DETECTING and DETECTING and RESPONDING to RESPONDING to signalssignals

RECEPTORSRECEPTORSReceptors:Receptors: Specialised structures capable of Specialised structures capable of

responding to specific stimuli by initiating signals in the responding to specific stimuli by initiating signals in the nervous system or triggering the release of a hormone.nervous system or triggering the release of a hormone.

Types of ReceptorsTypes of Receptors ChemoreceptorsChemoreceptors – These are stimulated by specific – These are stimulated by specific

chemicals in the external and internal environment. chemicals in the external and internal environment. MechanoreceptorsMechanoreceptors – These are stimulated by – These are stimulated by

anything that changes the shape of the receptor.anything that changes the shape of the receptor. PhotoreceptorsPhotoreceptors – These detect light. In some – These detect light. In some

animals they also detect colour and form images.animals they also detect colour and form images. ThermoreceptorsThermoreceptors – These detect external heat and – These detect external heat and

cold through receptors near the surface and internal cold through receptors near the surface and internal body temp. deeper in the body by receptors in the body temp. deeper in the body by receptors in the major arteries and hypothalamus.major arteries and hypothalamus.

Detecting Stimuli.Detecting Stimuli.

The intensity of a The intensity of a stimulus must be stimulus must be sufficient to reach the sufficient to reach the threshold of the threshold of the receptor. This is the receptor. This is the weakest stimulus to weakest stimulus to which the receptor can which the receptor can respond. Receptors respond. Receptors then stimulate then stimulate effectors to produce a effectors to produce a response.response.

RespondingRespondingResponses in animals are based on sensory Responses in animals are based on sensory information received from all parts of the body, information received from all parts of the body, often requiring coordination from different parts often requiring coordination from different parts of the body. Internal communication involved in of the body. Internal communication involved in homeostasis and regulation are carried out by homeostasis and regulation are carried out by the nervous and hormonal systems. the nervous and hormonal systems.

The nervous system carries messages rapidly The nervous system carries messages rapidly along nerve pathways.along nerve pathways.

The hormonal, (The hormonal, (ENDOCRINEENDOCRINE), system is a ), system is a slower system that releases specific chemicals slower system that releases specific chemicals into the bloodstream.into the bloodstream.

RESPONDING con’t.RESPONDING con’t. Misalignment detectorsMisalignment detectors:: These are detectors These are detectors

that detect when a particular factor is out of line. that detect when a particular factor is out of line. They monitor the precise factor of the internal They monitor the precise factor of the internal environment that is being controlled, eg. Oxygen environment that is being controlled, eg. Oxygen level in the blood or blood temperature in the level in the blood or blood temperature in the brain.brain.

Disturbance DetectorsDisturbance Detectors:: These warn of These warn of problems before they arise. They detect the problems before they arise. They detect the presence of external or other internal changes presence of external or other internal changes that may result in a change in the factor of the that may result in a change in the factor of the internal environment being controlled.internal environment being controlled.

Note: Disturbance and misalignment detectors Note: Disturbance and misalignment detectors allow for a more precise control of internal factors allow for a more precise control of internal factors than misalignment detectors acting alone.than misalignment detectors acting alone.

RESPONDING con’t.RESPONDING con’t. Effector organs include muscles and Effector organs include muscles and

glandular tissue.glandular tissue. Muscle cells can be Muscle cells can be stimulated to contract or can be inhibited stimulated to contract or can be inhibited restricting contraction. Glands secrete restricting contraction. Glands secrete biologically active substances such as hormones biologically active substances such as hormones and enzymes.and enzymes.

DirectionalityDirectionality is often an important aspect of is often an important aspect of responsiveness. Some environmental stimuli, responsiveness. Some environmental stimuli, particularly light and sound the direction from particularly light and sound the direction from which it comes is of the utmost importance. which it comes is of the utmost importance. Often the direction is determined by signals from Often the direction is determined by signals from a pair of sensory organs such as eyes and ears.a pair of sensory organs such as eyes and ears.

NERVES - NEURONSNERVES - NEURONSThe Nervous System:The Nervous System: The The nervous system is present in animals nervous system is present in animals but not plants and is characterized but not plants and is characterized by rapid response. by rapid response. It is composed of three It is composed of three complimentary systems:complimentary systems:

11 The Central Nervous System The Central Nervous System (CNS)(CNS) – the Brain and Spinal Cord – the Brain and Spinal Cord where most integration in the where most integration in the nervous system takes place.nervous system takes place.

