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.

Additional animationsCocaine effect

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.