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Broadly speaking, the nervous system can be split into an afferent or sensory division and an efferent or motor division.
The afferent division brings sensory information from the outside world into the brain.
Sensory information involves special senses - like vision, hearing, taste, and smell - as well as general somatic senses which make up the somatosensory system, which is involved in the sense of touch, proprioception, pain, and temperature. These sensations are felt by sensory neurons all over the body.
These sensory neurons have receptors that are classified according to the stimulus they respond to - there are mechanoreceptors for touch and proprioception, nociceptors for pain, and thermoreceptors for temperature.
Now, neurons are the main cells of the nervous system. They’re composed of a cell body, which contains all the cell’s organelles, and nerve fibers, which are projections that extend out from the neuron cell body. These are either dendrites that receive signals from other neurons, or axons that send signals along to other neurons.
Where two neurons come together is called a synapse, and that’s where one end of an axon sends neurotransmitters to the dendrites or directly to the cell body of the next neuron in the series.
The somatosensory pathways are made up of a relay of four neurons.
The first neuron is called the first order neuron or sensory neuron, which has the sensory receptors and converts stimuli from the outside world into an impulse that can be passed through a synapse to the next neuron in series.
Finally, the third order neuron takes the impulse to the fourth order neuron, which has its cell body further up in the sensory cortex of the brain.
Now let’s zoom into first order or sensory neurons. First order neurons are called pseudounipolar neurons, which means that they don’t have separate dendrites and axons; instead, there’s only one axon that extends out from the cell body, and it has two branches: a peripheral branch and a central branch.
Somatosensory receptors are specialized sensory nerves that respond to various stimuli, including vision, hearing, taste, and smell, as well as general somatic senses which make up the somatosensory system, which is involved in the sense of touch, proprioception, pain, and temperature. They are located throughout the body, including the skin, muscles, tendons, joints, and internal organs. There are several different types of somatosensory receptors, including: Mechanoreceptors which respond to stimuli such as touch, pressure, and vibrations. Thermoreceptors which respond to changes in temperature. Nociceptors which respond to harmful stimuli, such as extreme temperatures or tissue damage, transmitting pain signals to the brain. Proprioceptors which respond to changes in the position and movement of body parts. Chemoreceptors which respond to specific chemical signals such as taste buds.
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