Neurons and neural pathways

The cells of the nervous system. Structure and function of neurons to include dendrites, cell body and axons. Sensory, motor and inter neurons. Structure and function of myelin sheath in increasing the speed of impulse conduction. Axons are surrounded by a myelin sheath which insulates the axon and increases the speed of impulse conduction from node to node. 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. Neurons (nerve cells)   Each nerve cell has a cell body, containing the nucleus and organelles, and extensions called fibres (axons and dendrites) Axons carry impulses away from the cell body Dendrites carry impulses towards the cell body cell body dendrites axon Direction of nerve impulse

Sensory, motor and inter neurons. Types of neurons Name Function Structure Sensory neuron Transmits nerve impulses from sense receptors to the central nervous system Motor neuron Transmits nerve impulses from the central nervous system to effectors like muscles and glands Inter neuron Found inside the central nervous system. Transmits nerve impulses between sensory and motor neurons.

Layer of fatty material around the axon that insulates it. Structure and function of myelin sheath in increasing the speed of impulse conduction. Axons are surrounded by a myelin sheath which insulates the axon and increases the speed of impulse conduction from node to node 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. Myelin sheath Layer of fatty material around the axon that insulates it. There are gaps in the myelin sheath which are called nodes. The presence of myelin sheath increases the speed at which nerve impulses pass from node to node along the axon. Myelination This is the development of a myelin sheath round the axon. Myelination is not complete at birth but continues until adolescence. This is the reason a toddler is not as co-ordinated and does not respond as rapidly to stimuli as an older child or adult. Effect of diseases In some diseases, e.g. multiple sclerosis (MS), the myelin sheath becomes damaged resulting in loss of co-ordination.

Glial cells Glial cells Physically support neurons and produce the myelin sheath Glial cells also maintain a homeostatic environment around the neurones and remove debris by phagocytosis Glial cells There are several types of glial cell. They do not transmit nerve impulses but provide neurons with physical support. Some types of glial cell cause myelination of axons by surrounding them with tightly packed layers of plasma membrane Nucleus of glial cell Layers of membrane axon Other types of glial cells provide neurons with essential chemicals and keep the chemical composition of the fluid around the neurons constant (i.e. maintain a homeostatic environment around the neurons).

Neurotransmitters at synapses. Chemical transmission at the synapse by neurotransmitters to include vesicles, synaptic cleft and receptors. 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. Synapse   Synapse is a point where two nerve fibres meet / junction between two neurons Fibres do not touch at a synapse, there is a gap called a synaptic cleft between them Chemicals called neurotransmitters are released by the first neurone when a nerve impulse arrives, diffuse across the synaptic cleft and have an effect on the second neurone Two neurotransmitters are acetylcholine and noradrenaline Neurotransmitters bind to receptors on the membrane of the post-synaptic neurone Depending on the receptors, binding of the neurotransmitter can be excitatory or inhibitory If not enough neurotransmitter molecules are released, the impulse is not transmitted across the synapse Production and removal of neurotransmitter requires a lot of energy so pre-synaptic neurones contain a lot of mitochondria

Removal of neurotransmitter The need for removal of neurotransmitters by enzymes or reuptake. Neurotransmitters must be removed from the synaptic cleft to prevent continuous stimulation of post-synaptic neurones. Receptors determine whether the signal is excitatory or inhibitory. Synapses can filter out weak stimuli arising from insufficient secretion of neurotransmitters. Removal of neurotransmitter   Neurotransmitter is removed as soon as it has its effect to prevent continuous stimulation of the post-synaptic neuron. Some neurotransmitters, e.g. acetylcholine, are broken down by enzymes Other neurotransmitters, e.g. noradrenaline are reabsorbed by the pre-synaptic neurone Excitatory and inhibitory signals The signal generated in the post synaptic neuron may be excitatory, e.g. leading to an increase in muscle contraction, or inhibitory, e.g. decreasing muscle contraction. The effect of the signal generated by the neurotransmitter on the post-synaptic neurone depends on the receptors on the neuron. Filtering out weak stimuli A nerve impulse reaching a synapse will only generate an impulse in the post-synaptic neuron if it causes the release of a certain minimum number of neurotransmitter molecules. This minimum number of molecules is needed to bind to a sufficient number of receptors on the post-synaptic neuron membrane – this is called reaching the membrane’s threshold. The synapse filters out any stimuli that do not cause this threshold level of neurotransmitter to be released.

Summation of weak stimuli Summation of a series of weak stimuli can trigger enough neurotransmitter to fire an impulse. Neurotransmitters relay messages from nerve to nerve within and out with the brain. Neurones connect with other neurones, muscle fibres and endocrine glands at a synaptic cleft. Not enough neurotransmitter released from each pre-synaptic neuron to trigger an impulse in post-synaptic neuron Added effect of the neurotransmitter from pre-synaptic neurons is enough to trigger a nerve impulse in the post-synaptic neuron Summation of weak stimuli If a neuron receives neurotransmitters, each at a sub-threshold level, from several other neurons at the same time, the effect of these added together may be enough to trigger an impulse in the neuron. Location of synapses Neurotransmitter relay messages between nerve cells inside and outside the brain. Synapses connect neurons with other neurons but also occur between motor neurons and muscles and between motor neurons and glands including endocrine glands.

Converging neural pathways Function of converging, diverging and reverberating neural pathways. Converging neural pathways increase the sensitivity to excitatory or inhibitory signals. Diverging neural pathways influence several neurons at the same time. Neural pathways Converging neural pathways Nerve impulses from several sources arrive at the same neuron. This allows summation of the signals. For example rods are light sensitive cells in the retina of the eye that detect low light and release only a small amount of neurotransmitter. However several rods form a synapse with the same post-synaptic neuron resulting in summation and triggering an impulse in that neuron. Direction of nerve impulses rods post-synaptic neuron

Diverging neural pathways influence several neurons at the same time. Reverberating pathway neurones later in the pathway synapse with earlier ones sending the impulse back through the circuit. Diverging neural pathway An impulse from a single neuron is transferred to several neurons. Diverging pathways allow impulses to be sent at the same time from a common starting point to several locations, e.g. from the motor area of the brain to different muscles in the hand – this allows fine motor control of the fingers to perform tricky tasks. Direction of impulse Reverberating pathway In this pathway, neurons later in the pathway form synapses with those earlier in the pathway allowing impulses to be re-triggered and repeated. These pathways occur in the nervous control of breathing (where the impulses reverberate for a lifetime) and in pathways involved in short term memory Direction of impulse

Plasticity of response is created when new neural pathways are developed to create new responses, bypass areas of brain damage, to suppress reflexes or responses to sensory impulses Plasticity of response When brain damage occurs, e.g. following a stroke, the functions associated with the part of the brain damaged, for example speech, are lost. These functions may be regained after a time because other undamaged areas of the brain form new neural pathways that allow them to take on these functions – this is called plasticity of response. Suppression of reflexes The ability of the brain to block reflexes like blinking is an example of minor plasticity of response.