Central Nervous System

- Spinal Cord, Spinal Nerves & Spinal Reflexes

Chapter 13A

Central Nervous System

Central nervous system (CNS) is responsible for:Receiving impulses from receptorsIntegrating informationSending impulses to the effectors

It is composed of:BrainSpinal cord

Spinal Cord - Functions

Spinal cord has the following functions:1. Receive and send impulses: receives impulses from

receptors and sends impulses to the effectors.

2. Communication with the brain: has bundles/cables of nerve fibers (tracts) that take sensory impulses up to the brain or motor impulses down from the brain.

3. Movement: muscle contraction for basic movement is controlled by the spinal cord…although the initiation, the speed and the direction of movement is controlled by the brain.

4. Reflexes: simple reflexes are controlled by the spinal cord….pulling your finger back when you touch a hot plate.Complex reflexes are controlled by the brain… remembering not to touch a hot plate again!

Spinal Cord - Protection

Spinal cord is protected by bones and 3 connective tissue membranes called meninges.

From outside inside:1. Boney protection: vertebrae vertebral foramina align form vertebral canal

houses spinal cord.2. Dura mater: outermost tougher meninx.3. Arachnoid mater: middle avascular meninx.4. Pia mater: innermost meninx that sticks to the spinal cord.

Pia materArachnoidmater

Dura mater

Vertebra

Spinal cordMeninges

Meninges

Vertebralforamen

Spinal Cord –Spaces

There are spaces between the protective bones and the 3 meninges.

1. Epidural space – space between vertebrae and the dura mater- filled with adipose tissue.

2. Subdural space –space between dura mater and arachnoid-filled with interstitial fluid (no such space in healthy person; space appears when there is trauma or underlying pathological conditions).

3. Subarachnoid space – space between arachnoid and pia mater- filled with cerebrospinal fluid (CSF).

Spinal Cord

Lumbar puncture/Spinal tap: a sample of CSF is removed from subarachnoid space chemical analysis for diagnosis of certain diseases and disorders- like Meningitis, Multiple Sclerosis, CNS tumors etc..

Epidural anesthesia: an anesthetic shot given in the epidural space of lumbar region numbs specific spinal nerves in the immediate area of injection…usually used for child birth, surgery in the pelvic region and legs.

Meningitis: inflammation of meninges…bacterial or viral.If left untreated infection spreads to brain death.

Spina bifida: incomplete closure of the pedicles of the vertebrae spinal cord and nerves not well protected paralysis of lower extremities and pelvic organs….May be due to lack of vitamin B/folic acid during pregnancy.

Lumbar puncture/spinal tap

Spinal Cord – External Structure

During early childhood development,vertebral column and spinal cord grow.

Around age 4 or 5 years, spinal cord growth slows down while vertebral column continues to grow.

Therefore in an adult, spinal cord extends from the base of the brain ends at L1 or L2.

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The level of the spinal cord segments do notrelate exactly to the level of the vertebral bodies i.e. damage to the bone at a particular level e.g. L5 vertebrae does not necessarily mean damage to the spinal cordat the same spinal nerve level.

Spinal Cord – External StructureSpinal cord is not uniform along its length.The amount of gray matter is greatest in segments of spinal cord dedicated to the sensory & motor control of the limbs.

Two regions of the spinal cord are wider:Cervical enlargement: located in the cervical and upper shoulder region supplies nerves to theshoulder and upper limbs.

Lumbar enlargement: located in the thoracic region innervates pelvis and lower limbs.

Conus medularis: tapered, cone shaped ending of the spinal cord.Located in L1-L2 region.

Cauda equina: A group of nerves that originate fromthe conus medullaris includes lumbar (2nd-5th), sacral (1st to 5th) and coccygeal (1) spinal nerves.- Resemble horse’s tail.- Function:- send and receive messages between the lower limbs and the pelvic organs, which consist ofthe bladder, the rectum, and the internal genital organs.

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L1

L2

L4L5

L3

Spinal Cord – Anchoring

Spinal cord is located inside the vertebral canal.It needs to be anchored to prevent it from moving around:

Filum terminale: extension of pia mater from conus medullaris to anchor spinal cord down to the coccyx.Denticulate ligament: extensions of pia mater-attaches laterally to arachnoid and dura mater-stabilizes side-to-side movement of spinal cord within vertebral column.

Pia materArachnoidDura mater

Spinal Cord – Internal Structure

To understand the internal structure of the spinal cord…..you have to get the directions right!!!!

Here you see spinal cord inside a vertebra.Which side is anterior and which is posterior??

Anterior or Posterior??

Anterior or Posterior??

Anterior/ventral

Posterior/dorsal

Anterior/Ventral

Posterior/dorsal

Spinal Cord – Internal Structure

Anterior median fissure: a deep groove that runs along the anterior surface of the spinal cord.

Posterior median sulcus: a depression that runs along the posterior surface of the spinal cord.

The tissue can be divided into gray matter and white matter.Gray matter: in the shape of letter H….towards the center.White matter: around the gray matter….along the periphery.

