Spinal Cord and Spinal Nerves | Anatomy ppt

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Spinal Cord and Spinal Nerves ppt

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Human Anatomy, First Edition
McKinley & O’Loughlin

Chapter 16 : Spinal Cord and Spinal Nerves


The Spinal Cord

Link between the brain and the body.

Exhibits some functional independence from the brain.

The spinal cord and spinal nerves serve two functions:

pathway for sensory and motor impulses

responsible for reflexes

Structure of the Spinal Cord

Typical adult spinal cord

ranges between 42 and 45 centimeters (cm) (16 to 18 inches) in length.

In cross section

roughly cylindrical

slightly flattened both posteriorly and anteriorly.

External surface has two longitudinal depressions:

the posterior (dorsal) median sulcus

the anterior (ventral) median fissure

Regions of the Spinal Cord

The cervical region

continuous with the medulla oblongata

contains neurons whose axons form the cervical spinal nerves (8)

The thoracic region

attached to this region are the thoracic spinal nerves (12)

The lumbar region

contains the neurons for the lumbar spinal nerves (5)

The sacral region

contains the neurons for the sacral spinal nerves (5)

The coccygeal region

one pair of coccygeal spinal nerves arises from this region

Structure of the Spinal Cord

The spinal cord is shorter than the vertebral canal that houses it.

Conus medullaris:

tapered inferior end of the spinal cord

marks the official “end” of the spinal cord proper.

Cauda equina

Inferior to conus medularis

nerve roots (groups of axons) that project inferiorly from the spinal cord.

Filum terminale

Within the cauda equina

thin strand of pia mater

helps anchor the conus medullaris to the coccyx.


Structure of the Spinal Cord

The spinal cord is associated with 31 pairs of spinal nerves

Connect the CNS to:


effectors (muscle and glands)

Each side of the spinal cord contains:

8 cervical nerves (called C1–C8)

12 thoracic nerves (T1–T12)

5 lumbar nerves (L1–L5)

5 sacral nerves (S1–S5)

1 coccygeal nerve (Co1)

Arrangement and Functions of the Spinal Meninges

Are continuous with the cranial meninges.

Structures that encircle the spinal cord, listed from superficial to deep are:


epidural space

dura mater

subdural space


subarachnoid space

pia mater

Location and Distribution of Gray Matter

In the spinal cord, it is centrally located.

Its shape resembles a letter H or a butterfly.

The gray matter may be subdivided into the following components:

anterior horns

lateral horns

posterior horns

the gray commissure

Location and Distribution of White Matter

The white matter of the spinal cord is external to the gray matter.

Three regions.

Composed of tracts



A posterior funiculus:

lies between the posterior gray horns and the posterior median sulcus.

Location and Distribution of White Matter

Lateral funiculus.

Anterior funiculus

between the anterior gray horns and the anterior median fissure.

The anterior funiculi are interconnected by the white commissure.

Spinal Nerves

31 pairs

connect the CNS to:


muscles, glands

Each spinal nerve is mixed:

thousands of motor and sensory axons.

Sensory axons originate from receptors

Motor axons originate from the spinal cord.

Anterior root and posterior root unite within the intervertebral foramen

become a spinal nerve.

Spinal nerve is associated with the vertebra of the same number.

Rami of Spinal Nerves

Posterior (or Dorsal) ramus

Innervates muscles and skin of the back

Anterior Ramus

Largest branch

Forms plexuses

Innervates anterior and lateral trunk, upper and lower limbs

Rami communicantes

Autonomic nervous system (sympathetic)


A specific segment of skin supplied by a single spinal nerve.

All spinal nerves

innervate a segment of skin and are associated with a dermatome.

except for C1

Dermatome map:

sensory segments: skin of the body associated with a spinal nerve

Intercostal Nerves

Anterior rami of spinal nerves T1–T11.

Travel in the intercostal space sandwiched between two adjacent ribs

Nerve Plexuses

A network of interweaving anterior rami of spinal nerves.

nerve plexuses on both the right and left sides of the body.

Nerve plexuses then split into multiple “named” nerves that innervate various body structures.

Principal plexuses

cervical plexuses

brachial plexuses

lumbar plexuses

sacral plexuses.


A reflex is a response:

Rapid, automatic

involuntary reactions of effectors to a stimulus.


a stimulus

required to initiate a response to sensory input

a rapid response

requires that few neurons be involved

synaptic delay be minimal

an automatic response occurs the same way every time

An involuntary response requires no intent or pre-awareness of the reflex activity.

Reflexes usually can not be suppressed.

Awareness of the stimulus occurs after the reflex action

in time to correct or avoid a potentially dangerous situation.


Components of a Reflex Arc

The neural “wiring” of a single reflex.

Always begins at a receptor in the PNS

Sensory afferent

Communicates with the CNS.

May involve interneurons

Ends at a peripheral effector (muscle or gland)

Motor efferent

Ipsilateral and Contralateral  Reflex Arcs


both the receptor and effector organs of the reflex are on the same side of the spinal cord.


the sensory impulses from a receptor organ cross over through the spinal cord to activate effector organs in the opposite side


Monosynaptic Reflexes

The simplest of all reflexes.

No interneurons.

The patellar (knee-jerk) reflex is a monosynaptic reflex

physicians use to assess the functioning of the spinal cord.

tap the patellar ligament with a reflex hammer

muscle spindles in the quadriceps muscles are stretched.

Produces a noticeable kick of the leg.

Polysynaptic Reflexes

Have more complex neural pathways

exhibit a number of synapses

involve interneurons within the reflex arc.

Has more components

more prolonged delay between stimulus and response.


Stretch Reflexes

Monosynaptic reflex that monitors and regulates skeletal muscle length.

When a stimulus results in the stretching of a muscle, that muscle reflexively contracts.

The patellar (knee-jerk) reflex is an example of a stretch reflex.

The stimulus (the tap on the patellar tendon) initiates contraction of the quadriceps femoris muscle and extension of the knee joint.


Golgi Tendon Reflex

Prevents skeletal muscles from tensing excessively.

Golgi tendon organs are nerve endings located within tendons near a muscle–tendon junction.

activation of the Golgi tendon organ signal interneurons in the spinal cord, which in turn inhibit the actions of the motor neurons

The associated muscle is allowed to relax, thus protecting the muscle and tendon from excessive tension damage.


Reflex Testing in a Clinical Setting

Reflexes can be used to test specific muscle groups and specific spinal nerves or segments of the spinal cord.

Consistently abnormal reflex response may indicate damage to the nervous system or muscles.

A reflex response may be normal, hypoactive, or hyperactive.


Spinal Cord Development

The central nervous system forms from the embryonic neural tube.

Cranial and spinal nerves form from neural crest cells that have split off from the developing neural tube.

The cranial (superior) part of the neural tube expands and develops into the brain.

The caudal (inferior) part of the neural tube forms the spinal cord.



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