Spinal Region: Lecture #1The Spinal Cord

Lecturer: Sue Palfreyman

Reading

• Lundy: Chapter 13• Kandel: Chapter 16

Lundy-Ekman. Neuroscience: Fundamentals for Rehabilitation, 4th Edition. W.B. Saunders Company, 2013.

Kandel et al. Principles of Neural Science, 5th Edition. McGraw Hill, 2012.

Tortura & Derrickson. Principles of anatomy and physiology, 13th Edition. Wiley. 2012.

Overview

• External anatomy of the spinal cord• Internal anatomy of the spinal cord

– Gray matter– White matter

• Spinal cord tracts• Spinal nerves• Dorsal and Ventral Rami• Reflexes

Learning OutcomesAt the end of this session you should be able to….

• Describe the coverings of the cord, the internal and external anatomy of the spinal cord

• Describe the organisation and segmentation of the spinal cord • Explain why/how the white and gray matter changes with

placement along the cord• Name and locate the five ascending tracts and the six

descending tracts• Describe the relationship between spinal nerves and nerve

roots, rami, dermatomes and myotomes• Explain how the following reflexes work:

– Flexor/Withdrawal reflex– Crossed-Extensor reflex

INTRODUCTION AND REVIEW

What are the functions of the spinal cord?

What structures provide protection for the spinal cord?

Segmentation of the Cord

How many spinal segments are there?

And how many in each region?• Cervical: • Thoracic: • Lumbar: • Sacral:

• C0 (coccygeal):

Ref: Lundy-Ekman. Neuroscience: Fundamentals for Rehabilitation, 4th Edition. Pub: W.B. Saunders, 2013.

Ref: Tortura & Derrickson. Principles of anatomy and physiology, 13th Edition. Wiley. 2012.

Relationship of the Spinal Cord to the Vertebral Column

• Where is the spinal cord located in relation to the vertebral column?

• Where do spinal nerves emerge (in general terms)?

• Where do each of the following spinal nerves emerge?– C4– T6– L5– S2

Caudal End of the Spinal Cord

Explain the following terms:

• Conus medullaris

• Cauda equina

• Filum terminale

Cervical & Lumbar Enlargements

What and where are the cervical and lumbar enlargements?

EXTERNAL ANATOMY AND COVERINGS OF THE SPINAL CORD

External Anatomy

• Most surface features were reviewed in the previous section

• There is a prominent anterior median fissure which contains branches of the anterior spinal artery

• The less prominent posterior median sulcus contains a delicate layer of pia mater, called the posterior median septum, separating the posterior part of the cord into two halves

• Anterolateral and posterolateral sulci are small depressions where ventral and dorsal rootlets emerge

NB fissures are BIG and sulci are SMALL cf longitudinal fissure vs. central sulcus

Meningeal Coverings

• Spinal dura mater– Separated from vertebral canal by extra-dural space– Continuous with cranial dura mater superiorly– Inferiorly, extends as a fascial tube around pial part of filum

terminale and attaches to coccyx

• Arachnoid mater– Thin, delicate membrane– Close to, but not attached to, internal surface of dura– Delicate network of fibres run from arachnoid, across subarachnoid

space, to pia mater

• Pia mater– Vascular membrane firmly attached to outer surface of spinal cord

Ref: Tortura & Derrickson. Principles of anatomy and physiology, 13th Edition. Wiley. 2012.

Meningeal Coverings cont’d

• As spinal nerves exit the cord, they are surrounded by a tube of dura mater

• As the spinal nerve passes through he intervertebral foramen, the dural sheath blends with the epineurium, which adheres to the periosteum of the intervertebral foramen

• A tube-like extension of the arachnoid mater also surrounds the spinal nerve, to the point where the nerve enters the intervertebral foramen

Ref: Lundy-Ekman. Neuroscience: Fundamentals for Rehabilitation, 4th Edition. Pub: W.B. Saunders, 2013.

Meningeal Coverings of the Spinal Nerves

INTERNAL ANATOMY OF THE SPINAL CORD

General Arrangement

• Transverse section reveals white matter surrounds gray (unlike in the cerebrum)

• White matter consists of bundles of myelinated axons

• Two grooves divide cord into left and right sides• Anterior median fissure is deep, wide groove

on ventral/anterior side• Posterior median sulcus is shallow, narrow

groove on dorsal/posterior side

White & Grey Matter

• White matter contains the axons that connect different levels of the cord with each other and with the brain

• Grey matter consists of nerve cell bodies, dendrites, interneurons and neuroglia– Shaped like H or butterfly

• Grey commissure crosses centre and within this is the central canal (CSF-filled)

• The relative amounts of white and grey matter varies at different levels of the cord

Ref: Kandel et al. Principles of Neural Science, 5 th Edition. McGraw Hill, 2012.

