Coordination and response

• • The human nervous system consists of the:
  • central nervous system (CNS) - the brain and the spinal cord
  • peripheral nervous system (PNS) - all of the nerves in the body
  • It allows us to 
  • Make sense of our surroundings and respond to them 
  • Coordinate and regulate body functions
  • Information is sent through the nervous system as nerve impulses - electrical signals that pass along nerve cells known as neurones
  • A bundle of neurones is known as a nerve

  
• • There are three main types of neurone: 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)
• • Neurones have a long fibre (axon) 
  • This means that less time is wasted transferring the impulse from one cell to another
  • The axon is insulated by a fatty sheath with small uninsulated sections along it (called nodes)
  • This means that the electrical impulse does not travel down the whole axon, but jumps from one node to the next
  • Their cell body contains many extensions called dendrites
  • This means they can connect to many other neurones and receive impulses from them, forming a network for easy communication
• • 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

  
• The Reflex ArcVoluntary Responses

  • A voluntary response is one where you make a conscious decisionto carry out a particular action therefore it starts with your brain
  • An example is reaching out to pick up a cup of coffee
  • 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
  • Involuntary actions are usually ones which are essential to basic survival and are rapid, whereas voluntary responses often take longer as we consider what the consequences might be before doing it
• Reflex Responses

  • 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
  • This is an automatic and rapid response to a stimulus such as touching something sharp or hot
  • As it does not involve the brain, a reflex response is quicker than any other type of nervous response
  • This helps to minimise the damage to the body
• 1. The pin (the stimulus) is detected by a pain/pressure/touch receptor in the skin
  2. Sensory neurone sends electrical impulses to the spinal cord (the coordinator)
  3. Electrical impulse is passed on to relay neurone in the spinal cord 
  4. Relay neurone connects to motor neurone and passes the impulse on
  5. Motor neurone carries impulse to a muscle in the leg (the effector)
  6. The muscle will contract and pull the foot up and away from the sharp object (the response) 

  
• The reflex pathway

  
• Reflex action - a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors(muscles and glands)
• Synapses

  • Where two neurons meet or join, they do so at a junction called a synapse
  • Synapses allow junctions between neurones so are important in the nervous system being a connected network of neurones
  • Nerve impulses can transmit across synapses and be directed along the appropriate route by them eg. to the correct part of the brain
  • Think about the analogy of railway points that guide the trains onto the appropriate tracks based on that train's destination. 
• • The junction between two neurones is known as a synapse

  
• Structure of a synapse

  • vesicles containing neurotransmitter molecules
  • the synaptic gap
  • receptor proteins
• Synapses & Neurotransmitters:

  • Neurones never touch each other
  • The junctions (gaps) in between them are called synapses
  • Synapses ensure that impulses only travel in one direction, avoiding confusion within the nervous system if impulses were travelling in both directions
  • As this is the only part of the nervous system where messages are chemical as opposed to electrical, it is the only place where drugs can act to affect the nervous system - eg. this is where heroin works
• Electrical impulse

  1. Travels along the first axon

  2. Stimulates the second neurone to generate an electrical impulse that travels down the second axon
• Release of neurotransmitters

  1. Triggers the nerve-ending of the presynaptic neurone to release chemical messengers called neurotransmitters from vesicles which fuse with the presynaptic membrane

  2. Diffuse across the synaptic gap (or cleft) and bind with receptor molecules on the membrane of the second neurone (known as the postsynaptic membrane)
• Destruction of neurotransmitters


  To prevent continued stimulation of the second neurone which would cause repeated impulses to be sent
• Events at a synapse

  
• Sense Organs as Receptors

  • Receptors are groups of specialised cells
  • They detect a change in the environment and stimulate electrical impulses in response
  • Sense organs contain groups of receptors that respond to specific stimuli: light, sound, touch, temperature and chemicals

  
• Sense organs

  • Once the receptor cell in the sense organ has been stimulated, it generates an electrical impulse
  • This is passed on to a sensory neurone which carries the impulse to the central nervous system
  • Here a response will be decided on and the impulse will be passed to a motor neurone (via a relay neurone)
  • The motor neurone carries the impulse to the effector (muscle or gland)
  • The effector carries out the response
• Structure of the Eye

  • The eye is a sense organ containing receptor cells that are sensitive to light (rod cells) and colour (cone cells)

