Coordination, Response & Homeostasis

The Mammalian Nervous System

• The human nervous system consists of the central nervous system (CNS) – brain and spinal cord – and the peripheral nervous system (PNS) – all nerves in the body.
• It allows us to sense our surroundings, respond to them, and coordinate body functions.
• Information is sent as nerve impulses – electrical signals that pass along nerve cells called neurones.
• A bundle of neurones is known as a nerve.

Which of the following is NOT part of the central nervous system?

• Brain
• Spinal cord
• Sensory neurone
• None of the above

Types of Neurones

• There are three main types: sensory, relay, and motor neurones.
• Sensory neurones carry impulses from sense organs to the CNS.
• Relay neurones (intermediate) are inside the CNS and connect sensory to motor neurones.
• Motor neurones carry impulses from the CNS to effectors (muscles or glands).

A ____ neurone carries impulses from sense organs to the CNS, while a ____ neurone carries impulses from the CNS to effectors.

The Reflex Arc

• A reflex action is an automatic, rapid, involuntary response to a stimulus.
• It does not involve the brain as the coordinator – the spinal cord acts as the coordinator.
• The reflex arc pathway: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response.
• Reflex actions are protective and help minimise damage.

In a reflex arc, which neurone is found inside the spinal cord connecting sensory and motor neurones?

• Sensory neurone
• Relay neurone
• Motor neurone
• Effector

The Synapse (Extended)

• A synapse is the junction between two neurones where they meet but do not touch.
• The electrical impulse triggers release of chemical messengers called neurotransmitters from vesicles in the presynaptic neurone.
• Neurotransmitters diffuse across the synaptic gap and bind to receptors on the postsynaptic membrane, generating a new electrical impulse.
• Neurotransmitters are then destroyed to prevent repeated stimulation; synapses ensure impulses travel in one direction.

At a synapse, the electrical impulse causes the release of ____ from vesicles. These chemicals diffuse across the ____ and bind to receptors on the ____ membrane.

Sense Organs and the Eye

• Sense organs contain groups of receptor cells that detect specific stimuli.
• The eye is a sense organ containing light-sensitive receptor cells (rods and cones) in the retina.
• Key structures: cornea (refracts light), iris (controls pupil size), lens (focuses light), retina (contains receptors), optic nerve (carries impulses to brain).
• The fovea has the highest density of cone cells for sharp, coloured vision.

Which part of the eye contains light receptor cells?

• Cornea
• Iris
• Lens
• Retina

Hormones and the Endocrine System

• A hormone is a chemical substance produced by a gland and carried by the blood to target organs.
• Hormones alter the activity of specific target organs that have complementary receptors.
• The endocrine system consists of all hormone-producing glands.
• Important hormones: insulin, glucagon, adrenaline.

How are hormones transported around the body?

• Via nerves
• Via the blood
• Via the lymphatic system
• Via diffusion through tissues

Adrenaline – Fight or Flight

• Adrenaline is released from the adrenal glands in response to danger or stress.
• Effects: increased blood glucose concentration, increased heart and breathing rates, diverted blood flow to muscles, dilated pupils.
• These prepare the body for rapid action (fight or flight).

Which of the following are effects of adrenaline?

• Increased blood glucose concentration
• Decreased heart rate
• Dilated pupils
• Constricted airways

Homeostasis and Blood Glucose Control

• Homeostasis is the maintenance of a constant internal environment.
• Blood glucose concentration is controlled by negative feedback involving insulin and glucagon from the pancreas.
• When blood glucose rises, insulin is released – it stimulates liver and muscle cells to convert glucose into glycogen (storage).
• When blood glucose falls, glucagon is released – it stimulates conversion of glycogen back to glucose.

When blood glucose is high, the pancreas secretes ____ which causes the liver to convert glucose into ____. When blood glucose is low, the pancreas secretes ____ which causes the liver to convert ____ into glucose.

Temperature Control (Extended)

• Body temperature is maintained at about 37°C by the skin and brain.
• When hot: sweat glands produce sweat (evaporation cools), skin capillaries dilate (vasodilation) to lose heat, hairs lie flat.
• When cold: sweat production stops, capillaries constrict (vasoconstriction) to reduce heat loss, hairs stand up to trap insulating air, shivering generates heat.

What happens to skin capillaries when the body is too cold?

• They dilate (vasodilation)
• They constrict (vasoconstriction)
• They remain unchanged
• They disappear

Tropisms in Plants

• Plants respond to stimuli by growing towards or away from them – these growth responses are called tropisms.
• Phototropism: growth in response to light. Shoots grow towards light (positive phototropism); roots grow away from light (negative phototropism).
• Gravitropism: growth in response to gravity. Roots grow towards gravity (positive gravitropism); shoots grow away from gravity (negative gravitropism).
• The plant hormone auxin controls these responses by causing cell elongation.

In a shoot, auxin accumulates on the ____ side, causing cells to elongate more and the shoot to bend towards the ____. In a root, higher auxin concentration ____ cell elongation, so the root bends towards ____.