Control of heart rate and the different types of receptors

Changes to heart rate are controlled by a region in the brain. It has two centres:

• One that increases heart rate and is linked to the sinoatrial node by the sympathetic nervous system

• One that decreases heart rate and is linked to the sinoatrial node by the parasympathetic nervous system

Which one of these centres is stimulated depends on information received from two types of receptors:

• Chemoreceptors – detect chemical changes in the blood

• Baroreceptors – detect pressure changes in the blood

Control by chemoreceptors

These are found in the walls of the carotid arteries (the arteries that serve the brain). Their stimulus is pH changes. This is caused in the blood by carbon dioxide concentration. If this is high the pH if the blood falls to lower than normal. For example:

• Increased muscular activity so more carbon dioxide produced by respiration

• Blood pH is lowered

• Chemoreceptors are stimulated so increase the frequency of nerve impulses to the brain

• The centre in the brain that controls heart rate increases the frequency of the nerve impulses to the sinoatrial node via the sympathetic nervous system

• SA node then increases the heart rate and carbon dioxide can be removed from the blood faster

• pH then rises so the chemoreceptors reduce the frequency of the impulses to the brain

• The centre in the brain then reduces the frequency of the impulses to the SA node

• The SA node decreases heart rate

Control by baroreceptors

Blood pressure is higher than normal Blood pressure is lower than normal

• Pressure receptors transmit an impulse to the centre in the brain that decreases heart rate

• This then sends an impulse to the sinoatrial node

• This travels via the parasympathetic nervous system

• This then deceases the heart rate

• Pressure receptors transmit an impulse to the centre of the brain that increases heart rate

• This sends an impulse to the SA node

• This impulse travels via the sympathetic nervous system

• This increases the heart rate

Mechanoreceptors

Found in the skin and respond to deformation – see pacinian corpuscle

Summary Paragraph – Nervous Control

• The nervous system has two main divisions: the central nervous system (CNS) comprising the brain and spinal cord and the peripheral nervous system (PNS).

• The peripheral nervous system is made up of the motor nerves that carry impulses away from the CNS and sensory nerves that carry impulses towards the CNS.

• A spinal reflex is an involuntary response that involves the spinal cord. An example is the withdrawal reflex of a hand way from a hot object.

• The sequence of events begins with the heat from the hot object which acts as the stimulus.

• This is detected by receptors in the skin on the back of the hand which create a nerve impulse that then passes along a sensory neurone into the spinal cord.

• The impulse then passes to a reflex neurone in the central region of the spinal cord.

• The impulse then leaves the spinal cord via a motor neurone.

• This neurone stimulates the muscle of the upper arm to contract and withdraw the hand from the object.

• Structures such as muscles that bring about a response to a stimulus are called effectors.

The Pacinian Corpuscle

Features of receptors as illustrated by the Pacinian corpuscle:

• They are specific to a single type of stimulus – the pacinian corpuscle only responds to mechanical pressure not temperature or any other stimulus

• They produce a generator potential by acting as a transducer – all stimuli are forms of information but not information that the body can understand so the receptors need to convert this information into a form the body can understand, this is a nerve impulse. The stimulus is always some form of energy and a nerve impulse is electrical energy so the receptor converts the energy e.g. heat into a nerve impulse which is known as a generator potential.

How does a pacinian corpuscle work?

The neurone at the centre of the pacinian corpuscle has special sodium ion channels on it's membrane. These are called stretch mediated sodium channels.

• In the normal resting state these sodium ion channels are too narrow to allow sodium ions into the neurone

• When pressure is applied to the skin (the stimulus) the pacinian corpuscle changes shape and the membrane around it's neurone becomes stretched

• This widens the sodium channels allowing sodium ions to diffuse into the neurone

• This depolarises the membrane and produces a generator potential

• This in turn then creates an action potential (nerve impulse) that passes along the neurone and through passing to other neurones reaches the central nervous system

• A response is then coordinated from there

Reflex Arc – 3.5.1Diagram

What is a reflex arc?

It is a totally involuntary action in response to danger. This type of response doesn't involve any conscious thought and unless it is quicker to go through the brain, doesn't involve the brain at all. This type of response is known as a reflex action and the neurone pathway involved is called a reflex arc.

What set's this response apart from normal responses?

• It's a lot quicker

• It doesn't involve conscious thought

• You don't stop and consider the alternatives

• It often doesn't involve the brain

• It only involves 3 neurones

A reflex arc

This is the pathway of neurones that are involved in a reflex action. Only 3 neurones are involved. If it passes through the spinal cord the reflex arc is known as a spinal reflex.