Anterior median fissure

Posterior median sulcus

Superior viewAnterior/ventral side

Posterior/dorsal side

Gray matter

White matter

Spinal Cord – Internal Structure- Gray matter

Gray matter:- dominated by the cell bodies of neurons, neuroglia, and unmyelinated axons, surrounds the narrow central canal and forms a butterfly shape or letter H shape.

Horns:- the projections of gray matter toward the outer surface of the spinal cord 2 arms of the letter H.

Gray commissure- the cross bar that connects the gray horns/middle portion of the gray matter.

Central canal: located in the center of the gray commissure has CSF lined by ciliated ependymal cells to circulate CSF.

Gray commissure

Central canal

Posterior/Dorsal gray horn: posterior part of gray matterFunction: contains sensory nuclei- somatic and visceral -receives information from skeletal muscles and skin (somatic) and visceral organs (visceral) and relays it to CNS.

Spinal Cord – Internal Structure- Gray matter

Anterior side

Posterior sidePosterior/Dorsal gray horn

Dorsal root ganglion: contains the cell bodies of sensory neurons whose axons carry information to the spinal cord.

Dorsal root ganglion

Dorsal root: contains the axons of the sensory neurons whose cell bodies are in the dorsal root ganglion.

Dorsal root

Anterior side

Posterior side

Anterior side

Posterior side

What structure is within the dorsal root ganglion? Cell bodies of sensory neurons

Lateral gray horn: lateral side of gray matterFunction: contains visceral motor nuclei –sends commands to smooth muscle, cardiac muscle and glands.

Ventral root: connected to ventral horns of spinal cordContains axons of motor neurons send message out to effectors

Anterior side

Posterior side

Anterior/Ventral gray horn

Anterior/Ventral gray horn: anterior part of gray matterFunction: contains motor nuclei (somatic) –sends commands to skeletal muscles.

Anterior side

Posterior side

Anterior side

Posterior side

Ventral root

What types of neurons travel through the ventral root? Motor neurons

Lateral gray horn

Spinal Cord – Internal Structure- Gray matter

A single spinal nerve contains the axons of BOTH sensory and motor neurons. The sensory fibers enter the CNS through the dorsal root. The motor fibers emerge from the CNS via the ventral root.

What types of neurons travel through the dorsal root? Sensory neurons

The superficial white matter contains large numbers of myelinated and unmyelinated axons. The structural components of white mater are divided into columns.

1. The posterior white column- lies between the posterior gray horns and posterior median sulcus-has ascending tracts only- carry sensory information from the spinal cord to the brain.

* tracts-bundles of myelinated axons in CNS (brain and spinal cord)

2. The lateral white column- includes the white matter on either side of the spinal cord between the anterior and posterior columns- has both ascending (carry sensory information from spinal cord to brain) & descending tracts (convey motor information from brain to the spinal cord).

3. The anterior white column- lies between the anterior gray horns and the anterior median fissure-has both ascending & descending tracts.

Spinal Cord – Internal Structure- White matter

Posterior median sulcus

Posterior/dorsal horn

Posterior whitecolumn Lateral white

column

Anterior whitecolumn

Spinal Nerves

Nerves are part of peripheral nervous system (PNS).

Functions of the nerves:Bring sensory impulses to the spinal cord.Take motor impulses to the muscles and glands (effectors).

Spinal Nerves - Structure

Nerve organization is strikingly similar to muscle organization!

Remember:A muscle is made of fascicles each fascicle is made of many muscle fibers.Endomysium is connective tissue inside the fascicle.Perimysium is connective tissue around each fascicle.Epimysium is connective tissue that wraps around the entire muscle!

Nerve Muscle

Spinal Nerves - Structure

A nerve is the primary structure of the peripheral nervous system (PNS) that encloses the axons of peripheral neurons…made of fascicles each fascicle is made of a group of nerve fibers (axons).

Endoneurium: is connective tissue around nerve fibers (axons) inside the fascicle.Perineurium: is connective tissue around each fascicle.Epineurium: is connective tissue that wraps around all the fascicles to form a nerve!

Spinal Nerves - Number

There are 31 pairs of spinal nerves…a total of 62 nerves.

Spinal cord is located in the vertebral canal.

Spinal nerves exit vertebral column through intervertebral foramina.

Intervertebral foramen

Vertebralcanal

Spinal Nerves - Number

Posterior view

Spinal Nerves

All spinal nerves are mixed nerves….contain sensory and motor fibers.

Motor fiber

Spinal nerve

Sensory neuron

Sensory fiber

Motor neuron

Interneuron

Spinal Nerves - SupplyN V

C2–C3

C2C3

C4

C3

C4

C5

C5T1T2

T2

T2 T7

T3T4T5T6

T8T9T10T11T12

T2

T1

T2

T7

T3T4T5T6

T8T9T10T11T12 C6

C7C6

C7

C8

T1C8

L1L2

L3L4L5

L1

L2

L3

L4

L1

S2

S1

S2

S3S4

S5L5

L1

L2

L3L5

L4

S1

ANTERIOR POSTERIOR

KEYSpinal cord regions

= Cervical= Thoracic= Lumbar= Sacral

Spinal nerves go to skin, muscles and some of the internal organs.

Dermatomes: areas of the skin that is connected to a specific spinal nerve.

Myotomes: specific muscles that are supplied by a specific spinal nerve.