Biology A2:Unit 5-Responding to the Environment

Animals increase their chances of survival by responding to changes in their external environment.

They also respond to changes in their internal environment to ensure that the conditions are always optimal for their metabolism.

Plants also increase their chances of survival by responding to changes in their environment.

Any change in the internal or external environment is called a stimulus.

Receptors detect stimuli - They can be cells or proteins on cell-surface membranes.

Effectors are cells that bring about a response to a stimulus, to produce an effect.

Effectors include muscle cells and cells found in glands.

Receptors communicate with effectors via the nervous system and/or the hormonal system.

The nervous system is made up of a complex network of cells called neurones.

There are three main types of neurone:

Sensory Neurones: Transmit electrical impulses from receptors to the CNS.

Motor Neurones: Transmit electrical impulses from the CNS to effectors.

Relay Neurones: Transmit electrical impulses between sensory and motor neurones.

1. A stimulus is detected by receptor cells and an electrical impulse is sent along a sensory neurone.

2. When an electrical impulse reaches the end of a neurone, chemicals called neurotransmitters carry the information to the post-synaptic neurone.

3. The CNS processes the information, selects an appropriate response and sends impulses along the motor neurones to an effector.

The nervous system is split into two different systems:

1. Central Nervous System (CNS): Made up of the brain and the spinal cord.

2. Peripheral Nervous System (PNS): Made up of the neurones that connect the CNS to the rest of the body.

a. Somatic Nervous System (SNS): Controls conscious activities.

b. Autonomic Nervous System (ANS): Controls unconscious activities.

i. Sympathetic Nervous System (SyNS): Gets the body ready for action.

ii. Parasympathetic Nervous System (PaNS): Calms the body down.

Neurotransmitters are secreted directly onto the target cells - localised response.

Neurotransmitters are rapidly removed - response is short-lived.

Electrical impulses are fast - response is rapid.

The hormonal system is made up of glands and hormones:

a. Gland: A group of cells that are specialised to secrete a useful substance, such as hormones.                                                                                           b. Hormone: Are 'chemical messengers'. Many hormones are proteins or peptides.

Hormones are secreted when a gland is stimulated:

a. Glands can be stimulated by a change in concentration of a specific substance (e.g. Blood glucose).                                                                       b. They can also be stimulated by electrical impulses.

Hormones diffuse directly into and are transported by the blood plasma.

Each hormone will only bind to specific receptors on cell surfaces (target cells).

The hormones trigger a response in the target cells (the effectors).

Hormones are not released directly onto the target cells - slow response.

Hormones are not broken down quickly - response is long-lasting.

Hormones are transported all over the body - effect may be widespread.

Receptors are specific - they only detect one particular stimulus.

Some receptors are cells, others are proteins on cell-surface membranes.

Receptor cells communicate information via the nervous system by:

a. When a nervous system receptor is in a resting state, there is a difference in voltage across the cell-surface membrane - potential difference (PD).

b. The potential difference when a cell is at rest is called its resting potential.

c. When a stimulus is detected, the cell membrane is excited and becomes more permeable, allowing ions to move in and out - altering the PD.

d. The change in PD caused by a stimulus is called a generator potential (GP).

e. If a threshold level is reached by the GP it will trigger an action potential.

f. Action potentials are all of one size - the strength of a stimulus is measured by the frequency of action potentials.

Pacinian corpuscles are mechanoreceptors - they detect mechanical stimuli.

Pacinian corpuscles contain the end of a sensory neurone - sensory nerve ending.

The SNE is wrapped in several layers of connective tissue called lamellae.

When a Pacinian corpuscle is stimulated, the lamellae are deformed and press on the SNE.

This causes deformation of stretch-mediated sodium channels in the sensory neurone's cell membrane.

The sodium channels open and sodium ions diffuse into the cell, creating a generator potential.

If the generator potential reaches the threshold level, it triggers an action potential.

Photoreceptors are light receptors in your eye.

Light enters the eye through the pupil, the amount of light that enters is controlled by the muscles of the iris.

Light rays are focused by the lens onto the retina, which contains photoreceptors.

The fovea is the area of the eye where there are lots of photoreceptors.

Nerve impulses from the photorecptor cells are carried from the retina to the brain by the optic nerve, which is a bundle of neurones.

