Biol 2401 A&P

Biol 2401 A&P Lecture Notes Spinal Cord and Spinal Nerves Dr. Weis

SPINAL CORD AND SPINAL NERVES

Includes both the CNS (spinal cord) and PNS (spinal nerves)

Recall that the neural plate forms a tube that will close to form a hollow center called the central canal.
The cranial portion of the tube will develop into the various brain regions and the areas posterior to the hind brain (brain stem)
gives rise to the spinal cord. Neuroepithelial cells will give rise to any neural cell type.

Protection of the spinal cord

1. Vertebral foramen.....to allow passage of the spinal cord.

2. Three meningeal membranes

a. dura mater (outermost).... spinal dural sheath

b. arachnoid mater (middle)

c. pia mater (innermost) .. tightly bound to the neural tissue

The meningeal membranes are separated by spaces

1. Epidural Space

between the vertebrae and dura

contains loose connective tissue, adipose, blood vessels

cushions cord from vertebrae

2. Subdural space

between dura and arachnoid

contains lymphatic fluid

3. Subarachnoid space

between arachnoid and pia

contains CSF produced by the ependymal cells

GROSS ANATOMY OF THE SPINAL CORD

The spinal cord provides a two way conduction pathway to and from the brain.
The cord extends from the medulla oblongata of the brain (which gives rise to Cranial Nerves VIII to XII)
to the level of the first or second lumbar vertebrae (L1-L2) in the adult.

Here it tapers down to a cone shaped area called the CONUS MEDULARIS.

The caudal nerve roots will continue and extend down from the lower part of the cord and are collectively called the
CAUDA EQUINA (horse tail) due to its gross anatomical appearance. T
he final piece of the cord that extends into the sacral-coccygeal region is called the FILIUM TERMINALE.
There are two regions of enlargement (entumesence) in the cervical and lumbar regions that will give rise to the nerves in the cervical, brachial, lumbar, and sacral plexus.

Spinal nerves :

each contain thousands of nerve fibers

are mixed nerves (carry sensory and motor information)

are named according to their point of origin from the spinal cord.

The spinal cord can be divided into segments related to the vertebral region.

There are 8 pair of cervical nerves

12 pair of thoracic nerves

5 pair of lumbar nerves

5 pair of sacral nerves

1 pair of coccygeal nerves __________________________________

31 pair of spinal nerves

In the cervical region, the spinal nerves will exit the intervertebral foramen CRANIAL to their respective vertebrae for C1-C7.

Cervical nerve 8 will exit caudally to C7 and cranial to T1.

The remaining spinal nerves will exit caudal to their respective vertebrae. Therefore the first thoracic spinal nerve will exit behind the first thoracic vertebra (T1).

A plexus is formed by the interlacing network of the ventral branches (rami) of the spinal nerves from various vertebral regions

Cervical Plexus C1-C5 ventral branches phrenic nerve (C3-C5)

Brachial Plexus C5-T1 ventral branches axillary n. C5-C6

suprascapular C5-C6

radial n. C5-T1

musculocutaneous (C5-C7)

median/ulnar C8 T1

Lumbar Plexus L1-L4 ventral branches obturator n. L2 L4

femoral n. L2-L4

Saphenous L2-L4

Sacral Plexus L4-S3 ventral branches gluteal n. L4-S2

Sciatic n L4-S3

pudendal n S2-S4

note :: There are NO plexuses formed from the ventral rami of spinal nerves T2-T12.

SPINAL CORD HISTOLOGY :

On histological cross section, the spinal cord is designated :

central canal, median sulcus, median fissure, white matter,

gray matter (horns, commisure)

gray matter ::

The cord gray mater involves

1. reflexes

2. other motor functions

The posterior (dorsal) gray horn contains cell bodies of sensory neurons

The anterior (ventral) gray horn contains cell bodies of motor neurons

white mater :: in columns or funiculi --> posterior, lateral, anterior

The cord white matter involves messages that are carried

1. to the brain along ascending tracks

2. from the brain along descending tracks

will form multineuron pathways using interneurons.

Each spinal cord segment has two roots and a ganglia

A. Roots

dorsal root has sensory afferent fibers arising from sensory neurons in the dorsal root ganglia and
conducts impulses from peripheral receptors to the spinal cord

ventral root has motor efferent fibers arising from the anterior horn motor neurons.

Will extend to skeletal muscle

ANS efferents are also contained in the ventral roots

B. Ganglia

dorsal root ganglia contains the cell bodies of the sensory neurons

located between the pedicles of the vertebrae.

The dorsal and ventral roots will leave through the intervertebral foramen and join to form the spinal nerve.

