Chapters 14 and 17
Spinal Cord, Spinal Nerves and Autonomic Nervous System
 

Spinal Cord
  Gross Anatomy
     The spinal cord extends from the foramen magnum to the inferior border of the first lumbar vertebra. Visible features of the spinal cord include:
  Posterior median sulcus – shallow longitudinal groove on the dorsal surface.
  Anterior median fissure – deep crease on the ventral surface.
  Enlargements – along the length of the spinal cord the regions devoted to processing information associated with the limbs are enlarged. These include:
  Cervical enlargement (C5 – T1) supplies nerves to the upper limb and pectoral girdle.
  Lumbosacral (lumbar) enlargement (L2 – S3) – supplies nerves to the lower limbs and pelvic girdle.
  Conus medullaris – The conical end of the spinal cord.
  Filum terminale – slender thread of fibrous pia mater that extends from the tip of the spinal cord and anchors it longitudinally.
  Cauda equina – The spinal cord stops growing before the vertebral column. As a result the dorsal and ventral roots continue to grow as their exit points move further away. This collection of roots within the vertebral column at this point form the cauda equina.
  Spinal Cord Segments
     There are 31 spinal cord segments, 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal segments. Each segment is associated with the pair of spinal nerves that arise from it.
  Spinal Nerves
     The spinal nerves result from the fusion of dorsal and ventral roots. The roots themselves result from the fusion of rootlets that extend from either side of each spinal segment. The dorsal and ventral roots exit the vertebral column through the intervertebral  foramina. At the exit point there is a dorsal root ganglion that contains the cell bodies of the sensory neurons found in the dorsal root. The ventral root which contains motor neurons, fuses with the dorsal root distal to the ganglion to form the spinal nerve.
 
  Spinal Meninges
     The spinal meninges are specialized membranes that provide protection, physical stability and shock absorption within the bony vertebral canal. There are three meningeal layers:
  Dura mater
     The outer layer consists of dense irregular connective tissue. Between the dura mater and the inner wall of the vertebral canal there is an epidural space containing areolar connective tissue, adipose tissue and blood vessels.
     The dura mater is attached cranially to the periosteum at the margin of the foramen magnum and tapers distally to form the coccygeal ligament that attaches to the periosteum of the sacrum and coccyx.
  Arachnoid mater
     The arachnoid is the middle meningeal layer and consists of a simple squamous epithelium that attaches directly to the inside of the dura mater. It is separated by the subarachnoid space that contains cerebrospinal fluid (CSF) from the pia mater. The space between the arachnoid and pia mater is also spanned by collagen and elastic fibers that form bundles of fibers called arachnoid trabeculae.
  Pia mater
     The innermost meningeal layer consists of collagen and elastic fibers that adhere closely to the contours of the nervous tissue. Lateral extensions of the pia mater and arachnoid mater between the dorsal and ventral roots attach to the dura mater and are called denticulate ligaments. The denticulate ligaments prevent side to side and inferior movement of the spinal cord.
 
Sectional Anatomy of the Spinal Cord
        A narrow central canal runs through the center of the spinal cord.  
       Gray matter is deep within the cord in the shape of an “H” with the cross bar of the H lying over the central canal. The gray matter has:
  Posterior gray horns – posterior (dorsal) projections of gray matter that contain somatic and visceral sensory nuclei.
  Anterior gray horns – anterior (ventral) projections of gray matter that contain the cell bodies of somatic motor neurons.
  Lateral gray horns – lateral projections of gray matter in spinal segments T1 to L2 that contain visceral motor neurons.
  Gray commissures – forms the cross bar of the H and contains axons crossing over from one side to the other.
       White matter is superficial with respect to gray matter and is divided into regions by the posterior and anterior gray horns. The major regions are:
  Posterior white columns – between the posterior median sulcus and posterior gray horns.
  Anterior white columns – between the anterior median fissure and the anterior gray horns.
  Lateral white columns – between the anterior and posterior gray horns.
       Tracts
    Within the columns, collections of axons performing a similar function forms a tract. Tracts that carry sensory information to the brain are ascending tracts. Tracts that carry motor information to the spinal cord are descending tracts.
 
Spinal Nerves
     Peripheral nerves have three layers of connective tissue:
  Epineurium – is the outermost layer of connective tissue that is continuous with the dura mater. It also includes the connective tissue in between any fascicles (bundles) within the nerve.
  Perineurium – is the connective tissue sheath that surrounds a fascicle of axons.
  Endoneurium – is the delicate connective tissue fibers that surround individual axons.
 
