the cells of the nervous system and neurotransmitters at
TRANSCRIPT
The cells of the nervous system and neurotransmitters at
synapsesKey Area 3.4
Learning Intention
• Name and describe the components of a neuron
Cells of the Nervous System
• The nervous system consists of a complex network of nerve cells called neurons which receive and transmit electrical signals (nerve impulses) and glial cells which support and maintain the neurons
Video
Structure of a Neuron
node
Cell Body
• The cell body contains the nucleus and most of the cytoplasm. It is the control centre of cell metabolism
Cell body
Dendrites
• Dendrites are nerve fibres which receive nerve impulses and pass them towards a cell body.
Axon
• An axon is a single nerve fibre that carries nerve impulses away from a cell body.
Axon
The direction in which a nerve impulse travels is always:
dendritescell bodyaxon
DENDRITE
CELL BODY
AXON
interneurone
Learning Intention
• Compare the myelin content of adults and infants
Myelin
• Myelin is the fatty tissue that insulates an axon
• The presence of myelin greatly increases the speed at which impulses can be transmitted
Myelination
• Myelination is the process by which myelin develops round axon fibres
• Myelination is not complete at birth and so over the first two years of life many more neurons are myelinated
• As a result responses to stimuli in the first two years of life are not as rapid as there are slower impulses
Diseases of the myelin sheath
• Certain diseases destroy the myelin
sheath causing a loss of co-ordination.
• Examples of these are poliomyelitis &
Multiple Sclerosis.
• The myelin sheath becomes damaged or
destroyed.
• MS: Episodic attacks, and/or Progressive
loss of coordination over time.
• Polio: Asymmetric weakness or paralysis
of body parts, commonly limbs.
The role of Glial Cells• Glial cells do not transmit nerve impulses.
• Glial cells (some types) are responsible for
myelination.
• They lay down tightly packed layers of
plasma membrane around an axon.
• Other glial cells act as a support for
neurons. E.g. Producing chemicals the
neurons need to function.
Myelination Summary
• Axons are surrounded by a myelin sheath which insulates the axon and increases the speed of impulse conduction.
• Myelination continues from birth to adolescence. As a result responses to stimuli in the first two years of life are not as rapid or coordinated as those of an older child or adult.
• Certain diseases destroy the myelin sheath causing a loss of coordination.
Types of Neuron
Types of Neuron
• Sensory neurons, carry impulses into the Central Nervous System (CNS) from sense organs
• Interneurons, found in the CNS where they connect with other neurons
• Motor neurons, carry impulses out from the CNS to effectors such as muscles and glands
Learning Intention
• Describe the structure of a synapse and movement of neurotransmitter
Synapses
• The tiny gap between an axon ending of one neuron and the dendrite of the next neuron in the pathway is called a synapse
• The plasma membranes of the two neurons are very close but are separated by a space called the synaptic cleft
Video
Synapses
• The nerve cell before the synaptic cleft is called the presynaptic neuron
• The nerve cell after the synaptic cleft is called the postsynaptic neuron
• As well as connecting to other each, neurons can also connect with muscle fibers and endocrine glands
synaptic cleft
Neurotransmitter
• Impulses are relayed across synaptic clefts by chemicals called neurotransmitters.
• There are many neurotransmitters passed on at the synapse. Two examples are:
ACETYLCHOLINE
NORADRENALINE
Neurotransmitter
• Neurotransmitters are stored in vesicles at each presynaptic terminal. When a nerve impulse arrives, the vesicles fuse with the membrane and release neurotransmitter into the cleft
• The neurotransmitters then diffuse across the cleft and bind to receptors on the postsynaptic membrane
Neurotransmitter
• The vesicles containing neurotransmitters occur on one side of the synapse only.
• This means nerve impulses travel in one direction only.
Video 2
Video 1
Learning Intentions
• Explain the need for and methods of removal of neurotransmitters
Removal of Neurotransmitter
• Between impulses the transmitter molecules are rapidly removed from the synaptic cleft to prevent continuous stimulation of post-synaptic neurones
• There are 2 types of removal:
Re-uptake
Enzyme degradation
Re-uptake
• Noradrenaline undergoes reuptake by being reabsorbed directly into the presynaptic membrane that secreted it and is stored in a vesicle ready for use
Enzyme Degradation
• Acetylcholine is broken down by an enzyme into non active products which are then reabsorbed by the presynaptic neuron and resynthesised into active neurotransmitter
Removal and Summation Summary
• Synapses can filter out weak stimuli arising from insufficient secretion of neurotransmitters.
• Summation of a series of weak stimuli can trigger enough neurotransmitter to fire an impulse.
• Neurotransmitters must be removed from the synaptic cleft to prevent continuous stimulation of post synaptic neurones.
• Neurotransmitters are removed by enzymes or re-uptake.
Learning Intention
• State the difference between excitatory and inhibitory signals
Excitatory and Inhibitory Signals
• The type of alteration to a postsynaptic membrane that occurs following the binding of a neurotransmitter depends on the type of receptor present
• The signal generated is determined by the receptor and may be either excitatory or inhibitory
Excitatory and Inhibitory Signals
• Acetylcholine released into the cleft between a motor neuron and a skeletal muscle fibre binds to receptors that have an excitatory effect and make the muscle fibres contract eg peristalsis
• Acetylcholine released into a cleft between a motor neuron and a heart muscle fibre instead binds with receptors which have an inhibitory effect. This reduces the rate and strength of contraction of heart muscle
Neurotransmitter Summary
• Neurotransmitters relay messages from nerve to nerves within and outwith the brain.
• Neurons connect with other neurons, muscles fibres and endocrine glands at a synaptic cleft.
