

Single layer of light sensitive receptor cells- rods and cones- together with connecting neurones on the back of the eye

Function of retina

Gather information about the incoming light and relay it to the brain via the optic nerve

The actual image is formed in the brain- in the visual cortex

Define

Sensitivity

Acuity

Sensitivity- level of light needed for the cells to function

Acuity- ability to perceive detail

Rods vs cones- sensitivity and reasons why

Rods more sensitive than cones- can function even in dim light

1. The pigment n rod cells is more easily bleached (broken down) than than pigment in cones

2. Retinal convergence- many rod cells converge into one neurone so they can all contribute to the generator potential, making it more likely that the threshold will be reached. Each rod has its own connection. Any impulses generated by an individual cone are transmitted to the brain but any impulses generated by a rod are fed into a neurone along with impulses from many others

Diagram showing distribution of rods and cones in retina

Rod cells and cone cells

How does colour vision work

How are after images produced

Eg

look at blue for 30s then see yellow

look at red for 30s then see cyan

look at green for 30s then see magenta

Eg look at red

Red cones are stimulated. Pigment in them is bleached (red cones get ‘fatigued’ and can’t be stimulated again until the pigment is re-synthesised)

So when you look immediately at white red cones dont work but blue and green do so we see cyan

Summary rods v cones