15.1. Synapses and Muscles Flashcards
Synapse
a point at which two neurones meet but do not touch
- the synapse is made up of the end of the presynaptic
neurone, the synaptic clef and the end of the postsynaptic neurone
Synaptic Cleft
a very small gap between two neurones at a synapse
Action potentials at a Synapse
- known as cholinergic synapses.
1) When an action potential arrives at the presynaptic neurone it causes voltage-gated calcium ion channels to open
2) Calcium ions rapidly diffuse into the cytoplasm of the neurone, down the concentration gradient
3) The calcium ions cause vesicles containing neurotransmitter to move towards the presynaptic membrane, fuse with it and release its contents into the cleft
4) Neurotransmitter diffuse across the cleft and binds to complementary receptor cell proteins in the postsynaptic neurone
5) This causes sodium ions channels to open, so sodium ions flood into the cytoplasm of the neurone, depolarising the membrane of the postsynaptic neurone
6) This depolarisation sets up an action potential in the postsynaptic neurone
7) (sometimes) As neurotransmitter are not need anymore, enzymes break them down and there is neurotransmitter active reuptake back into the presynaptic neurone
Names of Neurotransmitter
- noradrenaline
- acetylcholine (ACh) - acetylcholinase breaks it down into choline and acetate and choline diffuses back in synapse where is combines with acetyl coenzyme A to form acetylcholine again
- both found found throughout the nervous system
Functions of Synapses in the Body
1) ensure one-way transmission.
2) allow integration of impulses.
3) allow the interconnection of nerve pathways
4) involved in memory and learning.
How synapses ensure one-way transmission
- This is because neurotransmitter is released on one side and its receptors are on the other.
- There is no way that chemical transmission can occur in the opposite direction.
How synapses allow integration of impulses
- One neurone may have synapses with many other neurones. This allows interconnection of nerve pathways from different parts of the body
- Each sensory neurone has many branches at the end of its axon that form synapses with many relay (intermediate) neurones.
- The cell body of each motor neurone is covered with the terminations of many relay neurones.
- Motor neurones only transmit impulses if the net effect
of the relay neurones is above the threshold at which it
initiates action potentials.
How synapses allow the interconnection of nerve pathways
- Synapses allow a wider range of behaviour than could be generated in a nervous system in which neurones were directly ‘wired up’ to each other.
- They do this by allowing the interconnection of many nerve pathways.
- individual sensory and relay neurones have axons that branch to form synapses with many different neurones
- this means that information from one neurone can spread out throughout the body to reach many
How synapses are involved in memory and learning.
if your brain frequently receives information about two things at the same time, say the sound of a particular voice and the sight of a particular face, then it is thought that new synapses form in your brain that link the neurones involved in the passing of information along the particular pathways from your ears and eyes
Striated Muscle
This type of muscle tissue makes up the many muscles in the body that are attached to the skeleton.
Striated Muscle Structure
- made up of thousands of “cells” called muscle fibres
- not normal cells, because each one contains several nuclei
- cells are supposedly called syncytium
- each muscle fibre is made up of many parallel myofibrils which contain several proteins like actin and myosin
- Have stripes which are produced by a very regular arrangement of many myofibrils in the sarcoplasm.
- The muscle fibres also contain many mitochondria, which supply ATP for muscle contraction
- The endoplasmic reticulum (sarcoplasmic reticulum) of the muscle fibre is very specialised, and forms T-tubules that run from the cell surface membrane right into the centre of the fibre.
- Cell surface membrane is called sarcolemma
Sarcomere Structure
- a length of myofibril (muscle fibre)
- contains collections of actin and myosin filaments
- actin molecules form thin filaments
- bundles of myosin molecules form thick filaments
- filaments are arranged in a very specific pattern to form sarcomeres
- has different lines - z and m
- has different bands - a, h and i
Sarcomere Lines
- Z Lines = vertical lines at edge of sarcomere
- M Lines = vertical imaginary lines in the middle down the myosin
Sarcomere Bands
HIA
- A Band = horizontal line where there is Myosin in total
- H Band = horizontal line where there is only Myosin
- I Band = horizontal line where there is only Actin
Muscle Contraction
- causes movement
- sarcomeres in each myofibril get shorter as the Z discs are pulled closer together.
- energy for the movement comes from ATP molecules that are attached to the myosin heads.
- each myosin head is an ATPase.
- when a muscle contracts, calcium ions are released from stores in the SR and bind to troponin.
- this stimulates troponin molecules to change shape
- the troponin and tropomyosin proteins move to a different position on the thin filaments, so exposing parts of the actin molecules which act as binding sites for myosin
- the myosin heads bind with these sites, forming cross-bridges between the two types of filament.
- the myosin heads tilt, pulling the actin filaments along towards the centre of the sarcomere.
- The heads then hydrolyse ATP molecules, which provide enough energy to force the heads to let go of the actin.
- The heads tip back to their previous positions and bind again to the exposed sites on the actin.
- The thin filaments have moved as a result of the previous power stroke, so myosin heads now bind to actin further along the thin filaments closer to the Z disc.
- This goes on and on, so long as the troponin and tropomyosin molecules are not blocking the binding sites, and so long as the muscle has a supply of ATP.
