2.84 + 2.85 nerve impulses --> responses

Understand that stimulation of receptors in the sense organs sends electrical impulses along nerves into and out of the central nervous system, resulting in rapid responses
                                                                     +Describe the structure and functioning of a simple reflex arc illustrated by the withdrawal of a finger from a hot object

1. Reflex actions

when a receptor is stimulated by a change, it sends an electrical impulse along the sensory nerves to the CNS, where the brain co-ordinates the response (decision making). Sometimes a very rapid response is need = reflex actions.

Reflex actions are rapid and happen without us thinking.

eg: pulling your hand away from a hot surface. (you do it without thinking about it)

• pain receptor detects a stimulus (heat)  
• sensory nerve sends the electrical impulse to the CNS, brain
• the motor nerve sends the signal to the effector 
• the effector (skin) produces a response (reflex +muscle contracts)
  
  

CNS Central Nervous System

recall that the central nervous system consists of the brain and spinal cord and is linked to sense organs by nerves

1. The nervous system allows to the body to respond to changes in the environment (stimuli)
2. This consists of: the brain, the spinal cord and all of the nerves that branch off from the spinal cord.   

3. The CNS refers to only the brain + spinal cord. 
4. Impulses from receptors pass along the sensory nerves until they reach the brain + spinal cord.
5. 2 ways in which the CNS handles these changes

• a response co-ordinated by the brain (slower way of responding)
• reflex actions (these are extra-rapid responses to stimuli that by-pass the brain)
  
  

2.82 Communication

Describe how responses can be controlled by nervous or by hormonal communication and understand the differences between the two systems.

1. Motor Nerve (neurone) (up to a meter long) Links our co-donator/receptor to our effector.

• has tiny branches at each end
• and a long fibre to carry the electrical implues/signals from the cell body to the Synaptic Knob (connected to the muscle) 
• in mammals, the axon is surrounded by a Schwann cell - which contain fat to increase the speed of nerve conduction. 

2. Endocrine System 

Endocrine Gland produces hormones 

• hormones can be Proteins or Steroids 
• hormones are secreted into the blood 
• they arrive at the "Target tissue/organ" 
• Hormones may have multiple targets and there for multiple effects. 

Nervous System, continued.

The nervous system is coordinated by the brain and allows the body to respond to changes in the environment. We take our responses in very quick "reflex" actions  in response to stimuli. (changes in the environment)

Receptors 

• are groups of specialised cells that detect stimuli, and turn them into "electrical" impluses. 
• they are located in the sense organs, (ear, skin, nose and eye)
• each are sensitive to particular kinds of stimulus. 

Skin: receptors sensitive to: touch, pain, pressure and temperature. 
Tongue: receptors sensitive to: chemicals in food.
Nose: receptors sensitive to: chemicals in the air. 
Eyes: receptors sensitive to: light 
Ear: receptors sensitive to: sound

Process:  A receptor is stimulated by a change --> a signal (electrical impulses)  is sent along the sensory nerves to the CNS (brain+spinal cord) --> the brain then co-ordinates (decision) --> impulses are then sent along the motor nerves to the effector where the response is produced.

Eg of Effectors 

• a muscle contracting to move the arm
• a gland releasing a hormone into the blood
• a muscle squeezing saliva from the salivary gland
• a muscle squeezing sweat from the sweat gland

2.77b Thermoregulation

Understand that homeostasis is the maintenance of a constant internal environment and that body water content and body temperature are both examples of homeostasis.

receptors, transform external energy into internal electrical impulses. 

1. Controlling body temperature:

Human enzymes usually work best at 37-38 degres.

2. Extremes of body temp are dangerous: 
 - high temp can cause dehydration, heat stroke and death if untreated 
- low temp can cause hypothermia and death if untreated


3. Body's temp is monitored by the brain, when your too cold or too hot the brain sends nerve impulses to the skin, which has 3 ways in increasing or decreasing the heat los from the body's surfaces.

a) Hair standing up on the skin traps more warm air (reduce heat loss) and less if they are lying flat (increase heat loss). Tiny muscles is what controls the hairs to do so. The capillary network which allows blood to move further away from the skin.

b) When the body is too hot, glands under the skin secret sweat onto the surface of the skin to increase heat loss by evaporation. When body is back to normal temperature, the skin stops secreting sweat. And the capillary network allows blood to move closer to the suface of the skin.

c) Blood vessels supplying blood to the skin can swell or dilate.

On a hot day: the blood vessels dilate (widen) it is called vasodilation and hairs are flat. This means more heat is carried by the blood to the skin, where it can be lost to the air. (sweat)

On a cold day: the blood vessels shrink down (swell), this is called vasoconstriction and hairs are raised. This reduces heat loss through the skin ounce the body temp is back to normal again.

2.77a Thermoregulation

Understand that homeostasis is the maintenance of a constant internal environment and that body water content and body temperature are both examples of homeostasis.

1. Homeostasis - Homeo = 'same' stasis = conditions

2. Homeothermic - Homeo = 'same' thermic = temp

3. Homeothermic eg: Mammels 
When environmental temp increases, their body temp stays constant.

4. Homeostasis

2.76 Sensitivity

Understand that organisms are able to respond to changes in their environment 

1. Characteristics of life = MRS GREN

S = Sensitivity (how you respond to changes in the environment).

2. Types of changes: Light levels, Temp levels and Chemical Levels.

3. To detect changes, organisms have receptors. 
4. To respond to changes, organisms have effectors.
5. Response = the organism is able to survive  the changes in the environment.