04.05 Spinal Cord

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Outline

Overview

  1. Function
    1. Two-way conduction pathway
      1. Sensory up
      2. Motor down
    2. Reflex center
      1. Somatic or spinal
        1. Usually related to skeletal muscle
      2. Visceral
        1. Usually related to smooth or cardiac muscle

Nursing Points

General

  1. Gross structure
    1. Begins at foramen magnum of skull
    2. Ends at L1 vertebra
    3. Meninges
      1. Meningeal dura mater
      2. Arachnoid
      3. Pia mater
    4. Conus medullaris – end of cord
    5. Cauda equina – last 9 pairs of spinal nerves
      1. Project out of sacrum
    6. Filum terminale
      1. Cord of pia mater which secures the conus to the coccyx
  2. Cross-sectional structure
    1. Oval shape
    2. Divided by:
      1. Posterior median sulcus
      2. Anterior median fissure
    3. Gray matter – “H” shaped in center – cell bodies and unmyelinated fibers
      1. Posterior horn
      2. Lateral horn—only in thoracic region
      3. Anterior horn
      4. Gray commissure
        1. Surrounds central canal
    4. White matter – external to gray matter – myelinated axons and dendrites
      1. Nerve tracts
        1. Funiculi (3) ((posterior, lateral, anterior))
          1. Ascending = sensory
          2. Descending = motor

References
Betts, J.G., et al. (2017). Anatomy and physiology. Houston, TX: OpenStax, Rice University. Retrieved from https://openstax.org/details/books/anatomy-and-physiology?Book%20details

Transcript

Okay guys, in this lesson we’re going to talk about the spinal cord and its structure and function.

So, the spinal cord functions as a two-way conduction pathway. It is literally what helps our brain control the rest of our body. We have both sensory pathways and motor pathways. We usually say “sensory up” and “motor down”. Sensory pathways take signals from the body to the central nervous system – so the brain and spinal cord. And motor pathways take signals back out to the body to produce some sort of action. When these are working together we call it the reflex arc. So let’s say your hand gets a signal that it’s touching something hot – it sends that sensory signal to the central nervous system, which interprets it and immediately sends a motor signal back down to the muscles in the arm to tell you to pull your hand away – that’s a reflex. We have somatic reflexes – which usually involve skeletal muscle like I just talked about. And we have visceral reflexes, which involve responses in organs and blood vessels that are involuntary. For example a signal that you’re too cold might have your blood vessels react by constricting in your skin to prevent heat loss. That’s a visceral reflex that is involuntary.

Looking at the gross structure of the spinal cord – we see the same meninges that we see around the brain. These are connective and epithelial tissue membranes that help protect the brain and spinal cord. The outermost layer is called the dura mater, the middle layer is the arachnoid layer, and the layer directly around the brain and spinal cord is called the pia mater. You can think DAP or PAD depending on which way you’re going. The spinal cord extends off the brainstem and comes out of the foramen magnum of the skull. That extends all the way down to the L1 vertebrae, which is right about here. That’s where you find what’s called the conus medullaris – basically it’s the end of the actual cord itself. After that, extending down through the lumbar vertebrae and into the sacrum is the cauda equina. It literally means horse tail – and it’s the last 9 pairs of spinal nerves. Basically they’re just loose as opposed to being wrapped in those meninges, that’s the only reason they have a different name. And lastly there’s a thin piece of white fibrous connective tissue called the filum terminale that connects the cord to the coccyx to keep it all in place.

Now let’s look at a cross section of the spinal cord to see a more microscopic structure. It’s an oval shape that is divided left and right by the posterior median sulcus and the anterior median fissure – a fissure is just a deeper groove than a sulcus. Then you’ll see there’s gray matter in the center in kind of an H shape and white matter around the outside. Gray matter contains out neuron cell bodies and the unmyelinated neurons – so remember unmyelinated means things move a little slower. White matter contains our myelinated axons and dendrites – so think myelin equals white. You can also see here the ganglion coming out of the spinal cord. Remember when we talked about the autonomic nervous system we talked about preganglionic and postganglionic. That’s where this happens. Pre is coming off the spinal cord into the ganglion – which remember is just a group of neuron cell bodies in one place. And post is coming off that and out to the body. Now you can see here, inside the gray and white matter there are specific regions or tracts for various sensory and motor pathways. Make sure you check out the spinal cord injury lesson to see what happens when certain parts of this spinal cord are damaged.

So let’s recap. The spinal cord is a two-way conduction pathway, with sensory going up and motor going down that helps our brain control the rest of our body. It goes from the skull down to L1 and then the cauda equina takes the last 9 pairs through the sacrum. Just like the brain, it is surrounded by meninges – the dura mater, arachnoid layer, and pia mater. It contains gray matter in the center which has our neuron cell bodies and unmyelinated axons, and white matter around the outside that has myelinated axons and dendrites. Remember specific parts of the spinal cord have certain tracts for sensory and motor pathways – so an injury would affect those specific tracts.

That’s it for our review of the spinal cord. Make sure you check out all of the images and resources attached to this lesson Now, go out and be your best selves today. And, as always, happy nursing!