The nervous system and The Reflex Arc

• Homeostasis involves automatic control systems. Like this...

  
• Homeostasis involves the nervous system. The nervous system consists of two parts. We have the central nervous system (CNS). Which is the brain and the spinal cord, and then we have other nerves running to and from the central nervous system.
• Receptors detect a stimulus, and send electrical impulses down neurones to the central nervous system. Neurones is another way of saying nerve cells. Like this...

  
• The central nervous system is the coordination centre. This now sends electrical impulses down other neurons to effectors. The effectors bring about a response. Like this...

  
• Effectors are usually a muscle which contracts or a gland which secretes a hormone. Like this...

  
• A key role of the nervous system is that it enables humans to react to their surroundings and co-ordinate their behaviour. One way that this is done is through the reflex arc.
• What happens when you touch a hot object. First the stimulus is detected by a receptor. In this case, the stimulus is heat and the receptor is in the skin. Electrical impulses now pass from the receptor along sensory neurone to the central nervous system. Like this...

  
• Sensory neurones are connected to receptors, and that allows us to sense our environment.
• At the end, of the sensory neurone is a junction called a synapse. At the synapse, a chemical is released. This chemical now diffuses across to a relay neurone in the central nervous system, where it triggers an electrical impulse. Like this...

  
• The electrical impulse now passes across the relay neurone and reaches another synapse. Once again a chemical is released. This chemical triggers an electrical impulse in a motor neurone. Like this...

  
• The electrical impulse now passes down the motor neurone to an effector. In this case the effector is a muscle. Like this...

  
• The muscle now contracts and pulls the hand away from the heat. Pulling the hand away is called the response. The central nervous system acts as the coordinator and in most cases, the brain actually makes decisions about what action to take. However, in this case of reflexes there is no decision making by the conscious part of the brain. The makes reflexes automatic and rapid. Because they are rapid, reflexes help to protect us from danger. Like this...

  
• The nervous system enables humans to react to their surroundings and to coordinate their behaviour.

  • Information is sent through the nervous system as electrical impulses – electrical signals that pass along nerve cells known as neurones.
  • A bundle of neurones is known as a nerve.
• The nervous system is made up of the CNS and the PNS (part 1):

  
• The nervous system is made up of the CNS and the PNS (part 2):

  
• The nervous system is made up of the CNS and the PNS (part 3):

  
• key terms associated with the nervous system topic (part 1):

  • Stimulus: A detectable change in the internal or external environment of an organism e.g. light, sound or temperature.
  • Receptor: A cell or organ which detects a stimulus.
  • Neurone: A nerve cell, specially adapted to carry electrical charges, called nerve impulses from one part of the body to another.
  • Sensory neurone: A neurone which carries nerve impulses from receptors to the central nervous system.
  • Motor neurone: A neurone which carries nerve impulses from the central nervous system to effectors.
• key terms associated with the nervous system topic (part 2):

  • Relay neurone: A neurone that acts as a coordinator, transmitting impulses from the sensory to the motor neurone in the spinal cord.
  • Effector: A cell, tissue, organ or organ system that responds to a stimulus.
  • Voluntary response: A nerve pathway which produces a conscious response to a stimulus.
  • Reflex response: A nerve pathway which produces an automatic response to a stimulus.
  • Synapse: A gap between two neurones.
• key terms associated with the nervous system topic (part 3):

  • Axon: A single long fibre within a neurone which carries a nervous impulse away from the cell body.
  • Electrical impulse: A signal which is passed through the nervous system as electrical charge to instigate a response.
  • Central nervous system (CNS) – The part of the nervous system which includes brain and spinal cord.
  • Peripheral nervous system (PNS) – All of the nerves in the body which extend from the CNS.
• The central nervous system (CNS) consists of the brain and spinal cord.

  • It is responsible for coordinating the response by directing the electrical impulse to the correct effector.
  • Voluntary responses move through the nervous system via the brain.
  • Automatic responses move through the unconscious part of the brain or the spinal cord.
  • The neurones that are found in the CNS are relay neurones.
• Table of senses and stimuli:

  
• Neurones reach all parts of the body to coordinate responses.

  • Each neurone is made up of the following key components:
  • The axon is the long fibre which transmits electrical impulses away from the cell body.
  • Axons are adapted to ensure fast transmission of the electrical impulse: 
  • A fatty layer called the myelin sheath may cover the axon to insulate it and increase the speed of the impulse.
• Some axons are very long (more than a metre in length) this reduces the number of synapses, and speeds up transmission.

  • Branched endings called dendrites, create links between neurones.
  • One neurone can link with many other neurones which increases the response from a stimulus.
• A neurone is a nerve cell:

  
• A synapse is a gap between two neurones found between the terminal end of the axon of one neurone and the membrane of a dendrite of another.

  • Electrical impulses cannot cross synapses, instead chemicals called neurotransmitters (such as dopamine and serotonin) transmit the impulse from one neurone to the next.
  • This is the only part of the nervous system where messages are chemical as opposed to electrical.
  • Neurotransmitters diffuse across the synapse which slows down the transmission of electrical impulses.
  • Synapses ensure that impulses only travel in one direction.
• A synapse is a gap between two neurones:

  
• The electrical impulse travels along the first axon.

  • When an electrical impulse arrives at the terminal end of the axon on the presynaptic neurone, chemical messengers called neurotransmitters are released from vesicles.
  • The neurotransmitters diffuse across the synaptic gap and bind with receptor molecules on the membrane on the dendrite of the second neurone (known as the postsynaptic membrane).
• This stimulates the second neurone to generate an electrical impulse that travels down the second axon.

  • The neurotransmitters are then destroyed or recycled to prevent continued stimulation of the second neurone which would cause repeated impulses to be sent.
• A voluntary response is one where you make a conscious decision to carry out a particular action therefore it starts with your brain.

  • Voluntary responses often take longer than involuntary responses as we consider what the response might be before doing it.
• An involuntary (or reflex) response does not involve the brain as the coordinator of the reaction and you are not aware you have completed it until after you have carried it out.

  • The passage of information in an involuntary response is called a reflex arc.
  • Involuntary actions are usually ones which are rapid and are essential to basic survival.
  • An example of a reflex action might be pulling your hand away from a hot object to prevent harm.
• The reflex arc is an involuntary response so it does not involve the conscious part of the brain as the coordinator of the reaction.

  • Awareness of a response having happened occurs after the response has been carried out.
  • Responses are therefore automatic and rapid - this helps to protect the body from harm.
• The neurones of the reflex arc:

  • There are three main types of neurone in a reflex arc: sensory, relay and motor.
  • Sensory neurones carry impulses from sense organs to the CNS (brain or spinal cord).
  • Relay neurones are found inside the CNS and connect sensory and motor neurones.
  • Motor neurones carry impulses from the CNS to effectors (muscles or glands).
• Here are three different types of neurone in the reflex arc:

  
• Sensory neurones are long and have a cell body branching off the middle of the axon.

  • Relay neurones are short and have a small cell body at one end with many dendrites branching off it.
  • Motor neurones are long and have a large cell body at one end with long dendrites branching off it.
• Reflex actions are automatic and rapid; they do not involve the conscious part of the brain:

  
• Reflex actions are automatic and rapid; they do not involve the conscious part of the brain:

  
• The independent variable is the variable that will be changed.
• The dependent variable is the variable that is being measured.