2.85 Reflex Arc

• Reflex arcs are fast responses to protect the body 
• Does not involve the brain as this increases the reaction time 
• Involuntary action

EG. 

Stimulus --> Hot object 

Receptor --> Heat sensor in the skin 

 Impulse travels to the spinal chord along the sensory neuron 

Impulse traveled from the spinal chord to the relay neuron 

Then to the motor neuron which carries the impulse to the muscle, making it move in response 

2.84 Electrical impulses

Stimulation of receptors in the sense organs send electrical impulses along nerves into and out of the central nervous system
The messages the nerves send are through electrical impulses, they pass along the axon of the neuron

2.83 Central nervous system

• Controls our actions and different parts of our body 
• Consist of he brain and spinal chord, they are linked to the sense and effector organs by nerves or neurons 
• Protected by the bones as it is important for the bodies functions, the brain is inside the skull, protecting it, the spinal chord is protected by the backbone 
• Motor neurons carry instructions from the CNS to the effector organs.

2.82 Communication

Nervous or hormonal systems can control responses

The Nervous System 

• Central nervous system -->Consists of the brain, spinal chord, linked by nerves 
• Uses information from the sensory organs to coordinate reflexes and other actions
• Stimulus --> Receptor --> coordinator --> effector --> response 
• An individual nerve cell is a neuron, it responds to a stimuli and can conduct electrical impulses. A bunch of neurons is called a nerve 
• Peripheral nervous system --> consists of nerves and nerve networks throughout the body 
• The nerve system is actually made up of bundles of axons. The axon is a thread like structure that conducts the electrical impulses and passes them on to other cells or the muscle fibres.
• In a motor neuron the connection between the axon and the muscle fiber is called the nerve-muscle-junction  
• One way of connecting the coordinator to the effector is through Schwann Cells. These contain fat that cover the axon, the fat forms the myelin sheath. The Myelin sheath also increases the speed of nerve conduction 

The Endocrine System 

• Involves the endocrine gland which produces hormones. EG. adrenal gland producing adrenaline and insulin. 
• The hormones are secreted into the blood and affect the organs with the correct receptors, sometimes known as the target tissues 

The difference between the two 

1. Nerve system travel along neurons 
2. Chemical messages travel in the blood 
3. Hormonal system sends messages slower, Chemical is very fast 
4. Chemical take longer to have effect
5. Sometimes, chemical effects are widespread throughout the body, unlike nerves 
6. Chemical effects last long 

External stimuli

External stimuli -Light, chemical, pressure -->  receptors (then changed to electrical impulse) --> to the coordinator then to the  --> effector as impulse

Coordination

Stimuli - Chemical, light, pressure, temperature
Chemical - taste and smell
light - Eyes - changes in light
Pressure - Ears -> equalizing the pressure --> touching
Sound - ears

2.77b Thermoregulation

1. Negative feedback loop, a method of control and constant conditions
2. this is achieved at 37 Degrees C
3. Receptor --> hypothalamus <-- responds to the stimulus
4. Body temp feats into the brain, then is compared to optimum figures, then goes into the effector
where the temperature can be altered
5. This feeds back to the hypothalamus on the basis of the input and new output will be produced

Skin - what controls the body temp
1. Sweat glands
2. capillary network - blood to move closer or further away from the skin
3. If body temperature increases the input hypothalamus stimulates responses in the skin that bring around cooling, e.g. Sweating or increase in blood flow to the surface and the blood vessels dilate (whiten) increasing the exchange of heat to the outside of the body with sweat and radiation, cooling the blood, returning to body temp.

4. When cold, the opposite happens, the blood moves further away from the skin as it wants to keep all the heat it can. Countered by shivering, Vasoconstriction and hair raised.

2.77a Thermoregulation

Homeostasis -
Homeo - 'same'
Stasis - 'fixed point'
1. the condition are kept the same/constant
2. Thermoregulation - keeping the same temperature - 37 degrees for humans
3. Homeothermic organisms (mammals) when env temp changes
their body temperature remains constant
4. Mammals carry out a process called thermoregulation
5. Mammals want to maintain a constant temp because it is optimum for rate of reaction (enzyme reaction)

2.76 Sensitivity

--> MRS GREN
S = sensitivity - the part where organisms respond to changes in the environment
Stimuli =
--> Light
--> pressure
--> Temperature
--> Chemical
- in order to be able to detect the changes in the environment, organisms have receptors
--> the sensory nerve
- in order to respond to the changes, organisms have effectors, like muscles and glands

- response ensures that the organism is able to survive the changes of the environment