Nervous system 2, Sensory neuron

Stimuli from all five senses are transmitted via sensory neurones. Sensory neurones carry sensations from the environment to the sensory cortex in the brain. This is the area of the brain which allows us to detect the presence and location of sensation from the body. In sensory neurones the nervous impulse is not generated in the cell body but in a peripheral sensory receptor. This is a specialised structure designed to detect a particular sensation, for example touch in the fingertips. When a sensation is detected the receptors generate a new nerve impulse which represents the external stimulus. Impulses from the neck and below, and from the back of the head, travel to the brain via spinal nerves and the spinal cord. Impulses from the front of the head, including the face, mouth, teeth and eyes do not travel via the spinal cord, but are relayed directly to the brain stem via the cranial nerves.

From the sensory receptor the impulse is transmitted along the dendrite of a sensory neurone to the cell body. This first fibre, between the receptor and the cell body, is referred to as a dendrite because it carries information towards the cell body. From a cell body in a spinal nerve, an impulse is carried into the spinal cord via the axon of a sensory neurone. After entering the spinal cord the impulse travels upwards, towards the brain in an ascending sensory pathway. The first sensory neurone will synapse with a second in the spinal cord or brain stem. All sensory neurones cross over (decussate) to the opposite side at the level of the spinal cord or brain stem. In the case of cranial nerve this crossing over (or decussation) occurs at the level of the brain stem. This means sensation from the left side of the body is experienced in the right side of the brain. Likewise, sensations from the right side of the body are experienced in the left hemisphere of the brain.

All sensory impulses then pass up through the brain stem to the thalamus where the second sensory neurone synapses with a third. The thalamus is an area of the brain with many interconnections, communicating between the brain and spinal cord. The thalamus acts as a relay station, coordinating communications between various components of the brain. A third sensory neurone from the thalamus finally carries the impulse into the sensory cortex of the brain where it is experienced as sensation.