2.83 - 2.85

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

The nervous system controls your actions. It coordinates different parts of your body so that they work together and are able to bring about the correct responses.
The nervous system consists of the brain and the spinal cord - both are made up of delicate nervous tissue.
The central nervous system is connected by different parts of the body by nerves which are made up of lots of neurons.
Sense organs are our receptors and the instructions are sent along sensory neurons to the central nervous system.

2.84 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

The messages that nerves carry are called nerve impulses - they are electrical signals. They pass very quickly along the axon of the neuron. Some of the axons have a fatty sheath around them that insulates the axon and makes the impulse travel along faster. The motor neuron carries signals from the central nervous system to the muscles and this controls the response. Sensory neurons carries signals into the central nervous system.
The electrical nerve impulses are very fast enabling a fast response and therefore known as 'rapid responses'

2.85 describe the structure and functioning of a simple reflex arc illustrated by the withdrawal of a finger from a hot object

A reflex is a type of response that are there to protect you where it happens very quickly without thinking. You do this automatically.

The flowchart of co-ordination in a reflex system is:

stimulus (hot flame) ---> receptor (in this case, fingertip - it has a pain sensor) ---> sensory neuron ---> coordinator (Central Nervous System) ---> Motor neuron ---> effector (muscles) ---> response

The stimulus is the hot flame, the receptor is the heat sensor in the skin. The impulse travels to the spinal cord along the sensory neuron and in the spinal cord, the impulse is passed on to the relay neuron. From this, the impulse then goes to the motor neuron and then into a muscles in the arm. The muscle is the effector and it contracts to remove the hand from the hot object. This action is known as the response.

2.82 Communication

1st Diagram
- example is a motor nerve. The orange part would be embedded in the spine and at the other end, it would connected to the effector (most likely a muscle)
- the electrical impulse (or the nerve impulse) is carried a long inside the nerve down the orange structure from the cell body to the end known as the synoptic knob where it connects to the muscle.
- this can be up to a metre long. A single cell is shown in orange.
- the long structure is known as the axon.
- in mammals, the axon is surrounded by the Schwann cell - contains a great deal of fat and form a Myelin Sheath.
- this is one way of linking our co-ordinator to our effector

2nd Diagram
- the endocrine system
- involves the endocrine gland that produces a chemical known as a hormone (they can be proteins or steroids). example of this is Adrenal gland
- the hormone is secreted into the blood. Adrenaline is secreted by the Adrenal gland into the blood and travels to the target tissue (or target organ) and then it will have an effect.
- hormones can have multiple targets and bring about multiple effects contrasting with the nerve system.

- a comparison between communication based on nerves and communication based on hormones needs to be drawn
e.g. the nerve impulses are fast
hormones are relatively slow

2.77b Thermoregulation

- Negative feedback loop = method of control or maintaining constant conditions. (in humans, this is the idea of our fixed point, our constant body temperature = 37 - 38 degrees)
- In order for it to work we have receptors of body temperature (internal conditions) = Hypothalamus - region of the brain
- Hypothalamus responds to a stimulus = the temperature of the body and the temperature of the blood.
- Body temperature feeds into the brain --> it is compared to the theoretical level of control. If the temperature needs to be altered, it is brought about through the action of the effector such as the skin.
- The response would either be an increase or decrease in the body temperature.
- This feedback to the hypothalamus on the basis of the input, a new output would be produced.

- One of the major components of the skin for the control of body temperature are:
+ sweat glands
+ Capillary network - it allows blood to move closer or further away from the surface of the skin.
- In the graph, x axis is time and the y axis is the 37/38 regulation point.

In a hot environment:
- If body temp. increases, the input to the hypothalamus stimulates responses in the skin which bring about cooling
- One of the responses is sweating.
- Another is when the blood flow to the surface of the skin increases. The blood vessels dilate and widen. This increases the exchange of heat to the outside of the body by processes such as the evaporation of sweat and radiation - vasodilation
- Hairs flat
- This brings the cooling of the blood and brings the body temp back to the fixed level.

In a cold environment:
- Our body temperature will fall. This feeds to the hypothalamus and switches on and brings about regulation to increase the body temperature such as shivering, raised hairs and vasoconstriction.

2.77a Thermoregulation

- Homeostasis = Homeo (refers to the idea that something being the same) stasis (refers to a fixed point or a set of conditions) --> The idea that conditions are kept the same or constant.
- Homeothermic = maintaining the same temperature.
- Some organisms (e.g. mammals) when the environmental temperature either increases or decreases, then their body temperature remains constant.
- These are Homeothermic organisms that carry out the process of Thermoregulation (an example of Homeostasis)
- Other organisms body temperature vary with the environmental temperature.
- Mammals maintain a constant body temperature because:
+ Enzymes work best at the optimum temperature.
+ The optimum temperature for the enzyme reaction is the approx. the same as the temperature that which the mammals maintain their body conditions.

2.76 Sensitivity

- Characteristics of life:
M.R.S.G.R.E.N.
- S = Sensitivity. This is the characteristic in which organisms respond to changes in the environment.
- Type of change in the environment:
+ changes in light levels
+ Temperature
+ Pressure levels
+ Chemicals
- To be able to detect the changes in the environment, organisms need to have receptors. In order to respond to the changes in the environment, organisms need to have effectors (eg. muscles and glands)
- The response that ensures that the organisms is able to survive the changes in the environment.