The 3 neurones involved are:

• a sensory neurone – red

• A relay neurone – blue

• A motor neurone – green

Examples of reflex actions

• Removing your hand from something hot

• Removing your foot from a pin

• Knee jerk when your knee is hit with something

Stimulus and Response - 3.5.1

Behaviour Definition Purpose Example Advantage

Taxes A simple response whose direction is determined by the direction of the stimulus

To move away from an unfavourable stimulus (negative taxis) or towards a favourable one (positive taxis)

Single celled algae will move towards light (positive phototaxis)

Increases chances of survival by moving towards the favourable factors and away from unfavourable ones

Kineses A kineses is a increase in random movement designed to bring the organism back into favourable conditions

To increase the chances of being in favourable conditions for survival

Woodlice – do this in order to spend most of their time in the dark moist conditions which are good for survival

Increases the chances of the organism getting out of unfavourable conditions and into favourable ones

Tropisms A tropism is a growth movement of a plant in response to a directional stimulus

To grow towards a positive stimulus and away from a negative one

Plant shoots grow towards light (positive phototrophism) so their leaves are in the best position to capture light for photosynthesis

The plant orientates itself in the best possible position for growth so helps its survival

Reflex Action An involuntary action to help survival

Helps to move the body or parts of it away from a negative stimulus which could hurt us.

Moving your hand away from a hot flame

These split second responses help to keep the body from harm by moving us away from unfavourable stimuli.

The autonomic nervous system

Autonomic means self governing. This system controls the involuntary activities of internal

muscles and glands.

It has two branches which are antagonistic to each other i.e. they work do the opposite of

each other so if one branch contracts a muscle the other will relax it. Which branch is

stimulated depends on information received by receptors.

The two branches are called sympathetic and parasympathetic:

• The sympathetic nervous system in general stimulates effectors such as muscles

so speeds up any activity. It acts like an emergency controller by helping us cope

with stressful situations by heightening our awareness and preparing us for activity

(the flight or fight response)

• The parasympathetic nervous system in general inhibits effectors so slows down

our activity. It controls activity in normal resting conditions, it's concerned with

conserving energy and replenishing the body's reserves

Depolarisation and Establishing an Action Potential

Depolarisation

This is when the charge on the axon (nerve fibre) is reversed. The charge between the tissue fluid on the outside of the axon and the cytoplasm inside is normally negative. However when a stimulus is received from a receptor (such as a pacinian corpuscle) or another nerve cell this charge reverses and it becomes positive.

This is because the energy received from the receptors causes sodium gated channels to open on the axon membrane. When this happens and depolarisation occurs an action potential is produced

Action potential

(numbers refer to the numbers on the diagram)

1. At resting potential some potassium voltage gated channels are open (the ones that are always open) but the sodium voltage gated channels are closed

2. The energy of the stimulus causes some sodium voltage gated channels to open and sodium ions flood into the axon from the outside by diffusion. Being positively charged they reverse the potential difference across the membrane causing it to become positive

3. As sodium ions diffuse into the axon through the voltage gated channels more channels open causing an even greater influx of sodium ions (see the all of nothing principle below)

4. Once the action potential of around +40mV has been established the voltage gated sodium channels close preventing any more sodium ions from diffusing in. The voltage gated potassium channels begin to open

3

1

2

4

5

6

5. More and more potassium gated channels open and this causes the potassium ions to flood out as there is no electrical gradient to stop them. This causes repolarisation of the axon.

6. The outward diffusion of these potassium ions causes a temporary overshoot of the gradient, with the inside of the axon more negative relative to the outside than it should be. This is called hyperpolarisation. The gates on the potassium channels closes at this point and the action of the sodium potassium pumps begins to have an effect. The sodium potassium pump pumps potassium ions in and sodium ions out and the resting potential is reached. The axon is repolarised

Hyperpolarisation

This occurs during the repolarisation stage. This overshoot is very important as it stops the nerve being stimulated for a small amount of time (less than a second) this produces discrete (separate) impulses that the CNS can understand and act upon. The frequency of the impulses therefore determines the strength of the stimulus not the amount of polarisation (see all or nothing principle)

All or nothing principle

This means that nerve cells act on an all or nothing basis. They are either stimulated and produce an action potential or they don't.

This means they will always produce the same action potential each time and the energy of the stimulus opens enough sodium voltage gated channels or it doesn't. There is no smaller action potentials, the nerve will produce the same one each time if the stimulus provides enough energy.

