The nervous system of the human is broken down into two categories we have the central nervous systemwhich
now the somatic nervous system consists of two divisions we have the motor division that consists of only motor neurons also known as efferent neurons and we have the sensory division which consists of only sensory neurons also known as a ferret neurons now recall that a motor neuron is basically a neuron that accepts an electrical signal from the central nervous system and sends it away to some type of target organ or gland muscle tissue and so forth while the sensory neuron picks up that electrical signal from some type of outside stimulus and sends that electrical signal to the central nervous system where it’s integrated as well as processed.
now the somatic nervous system is responsible for innovating and controlling skeletal muscle skeletal tissue which basically means the somatic nervous system is ultimately responsible for voluntary movement so if I want to actually for example bend and extend my arm this requires the somatic nervous system so it’s the brain that actually creates or initiates that electrical signal but it’s the somatic nervous system that actually extends my arm as we’ll see in just a moment so let’s begin with our motor division so let’s discuss the motor neurons found in the somatic nervous system so as I mentioned earlier motor neurons are those neurons that accept our signals from the central nervous system so the brain or the spinal cord and send these signals to the target tissue gland organ muscle etc in the case of the somatic nervous system the target organ is always our skeletal muscle so in a somatic nervous system the dendrite and a cell body of the motor neuron always begin always originate in the spinal cord so let’s take a cross-section of the spinal cord as shown in the following diagram we have the white matter and we have the gray matter so the cell body and the dendrites of the motor neuron in the somatic nervous system always begin within our spinal cord so let’s suppose I want to actually move my bicep what happens is our brain my brain initiates creates that electrical signal in a form of an action potential and that action potential travels and eventually ends up on
the dendrites of our somatic motor neuron and that then DRI picks up that signal and once the signal is picked up by the dendrites it sends it through the axon of our body so the axon of the somatic motor neuron now notice the body of this neuron the axon not the body the cell body is within the central nervous system but notice our axon of the motor neuron is located entirely in the peripheral nervous system so our electrical signal the action potential travels and eventually ends up at our axon terminal now the axon terminal is right next to our cell membrane of the muscle cell, in this case, let’s say it’s the skeletal muscle found inside my bicep so if we zoom in on the synapse
this is basically our neuromuscular junction it’s the synapse between our neuron the motor neuron and our cell membrane of that muscle so if we zoom in we basically get the following picture so we have the axon terminal of our somatic motor neuron and we have the cell membrane of the skeletal muscle inside our biceps muscle so notice that the cell membrane contains these receptor proteins and the axon terminal contains these synaptic vesicles that carry special neurotransmitters now in the case of the motor division of the somatic nervous system.
these neurotransmitters are always acetylcholine so as the action potential travels and eventually ends up on the axon terminal it basically causes the release of these synaptic vesicles and that releases our acetylcholine the cetyl choline eventually bind onto these receptor proteins that create an action potential and that action potential ultimately causes the contraction of our biceps muscle so in this way it’s the brain that generates that electrical signal but it’s the motor neuron of the somatic nervous system that actually causes that movement that voluntary movement in the first place now one more thing we have to notice about the motor neurons is when they actually leave when they actually exit the spinal cord they always exit from the front side from the ventral side of our spinal cord so if this is the back and this is the front notice they always leave from the front side of our from our spinal cord now one more thing we have to notice is we have a single axon leaving this spinal a spinal cord and the axon eventually ends up exactly at our effector targets so our effector organ in this case our biceps tissue the biceps muscle so basically we have a single axon traveling all the way to the target tissue the target muscle now let’s move on to our sensory neuron so sensory neurons are those neurons that connect to receptors these receptors basically pick up stimuli and this stimulus is transformed into an electrical signal by those sensory neurons of the somatic nervous system so these somatic sensory neurons then carry our electrical signal via a single axon as in this case and they carry that signal to the backside which is our dorsal side of our spinal cord so the motor neurons leave from the front side the ventral side but the sensory neurons pick up those signals and carry them through the back side into our spinal cord the back side is the dorsal side now one major difference between the sensory neurons and the motor neurons of our somatic system is the cell body of our motor neuron is found inside our spinal cord but the cell body here is found close in the backside so close to the backside of our spinal cord so this region where we find the cell body of the sensory neuron is known as the dorsal or backside so dorsal root ganglia where ganglia simply means neurons outside of the spinal cord and the brain so let’s see how this signal actually takes place so let’s suppose I apply pressure onto my finger as I apply the pressure the dendrite of our sensory neuron contains special pressure receptors and when I apply these receptors are basically these receptors basically create a force that force creates the oscillation the movement of our ions and that ultimately creates our electric current the action potential and so the potential travels our electric current travels through the axon eventually that creates an action potential at this section the axon terminal which is found inside our spinal cord and then that sends the electric electric signal up to the spinal cord into the brain and the brain basically senses that pressure now sensory neurons and motor neurons so these pathways that we discussed so far involve voluntary movement so if I want to move my hand if I want to extend it or bend it this is basically a result of the somatic nervous system
the more division and the sensory division now we are in complete control of this voluntary motion but the somatic nervous system doesn’t only control voluntary movement of skeletal muscle it also controls reflex arts or reflex arts that we are not in control of so reflex arts are basically those are reflexes those responses that we have no control over so they simply take place and we cannot do anything about them so the somatic nervous system is also responsible for reflex arcs these are quick and automatic responses that cannot be controlled voluntarily and they basically are result of outside stimuli so let’s suppose I place my hand on a hot stove so without even knowing my hand will basically move away now eventually I’m going to feel that but initially I have no control over the fact that my hand will automatically move and this is known as a reflex arc and our somatic nervous system controls this reflex arc and involves both sensory and motor neurons so we have two types of reflex arcs we have monosynaptic and polysynaptic so monosynaptic simply means we have a sink synapse between our motor neuron and a sensory neuron but polysynaptic means we have more than one synapse so to see what we mean let’s take a look at these two diagrams so in diagram one this is our spinal cord so let’s suppose my stimulus is that hot stove so I place my hand on the hot stove that stimulus is transformed into an electrical signal that is picked up by our sensory neuron of the somatic nervous system it travels from the backside the dorsal side of our spinal cord and let’s say it basically synapses first with an inter neuron if it’s in APS’s with an inter neuron that means we have one synapse here now eventually the inter neuron shown in brown found inside the spinal cord will synapse with our motor neuron of a somatic nervous system and that electrical signal will basically travel back into my hand and it will tell my hand to basically move that hand so this is known as a polysynaptic reflex because it contains 1/2 synapses so more than one synapse.