Nerve Cells and Synapses: Grade 9 Understanding for IGCSE Biology 2.88 2.89
There is very little in the iGCSE specification about nerve cells and synapses. This is a shame since neuroscience is going to be one of the massive growth areas in Biology in the 21st century. There is a syllabus point about reflex acs and I draw your attention to this blog post about that: https://pmgbiology.wordpress.com/2014/04/22/a-simple-reflex-arc/
But in this new post I am going to give you a tiny bit more detail about the types of nerve cells (neurones) that you might encounter, together with an explanation about the most important component of the nervous systems: the chemical synapse.
Neurones are the cells in the nervous system that are adapted to send nerve impulses. You won’t fully understand what the nerve impulse is until year 13 but it is correct so that it is a temporary electrical event that can be transmitted over large distances within a cell with no loss of signal strength. The upshot of this is that neurones can be very long indeed…..
There are three basic types of neurone that are grouped according to their function:
Motor neurones (efferent neurones) take nerve impulses from the CNS to skeletal muscle causing it to contract
Sensory neurones (afferent neurones) take nerve impulses from sensory receptors into the CNS
Relay (or sometimes Inter) neurones are found within the CNS and basically link sensory to motor neurones.
These three types of neurone also have different structures although many features are shared….
This is a diagram of a generalised motor neurone: I know it is a motor neurone since the cell body is at one end of the cell. The cell body contains the nucleus, most of the cytoplasm and many organelles. Structures that carry a nerve impulse towards the cell body are called dendrites (if there are lots of them) and a dendron if there is only one. The axon is the long thin projection of the cell that takes the nerve impulse away from the cell body. The axon will finish with a collection of nerve endings or synapses.
Neurones can only send nerve impulses in one direction. In the diagram above these two cells can only send impulses from left to right as shown. This is due to the nature of the junction between the cells, the synapse (see later on….)
The diagram above shows a sensory neurone. You can tell this because it has receptors at one end collecting sensory information to take to the CNS. The position of the cell body is also different in sensory neurones: in all sensory neurones the cell body is off at right angles to the axon/dendron.
You can see from the diagrams that motor and sensory neurones tend to be surrounded by a myelin sheath. Myelin is a type of lipid that acts as an insulator, speeding up the nerve impulse from around 0.5m/s in unmyelinated neurones to about 100 m/s in the fastest myelinated ones. The myelin sheath is made from a whole load of cells (glial cells) but there are gaps between glial cells called nodes of Ranvier. These will become important in Y12/13 when you study how the impulse manages to travel so fast in a myelinated neurone.
Relay neurones, also known as interneurones, have a much simpler structure. They are only found in the CNS, almost always unmyelinated and have their cell body in the centre of the cell.
The diagram above shows the three types of neurone and indeed how they are linked up in a simple reflex arc. The artist hasn’t really shown the interneurone structure very well, but it was the best I could find just now…..
Nerve cells are linked together (and indeed linked to muscle cells) by structures known as synapses. There are a lot of synapses in your nervous system. The human brain contains around 100 billion neurones and each neurone is linked by synapses to around 1000 other cells: a grand total of 100 trillion synapses. 100 000 000 000 000 is a big number.
The big idea with synapses is that the two neurones do not actually touch. There is a tiny gap called the synaptic cleft between the cells. The nerve impulse does not cross this tiny gap as an electrical event but instead there are chemicals called neurotransmitters that diffuse across the synaptic cleft.
The nerve impulse arrives at the axon terminal of the presynaptic neurone. Inside this swelling are thousands of tiny membrane packets called vesicles, each one packed with a million or so molecules of neurotransmitter. When the impulse arrives at the terminal, a few hundred of these vesicles are stimulated to move towards and then fuse with the cell membrane, releasing the neurotransmitter into the synaptic cleft. The neurotransmitter will diffuse rapidly across the gap and when it reaches the post-synaptic membrane, it binds to specific receptor molecules embedded in the post-synaptic membrane. The binding of the neurotransmitter to the receptor often causes a new nerve impulse to form in the post-synaptic cell.
These chemical synapses are really beautiful things. They ensure the nerve impulse can only cross the synapse in one direction (can you see why?) and also they are infinitely flexible. They can be strengthened and weakened, their effects can be added together and when this is all put together, complex behaviour can emerge. I am going to exhibit some complex behaviour now by choosing to take my dogs for a walk… And it all happened due to synapses in my brain!