22 The Autonomic Nervous System The Autonomic Nervous System – – includes nerves involved in includes nerves involved in unconscious/involuntary responses.unconscious/involuntary responses.

33 Peripheral Nervous SystemPeripheral Nervous System – – includes sensory nervesincludes sensory nerves and motor and motor nerves.nerves.

Types of Nerve CellsTypes of Nerve CellsNerve Cells:Nerve Cells: There are There are Three main types of nerve Three main types of nerve cells. They are:cells. They are:

Sensory neuronsSensory neurons – these conduct – these conduct messages from the receptors to messages from the receptors to the CNS.the CNS.

Intermediate/Connector or Intermediate/Connector or Inter-neurons Inter-neurons – these relay – these relay impulses from the sensory to the impulses from the sensory to the motor neurons. They are found in motor neurons. They are found in the CNS.the CNS.

Motor neuronsMotor neurons – these relay – these relay messages away from the CNS to messages away from the CNS to the effector organs, glands & the effector organs, glands & muscles.muscles.

A Typical Nerve CellA Typical Nerve CellNeurons:Neurons: All neurons are made up of three main parts: All neurons are made up of three main parts:

Cell bodyCell body – contains the nucleus and the cytoplasm of the cell. – contains the nucleus and the cytoplasm of the cell. Messages received by the dendrites are sent to the cell body.Messages received by the dendrites are sent to the cell body.

AxonAxon – an elongated section of the cell body that conducts – an elongated section of the cell body that conducts impulses away from the cell body and transmits messages to impulses away from the cell body and transmits messages to other cells. Axons vary in length and branching.other cells. Axons vary in length and branching.

DendriteDendrite – Fine branching extensions of the neuron that conduct – Fine branching extensions of the neuron that conduct impulses toward the cell body and away from other cells.impulses toward the cell body and away from other cells.

Electrical InsulationElectrical InsulationMyelinMyelin – rich in fats, forms an electrical – rich in fats, forms an electrical insulating layer around the axon, thus insulating layer around the axon, thus increasing the speed of impulse conduction.increasing the speed of impulse conduction.Schwann cellsSchwann cells – cells outside the CNS that – cells outside the CNS that form a tightly wrapped myelin sheath.form a tightly wrapped myelin sheath.Node of RanvierNode of Ranvier – gaps in the myelin sheath – gaps in the myelin sheath along the axon. The sheath prevents ion flow along the axon. The sheath prevents ion flow across the neuron membrane and forces the across the neuron membrane and forces the impulse to flow from node to node. In this way impulse to flow from node to node. In this way impulses jump along the axon.impulses jump along the axon.AxonAxon – speed of impulse travel is partly – speed of impulse travel is partly dependent on the diameter of the axon. The dependent on the diameter of the axon. The larger axon increases speed of conduction. (eg. larger axon increases speed of conduction. (eg. squid have giant axons with very rapid squid have giant axons with very rapid conduction speeds).conduction speeds).

Nerve Impulse – Action Nerve Impulse – Action PotentialPotential

The action potentialThe action potential

When chemicals contact the surface of a neuron, they When chemicals contact the surface of a neuron, they change the balance of change the balance of ionsions (electrically charged atoms) (electrically charged atoms) between the inside and outside of the cell membrane.  between the inside and outside of the cell membrane.  When this change reaches a threshold level, this effect When this change reaches a threshold level, this effect runs across the cell's membrane to the axon.  When it runs across the cell's membrane to the axon.  When it reaches the axon, it initiates the reaches the axon, it initiates the action potentialaction potential..

The surface of the axon contains hundreds of The surface of the axon contains hundreds of thousands of miniscule mechanisms called thousands of miniscule mechanisms called ion ion channelschannels. When the charge enters the axon, the ion . When the charge enters the axon, the ion channels at the base of the axon allow positively channels at the base of the axon allow positively charged ions to enter the axon, changing the electrical charged ions to enter the axon, changing the electrical balance between inside and outside.  This causes the balance between inside and outside.  This causes the next group of ion channels to do the same, while other next group of ion channels to do the same, while other channels return positive ions to the outside, and so on channels return positive ions to the outside, and so on all the way down the axon.  all the way down the axon. 

