Sensory Spinal Pathways

Sensory Pathways

This is a great picture to start talking about sensory pathways. First, let’s look at the picture on the right. It shows a sensory pathway running from your fingertip through a spinal nerve. Next, it passes through a dorsalRelating to the back side of the body. root ganglionA cluster of neuron cell bodies located in the peripheral nervous system (PNS). and the dorsal root. Then it enters into the dorsal horn of the Gray matter in this transverse section A cut or slice of the body or an organ for study. of the spinal cordThe central nervous system structure that relays signals between the brain and body.. You can see that here in the gray matterThe inner portion of the spinal cord composed mostly of neuron cell bodies and synapses. the original sensing neuronsThe functional cells of the nervous system that transmit signals. synapseThe junction between two neurons where communication occurs. with other neurons. These neurons seem to disappear into the top of this picture.

It is these disappearing neurons that actually extend up into the brain and are better seen on the left diagram. There are two different pathways 1 red 1 blue and there are a couple places where synapses happen. So, one neuron does not carry sensory information all the way to the brain. That’s quite rare. What is more common is the example that we are going to use. We have been using one unipolar neuronA neuron with one process that splits into peripheral and central branches, common in sensory neuron with its cell bodyThe central part of a neuron containing the nucleus and organelles. and the dorsal root ganglion. It enters the spinal cord and then synapses with another neuron in the dorsal horn of the gray matter. It is that second neuron that will then bring information up the spinal cord. It transfers data to the brain’s phone operator, the section called the thalamusThe brain’s relay center, sending sensory information to the cerebral cortex..

Look closely at the diagram on the left. You can see that in the thalamus, the neurons synapse again with other neurons. These neurons then bring the information to a very specific area in the brain. This area evaluates the incoming information. We have been drawing diagrams like this. And now we have to get more specific to how these types of pathways actually work. Understanding these diagrams is very important. You need to differentiate between unipolar and multipolar neurons. Be careful to see where the neurons synapse.

Specific Sensory Pathways

The spinothalamic pathway carries various types of information. Free nerve endings in the somatic tissues take these data. The spinothalamic tract also carries conscious information. We haven’t yet gotten to the brain. It’s important to know that the thalamus is like a conscious gateway to your brain. Any sensation you feel in your skinThe body’s largest organ, providing protection and regulation., muscle, or bones is likely sent to your thalamus. This uses the spinothalamic pathway.

The spinothalamic pathway will prioritize pain over other sensations such as temperature and pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange.. From an evolutionary perspective, this function saves lives. Something causing you pain is probably bad for you. Spinal cord stimulators can be implanted with leads in the thoracic regionThe part of the spinal cord corresponding to the chest.. You can see this in the picture on the right. The generator implanted in your love handle produces a very small pulse. This pulse disrupts action potentials rising up the spinothalamic tract. This small pulse is enough to stop the sensation of pain from rising to the conscious thalamus. However, it’s not enough to prevent sensations of temperature and pressure. I have one of these it changed my life.

Another incoming pathway is the spinocerebellar pathway. This pathway takes information from many of your proprioceptorsSensory receptors that monitor the position and movement of the body.. They are set in your joints, tendons, muscles, and ligaments. The spinal cerebellar pathway carries unconscious information to your cerebellumThe brain region responsible for balance, coordination, and fine motor control. or the little brain at the back of your headRounded proximal end that fits into the acetabulum of the hip bone.. Again, I know we haven’t covered the brain. It’s important to understand that the cerebellum is involved in many unconscious postural adjustments. These adjustments are made by your lower limbs.  This means that when somebody walks past you and bumps into you, your body makes unconscious adjustments. These adjustments ensure that you don’t fall down. Your cerebellum is using the proprioceptive information carried by the spinocerebellar pathway period the name again is very nice because it tells you the origin the spineProminent ridge on the posterior scapula dividing it into supraspinous and infraspinous fossae. and the destination the cerebellum period

Specific Sensory Pathways

The first sensory neuron that has an action potentialA rapid, temporary electrical charge that travels along neurons, allowing signal transmission. generated by incoming information is usually a unipolar neuron. Its cell body is in the dorsal root ganglion. It synapses with another neuron in the Gray matter of the spinal cord. This is always called the neuron of the 1st order. It is the first neuron in this sequence to have an action potential. Of course, the second neuron is called the neuron of the 2nd order. The third one is called the neuron of the 3rd order. This pattern continues on and on.

This slide shows you three different representations of the same type of information. The diagrams on the left are color-coded. They show you the neurons of their orders. These diagrams demonstrate how neurons bring information into the spinal cord and up into the brain. The middle picture details the locations where these neurons synapse. It shows how they use the white matterThe outer portion of the spinal cord made of myelinated nerve fibers that transmit signals. columns to rise up the spinal cord. I’m not a fan of the picture on the right but that doesn’t mean that you might not be. The picture on the right is a schematic and text representation of these first second third and 4th order neurons. This diagram illustrates two specific pathways of neurons. They run from inferiorBelow or toward the lower part of the body. to superiorAbove or toward the upper part of the body. in the spinal cord. These pathways carry specific types of information.

The medialToward the midline of the body limescale pathway intakes information concerning your posture. Proprioceptors in your joints can sense their degree of flexion or extension. They send this information to unconscious parts of your brain. The ventralRelating to the front or belly side of the body. spinothalamic tract senses general or crude touch information. Mechanoreceptors in your fingertips connect to neurons of the 1st order in this pathway. They also connect in other areas of your body. I like the name of this pathway because it tells you the origin and the destination of the pathway. The origin is in the spine. The destination is in the thalamus. This means that this is an ascending pathway curing sensory information.

Dermatomes are Clues

Fortunately for physical therapists and occupational therapists, each spinal nerve leaves from a specific level of the spinal cord. It has one branch that innervates the skin. The extent of the skin that is innervated is shown on the picture on the right. These are what are called dermatomes. Pain in certain dermatomes can provide clues. They can help locate which spinal nerve or level of the spinal cord is experiencing pain. For example, a patient might describe a pressure sensation on the side of their right knee. This could indicate a herniation at the L4L5 level.

A patient could experience pain that runs down the arm and into the thumb. This could indicate an issue with the C6C7 vertebra. The problem with dermatomes is that they depend on the precision of your patient’s ability to report their pain. Pain is a symptom reported by the patient not a sign that can be observed by the medical practitioner. Complicating things is that pain sensations can be felt differently such as burning aching stabbing types of pain. So although dermatomes can be used as clues, they are only one part of a very complex picture. This picture has to be painted for spinal injuries.