Response to stimuli

Stimulus: a detectable change in the internal or external environment of an organism that provides a responce in an organism

Stimuli are detected by receptors. These transport the energy of a stimulus into some form of energy that can be processed by the organism and leads to a responce. 

The response is carried out by effectors, recieving it from a coordinator

stimulus -> receptor ->  coordinator -> effector -> response 

Taxes

A simple responce whose direction is determined by the direction of the stimulus. The organism responds directly to environmental changes by moving its whole body either towards a favourable stimulus or away from an unfavourable one.

Kinesis

A form of responce in which the organism does not move towards or away from a stimulus. Instead the more unpleasent the stimulus, the more rapidly it moves and changes direction. Its random movements. 

Tropisms

A growth movement of part of a plant in response to a directional stimulus.

• plants shoots are +vely phototropic
• plant roots are -vely phototropic and +vely geotropic
• plant roots are +vely hydrotropic

The nervous system has two main divisions:

• the central nervous system: made up of the brain and spinal cord
• the peripheral nervous system: made up of pairs of nerves that originate from either the brain or the spinal cord

the peripheral nervous system is then divided into two:

• sensory neurones: carry nerve impulses from receptors towards the CNS
• motor neurones: carry nerve impulses away from the CNS to affectors

 the motor neurones are further subdivided:

• voluntary nervous system: carries nerve impulses to body muscles and is under voluntary concious control
• autonomic nervous system: carries nerve impulses to glands smooth muscle and cardiac muscle and is under involuntary subconcious control

the spinal cord: a column of nervous tissue that runs along the back and lies inside the vertebral column for protection. Emerging at intervals along the spinal cord are pairs of nerves.

Reflex arc

• an involuntary responce 
• a involunatary responce to a sensory stimulus is called a reflex
• the pathway of neurones involved in a reflex is known as a reflex arc
• reflex arcs involve just 3 neurones:sensory, intermediate and motor

stimulus -> receptor -> sensory neurone -> intermediate -> motor -> effector -> response

Importance of reflex arc: They are involuntary therefore the brain isn't overloaded with situations in which the response is always the same. They protect the body from harmful stimuli, they are effective and do not have to be learned. They are fast, because the neurone pathway is short with very few synapses (which are the slowest link in a neurone pathway).

The autonomic nervous system

2 divisions:

• the sympathetic nervous system: stimulates effectors and so speeds up activity. It stimulates effectors when we exercise strenuously or experience powerful emotions. Helps us cope with stressful situations 
• the parasympathetic nervous system: inhibits effectors and so slows down any activity. Controls activities under normal resting conditions. Its concerned with conserving energy and replenishing the bodys reserves

these nervous systems are antagonistic

control of heart rate

Changes to the heart rate are controlled by a region of the brain called the medulla oblongata. This has two centres: 

• a cente that increases heart rate, linked to the SAN by the sympathetic nervous system
• a cente that decreases heart rate, linked to the SAN by the parasympathetic nervous system

which of the centres is stimulated depends of the info the recieve from 2 types of recepetor, which respond to one of the following:

• chemical changes in the body
• pressure changes in the body

Chemoreceptors are found in the wall of the carotid arteries (the arteries that serve the brain). They are sensitive to changes in the pH of the blood that result from changes in CO2 conc. Process:

• higher conc of CO2 = lower pH
• the chemoreceptors in the walls of the carotid arteries and aorta detect this and increse the freq of nervous impulses to the medulla oblongata which increases heart rate
• this centre increses the freq of impulses via the sympathic nervous system to the SAN to increase heart rate
• increased blood flow lowers pH therefore the chemoreceptors reduce the freq of nerve impulses to the medulla oblongata 
• the medulla oblongata then reduces the freq of impulses to the SAN snf decreases heart rate to normal

Pressure receptors occur in the walls of the carotid arteries and aorta. They operate as follows:

• when blood pressure is higher than normal:  they transmit a nervous impulse to the medulla oblongate that decreases heart rate. This centre sends impulses via the parasympathetic nervous system to the SAN, which decreases heart rate.
• when blood pressure is lower than normal: they transmit a nervous impulse to the medulla oblongata that increases heart rate. This centre sends a impulse via the sympathetic nervous system to the SAN which increases heart rate