Peripheral Distribution of Spinal Nerves
     After the dorsal and ventral roots of the spinal segment fuses the spinal nerve divides into several branches:
  Dorsal  ramus – is the branch that innervates the skin and muscles of the neck and back.
  Ventral ramus – is the larger branch  that innervates the ventrolateral body surface and the limbs.
  Rami communicantes (sing. ramus communicans) - the thoracic and first two lumbar spinal nerves have rami communicantes (communicating branches) associated with the autonomic nervous system.
 
Dermatomes
     The sensory fibers of each spinal segment collect information about a specific region of the body surface called a dermatome. Loss of sensation in a dermatome can be used to diagnose nerve or spinal cord damage.
 
Nerve Plexuses
     In the spinal segments controlling the skeletal muscle of the neck and the limbs, the ventral rami do not remain separate. Instead, the fibers of neighboring ventral rami fuse and divide to form interwoven networks of nerves called nerve plexuses. There are four nerve plexuses:
    Cervical Plexus – forms from the ventral rami of spinal nerves C1 – C4 and some fibers from C5. The phrenic nerve innervates the diaphragm and arises from this plexus.
    Brachial Plexus – is a large and complex plexus that innervates the pectoral girdle and upper limb with contributions from the ventral rami of C5 to T1. The nerves that arise from this plexus include the axillary, musculocutaneous, median, ulnar and radial nerves.
    Lumbar Plexus – is formed by the ventral rami of T12 – L4. The femoral nerve is a major nerve that arises from this plexus.
    Sacral Plexus – contains fibers from the ventral rami of L4 – S4. The sciatic nerve is a major nerve of this plexus.
 
Reflexes
     A reflex is an immediate involuntary response to a stimulus that never varies. The pathway that impulses follow to produce the response is called the reflex arc. The reflex arc can be divided into the following steps:
  Step 1 – Activation of a receptor by a stimulus.
  Step 2 – Information is carried to the CNS in the form of an impulse that travels along a sensory (afferent) fiber.
  Step 3. The information is processed within the CNS by interneurons.
  Step 4. Motor neurons are stimulated to conduct an impulse along an efferent fiber to the periphery.
  Step 5. The impulse arrives at an effector organ that produces the response.
  Monosynaptic reflexes
     The simplest reflex arc involves only two neurons, a sensory and motor neuron with a single synapse between them. The stretch reflex is a simple monosynaptic reflex which enables a muscle to automatically adjust tone in response to stretching. The most familiar stretch reflex is the patellar (knee-jerk) reflex. A sharp tap on the patellar ligament activates stretch receptors in the quadriceps femoris muscle. The monosynaptic reflex arc quickly produces a contraction in the same muscle that results in a noticeable kick.
  Polysynaptic reflexes
     These reflexes involve interneurons and can be very complicated depending on the number of interneurons. The withdrawal reflex from a painful stimulus is an example of a polysynaptic reflex.
 
Autonomic Nervous System
     The autonomic nervous system coordinates vital functions associated with cardiovascular, respiratory, digestive, excretory and reproductive functions that are out of our voluntary control. Two neurons, connected in series, sends out the motor command. The first neuron is preganglionic and has its cell body within the central nervous system. The second neuron in the chain is ganglionic and has its cell bodies outside of the central nervous system in a ganglion.
  Subdivisions of the ANS
     There are two major subdivisions of the ANS:
  Sympathetic Division
     This division is called the “fight or flight” division and prepares the body for exertion, or stress by increasing tissue metabolism.
  Parasympathetic Divison
     This division is called the “rest and repose” system and promotes activities that conserves or increases energy such as digestion.
 
  Sympathetic Division
     The sympathetic division is also called the thoracolumbar division because it preganglionic neurons are located in the spinal cord segments T1 to L2 in the lateral gray horns. The preganglionic neurons synapse with ganglionic neurons in ganglia near the vertebral column. These include:
  Sympathetic chain ganglia – these ganglia lie lateral to the vertebral column and control effectors in the body wall, head and neck, limbs, and inside the thoracic cavity.
  Collateral ganglia – these ganglia lie anterior to the vertebral column and innervate effectors in the abdominal pelvic cavity.
  Adrenal medulla – the core of the adrenal gland represents a modified sympathetic ganglion.
 
  Parasympathetic Division
     This division is also called the craniosacral division   because preganglionic neurons are located in the brainstem and in the spinal cord segments S2 – S4. The preganglionic neurons of the brainstem are associated with cranial nerves III, VII, IX and X.
     The ganglionic neurons in the peripheral ganglion are located very close to or within the target organs.