T9-T12

Ref: Tortura & Derrickson. Principles of anatomy and physiology, 13th Edition. Wiley. 2012.

Grey Matter

• Divided into three horns:– Posterior/Dorsal horn: processes sensory information (cell

bodies in dorsal root ganglia)

– Lateral horn (T1-L2 only): contains cell bodies of preganglionic sympathetic neurones (a comparable are in S2 -4 includes cell bodies of preganglionic parasympathetic neurones)

– Anterior/Ventral horn: processes motor information (contain somatic motor cell bodies)

• Grey matter has been classified into ten regions based on the size, shape and distribution of the neurones in each area– These regions are called laminae (Rexed’s laminae)

• Grey commissure connects grey matter of left/rights sides

White Matter

• White matter is also organised into regions:– Posterior/Dorsal column– Lateral column– Anterior/Ventral column

• Columns are also called fasciculi and funiculi in the cord

• White commissure connects white matter of left/rights sides

Ref: Tortura & Derrickson. Principles of anatomy and physiology, 13th Edition. Wiley. 2012.

Spinal Cord Tracts

• Each column contains distinct bundles of axons that carry similar information to their destination– These bundles are called tracts (“nerves” are

bundles of axons in the PNS)

• Sensory (ascending) tracts carry nerve impulses from receptors in periphery toward the brain

• Motor (descending) tracts carry nerve impulses from the brain to effectors in the periphery

Spinal Cord Tracts cont’d

• Names of most of the tracts indicate: – The white column in which the tract is located, – The structure in which the axons making up the

tract originate, and – The structure in which they terminate

For example:

Lateral corticospinal tract

Lateral white column Axons originate in the

cerebrum/cortex

Terminate in the spinal cord

Will the tract be afferent or efferent???

Spinal Cord Tracts

• NB: At the moment you only need to know about the concept of the spinal cord tracts

• You will need to know about these tracts in detail later in the course (and importantly, after the mid-semester test) in relation to the somatosensory NS and the motor system

• The next few slides summarise much of the information that you will need to know on the tracts is but for now this is FYI only

Spinal Cord Tracts

www.baileybio.com/plogger/images/biology/powerpoint_-nervous_system/spinal_cord_tracts.jpg

Ascending Tracts

1. Lateral spinothalamic: coarse touch, pain, temperature

2. Anterior spinothalamic: coarse touch, pressure

3. Fasciculi gracilis/cuneatus: discriminating touch, proprioception

4. Spinocerebellar: subconscious kinesthesia

5. Spinotectal: touch that triggers visual reflexes (termination: superior colliculus)

Major Ascending Tracts

Name Function Location Origin Termination

Lateral Spinothalamic

Lateral white columns

Anterior Spinothalamic

Fasciculi Gracilis and Cuneatus

Spinal ganglia on same side

Anterior & Posterior Spinocerebellar

Spinotectal Superior colliculus (midbrain)

Descending Tracts

1. Lateral corticospinal: voluntary movement (contralateral)

2. Anterior (medial) corticospinal: voluntary movement (ipsilateral)

3. Reticulospinal: maintaining posture during muscle movement (origin: reticular formation)

4. Rubrospinal: transmit impulses that coordinate body movements and maintenance of posture (origin: red nucleus of midbrain)

5. Tectospinal: head and neck movement related to visual reflexes (origin: superior colliculus)

6. Vestibulospinal: coordination of posture and balance (vestibular nucleus in pons/medulla ob.)

Major Descending TractsName Function Location Origin Termination

Lateral corticospinal (crossed pyramidal)

Lateral or anterior grey columns

Anterior corticospinal (direct pyramidal)

Reticulospinal Anterior white columns

Rubrospinal Red nucleus (midbrain)

Tectospinal

Vestibulospinal

Ref: Lundy-Ekman. Neuroscience: Fundamentals for Rehabilitation, 4th Edition. Pub: W.B. Saunders, 2013.

Organisation of Fasciculus Gracilis/Cuneatus

SPINAL NERVES, DERMATOMES AND MYOTOMES

Spinal Nerves - PNS

• Spinal nerves are parallel bundles of axons wrapped in several layers of connective tissue

• Connect CNS to sensory receptors, muscles, glands

• 31 pairs named/numbered according to which level of vertebral column they emerge

• Spinal nerves are mixed (motor & sensory) and exist briefly as they pass through the intervertebral foramen – this marks the division between CNS & PNS

Spinal Nerve Roots

• Roots connect each spinal nerve to a “segment” of cord (note that the cord is actually continuous)

• For convenience the naming of spinal nerves is based on the segment in which they are located

• Dorsal /posterior roots contain afferent/sensory nerve fibers ONLY and conduct nerve impulses from the periphery (skin, muscles, internal organs) to the CNS (spinal cord and brain)– Swelling is dorsal root ganglion: cell bodies of the sensory

neurons from the periphery

• Ventral /anterior roots contain efferent/motor nerve fibers ONLY and conduct nerve impulses from the CNS to the peripheral effectors (muscles/glands)

Ref: Tortura & Derrickson. Principles of anatomy and physiology, 13th Edition. Wiley. 2012.

