Synapses
A synapse is a junction between an axon terminal of one neuron and the dendrites/cell body of the next neuron. Generally, synapses are between the axon ending of one neuron and the dendrites and cell bodies of the next neurons and are known as axodendritic synapses. There are two different types of synapses: electrical and chemical.
Electrical synapses
These synapses are found in all nervous systems, including the brain. This is where the membranes of the two neurons which are communicating come very close together at the junction and are linked together by an intracellular process called the gap junction. The junction contains paired protein channels of the junction and each channel pair forms a pore.
As the pore of a channel is larger than the pore of a gated ion channel, a very low resistance pathway is therefore formed and ions can diffuse and flow directly from one neuron to the next. Transmission across the synapses via electrical communication is very fast.
Chemical synapses
Chemical synapses are released and received by neurotransmitters which open and close different ion channels that influence the permeability of the membrane and therefore, the membrane potential.
The space between the synapses of pre and post synaptic neurons is a lot greater than the space between electrical synapses and is known as the synaptic cleft. Within the presynaptic terminal are there are tiny membrane bound sacs known as synaptic vessels containing thousands of neurotransmitters. These chemical agents act as messengers to transmit signals between both of the neurons. These chemical synapses can actually prevent a nerve impulse from being transmitted from one neuron to the next. A nerve impulse will reach the axon terminal and trigger a release of neurotransmitters. Calcium gates will open in the presynaptic axon terminal to and the neurotransmitter will be released due to increased intracellular levels of ionic calcium. They will then cross the synaptic cleft and bind to receptors in the postsynaptic membrane to cause changes in the membrane permeability
Neurotransmitters
Neurotransmitters are chemicals which relay information or messages to carry a signal across a synapse which are stored in sac like structures at the end of an axon. When the nerve impulse reaches the end of the axon, the sac/vesicle releases a neurotransmitter into the synapse. These neurotransmitters diffuse across the synapse and bind to the receptors in the next cell.
Neurotransmitters change the permeability of the cell membrane to different sodium and potassium ions. This assists to set up the action potential and transmit the next impulse across to the adjacent neuron. Neurotransmitters can be stimulated or blocked by different drugs or toxins that may be taken.
Neural Circuits
Neurons are organised into billions of networks which are known as the neural circuit.
Reflex Arcs
This is the path followed by a nerve pathway in response to a particular stimulus and produces involuntary reactions. The reflex arc includes five functional components:
- Sensory receptor - this is the distal end of a sensory neuron which will respond to a particular stimulus by producing a graded potential called a generator
- Sensory neuron - the nerve impulse formulates from the sensory receptor along the axon of the sensory neurons to the axon terminals (integrating centre) which are located in the grey matter of the spinal cord
- Integrating Centre – different regions in the grey matter act as an integrating centre and will translate impulses to other neurons
- Motor Neuron – the integrating centre will send impulses out of the CNS along a motor neuron to the part of the body where it may be needed
- Effector – this is the area of the body that will react to the motor nerve impulse
The human nervous system consists of billions of nerve cells - or neurons, plus supporting - neuroglial cells. Neurons are able to respond to stimuli such as touch, sound, light, and so on, conduct impulses, and communicate with each other and with other types of cells like muscle cells. The nucleus of a neuron is located in the cell body.
Motor skills includes such things as locomotion - running, walking, leaping, climbing, manipulation - throwing, kicking, catching, bouncing, and stability - bending, twisting, rolling, dodging. Mastery of a range of motor skills, combined with good overall physical fitness, is critical to the healthy development of any adult or child and forms the basis athletic competence.