Action PotentialAction PotentialGreat Website: http://outreach.mcb.harvard.edu/animations/actionpotential.swf

Action Potential – movement of Action Potential – movement of ionsions

The SynapseThe SynapseSynapse:Synapse: Neurons never touch Neurons never touch

each other. There is a gap or each other. There is a gap or junction between one neuron junction between one neuron and the next, known as a and the next, known as a synapse. The synapse synapse. The synapse consists of the end of the consists of the end of the axon of one neuron and the axon of one neuron and the start of a dendrite of another start of a dendrite of another neuron. The Axon releases a neuron. The Axon releases a chemical called a chemical called a neurotransmitter into the neurotransmitter into the synapse, which diffuses synapse, which diffuses across to the dendrites of across to the dendrites of the other neuron. Receptors the other neuron. Receptors on the dendrites combine on the dendrites combine with the neurotransmitter with the neurotransmitter and trigger a nerve impulse and trigger a nerve impulse in the next neuronin the next neuron

Vesicles

Neurotransmitters

Postsynaptic membrane

Presynaptic membrane

Receptor sites

Axon

Mitochondria

HORMONES – The Endocrine HORMONES – The Endocrine SystemSystem

The Endocrine SystemThe Endocrine System consists of ductless consists of ductless (endocrine) glands – specialised cells that (endocrine) glands – specialised cells that secrete hormones directly into the blood stream.secrete hormones directly into the blood stream.

HormonesHormones are specialised chemicals produced are specialised chemicals produced in minute amounts that are involved in the in minute amounts that are involved in the regulation of many body processes. They regulation of many body processes. They circulate in the bloodstream but can only be circulate in the bloodstream but can only be detected by specific receptors on particular detected by specific receptors on particular cells.cells.

Most hormones only affect the production of Most hormones only affect the production of enzymes, or structural proteins that affect enzymes, or structural proteins that affect growth, development, reproductive cycles and growth, development, reproductive cycles and other processes in specific organs or tissues.other processes in specific organs or tissues.

HORMONES IN ACTIONHORMONES IN ACTION

Hormonal response may be slow acting Hormonal response may be slow acting but its effects may be long lasting.but its effects may be long lasting.

The Hypothalamus gland in the brain is The Hypothalamus gland in the brain is the main control centre that regulates the main control centre that regulates hormones by sending nerve or hormones by sending nerve or hormonal messages to the Pituitary hormonal messages to the Pituitary gland.gland.

The Pituitary gland, in turn passes The Pituitary gland, in turn passes “messages” via hormones to target “messages” via hormones to target tissues around the body.tissues around the body.

The Endocrine GlandsThe Endocrine Glands

Comparison of Hormone TypesComparison of Hormone TypesType Relative

SizeMovement Examples

Fatty-acid hormones

Small Lipid-soluble, so they pass directly through plasma membranes.

Steroid hormones: Testosterone, oestrogen, progesterone

Amino-acid hormones

Larger Water-soluble, so they bind to receptors on plasma membranes. This activates the second messenger mechanism, cyclic AMP, which causes the change within the cell.

Insulin, glucagon, adrenaline, thyroxine, oxytocin, ADH (anti-diuretic hormone, growth hormone

Hormones in ActionHormones in Action

Neural or hormonal

stimulation

Endocrine gland

Carried by bloodstream around the body

Hormone fuses to specific receptors on target cells

Target Cell

Response

Hormone

ReceptorHormone

Hormone secreted

Hormones con’t.Hormones con’t.PheromonesPheromones are chemical signals released outside the body. They are chemical signals released outside the body. They target organisms of the same species and are most commonly used to target organisms of the same species and are most commonly used to attract mates or mark territory.attract mates or mark territory.

Comparing The Nervous And Endocrine Comparing The Nervous And Endocrine SystemsSystemsNervous System Hormonal System

Type of message

Electrochemical impulse Chemical messenger

Speed of message

Rapid Slow

Transmission Nerves / Neurons Bloodstream

Duration of response

Short Long lasting

Target Very specific in target neurons, muscles or glands

More general, to target tissues or organs in the body

REGULATION IN PLANTSREGULATION IN PLANTS Plants are able to adjust their growth and Plants are able to adjust their growth and

development in response to the environment.development in response to the environment. When a plant responds to an external stimulus the When a plant responds to an external stimulus the

plant exhibits a plant exhibits a TROPISMTROPISM. . Growth towards a stimulus is called a Positive Growth towards a stimulus is called a Positive

Tropism.Tropism. Growth away from the stimulus is called a Negative Growth away from the stimulus is called a Negative

Tropism.Tropism.

StimulusStimulus TropismTropism

LightLightPhototropismPhototropismGravityGravity GeotropismGeotropismTouchTouch

ThigmotropismThigmotropismWaterWater HydrotropismHydrotropism

Plants: Sensing and RespondingPlants: Sensing and RespondingStimuliStimuli

Plants do not monitor their internal environment, as do animals.Plants do not monitor their internal environment, as do animals. They are however sensitive to a number of environmental factors, both They are however sensitive to a number of environmental factors, both

physical and chemical factors.physical and chemical factors.

