2.85 Reflex

• Reflex is the simplex type of response. It is very quick and is involuntary (automatically happen without you being able to consciously controlling it). 
• Reflex reactions are often for self-protection, such as blinking, sneezing, or pulling your hand away from a hot object. 

• stimulus --> receptor --> sensory neurone --> relay neurone in CNS --> motor neurone --> effector --> response

• The small nerve called relay nerve in the spinal cord directly connects sensory nerve to the motor nerve so the response can take place. 
• They know which relay nerve to target because it is encoded within our DNA.

Example :The reflex arc for touching hot object:

Stimulus (pain sensor responding to heat) --> sensory neuron --> spinal cord --> relay neuron --> motor neuron --> effector (muscles) --> response (pulling your hand away from the heat source)

(credits to Michelle)

2.84 Electrical Impulses

• The messages that nerves carry are called the nerve impulses and they are electrical signals.
• They pass along quickly along the axon of the neuron.
• Some axons have a fatty sheath around them which insulates the axon and allows the impulse to travel faster along the axon.

Diagram showing how it works:

(credits to Daniel)

2.83 Central Nervous System

• The nervous system controls your action and coordinates different parts of the body.
• The central nervous system consist of the brain and the spinal cord where they are both made of delicate nervous tissue .They are both protected where the brain is protected inside the skull and the spinal cord is protected inside your backbone.
• The peripheral nerves connects organs to the central nervous system. Each nerves is made up or nerve cels or neurones. 
• Sense organs are our receptors where they send messages to the central nervous system and are sent along the sensory neurons. 
• When the central nervous system sends messages telling effectors what to do, the message is sent along the motor neuron. 

2.82 Communication

1. Nervous system

• The cell body of the nerve would be embedded to the spine
• On the other end (synaptic knob) of the nerve would be connected to the effector which in this case is the muscle fibers
• The electrical impulse or the nerve impulse is carried inside the walls from the cell body to the synaptic knot where it connects to the muscles through the axon which can be as long as one meter and only one cell wide.
• In mammals, the axon can be surrounded by another kind of cell known as the schwann cell - contains a great deal of fats and formed a myelin sheath, which helps increase the speed of nerve conduction and is one way of connecting the coordinator to the effector.

2. Hormone - Endocrine system

• Endocrine gland produces chemical known as hormone which can be proteins or steroids. 
• Example adrenal gland which produces adrenaline. The hormone is secreted into the blood and travels through the stream and arrive at target tissue/ organ that will have an effect on.
• Hormones can have multiple target and brings about multiple effects.

Difference between Nerves and Hormones:

• Nerves transfer impulses much faster than hormone does
• Nerve impulses re sent through neurons whereas hormones are sent through blood.
• Nerve impulses enables body to response to external environment and the hormones enable body to respond to internal environment

2.77a Thermoregulation

Homeostasis - Conditions are kept at the same or constant throughout

Homeothermic - Temperature are kept the same or constant

Organisms such as mammal keeps their body temperature constant despite changes in the temperature of the environment. These organisms are called homeothermic organisms. 

Mammals always keep their body temperature constant because enzymes have a point of temperature which is called optimum temperature (maximum) . They would try to keep their body temperature as close to the optimum temperature as possible. 

credits to michelle

2.76 Sensitivity

Sensitivity is the characteristic in which organism respond to changes in their environment.


The different types of stimuli are:
- Light                     - Temperature         -Pressure          - Chemical


To detect the changes in the environment, organisms are required to have receptors 
The response is produced by effectors which are muscles or glands. 


It is the response that ensures the organism is able to survive the changes in the environment.