2.85 - Reflex arc

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

stimulus - the hot flame 

receptor - heat sensor in the skin 

- The impulse travels to the spinal cord along the sensory neurone 

- In the spinal cord the impulse is passed on to the relay neurone 

- This passes the impulse on to the motor neurone 

- The motor neurone carries the impulse to a muscle in the arm. The muscle is the effector. 

- The muscle contracts to remove the and from the hot object. This action is the response. 

There is a total of 3 synapses in this reflex arc: 

- sensory neuron and relay neurone 

- motor neurone and muscle 

2.84 - Receptors and Responses

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

Nerve cells are different from other cells, they do have a cell membrane, cytoplasm and nucleus, but they are a different shape. Part of the cell is stretched out to form the axon. The axon can be over a meter long. 

The messages that nerves carry are called nerve impulses. They are electrical signals. They pass very quickly along the axon of the neurone. 

An impulse travels along the axon which each impulse being separate from the next. They travel along one after another. 

Some axons have a fatty sheath around them. This insulates the axon and makes the impulse travel along faster. 

In multiple sclerosis the fatty sheath breaks down. Impulses slow down or may even stop. People with this disease gradually lose the use of their muscles because the messages never reach them. 

Synapses

The end of one neuron is not connected to the next making a small gap between them. This gap is called a synapse. 

When an impulse reaches the end of an axon, a chemical is produced. The chemical diffuses across the gap which starts off an impulse in the next neurone. 

Only one end of a neurone can make this chemical. So synapses make sure an impulse can only travel in one direction.

Synapses have 2 other functions:  

- A resistor: it may take a number of impulses before enough chemical is made to start the impulse in the next neurone. 

- a junction box: one neurone may pass on its impulse to a number of other neurones 

Our synapses are easily affected by drugs. Some drugs can block them whereas others can make them work too quickly. Alcohol is thought to affect synapses in the brain and this can slow down peoples reactions. 

2.83 - Central nervous system

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


- Your nervous system sends electrical messages along nerves to and from different parts of your body it 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 main parts of the nervous system are the brain and the spinal cord, together they are called the central nervous system and are both made of delicate nervous tissue.
- The brain is protected inside the skull and the spinal cord is protected inside your backbone 


The central nervous system is connected to different parts of the body by nerves, each nerve is made up of lots of neurones (nerve cells) 


Sense organs are our receptors - they send messages to the central nervous system telling it what has happened. These messages are sent along sensory neurones. 


Muscles and glands are our effectors. They central nervous system sends messages telling them what to do. These messages are sent along the motor neurones 

2.82 - Communication

first way 
- Motor nerve (orange part) would be embedded in the spine 
- other end would be connected to the effector - most likely a muscle 
- electrical impulse (nerve impulse) - carried along inside the nerve from the cell body to the synaptic knob 
- travels down the long structure, known as the axon (can be up to a meter long)
- in mammals the axon is surrounded by the schwann cell (contains a big deal of fat) and forms a myelin sheath - effect is to increase the speed of nerve conduction 
- one way of linking the coordinator (receptor) to the effector

second way 
- endocrine system - involves the endocrine gland which produces a hormone (protein or steroid) - adrenal gland 
- hormone is secreted into the blood (travels through the blood) - adrenaline into the blood 
- travels through blood stream to the target tissue (organ) which it will have an effect on 
- hormones can have multiple targets which bring about multiple effects 

comparison between nerves and hormones 
nerve impulses are fast - hormones are relatively slow 

2.77b - Thermoregulation

- negative feedback loop - method of control and maintaining constant conditions 
- in the case of humans our fixed point is our body temperature (37/38 degrees) 
- in order for it to work we have receptors of body temperature - hypothalamus (region of the brain)
- responds to a stimulus (temp of body and blood) 
- fixed point that we regulate around and try to maintain is our body temperature 
- body temperature feeds into the brain and is then compared to the theoretical fixed point (37/38degrees)
- if need to be increased or decreased then it is brought about by the action of the effectors (e.g. skin) 
- response is either a increase or decrease in body temperature 
- this would feedback to the hypothalamus and based on the input, a new output would be produced 

- main components of skin to control the body temperature - sweat glands and capillary network (allows blood to move closer or further away from the surface of the skin

- x axis - time 

- y axis - 37 and 38 degrees celsius (regulation points) 
if body temperature increases - input into hypothalamus, stimulates responses in the skin for cooling - sweating and increased blood flow to the surface of the skin
- blood vessels dilate (widen) and so more blood flows to the surface 
- this increases exchange of heat to outside of body by evaporation of sweat and radiation
- This brings about the cooling of the blood and returns body temperature to the fix level 


- If body temp falls, this feeds to the hypothalamus and switches on regulations to increase the body temperature - shivering, raised hairs 
- Forces blood to travel deeper through tissues and reduces heat exchange with external environment 
- temperature of the blood varies up and down around the fixed point making the efficiency of the system depend on how far they deviate from the fixed point 

2.77a - Thermoregulation

- Homeo - 'same' 
- Stasis - 'fixed point/conditions' 
- Homeostasis - Conditions kept the same/constant 
- when applying homeothermic - it is to do with having the same temperature 
- organisms (e.g. mammals) - when the environmental temperature decreases or increase, then their body temperature remains constant - these are homeothermic organisms - carry out the process - thermoregulation 
- other organisms body temperature, varies with the environmental temperature 


2.9 - recall - rates of reactions 
- maximum rate of reaction is achieved by an optimum temperature for that enzyme 


- both graphs are related as the optimum temperature for the enzyme is approximately the same as the temperature at which the mammals maintain their body conditions 

2.76 - Sensitivity

MRS GREN 
- S - Sensitivity - organisms respond to changes in the environment 
- Changes: light levels, temperature, pressure levels, chemicals 
- to detect changes, the organism needs to have receptors
- to respond to the change, the organism has to have effectors (such as muscles/glands)
- response shows that the organism is able to survive the changes in the environment