Physical Factors:Physical Factors: include direction and wavelength of light, day/night include direction and wavelength of light, day/night length (photoperiod), gravity, temperature and touch.length (photoperiod), gravity, temperature and touch.Chemical Factors:Chemical Factors: include water, carbon dioxide and specific chemicals, include water, carbon dioxide and specific chemicals, for example ethylene gas (which ripens fruit).for example ethylene gas (which ripens fruit).

Directionality is often important in plant sensing. Eg. Shoots growing Directionality is often important in plant sensing. Eg. Shoots growing towards light, Roots responding to gravity by growing down.towards light, Roots responding to gravity by growing down.

RespondingResponding Growth in plants is triggered by environmental factors. When the direction Growth in plants is triggered by environmental factors. When the direction

of the growth is related to the direction from which the stimulus comes the of the growth is related to the direction from which the stimulus comes the response is called aresponse is called a tropism tropism. See previous lesson for various Tropisms.. See previous lesson for various Tropisms.

Summary: Plant Hormonal Summary: Plant Hormonal Responses Compared To Responses Compared To AnimalsAnimals Hormonal responses in plants are relatively simple.Hormonal responses in plants are relatively simple. Plants have no endocrine system like animals.Plants have no endocrine system like animals. Hormone secreting cells are not organised into Hormone secreting cells are not organised into

specialised glands.specialised glands. Plant hormones are generally produced by cells Plant hormones are generally produced by cells

receiving appropriate stimuli.receiving appropriate stimuli. Plant hormonal responses are much slower.Plant hormonal responses are much slower. Plant hormones are distributed through:Plant hormones are distributed through:

1.1. From cell to cellFrom cell to cell2.2. Transport pathways – usually the phloemTransport pathways – usually the phloem3.3. Even through the air.Even through the air.

FLOWERINGFLOWERING

Plants flower, develop fruit and become dormant Plants flower, develop fruit and become dormant at the most favourable times of the year. These at the most favourable times of the year. These are controlled by the daily light length, or are controlled by the daily light length, or PHOTOPERIODPHOTOPERIOD. A response to the photoperiod is . A response to the photoperiod is called a called a photoperiodism.photoperiodism. It is the night-length that It is the night-length that stimulates flowering.stimulates flowering.

Short-day plants:Short-day plants: long nights. Produce flowers long nights. Produce flowers when the photoperiod is less than a critical value. when the photoperiod is less than a critical value. So flowering is prevented if the hours of daylight So flowering is prevented if the hours of daylight are too long. These plants usually flower in late are too long. These plants usually flower in late summer, autumn or winter.summer, autumn or winter.

Long-day plants:Long-day plants: short nights. Will not flower until short nights. Will not flower until the hours of daylight exceed a threshold value. the hours of daylight exceed a threshold value. They tend to flower in late spring or early They tend to flower in late spring or early summer.summer.

In day neutral plantsIn day neutral plants, the length of the , the length of the photoperiod is unimportant. A signal from the photoperiod is unimportant. A signal from the leaves, possibly a hormone called florigen, causes leaves, possibly a hormone called florigen, causes development of buds.development of buds.

DORMANCY & VERNALISATIONDORMANCY & VERNALISATION

DORMANCYDORMANCY Some plants become dormant prior to winter. Some plants become dormant prior to winter.

Lower Temps and shortening days trigger Lower Temps and shortening days trigger changes that involve loss of chlorophyll from changes that involve loss of chlorophyll from leaves and withdrawal of nutrients from leaves leaves and withdrawal of nutrients from leaves into stems and roots. Abscisic acid is largely into stems and roots. Abscisic acid is largely responsible for bud dormancy.responsible for bud dormancy.

Dormancy is broken by substantial rainfall, Dormancy is broken by substantial rainfall, intense heat (fire) to break seed coat, extended intense heat (fire) to break seed coat, extended exposure to cold or light or chemicals found in exposure to cold or light or chemicals found in the digestive tracts of animals.the digestive tracts of animals.

VERNALISATIONVERNALISATION Some plants require exposure to cold before they Some plants require exposure to cold before they

can complete their life cycle. Vernalisation is the can complete their life cycle. Vernalisation is the period of winter cold that stimulates flowering in period of winter cold that stimulates flowering in many plants.many plants.