AQA Biology Unit 5 - Chapter 9, Response to stimuli

Stimulus - A detectable change in the internal or external environment of an organism, that produces a response in the organism

The ability to respond to a stimulus increases the chance of survival

Stimuli are detected by cells or organs called receptors

When the receptor detects a stimulus, the response is initiated and carried out by effectors

The coordinator connects the receptor and effector, allowing information to pass between them

Stimulus - Receptor - Coordinator - Effector - Response

Taxes - a response which occurs in a particular direction based on the direction of the stimulus. A motile organism will move towards a favourable stimulus (positive taxis) and away from an unfavourable one (negative taxis)

Kinesis - a response in which the stimulus doesn't affect the direction the organism moves, but in which the organism will move more rapidly and change direction more often if it detects and unfavourable stimulus. This increases the chance of moving away from the stimulus. It is often seen when the stimulus is not particularly directional and clear-cut, such as humidity or temperature

Tropism - a response which affects the growth of a plant in relation to a directional stimulus. In most cases the plant will grow towards a favourable stimulus (positive tropism) and away from an unfavourable one (negative tropism)

The Nervous System is made up of two major divisions -
Central Nervous System (CNS) - made up of the brain and spinal cord
Peripheral Nervous System (PNS) - made up of pairs of nerves, originating from either the brain or the spinal cord

The PNS is divided into -
Sensory Neurones - carry nerve impulses from receptors towards the CNS
Motor Neurones - carry nerve impulses away from the CNS towards effectors

The motor nervous system can be divided into -
Voluntary Nervous System - carries nerve impulses to body muscles, this is under voluntary / conscious control
Autonomic Nervous System - carries nerve impulses to glands, smooth muslce and cardiac muscle, and this is not under voluntary control (it is involuntary / subconscious)

Reflex - an involuntary response to a sensory stimulus

Reflex arc - the pathway of neurones involved in a reflex. Reflex arcs involve just three types of neurone, one of which is the spinal neurone

The main stages of a spinal reflex arc, such as withdrawing a hand from a hot object are -

Stimulus - heat from the hot object
Receptor - temperature receptors in the skin on the hand, which create a nerve impulse in the sensory neurone
Sensory Neurone - passes the nerve impulse to the spinal cord
Intermediate Neurone - links sensory and motor neurones in the spinal cord
Motor Neurone - carries nerve impulse from spinal cord to a muscle in the arm
Effector - the muscle in the arm, which is stimulated to contract
Response - hand pulls away from the hot object

The Autonomic Nervous System
Controls the involuntary activities of internal muscles and glands. Made up of -

Sympathetic Nervous System - stimulates effectors, speeding up activity
Parasympathetic Nervous System - inhibits effectors, slowing down activity

The sympathetic and parasympathetic nervous systems are antagonistic - they oppose each other. E.g. if one system contracts a muscle, the other relaxes it

An example of this is the control of heart rate...

Changes to the heart rate are controlled by the medulla oblongata, which has two centres -

A centre which increases heart rate, which is linked to the sinoatrial node by the sympathetic nervous system
A centre which decreases heart rate, which is linked to the SA node by the parasympathetic nervous system

Which one of these is stimulated depends on the information they receive from two types of receptor (chemoreceptors and pressure receptors) which respond to chemical and pressure changes in the blood

Chemoreceptors

These are found in the wall of the carotid arteries (arteries that serve the brain)
They are sensitive to changes in the pH of the blood, resulting from changes in the concentration of carbon dioxide (high concentration = low pH)

• Carbon dioxide concentration increases, pH decreases
• Chemoreceptors detect change in pH, and increase the frequency of nervous 
  impulses sent to the centre in the medulla oblongata that increases heart rate
• The centre increases the frequency of impulses sent to the sinoatrial node via
  the sympathetic nervous system, leading to an increase in heart rate
• This leads to increased blood flow, resulting in more CO2 being removed by the
  lungs, returning the CO2 concentration of the blood back to normal
• This causes pH to increase to normal level, which is detected by chemoreceptors
• Chemoreceptors reduce the frequency of the nerve impulses sent to the medulla 
  oblongata, which in turn reduces the frequency of nerve impulses sent to the
  sinoatrial node, which therefore decreases the heart rate back to normal

Chemoreceptors - Summary of Process

Increased muscular / metabolic activity...

More CO2 produced by tissues (due to increased respiration)...

Blood pH is lowered (more acidic)...

Chemoreceptors increase frequency of impulses sent to medulla oblongta...

Centre in medulla oblongata that increases heart rate increases frequency of impulses sent to the sinoatrial node via the sympathetic nervous system...

SA node increases heart rate...

Increased blood flow removes CO2 from the blood faster...

CO2 levels return to normal 

Pressure Receptors

Also found in the walls of the carotid arteries and the aorta

When blood pressure is higher than normal:
Pressure receptors transmit a nervous impulse to the centre in the medulla oblongata that decreases heart rate. This centre sends impulses via the parasympathetic nervous system to the SA node, which in turn causes a decrease in heart rate

When blood pressure is lower than normal:
Pressure receptors transmit a nervous impulse to the centre in the medulla oblongata that increases heart rate. This centre sends impulses via the sympathetic nervous system to the SA nose, which in turn causes an increase in the heart rate