2(j) Coordination and Response in humans

Syllabus statements 2.80-2.82, 2.86-2.94

Stimulus

A change in the environment that can be detected by an organism, e.g.

• light

• temperature

• sound

• touch

• chemicals

How do organisms respond to stimuli and why is this necessary?

• They need to respond to stimuli in order to survive.

• Organisms have receptors that allow them to detect stimuli.

• In humans, receptors are found in sense organs

• In plants, specialised cells detect stimuli such as light, gravity or water

• They then coordinate the required response.(e.g. move away, secrete chemicals,…)

• The response increases the organisms chance of survival

Homeostasis

The maintenance of a constant internal environment.

• body temperature (37^oC in humans to provide an optimum T^o for enzymes)

• body water content (ensures that cells function correctly, preventing them from bursting or shrivelling due to osmosis)

• pH level

• CO2 level

• glucose level

What 3 elements does a coordinated response require? + their definition

• A stimulus, a change in the environment that can be detected by an organism(light, temperature, touch,…)

• Receptor cells to detect the stimulus and to transduce the stimulus energy into electrical impulses(found in sense organs in humans, such as the skin on the fingertip, eyes,…)

• Effector cells to bring about the required response to the stimulus(usually muscle cells or a gland)

2 Systems involved in coordination&response in humans

• Nervous(hormonal) system

• Endocrine system

Both systems require a stimulus, receptor and effector and chemicals are involved in both.

Key differences between the nervous and endocrine(homonal) system

• The nervous system is made up of nerve cells(neurons) that carry electrical impulses around the body.

• The endocrine system consists of glands that produce hormones, which are carried by the bloodstream (plasma) and stimulate changes in the body.

• The nervous systems brings about quick responses that last for a short time.

• The endocrine system brings about slower responses that last for a longer time.

2 parts of the nervous system:

• CNS (Central Nervous System)

• PNS (Peripheral Nervous System)

CNS and PNS (components and how they are linked)

• The CNS consists of the brain and the spinal chord.

• The CNS is linked to sense organs by neurons(nerves).

• The neurons form the PNS.

Neurons (+Draw and label one)

Specialised nerve cells that carry electrical impulses (at high speeds).

3 types of neurons

• Sensory neuron(from receptor to CNS)

• Relay neuron(CNS)

• Motor neuron(from CNS to effector)

(2.88) Describe the nevous system pathway(including different types of neurons).

1. A stimulus is detected by a receptor in a sense organ.

2. The receptor transduces the stimulus energy into an electrical impulse.

3. This impulse is transmitted from the receptor to the CNS along sensory neurons.(The impulse is transmitted within the CNS to the brain along relay neurons)

4. The CNS(brain) coordinates this information and sends another impulse in response.

5. This impulse is transmitted to an effector along motor neurons.

in the exam a specific scenario will be given. Substitution of a specific stimulus, receptor and effector is needed.

Reflex action and reflex arc

A reflex action is a subconscious(automatic)and extremely quick response to a dangerous stimulus. It does not involve the brain. Instead, the impulses pass directly from sensory neurons, along relay neurons in the CNS(spinal cord), to motor neurons. This nervous pathway is known as a reflex arc.

Examples:

• Dilation&Constriction of pupils

• Contraction of muscles

• Salivation

(2.90) Describe the reflex arc : withdrawal of a finger from a hot object

1. A stimulus(heat) is detected by a receptor (thermoreceptors in fingertips).

2. The receptor converts/transduces the stimulus energy into an electrical impulse.

3. This impulse is transmitted from the receptor to the CNS along a sensory neuron.

4. The impulse is passed to a relay neuron in the CNS.(Pain message is transmitted to the brain)

5. The impulse is transmitted from the CNS to an effector along a motor neuron(arm muscle).

6. The effector (arm muscle) brings about the required response, in this case it contracts to pull the hand away from the flame.

(2.89) Synapse and the role of neurotransmitters

A synapse is the gap between two neurons.

• When an electrical impulse is carried along an axon,

• it triggers the nerve-endings to release a chemical called neurotransmitter so that the signal can travel across the synapse.

• The neurotransmitter diffuses across the synapse and binds to a receptor molecule on the next neuron.

• This stimulates the second neuron to transmit the electrical impulse.

(2.91) Label the structure of the eye

Cornea

the transparent outer part of the eye

• refracts light to reach the retina

Conjunctiva

• protects eye surface

Iris

the coloured part of the eye that does not allow light go through

• controls how much light enters the pupil

• has circular and radial muscles to constrict or dilate the pupil

Pupil

• Hole allowing light to enter the eye

Lens

transparent, biconvex disc that attaches to ciliary muscles by suspensory ligaments(悬韧带）

• focuses light onto retina

Retina

• contains light receptors

Fovea

• Area of retina that prpvides the clearest vision

Optic nerve

• carries impulses from the eye to the brain

Ciliary muscles

ring of muscle around the eye

• involved in changing the shape of the lens

Suspensory ligaments

• hold the lens in place

• involved in changing the shape of the lens

Accomodation of the eye - distant object

• Ciliary muscles relax. (causing…)

• Suspensory ligaments tighten.

• Lens is pulled thinner(less convex)

Accomodation of the eye - near object

• Ciliary muscles contract.

• Suspensory ligaments slacken

• The lens fattens(more convex)

Response of the eye in bright light

• Circular muscles contract.

• Radial muscles relax.

• The pupil constricts(becomes smaller).

• This reduces the amount of light entering the eye to avoid damage to the retina.

Response of the eye in dim light

• Circular muscles relax.

• Radial muscles contract.

• The pupil dilates(becomes bigger).

• This allows as much light as possible to enter the eye, falling onto the retina in order to maximise vision (create a better image).