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The Internal Environment
Week 9
External and Internal Environments
The internal environment is the interstitial fluid through which cells exchange substances
External and Internal Environments
Organisms are able to regulate their internal conditions despite the changing conditions of the external
environment by constant movement of material across membranes.
Different environments
Type oforganis
m
Externalenvironment
Tolerance
Internal environmen
t
Control
Unicellular
General environment
Relatively high, little mobility
• Contents of the cell.
• Closely related to external environment.
Tolerate, or move away
Multicellular
General environment
Variable, but high ability to maintain constant internal environment
• Extracellular fluid.
• Often very different from external environment.
High regulation
Homeostasis: keeping
within limits
Structure of the nervous
system
TheBrai
n
Having a brain wave
• A living brain shows continuous electrical activity which can be measured.
• A range of different ‘brain wave’ patterns is shown.
• A person is declared ‘brain dead’ when no electrical activity can be measured.
Stimulus-Response ModelStimulus Recept
orTransmission
Control Centre
Effector
ResponseTransmission
Stimulus-response model
Set points and optimum conditions are maintained through negative feedback
Negative Feedback
Negative Feedback
Positive feedback mechanisms• Positive feedback system
in which oxytocin produced by the posterior pituitary gland stimulates contraction of the uterus and also stimulates the pituitary gland to produce even more of the hormone.
• Note the positive feedback nature of the inputs.
Homeostasis:
hormones in action• Detecting and
counteracting change.• The two interrelated
stages of homeostasis.• Relies on negative
feedback systems.• If a variable slightly
overshoots the optimal as a result of effector action, the counter negative feedback system will respond to correct the overshoot.
• Occurs continuously in the body so that optimal levels of variables are maintained
Temperature Regulation
Temperature RegulationStimulusHigh CO2 levels in
blood
ReceptorMedulla and aortic and
carotid bodiesControl CentreBrain
EffectorsDiaphragm and
intercostal muscles
ResponseFaster, deeper
breathing
Transmission
Transmission
Examples of factors that animals detect
Environment
Physical factors Chemical factors
Internal Temperature, blood pressure, stress on bones and muscles
Concentrations of O2, CO2, water, ions, wastes
External Light, temperature, gravity, sound
Food, O2, CO2, water, otherorganisms, other chemicals
Types of sensory receptors and their stimuli
Type of receptor
Types of stimuli detected
Example
Chemoreceptor
O2, CO2, pH, ions, signalling molecules
Receptors on dendrites of nerve cells
Photoreceptor Light and infrared radiation
Rods and cones in the eye
Mechanoreceptor
Sound, touch, pressure and gravity
Geotropism receptors that direct roots to grow down
Thermoreceptor
Heat and cold Receptors in skin sensing changes in temperature
Neurons
Neurons
•A typical motor neuron.
•Note the cell body containing the nucleus, with many branching projections, called dendrites, and the single axon that ends with many synaptic terminals that allow it to communicate with other neurons.
3 types of neuron
Neurons
(a) affector (or sensory) neuron(b) effector (or motor) neuron(c) connecting neuron (or interneuron)
Different kinds of neurons:
Neurone animation – reference to drug use
Neurons
Which type of neuron is located completely within the CNS?
Relationship between different kinds of neurons.
Transmission of an impulse along and between nerve
cells
Synapse
Communication by neurotransmitters
•Notice the gap between the axon of one neuron and the one to which it joins.
•Transmitter substances are produced in vesicles near the end of the axon.
Synapses
Signal transduction across a synapse
Reflex Arc
Communication by neurohormones(a) Electrical
impulses transmit signals along a neuron. Neurotransmitters are secreted at the axon and diffuse across a small gap to deliver a signal to the target cell, another neuron.
(b) Similar to part (a), except the target is striated muscle tissue.
(c) Some neurons release chemicals called neurohormones into the blood. The blood carries the appropriate signal to the target organs that then respond.
Communication by neurohormones
• Neurons in the hypothalamus in the brain secrete neurohormones into blood capillaries that become associated with the pituitary.
• Signals received by cells of the pituitary are transduced and the pituitary responds by producing a hormone as instructed. This hormone enters the bloodstream and is transported to its target cells.
Network of nerve cells
Network of nerve cells
Network of nerve cells
Neuron function impededThyrotoxic goitre
A goitre is caused by the enlargement of the thyroid gland and may vary in size from a small
lump to an enormous swelling.
Toxins can overwhelm
neuron transmissio
n• Many animal toxins
act on the nervous system, particularly at neuro-muscular synapses.
• Some prevent the passage of nerve impulses along a nerve.
• Others act on one or both sides of the neuro-muscular synapse.
Chapter 5 and 6 Reviews• Chapter 5
– Biochallenge (p.162), all questions– Chapter Review (p.163), Q. 2-6, 8-9
• Chapter 6– Biochallenge (p.196), all questions– Chapter Review (p.197), Q. 2-3, 6-8
You can start this work as we have covered some of the content, but you will not be able to do all
questions. You will be submitting this work once we have finished Week 10.