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Spinal Cord, Peripheral Nervous System

Notes

Spinal Cord, Peripheral Nervous System

Sections







Summary

Overview

Vertebral column

  • Encases and protects the spinal cord.

Spinal cord

  • Connects the brainstem to the peripheral nervous system.

Peripheral nervous system

  • Connects the nervous system to the rest of the body and the environment.

Autonomic nervous system

  • Facilitates states of action and rest.

Spinal Cord

Overview

See: Spinal cord

  • Central canal is a continuation of the cerebral spinal fluid system.
  • Butterfly-shaped gray matter horns house neuronal tissue.

Gray matter

  • Posterior gray matter horns house sensory cells.
  • Intermediate zones house autonomic cells.
  • Anterior horns house motor cells.

White matter columns (aka funiculi)

  • Posterior comprises the posterior column pathway, which carries large sensory fibers for proprioceptive sensation (vibration, two-point discrimination, and joint position).
  • Lateral notably comprises the corticospinal tract, which carries motor fibers.
  • Anterior notably comprises the anterolateral system (aka spinothalamic tract), which carries small fiber sensory pathways that carry pain, temperature, and itch sensation.

Clinical Correlation - Spinal Cord Compression

Peripheral Nervous System

Nerve Roots

Posterior root (the sensory root)

  • Enters the posterior horn and contains the posterior root ganglion (aka dorsal root ganglion or DRG) ⎯ it houses the cell bodies of sensory nerves.
  • Sensory fibers project from the periphery through the posterior root to the posterior horn of the spinal cord.

Clinical Correlation - Neuropathy

Anterior root (the motor root)

  • Emerges from anterior horn cells in the anterior gray matter.
  • Motor fibers project from the anterior horn of the spinal cord through the ventral root to the neuromuscle junction.

Mixed spinal nerve

  • Mixed spinal nerve forms where nerve roots meet just distal to the intervertebral foramen.

The neuromuscle junction

  • Muscle fibers
  • Neurotransmissions pass across the neuromuscular junction to stimulate muscle fibers.
  • Neuromusclular junction disorders (such as Myasthenia Gravis) cause fatigable weakness.

The peripheral nerve sensory receptor

  • Detects sensory impulses from the musculoskeletal system and skin.
  • Typical peripheral neuropathies affect the distal ends of the nerves first.

Autonomic nervous system

Sympathetic nervous system

  • Contains paravertebral sympathetic chain ganglia.
  • The sympathetic nervous system is called upon in states of ACTION, meaning, physical and psychological stress — it produces the so-called "fight-or-flight" response.

Parasympathetic nervous system

  • The parasympathetic nervous system is active when the body is in a REST state, such as when sleeping or eating.

VERTEBRAL COLUMN

Spinal levels

From superior to inferior:

  • Cervical
  • Thoracic
  • Lumbosacral & coccygeal

Select key features

  • Cervical roots
  • Cervical plexus and brachial plexus (which are mixtures of different cervical and thoracic spinal nerve levels).
  • At the thoracic level, no plexuses exist (the spinal nerves do not mix with other spinal levels).
  • At the most inferior aspect of the spinal cord (the conus medullaris) aligns with the L2 vertebrae; therefore, the spinal cord level is always lower than the surrounding vertebral level.
  • The remaining lumbosacral nerve roots form the cauda equina (which means "tail of the horse"), which is the collection of nerve fibers that traverses the lower (caudal) spinal canal.
  • The lumbosacral plexus.

Clinical Correlation - Cauda Equina Syndrome

Anatomy

See: Vertebral column

  • Vertebral arch lies posteriorly.
  • Pedicle attaches to the vertebral body.
  • Vertebral body lies anteriorly
  • Intervertebral space: jelly-like vertebral disc.
  • Intervertebral lies foramen between them; vertebral pedicles stretch above and below the vertebral foramen.

Spinal Meninges

See: The spinal meninges

  • Dura mater and arachnoid mater lie between the spinal cord and vertebral column.
  • The subarachnoid space lies between the arachnoid materand the spinal cord; during lumbar puncture cerebrospinal fluid (CSF) is drained from the subarachnoid (from the lumbar cistern).
  • The epidural space lies between the vertebral arch and the dura mater; it is a common site for hematoma, infection, and spread of neoplastic disease.

Clinical Challenge Question

Use the following clinical case to further reinforce your general understanding of nervous system localization.

Localization Question

Full-Length Text

Introduction

Here, we'll learn about the spinal cord and peripheral nervous system.

To begin, start a table.

Denote that we will address...
The spinal cord: the spinal cord and also the spine (itself) (the vertebral column)

The peripheral nervous system: the nerve roots, peripheral nerves, neuromuscle junction, and sensory receptors

The autonomic nervous system: the sympathetic and parasympathetic components.

Spinal Cord

First draw the ovoid outline of the spinal cord.
Include the central canal, which is a continuation of the cerebral spinal fluid system.

Denote that the spinal cord connects the brain and the peripheral nervous system -- it is an intermediary between these two parts of the nervous system. As we recall, the spinal cord, itself, is part of the central nervous system, along with the brain.

Now denote that the vertebral column encases the spinal cord in bony protection.