Coverings of Spinal NervesThe diagrams here and on the next slide illustrate the structure of a nerve – not directly part of the syllabus for neuroscience but hopefully revision from bioscience last year.

Spinal Nerve Rami

• Shortly after passing through its intervertebral foramen, the (mixed) spinal nerve divides into several branches, known as rami

• (Posterior) Dorsal ramus: serves deep muscles and skin of dorsal surface of trunk

• (Anterior) Ventral ramus: serves muscles and structures of the upper and lower limbs and the lateral and ventral trunk

• Spinal nerves also give off a meningeal branch– Re-enters spinal canal through intervertebral foramen– Supplies vertebrae, ligaments, blood vessels, and meninges

• Rami communicantes: components of the autonomic system

Ref: Tortura & Derrickson. Principles of anatomy and physiology, 13th Edition. Wiley. 2012.

Ref: Tortura & Derrickson. Principles of anatomy and physiology, 13th Edition. Wiley. 2012.

Dermatomes & Myotomes

• Dermatome: detailed mapping of the skin surface has revealed a close relationship between the spinal origin of each spinal nerve and the region of the body it innervates

• Myotome is a skeletal muscle of group of muscles that receives motor axons from a given spinal nerve

• Useful for health practitioners – can provide information about areas of injury to the cord or spinal nerves

Dermatomes of the Lower LimbRef: Moore & Dalley, Clinically Oriented Anatomy, 5th edn

Dermatomes of the Upper LimbRef: Gilroy, Mac Pherson & Ross (2008) Atlas of Anatomy

Ref: Tortura & Grabowski (2000) Principles of Anatomy & Physiology 9th edn. Pub Wiley

REFLEXES

Reflexes

• Intrinsic and acquired• Learned or acquired reflexes result from

practice/repetition – eg changing gear in car• Intrinsic reflex is a rapid, predictable motor response to a

stimulus (unlearned, involuntary)• Evolutionarily adapted behaviour to keep body intact,

upright, and alive (and undamaged)• Many reflexes occur without involvement from higher

cognitive structures (thinking), but the brain is “advised” of spinal reflexes and can facilitate, inhibit, or adapt depending on circumstances (hot pot – you know you dropped the pot!)

• Other reflexes go on unnoticed (accommodation)

Reflex Arc• Signal conduction route

to and from the CNS• Commonly involves 3-

neuron relay– Afferent neuron (from

sensory receptor)– Interneuron (within the

spinal cord)– Efferent neuron (to

effector organ – usually muscle)

• Can be ipsilateral or contralateralRef: Anatomy & Physiology, Patton & Thibodeau

Ref: Tortura & Derrickson. Principles of anatomy and physiology, 13th Edition. Wiley. 2012.

Reflex Arcs

• E.g., Withdrawal reflex• Serial processing mediated by spinal cord• Parallel processing (simultaneously) of sensory

aspect – conscious awareness• Reflex arcs five essential components

– Receptor: nociceptor in skin– Sensory neuron: afferent impulse to CNS– Integration center (within CNS): could be single synapse

between sensory and motor neurons– Motor neuron: efferent impulse– Effector: muscle fiber/gland etc that responds

Reflexes

• Reflexes classified as:– Autonomic reflexes if activate visceral effectors– Somatic reflexes if activate skeletal muscle– Spinal reflexes are mediated by spinal cord

• Important clinically to assess condition of NS

• Flexor (withdrawal) reflex• Crossed-Extensor reflex

Flexor/Withdrawal Reflex

• Initiated by painful stimulus• Ipsilateral and polysynaptic (required when

several muscles are needed to make the withdrawal)

• Flexor muscles contract and withdraw limb• Can be overridden by descending signals

from the brain, such as when expecting a prick when a needle is required

Ref: Tortura & Derrickson. Principles of anatomy and physiology, 13th Edition. Wiley. 2012.

Crossed-Extensor Reflex

• Often accompanies flexor reflex in weight-bearing limbs (step on a tack)

• Important in maintaining balance• Ipsilateral flexor/withdrawal reflex AND

contralateral extensor reflex (mediated by ascending and descending interneurons within the spinal cord)

• Extensor muscles contract and extend opposite leg

Withdrawal Reflex and Crossed-Extensor ReflexRef: Tortura & Derrickson. Principles of anatomy and physiology, 13th Edition. Wiley. 2012.