• Neurotransmitters are stored in vesicles and released into the cleft on arrival of an impulse. They diffuse across the cleft and bind to receptors on nerve endings.
• The receptor determines whether the signal is excitatory or inhibitory.
Learning Intention
• Describe the filtering of weak stimuli and summation
Weak Stimuli
• A nerve impulse is only transmitted across a synapse if there is release of enough neurotransmitter
• A critical number of neurotransmitter molecules is needed (threshold) to affect a sufficient number of receptors on the postsynaptic membrane
• This means weak stimuli are filtered out because not enough transmitter molecules reach the next neurone
Summation
• A series of weak stimuli can combine to trigger enough neurotransmitter to fire an impulse in the post-synaptic neuron, a process known as summation.
Learning Intentions
• Describe the role of endorphins
• Describe the role of dopamine and the reward pathway
Endorphins
• Endorphins are neurotransmitters that stimulate neurones involved in reducing the intensity of pain
• Increased levels are also connected with euphoric feelings, appetite modulation and release of sex hormones.
Endorphins
• Endorphin production increases in response to:
severe injury
prolonged and continuous exercise
stress
certain foods
Dopamine and Reward Pathway
• The reward pathway involves neurones which secrete or respond to the neurotransmitter dopamine
• Dopamine induces the feeling of pleasure and reinforces particular behaviours
• The neurons of the reward pathway are located in the mid-brain below the cortex, and link to the areas at the base of the cortex and in the frontal areas of the cortex.
• The reward pathway is activated on engagement of beneficial behaviours, egeating when hungry
Dopamine and Reward Pathway
Endorphins Summary
• Endorphins are neurotransmitters that stimulate neurones involved in reducing the intensity of pain.
• Increased levels are also connected with euphoric feelings, appetite modulation and release of sex hormones.
• Endorphin production increases in response to severe injury, prolonged and continuous exercise, stress and certain foods.
Dopamine Summary
• The reward pathway involves neurones which secrete or respond to the neurotransmitter dopamine.
• The reward pathway is activated on engagement of beneficial behaviours, egeating when hungry.
• Dopamine induces the feeling of pleasure and reinforces particular behaviours.
Learning Intentions
• Discuss the causes, symptoms and treatment of neurotransmitter disorders
• Describe the mode of action of agonist, antagonist and inhibitor drugs
Neurotransmitter Disorders
• Many drugs used to treat neurotransmitter related disorders are similar to neurotransmitters.
• Neurotransmitter disorders include
Alzheimer’s disease
Parkinson’s disease
Schizophrenia
Agonists and Antagonists
• Agonists bind to and stimulate receptors mimicking the neurotransmitter and triggering a normal cellular response
• Antagonists bind to specific receptors blocking the action of the neurotransmitter.
Inhibitors
• Other drugs inhibit the enzymes (e.g. cholinesterase) which degrade neurotransmitters (e.g. acetylcholine)
• Or inhibit re-absorption of the neurotransmitter (e.g. noradrenalin)
Neurotransmitter Disorder Summary
• Many drugs used to treat neurotransmitter related disorders are similar to neurotransmitters.
• Agonists bind to and stimulate receptors mimicking the neurotransmitter. Antagonists bind to specific receptors blocking the action of the neurotransmitter.
• Other drugs inhibit the enzymes which degrade neurotransmitters or inhibit re-uptake.
Learning Intention
• Describe the mode of action of recreational drugs
Recreational Drugs
• Many recreational drugs affect neurotransmission in the reward circuit of the brain.
• They cause changes in neurochemistry leading to:
changes in mood
cognition
perception
behaviour
Recreational Drugs
• Recreational drugs interact with neurotransmitters in different ways, they can:
stimulate the release of neurotransmitters
imitate the action (agonists)
block their binding to receptors (antagonists)
inhibit their re-uptake
inhibit their breakdown by enzymes
Recreational Drugs Summary
• Recretionals drugs can also act as agonists or antagonists.
• They affect neurotransmission at synpases in the brain altering an individual’s mood, cognition, perception and behaviour.
• Many recreational drugs affect neurotransmission in the reward pathway of the brain.
• Drug additction is caused by repeated use of drugs that act as antagonists.
• Drug tolerance is caused by repeated use of drugs that act as agonists
Learning Intention
• Explain drug desensitisation and sensitisation
Drug Addiction
• Drug addiction can be defined as a chronic disease that causes the sufferer to compulsively seek out and use the drug regardless of the consequences
Drug Desensitisation
• Repeated use of a drug that acts as an agonistresults in neurotransmitters (egthose that promote dopamine release) being repeatedly stimulated and increased feelings of wellbeing and euphoria
Drug Desensitisation
• The nervous system compenstates for overstimulation of these receptors by reducing their number and those receptors left become less sensitive to the agonist drug
• A larger dose is needed to stimulate the reduced number of less sensitive receptors in order to gain the original effect – this is called tolerance
Tolerance
Drug Sensitisation
• Repeated use of a drug that acts as an antagonist by blocking neuroreceptorsprevents normal neurotransmitter from acting on them.
Drug Sensitisation
• The nervous system compensates for the reduced stimulation of the receptors by increasing their number and the receptors become more sensitive to the antagonist drug
Addiction
• Sensitisation results in other psychological changes, which transform ordinary sensations of ‘wanting’ into addiction
Remember
• Desensitisation is a decrease in the number and sensitivity of receptors as a result of exposure to drugs that are agonists and leads to drug tolerance.
• Sensitisation is an increase in the number and sensitivity of neurotransmitter receptors as a result of exposure to drugs that are antagonists and leads to addiction