Next, include the butterfly-shaped gray matter horns, which house neuronal tissue.

Show that from posterior to anterior they divide into the:

  • Posterior gray matter horns, which house sensory cells.
  • Intermediate zones, which house autonomic cells.
  • Anterior horns, which house motor cells.

We'll show the innervation of these gray matter horns momentarily.

But first, include the intervening white matter columns (aka funiculi:

  • The posterior funiculus, which comprises the posterior column pathway.
    • It carries large sensory fibers for proprioceptive sensation (vibration, two-point discrimination, and joint position).
  • The lateral funiculus, which notably comprises the corticospinal tract, which carries motor fibers.
  • The anterior funiculus, which notably comprises the anterolateral system (aka spinothalamic tract), which carries small fiber sensory pathways that carry pain, temperature, and itch sensation.

Peripheral Nervous System

Now, let's draw the proximal portion of the peripheral nervous system.

Denote that the nerve roots connect the spinal cord and the peripheral nerves.

Show that the dorsal root (the posterior, sensory root) enters the posterior horn and indicate its dorsal root ganglion (aka, DRG or posterior root ganglion). Show that it houses the cell bodies of sensory nerves.

Then, show that the ventral root (the anterior, motor root) emerges from the anterior spinal cord and meets the dorsal root ganglion to form the mixed spinal nerve.

These nerve roots meet just distal to the intervertebral foramen.

Next, draw representative muscle and include a sensory receptor and a neuromuscle junction.

Denote that peripheral nerves connect nerve roots to the periphery. The neuromuscle junction translates nerve impulses to the muscles. The sensory receptors provide sensory input to the peripheral nerves.

Show that sensory fibers pass as the mixed spinal nerve before entering the dorsal nerve root and innervating cells in the posterior gray matter horn of the spinal cord.

As a clinical pearl, typical peripheral polyneuropathies will affect the distal ends of the nerves first, so the toes are typically the first area of numbness in neuropathy.

Next, show that motor fibers emerge from the anterior gray matter horn of the spinal cord and pass through the ventral nerve root and then join the mixed spinal nerve.

They innervate the neuromuscle junction and then neurotransmission passes across the neuromuscular junction to stimulate muscle fibers.

As a clinical pearl, neuromuscular junction disorders, most notably myasthenia gravis, cause fatigable weakness, meaning the more the muscle is used the more evident the weakness.

Autonomic Nervous System

Now, let's layer-in the autonomic nervous system, which divides into sympathetic and parasympathetic systems.

Denote that the sympathetic nervous system is called upon in states of ACTION, meaning, physical and psychological stress — it produces the so-called "fight-or-flight" response.

Draw a representative sympathetic ganglion as part of the paravertebral chain. Show that fibers emanate from the spinal cord and synapse in the sympathetic paravertebral chain ganglia and then, as an example, project to the heart.

Denote that the parasympathetic nervous system is active when the body is in a REST state, such as when sleeping or eating.

Show parasympathetic fibers synapse in cardiac ganglia in the heart and show postganglionic fibers then innervate the heart, itself.

As we can see, as a general rule, sympathetic fibers synapse far from their target organ: they synapse in the sympathetic paravertebral chain ganglia before passing to the heart. Whereas parasympathetic fibers synapse near to or within their target organ: here, they synapse in cardiac ganglia of the heart before projecting a short distance into the heart muscle.

Spine

Draw a longitudinal (sagittal) view of the spinal canal as a simple line (underneath a skull) and show the general curvature of the spinal.

Indicate the spinal levels from superior to inferior as:

  • Cervical
  • Thoracic
  • Lumbosacral & coccygeal

There are 31 spinal nerves and 33 vertebrae.

The most inferior aspect of the spinal cord (the conus medullaris) aligns with the L2 vertebrae (the lumbar 2 vertebrae); therefore, the spinal cord level is always lower than the surrounding vertebral level.

The remaining lumbosacral nerve roots below this level form the cauda equina (which means "tail of the horse"), which is the collection of nerve fibers that traverses the lower (the caudal) spinal canal.

Finally, in horizontal (axial) view, re-draw the spinal cord.

Show that the vertebrae form an arch, posteriorly, and a vertebral body, anteriorly, around the spinal cord at every level.

In between the vertebrae is an intervertebral space that is filled with a jelly-like intervertebral disc.
As well the gap between the vertebra above and below, form intervertebral foramen through which the spinal nerves exit.

Spinal Meninges

Finally, let's include the spinal meninges.

First, let's draw the layers and then we will include the intervening spaces.
From outside to inside, draw the dura mater (show the dural root sheath encases the nerve root), the arachnoid mater, and show that pia mater, the innermost layer, directly adheres to the spinal cord and nerve roots.

Then show that epidural space lies between the vertebral arch and the dura mater; it is a common site for hematoma, infection, and spread of cancerous (neoplastic) disease.

Show that subdural space lies between the dura mater and arachnoid mater.

Finally, show that subarachnoid space lies between the arachnoid mater and the spinal cord; during lumbar puncture cerebrospinal fluid (CSF) is drained from the subarachnoid (from the lumbar cistern, typically).