ByAndy Hubbert 13LD

Caffeine and Adenosine

Mechanism of action

● Acts in the brain● Acts as an antagonist to adenosine which is a

neurotransmitter in the brain● Adenosine normally inhibites nerve responses for

example during sleep● This means it decreases the chances the postsynptic

nerve will fire an action potential● Caffeine stops this from happening so acts as a

stimulent and increases alertness

Cocaine Structure

Mechanism of action

● Binds to the dopamine transport protein● This prevents dopamine being reabsorbed by the

presynaptic neurone● This leads to a build up of dopamine in the synaptic

cleft● This leads to prolonged effects of dopamine as the

postsynaptic neurone is constantly stimulated● This leads to the high that users get

Mechanism of action

Nervous and Hormonal Systems

Comparison of main features

Hormonal System Nervous System

Communication is by chemicals called hormones

Communication is by nerve impulses

Transmission is by the blood system Transmission is by neurones

Transmission is relatively slow Transmission is very rapid

Hormones travel to all parts of the body but only target organs respond

Nerve impulses travel to specific parts of the body

Response is widespread Response is localised

Response is slow Response is rapid

Response is often long lasting Response is short-lived

Effect my be permanent and irreversible Effect is temporary and reversible

Chemical Mediators

These are chemicals that are released from cells (typically infected or injured cells) and have an effect on the cells in the immediate vicinity

Two examples:

• Histamine – stored in certain white blood cells and released following injury or in response to an allergen (such as pollen) it causes dilation of small arteries and increased permeability of capillaries. This leads to localised swelling redness and itching

• Prostaglandins -Found in cell membranes and cause dilation of small arteries and increased permeability of capillaries. They are released following injury and affect neurotransmitters and in doing so affect the pain sensation

(c) myrevisionnotes

Plant growth factors and control of tropisms by IAA

Plants need to respond to a variety of factors to survive:

• Light – stems grow towards the light (positively phototrophic)

• Gravity – roots grow towards the pull of gravity to firmly anchor the plant in the soil (positively geotrophic)

• Water – almost all plants grow towards water (positively hydrotrophic)

Plants don't have a nervous system so respond to these stimuli by means of plant growth factors. These are called plant growth factors because:

• They exert their influence by affecting growth

• They are made by cells located throughout the plant not in particular organs

• They may affect the tissue that releases them rather than acting on a distant organ

Plant growth factors

Plant growth factors are produced in small quantities and have their effects close to the tissue that produces them. An example of a plant growth factor is IAA which has many effects, one of them is to cause plant cells to elongate

Control of tropisms by IAA

A tropism is a growth movement of a plant in response to a directional stimulus. In a young shoot this can be observed as it will bend towards the light that is directed at it from one side. This is due to IAA and the following sequence of events:

1. Cells in the tip of the shoot produce IAA which is transported down the shoot

2. IAA is initially transported down all sides of the shoot

3. Light causes the movement of IAA from the light side to the shaded side of the shoot

4. A greater concentration of IAA builds up in the shaded side of the shoot

5. IAA causes the elongation of cells and there is a greater concentration of it on the shaded side so the cells on the shaded side elongate more

6. The shaded side of the shoot grows faster so the shoot bends towards the light

Resting Potential

• Intrinsic proteins in the phospholipid membrane of the nerve cell contain channels which can be opened or closed to allow sodium and potassium ions in and out of the cytoplasm. These are called gated channels. Others are open all the time allowing the ions to diffuse through them unhindered.

• Some proteins called sodium potassium pumps actively transport potassium ions into the cytoplasm and sodium ions out at the same time

As a result of these controls the inside of the axon is negatively charged relative to the outside and this is the resting potential. It varies in different nerve cells. When the axon is at the resting potential it is polarised.

How is this resting potential established in the first place?

It is all down to the movement of ions. Both sodium and potassium ions carry the 1+ charge. Sodium ions are actively transported out of the cell and potassium ions are transported in. These are both transported by the sodium potassium pump.

However more sodium ions are transported out for potassium ions transported in (3 sodium for every 2 potassium). There are therefore more sodium ions outside of the neurone than inside and more potassium ions inside than outside.

The gated channels then open. More open for potassium ions than sodium ions so as a result the membrane allows lots more potassium ions out of it than sodium ions in. This

again increases the difference in charge between the positive outside and less positive inside of the cell.

This sets up an electrical gradient. This means that after the initial exit of potassium ions from the cytoplasm of the cell it becomes harder for more to diffuse out. This is because they are positively charged and the fluid outside the neurone is positively charged. This means they are repelled (like magnets). However the cytoplasm is less positive so they are attracted to this.

This results in no net movement of ions and establishes the resting potential.

The Structure of a Neuron

Neurones are adapted to carry electrochemical charges called nerve impulses. Each

neurone comprises of a cell body that contains a nucleus and large amounts of rough

endoplasmic reticulum which are used in the production of proteins and neurotransmitters.

Extending from the cell body is a long fibre called an axon and smaller branched fibres

called dendrons.

Axons are surrounded by Schwann cells which protect and provide electrical insulation as

their membranes are rich in myelin.

There are 3 main types of neuron. Those that carry nerve impulses to an effector are

called motor neurones, those that carry impulses from a receptor are called sensory

neurones and those that link the other two types in the spinal cord are called reflex

neurones.

(c) myrevisionnotes