THEREFORE, all sensory information comes through the dorsal root ganglion and the ventral root is for motor
either directly to skeletal muscle or synapses with other neurons in the sympathetic chain (ANS) for further response of involuntary muscle.

For the peripheral nervous system, we can now summarize :

The spinal nerve is quite short and will divide into branches called RAMUS

The dorsal ramus provides sensory and motor to the skin and muscles of the back

The ventral ramus provides ventrolateral innervation.

Remember the plexuses that are formed from the internetworking of the ventral rami in the cervical, brachial, lumbar & sacral regions

Each pair of spinal nerves monitors a specific region of the body known as a DERMATOME
and these are useful in determining possible injury to a spinal nerve, by loss of skin sensation.
All spinal nerves, except C1, participate in dermatomes.

There is some overlap to dermatome regions (50%), but maps of the trunk and appendages can be drawn to show the sensory innervation regions of the spinal nerves.

SPINAL NERVE REFLEXES

Dorsal roots -----> sensory cell bodies of peripheral sensory nerve in the dorsal root ganglion

Ventral roots -----> motor cell bodies in ventral horn

Afferent neurons propagate an impulse into the spinal cord by way of the spinal nerve --> dorsal root ganglia --> dorsal gray column in the dorsal horn.

Afferent fiber may synapse upon

1. Interneuron

2. Lower motor neuron (LMN)

Interneurons can be located in the dorsal, intermediate or ventral horn. Its axons may

1. synapse onto ventral horn neurons

on the same side (ispilateral)

on the opposite side (contralateral)

on both sides (bilateral)

2. synapse with other interneurons that will project to ventral horn neurons

3. ascend or descend to other segments of the spinal cord or to the brain stem

before a synapse is made.

In the ventral horn, with its cell bodies of efferent neurons, the axons exit in the ventral root of the spinal nerve to reach effector organs.

REFLEX :: a rapid, predictable motor response to a stimulus

1. inborn --> unlearned, involuntary

2. learned --> acquired through practice, repitition

Two types based on anatomy :

Monosynaptic

Polysynaptic

REFLEX ARC

components

a. receptor.....site of stimulus

b. sensory neuron... afferent impulses to CNS

c. Integration center...

(+) multisynaptic, involve interneurons

(-) simple, monosynaptic

d. motor neuron... efferent impulses

e. effector organ... muscle contraction or gland secretion

Spinal Reflexes are somatic reflexes mediated by the spinal cord

Types ::

A. Stretch Reflex (Superficial Tendon)

* muscle spindle --> modified skeletal muscle fibers called intrafusal muscle fibers

1. nuclear bag

2. nuclear chain

associated with sensory afferents

1. type Ia fibers

2. type II fibers

and associated with motor efferents

1. gamma

2. alpha

stretching and exciting muscle spindle will affect the alpha motor to the spinal cord.

This will cause the relaxing of the antagonists to prevent resistance against the shortening of the stretched muscle

The stretch reflex is needed to maintain normal muscle tone and activity

Example : patellar reflex (knee jerk reflex)

Biceps reflex, triceps reflex, achilles tendon reflex

B. Deep Tendon Reflex

* golgi tendon receptors

Purpose : help insure smooth onset & termination of muscle contraction, cause muscle relaxation and lengthening

important in activities involving rapid switching between flexion & extension

C. Flexor Reflex (withdraw)

initiated by painful stimulus

ipsilateral, polysynaptic

D. Crossed Extensor

ipsilateral withdraw with contralateral extensor

polysynaptic (interneurons)

important in maintaining balance

E. Superficial

cutaneous stimulation

examples ::

plantar reflex ...... to check L4-S2

abdominal reflex..... to check T8-T12

In summary ::

the simplest reflexes are the myotatic reflexes.

no interneuron

only sensory and motor (2 neuron) pathway

tendon reflex :: patellar, triceps, biceps

A multisynaptic (polysynaptic) reflex ::

has interneurons (one or more)

3(+) neuron pathway -> sensory, interneuron(s), motor

The sensory signal can follow two paths

> locally in spinal cord

> higher levels..... brain stem cortex for conscious sensory experience

The spinal white matter

1. Contain primarily the dendrites and myelinated axons

that carry messages to and from the brain

2. Divided into regions that have a similar purpose

3. Create spinal tracks :

ascending --> sensory

descending -> motor

Most ascending tracks are located in dorsal and lateral funiculus (column)

Most descending tracks are located in medial and ventral funiculus (column)

Some tracks will stay on the same side, and some will cross.

(this applies to both ascending and descending tracks)

The name implies the origin and destination, for example

origin..... spinal

destination... cerebellar

will be called the spinocerebellar track, an ascending track for proprioception

spinal track origin.... vestibular

spinal track destination... spinal

will be called the vestibulospinal track

a descending track for involuntary regulation of balance

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