Where the optic nerve leaves the eye is called the blind spot, there are no photoreceptors so it is not sensitive to light.

Photoreceptors convert light into an electrical impulse:

1. Light enters the eye, hits the photoreceptors and is absorbed by light-sensitive pigments.

2. Light bleaches the pigments, causing a chemical change and increasing the membrane's permeability to sodium.

3. A generator potential is created and if it reaches the threshold, a nerve impulse is sent along a bipolar neurone.

4. Bipolar neurones connect photoreceptors to the optic nerve which takes impulses to the brain.

5. The human eye has two types of photoreceptor - rods and cones.

6. Rods are found in the peripheral parts of the retina, cones at the fovea.

7. Rods give monochromatic vision, cones give trichromatic vision

8. Red-, green- and blue-sensitive  cones are stimulated in different proportions to give different colours.

Rods are more sensitive, but cones allow you to see more detail.

1. Rods:

a. Sensitivity: Very sensitive to light, they fire action potentials in dim light.

This is because many rods join one neurone, so many weaker generator potentials combine to reach the threshold level.

b. Acuity: Low visual acuity because many rods join the same neurone.

Light from two objects close together cannot be distinguished.

2. Cones:

a. Sensitivity: Less sensitive - can only fire APs in bright light.

One cone to one neurone - more light needed to reach the threshold.

b. Acuity: High visual acuity - one cone to one neurone so two light stimuli can be distinguished.

In a neurone's resting state, the outside of the membrane is positively charged, as there are more positive ions outside than inside.

This means the membrane is polarised - there is a difference in voltage across it.

The difference in voltage across a membrane when it is at rest is called the resting potential - approximately -70mV.

The resting potential is created and maintained by sodium-potassium pumps and potassium ion channels.

Sodium-Potassium Pump: i. Three sodium ions moved out.

ii. Two potassium ions moved in.

iii. Moved by active transport. (ATP need to do this)

Potassium Ion Channels: i. Allow facilitated diffusion of potassium ions out.

ii. K+ ions move down their concentration gradient.

Maintenance of resting potential:

a. Na+/K+ pumps move Na+ ions out of the neurone, but the membrane is  not Na+ permeable so they cannot diffuse back in.

This creates a Na+ ion electrochemical gradient.

b. Na+/K+ pumps move K+ ions into the neurone, but the membrane is K+ permeable so they diffuse back out through K+ ion channels.

A stimulus triggers sodium ion channels to open. If the stimulus is big enough, it will cause a rapid change in potential difference. The sequence is called an action potential:

1. Stimulus: Excites the membrane, causing Na+ channels to open.

The membrane becomes more permeable to Na+, which diffuses down the electrochemical gradient.

This makes the inside of the cell less negative.

2. Depolarisation: If potential difference reaches the threshold (~-55mV) more Na+ channels open and more Na+ diffuses into the neurone.

3. Repolarisation: Potential difference: ~+30mV, Na+ close, K+ open.

Membrane is more permeable to K+, so ions diffuse out fast.

This starts to get the membrane back to resting potential. 

The sequence of an action potential is as follows:

4. Hyperpolarisation: K+ ion channels are slow to close, so there is an 'overshoot'.

The potential difference becomes more negative than the resting potential. (i.e. < -70mV)

5. Resting potential: Ion channels are reset.

The Na+/K+ pump returns the membrane to its resting potential and maintains it.

**After an action potential, the neurone cell cannot be excited again straight away.

This is because the ion channels are recovering and they cannot be made to open.

Na+ ion channels are closed during repolarisation and K+ ion channels are closed during hyperpolarisation.

This period of recovery is called the refractory period.

When an action potential happens, some of the sodium ions that enter the neurone diffuse sideways.

This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part.

This causes a wave of depolarisation to travel along the neurone.

The wave moves away from the parts of the membrane in the refractory period because these parts cannot an action potential.

During the refractory period, ion channels are recovering and cannot be opened.

So the refractory period acts as a time delay and ensures that action potentials do not overlap but pass along discrete impulses.

The refractory period also ensures action potentials are unidirectional.

If the threshold level is reached, an action potential will fire, if it is not reached, it will not fire.

Bigger stimuli will cause more frequent action potentials.