Thoughts on #TLAB14
It has been a few days since the TLAB14 conference at Berkhamstead School and I have had time to reflect and think about some thoughts on the day. The first thing to say was that the whole experience was uplifting, inspiring and thought-provoking. It is so easy in our job to get so obsessed with our particular goldfish bowl and spending just a day with a group of great educators from a variety of schools, maintained and private, primary and secondary, was a brilliant way to remind me of what is so great about our job. I would recommend this conference unreservedly to all teaching colleagues. It was one of the best days of CPD I have experienced in two decades of work.
The first keynote speaker was Elise Foster @elisefoster, the co-author of a book called “The Multiplier Effect: Tapping the Genius inside Schools” The basic idea of her presentations was that some leaders act as multipliers, getting twice as much productivity out of employees (and students) by not micromanaging them, giving them true responsibility and holding them accountable. Too often in my experience the school leaders I have worked for are Diminishers, certain that they alone have the right answers and that their role is to use their intellectual skills to come up with more and more of them. Diminishers need to retain control over every part of an organisation and although often they manage in this way with the very best of intentions, their actions result in team members just retreating further and further into their shell. It was difficult to listen to certain sections of the talk, mostly because I could so easily recognise my own behaviours in the classroom when I can act in this way, but also because I recognise all too well the effects of this kind of leadership on my own performance.
I then attended three great workshops. These were small group activities that I had selected from a menu of options when I signed up for the conference. The first was on ICT teaching and led by Drew Buddie @digitalmaverick. He gave a presentation on how his school uses the 4 Rs as a theme to direct all their teaching. These were Resilience, Resourcefulness, Reflectiveness and Reciprocity. This might sound like a dry topic but actually we all enjoyed a whirlwind tour through a load of IT resources and I came away with many new websites to explore and loads to think about. To just give you one example, if you are searching for images on FlickR try taggalaxy.com : it is an amazing site.
I then spent an hour with Sarah-Jayne Blakemore, Professor of Cognitive Neuroscience at UCL (@sjblakemore) She gave me a brilliant presentation on current understanding about the development of the pre-frontal cortex in adolescents. I knew a little about this from a presentation given in Eton a few years ago but the research findings in the past few years do make you question some of our entrenched attitudes towards adolescents. The importance of risk-taking behaviours was stressed together with interesting ideas about peer influence, sociability and the developmental progression of this region of the cortex. Research into this is still in it infancy but already there is plenty to learn. Quite how we as educators should apply this to our practice with adolescents is less clear. Thoughts and comments would be much appreciated if anyone out there has the answer!
My third and final workshop was with Daniel Muijs (@ProfDanielMuijs) who is Professor of Education at the University of Southampton. He gave a witty and entertaining talk on the current state of his research into measuring teacher effectiveness. This reminded me a few important things about my own practice and indeed it was good to be reminded how important the role of the teacher is in life outcomes of our students. I think it is too easy to get fixed with the ideas that pupils from higher socio-economic groups just coast through the education system with never a problem and his workshop was a brilliant way to learn about how teacher effectiveness can be measured (no mention of taking classes into a special classroom elsewhere on the site to film them……..) I loved hearing the idea that the toolkits used in the US to measure effectiveness do not work in the UK because they are not sensitive enough to discriminate between all the high-performing UK educators. A fact to remember when our profession is being bashed in the media for its inefficiencies.
Often on a conference like this, I am tempted to sneak away before the closing address. There was no thought of that at #TLAB14 and I am very glad I didn’t. The final 45 minutes was a truly inspiring talk from Dr Andy Williams, Executive Headteacher at Holmfirth High School and North Huddersfield Trust School. He took over as head of a “failing school” ear-marked for closure with the prospect of a brand new school (NHTS) to replace it. As things worked out the new school was never built, so having closed the failing school, he then had the same students, the same teachers all working in the same building starting again a few weeks later. Now I work in an institution that is old, very old…. This has some great advantages at times but there are many times each week when it is hard not to wish to be able to do just what Andy was forced to confront. I wouldn’t want to change the pupils, the staff or the buildings here but it would be brilliant to be able to start with a brand new identity and construct the school of which we could all be proud. Andy was a great speaker and clearly a “Multiplier” in his management of staff and students. He spoke with passion and clarity about the importance of teaching in changing the lives of young people and I left inspired once again.
If you only are prepared to attend one conference a year about education, I would urge you to go to #TLAB. The day is run by a group of inspirational leaders and there was little you could fault. I will certainly be back in 2o15.