  
•  Function of the Eye:

  • cornea - refracts light
  • iris - controls how much light enters the pupil
  • lens - focuses light on retina
  • retina - contains light receptors, some sensitive to light of different colours
  • optic nerve - carries impulses to the brain

  
• The Blind Spot

  • At the point where the optic nerve joins the retina, there are no light-sensitive rod and cone cells on that part of the retina
  • Light falling onto that part of the retina will not result in an image being detected
  • the brain 'fills in' from surrounding light so we don't see a black hole where no light has fallen
  • This causes a blind spot, where we cannot detect an object in our peripheral vision even if it is there
• The Pupil Reflex

  • This is a reflex action carried out to protect the retina from damage
  • In dim light the pupil dilates (widens) in order to allow as much light into the eye as possible
  • In bright light the pupil constricts (narrows) in order to prevent too much light entering the eye and damaging the retina

  • In dim light, the pupil dilates (becomes larger) to allow more light to enter the eye to improve vision. 
  • In bright light, the pupil constricts (gets smaller) to allow less light to enter the eye to protect the retina from damage.

  
• • The pupil reflex is an example of a pair of antagonistic muscle groups acting together
  • They work together to regulate the amount of light entering the eye
  • The muscles that work antagonistically are the radial muscles and the circular muscles of the eye
  • When one set of muscles contracts, the other relaxes

  
• Accommodation: The function of the eye in focusing on near and distant objects

  • The way the lens brings about fine focusing is called accommodation
  • The lens is elastic and its shape can be changed when the suspensory ligaments attached to it become tight or loose
  • The changes are brought about by the contraction or relaxation of the ciliary muscles
• • When an object is close up:
  • The ciliary muscles contract (the ring of muscle decreases in diameter)
  • This causes the suspensory ligaments to loosen
  • This stops the suspensory ligaments from pulling on the lens, which allows the lens to become fatter
  • Light is refracted more

  
• • When an object is far away:
  • The ciliary muscles relax (the ring of muscle increases in diameter)
  • This causes the suspensory ligaments to tighten
  • The suspensory ligaments pull on the lens, causing it to become thinner
  • Light is refracted less

  
• • There are two types of receptor cells in the retina:
  • Rods, which are sensitive to dim light 
  • Cones, which distinguish between different colours in bright light
  • There are 3 types of cone cells which are sensitive to different colours of light (red, blue and green)
  • The fovea is an area on the retina where almost all of the cone cellsare found
  • Rod cells are found all over the retina, other than the area where the optic nerve attaches to the retina - there are no light-sensitive cells at all in this area, and so it is known as the blind spot
• Night vision is black and white because the low light intensity isn't enough to stimulate cone cells, so only rod cells are stimulated
• Cone cells can detect colour
• If you look directly at a dim star at night, it disappears, but reappears if you look slightly to one side of it
• Never look directly at the Sun
• Looking directly at a dim star

  Light falls on the fovea, which has more cones so fewer rods
• Looking slightly off to the side

  Light falls away from the fovea, onto more rod cells
• The star reappears in your peripheral (edge) vision
• What is a Hormone?

  • A hormone is a chemical substance produced by a gland and carried by the blood
  • The hormone alters the activity of one or more specific target organs i.e. they are chemicals which transmit information from one part of the organism to another and bring about a change
  • The glands that produce hormones in animals are known collectively as the endocrine system

  
• Transport around the body

  • Endocrine glands have a good blood supply as when they make hormones they need to get them into the bloodstream (specifically the blood plasma) as soon as possible so they can travel around the body to the target organs to bring about the response
  • Hormones only affect cells with target receptors that the hormone can bind to. These are either found on the cell membrane, or inside cells. Receptors have to be complementary to hormones for there to be an effect. 
  • The liver regulates levels of hormones in the blood; transforming or breaking down any that are in excess. 

  
• Important hormones in the human body:

  
• Comparison of Nervous & Hormonal Control

  
• Glucagon

  • Blood glucose levels are controlled by a negative feedback mechanism involving the production of two hormones - insulin and glucagon
  • Both hormones which control blood glucose concentration are made in the pancreas
  • Insulin is produced when blood glucose rises and stimulates liver and muscle cells to convert excess glucose into glycogen to be stored
  • Glucagon is produced when blood glucose falls and stimulates liver and muscle cells to convert stored glycogen into glucose to be released into the blood