# Coordination of Responses ## Responding to change and stimuli - Organisms **must respond** to changes in their environment to survive - They can only survive if they are successful at: - Finding favourable conditions for living - Finding food - Avoiding being eaten - If these vital requirements are not met, a species will **die out or go extinct** - Think of a mouse in a field - it needs to find seeds and insects while constantly watching for hawks circling overhead - Responses to change can vary in **complexity** depending on the organism and specific circumstances

Response: an action or change in behaviour that occurs as a result of a stimulus

### Detection and coordination - Responding to change requires **detection** - Detection involves a **stimulus being detected by a receptor cell** - There are different types of receptors: - Some receptor cells produce **electrical activity** in nerve cells in response to stimuli - Other receptor cells **secrete messenger chemicals** such as hormones in response to stimuli - The impulses or hormones sent by receptor cells travel to a **coordinator** - From the coordinators, the impulse is transported to the specific **effector** that will produce the appropriate response

The sequence is: **Stimulus → Receptor → Coordinator → Effector → Response**

## The 'fight-or-flight' response - An animal may produce a 'fight-or-flight' response in situations where there is a **high level of stress, fear or aggression** induced by environmental stimuli - 'Fight-or-flight' responses are **rapid** and can be **crucial for preserving life** ### Example pathway Using the example of a mouse detecting a hawk: 1. A sudden shadow from a hawk (the **stimulus**) is detected by the receptors in the mouse's eye 2. The receptor cells send an **impulse** along the nerves and to the brain (**coordinator**) 3. The brain sends an impulse to the leg muscles (**effectors**) of the mouse so it can run to cover (**response**) This sequence of detection, coordination and action results in a 'fight-or-flight' response that saves the mouse's life ## Coordination of the nervous system and endocrine system - There are **two important coordination systems** in the body: - The **nervous system** - The **endocrine system** - Both systems are involved in the 'fight-or-flight' response - The nervous and endocrine systems work together in a **complementary manner** to coordinate this fast response ### The sympathetic nervous system - The **sympathetic nervous system** is responsible for coordinating many of the responses to danger - Its actions are supported by the effect of two hormones: **adrenaline and cortisol** (both secreted from the adrenal glands) - The **initial part** of the response is controlled by the **nervous system** - The response is **continued** by the **endocrine system** ## Mechanism of the 'fight-or-flight' response 1. **Sensory neurones detect** environmental stimuli associated with danger and send impulses to the brain 2. The **amygdala** (a small region of the brain located in the cerebrum) sends impulses to various other parts of the brain, including the hypothalamus 3. The **hypothalamus** is stimulated to send impulses via the **sympathetic nerves** to the **adrenal glands** 4. This causes the **adrenal medulla** to secrete the hormone **adrenaline** 5. Adrenaline stimulates target organs and tissues to **increase sensory awareness**, making the organism more alert and improving its ability to respond to danger 6. At the same time, the hypothalamus also releases a **peptide hormone** that stimulates the **anterior pituitary gland** to release **ACTH** (adrenocorticotropic hormone) 7. ATCH is transported to the adrenal glands via the bloodstream 8. This causes the **adrenal cortex** to secrete the hormone **cortisol** 9. Cortisol stimulates target organs and tissues to **increase blood pressure and blood glucose**, ensuring the tissues have sufficient glucose and oxygen needed for rapid response 10. Cortisol also **suppresses the immune system**

The nervous system provides the rapid initial response, while the endocrine system maintains and sustains the response

## The effects of adrenaline - The hormone **adrenaline** is secreted from the adrenal glands (and sometimes the medulla oblongata) - It is often during times of **stress or aggression** that this hormone is secreted, which is why it is sometimes referred to as the **'fight-or-flight' hormone** - Adrenaline is transported via the **bloodstream** and it has a **rapid effect** on cells ### Range of effects Adrenaline can have effects on a number of different cell types: | Target | Effect | |--------|--------| | Eyes | Stimulates the muscles in the irises to contract, causing the **pupils to dilate** | | Airways | Increases the diameter of the bronchioles by **relaxing smooth muscle**, increasing airflow to the alveoli | | Blood vessels | Decreases blood flow to the gut and skin via **vasoconstriction**; increases blood flow to the brain and muscles via **vasodilation** | | Blood pressure | **Increases** due to increased resistance from vasoconstriction | | Heart | **Heart rate and stroke volume increase** (stroke volume = volume of blood pumped per beat) | | Liver | Stimulates the **breakdown of glycogen into glucose**, causing the blood glucose concentration to increase |

Adrenaline redirects resources away from non-essential functions (like digestion) towards essential survival functions (like muscle movement and alertness)

### Example scenario Imagine a deer grazing peacefully in a forest. Suddenly, it hears a twig snap nearby - a wolf is approaching: - Adrenaline floods its system - Its pupils dilate to see better in the dim forest light - Its airways open wider to take in more oxygen - Blood rushes from its stomach to its leg muscles - Its heart pounds faster, delivering oxygen-rich blood rapidly - Glucose floods into its bloodstream from liver stores - Within seconds, the deer is ready to sprint at maximum speed ## The second messenger model and adrenaline - When adrenaline is secreted it **increases the concentration of blood glucose** - It does this by binding to different receptors on the surface of liver cells that activate the same enzyme cascade that occurs when glucagon binds to its specific receptors ### The cascade mechanism 1. Adrenaline binds to **specific receptors** on the membrane of liver cells 2. This causes the enzyme **adenylyl cyclase** to change shape and become **activated** 3. Active adenylyl cyclase catalyses the conversion of **ATP to the second messenger, cyclic AMP (cAMP)** 4. cAMP binds to **protein kinase A enzymes**, activating them 5. Active protein kinase A enzymes activate **phosphorylase kinase enzymes** by adding phosphate groups to them 6. Active phosphorylase kinase enzymes activate **glycogen phosphorylase enzymes** 7. Active glycogen phosphorylase enzymes catalyse the **breakdown of glycogen to glucose** - This process is known as **glycogenolysis**

Second messenger: an intracellular signalling molecule that amplifies the signal from a hormone (the first messenger)

### Signal amplification - The enzyme cascade described above **amplifies the original signal** from adrenaline - Results in the **releasing of extra glucose** by the liver to increase the blood glucose concentration back to a normal level - Each adrenaline molecule can stimulate many molecules of cAMP, which in turn activate many enzyme molecules - This creates a **cascade effect** that rapidly mobilises large amounts of glucose ```text 1 adrenaline molecule ↓ Many cAMP molecules ↓ Many protein kinase A molecules ↓ Many phosphorylase kinase molecules ↓ Many glycogen phosphorylase molecules ↓ THOUSANDS of glucose molecules released ``` ### Adrenaline in muscles - Adrenaline also stimulates the **breakdown of glycogen stores in muscle** during exercise - The glucose produced **remains in the muscle cells** where it is needed for respiration - This ensures muscles have an immediate supply of energy for rapid contraction