B6 - The Nervous System

THE NERVOUS SYSTEM

Stimulus = A change in the environment

Central Nervous System ( Verterates) = Brain and Spinal Cord only

Peripheral Nervous System (Mammals) = The CNS that is connected to the body by sensory and motor neurones

Sensory Neurones = Carry impulses from receptors to CNS

Motor Neurones = Carry impulses from CNS to effectors

Receptors = Detect stimuli

Effectors = Bring about the change

NEURONES & SYNAPSES

NEURONES & SYNAPSES

Neurones = Nerve cells

Axons = Made from nerve cell cytoplasm stretched out into a long fibre and surrounded by cell membrane

When stimulated neurones transmit info around the body as ELECTRICAL IMPULSES

Some axons are surrounded by a FATTY SHEATH which acts as an ELECTRICAL INSULATOR which shields the neurone from neighbouring cells, which speeds up the electrical impulse

(Electrical impulses carry info around the body quickly so the response can happen fast but it means that they're short-lived)

(Hormones are also used to carry info around the body as they are produced in glands and travel around the body. These responses are more slow but longer lasting)

Synapse = The gap between two neurones

1. When an electrical impulse reaches the end of a neurone it triggers the release of TRANSMITTER CHEMICALS into the synapse

2. The transmitter chemcials DIFUSE across the SYNAPSE and bind to receptor molecules on the membrane of the next neurone

3. Only specific transmitter chemicals can bind to the receptor molecules on the neurone

4. When the chemicals bind to the right receptor they trigger a new electrical impulse in the next neurone

Drugs can interfere with the transmission of impulses across a synapse.

ECSTASY= Blocks sites in the brain's synapses where serotonin ( a transmitter chemcial is removed)

Serotonin affects things like pain, agression , mood and appetite.

So if the serotonin can't be removed, the concentration of it increases which affects a persons mood 

Ecstasy is often described as having mood - enhancing effects becaue of the INCREASE IN SEROTONIN

REFLEXES

Reflexes = Rapid, automatic responses to CERTAIN stimuli

They're INVOLUNTARY

Reflex Arc = The route taken by the information in a reflex (receptor to effector ) 

1. The neurones in reflex arcs go through the spinal cord or through an unconscious part of the brain

2. When a stimulus is detected by the receptors an impulse is sent along a sensory neurone to the CNS

3. The sensory nurone then passes on the message to a new neurone called the relay neurone

4. The relay neurone passes the imulse to a motor neurone

5. The impulse is then passed to the EFFECTOR (muscle etc)

An impulse ALWAY takes the same, direct route through the reflex arc so no information is ever processed which is why reflexes are involuntary and rapid.

Simple animals = Have no brain and rely entirely on simple reflex actions (jellyfish)

Simple reflexes cause these animals to respond in order to help them to survive

FINDING FOOD

SHELTERING FROM PREDATORS

Newborn babies also have reflexes that are lost as they develop:

• They automatically suckle from their mothers
• They'll grasp when their palms are touched
• They'll try to take steps when their feet are put on a flat surface

MODIFYING AND LEARNING REFLEXES

When picking up a hot object, the reflex response is to drop the plate because of the temperature

But dropping the plate might not be a good idea so the reflex response is modified

The response can be overridden by a neurone between the BRAIN and the MOTOR NEURONE of the reflex arc.

Conditioning Reflexes = Learning to produce a response to a secondary stimulus

Pavlovs Dogs:

• Pavlov noticed that the dogs produced saliva every time they smelt food (simple reflex in response to a primary stimulus)
• He experimented by ringing a bell before the dogs were given food
• After a while, the dogs started to produce saliva when the bell was rung
• The dogs responded to the secondary stimulus which means it was a cinditioned reflex 
• The final response had no direct connection to the secondary stimulus

BRAIN DEVELOPMENT

Neurone connections occur through new experiences

Connections form as a child experiences new things - when a neurone is stimulated by the experience it ranches out, connecting cells that were previoulsy unconnected

This means that as we grow , we form a huge network of neurones

When experiences are repeated the pathways that the nerve impulses travel along become strengthened.

Strenghtened pathways are more likely to TRANSMIT IMPULSES than others

It's harder for old people to learn new skills such as learning a new language because it becomes harder for pathways to form 

LEARNING SKILLS AND BEHAVIOUR

The ability to communicate by language depends on a child hearing other people speak.

It is thought that they must hear during a certain critical peiod - if a child hasn't learnt to talk by the age of 20 they probably never will.

Evidence to back this up comes from FERAL CHILDREN 

If the neuron pathways don't form at a young age, it's likely that they never will

THE CEREBAL CORTEX:

• Outer part of the brain
• Intelligence, memory, language and consciousness

1. STUDYING PATIENTS WITH BRAIN DAMAGE

• Can tell scientists about what the damaged part of the brain does
• Can tell scientists about certain areas of the brain

2. ELECTRICALLY STIMULATING THE BRAIN

• The brain can be stimulated electrically by pushing a tiny electrode into the tissue and giving it a small zap of electricity
• By observing what stimulating different parts of the brain does, it's possible to get an idea of what those parts do

3. MRI SCANS

• Produces detailed pictures of the brain structure
• Scientists use it to find out what areas of the brain are active when people are doing certain things

Short Term Memory = Lasts for anything from a few seconds to a few hours. It's used for information you're thinking about at the moment

Long Term Memory = Stored for days, months or even years

Humans are more likely to remember stuff when they see a patters in the information

You're also more likely to remember things if it's associated with a strong stimuli (bright lights, colours, strong smells, noises)

It also helps if the info is repeated over a set period of time