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Central Nervous System

Brain and Spinal Cord

Learn and Understand

• Brain function is both localized and lateralized but information sharing is key to success

• Spinal cord also exhibits localization

• Nature has physically and chemically protected the brain and spinal cord

• Cerebral cortex is the seat of consciousness, most other areas coordinate with the cortex subconsciously

• Each sense is mapped to a particular location of the cortex

• Superior and anterior portions of the cerebrum represent more “advanced” areas; best developed in the primates and humans, in particular

Comparative Vertebrate Brains

Cephalization

• Similarities in location, form, and function

• Areas associated with rationality, use of hands

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Regions and Organization

Adult brain regions

1. Cerebral hemispheres

– five lobes, basal nuclei, nerve tracts

2. Diencephalon

– thalamus, hypothalamus, epithalamus

3. Brain stem

– midbrain, pons, and medulla

4. Cerebellum

– hemispheres and subdivisions

Ventricles span the first three regions

Septumpellucidum

Inferiorhorn

Lateralaperture

Lateralventricle

Anteriorhorn

Interventricularforamen

Thirdventricle

Cerebral aqueduct

Fourth ventricle

Central canal

Posteriorhorn

Inferiorhorn

Medianaperture

Lateralaperture

Anterior view Left lateral view

Ventricles of the Brain

• Filled with cerebrospinal fluid (CSF) produced by ependymal cell lining

• CSF slowly flows from space to space before being reabsorbed into blood

Protection of the Brain

1. Bone (skull)

2. Protective Membranes (meninges)

3. Watery cushion (cerebrospinal fluid)

4. Selective membrane (Blood brain barrier)

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Figure 12.22 Meninges: dura mater, arachnoid mater, and pia mater.

Skin of scalp

Periosteum

Bone of skullDura mater• Periosteal layer• Meningeal layer

Arachnoid mater

Pia mater

Arachnoid villus

Blood vessel

Falx cerebri(in longitudinalfissure only)

Superior sagittalsinus

Subduralspace

Subarachnoidspace

2. Meninges

• Cover and protect CNS

• Protect blood vessels and enclose venous sinuses

• Contain cerebrospinal fluid (CSF)

• Form partitions in skull

• Three layers

– Dura mater

• Strongest meninx

– Arachnoid mater - Middle layer with weblike extensions• Subarachnoid space contains CSF and largest blood vessels of brain

• Arachnoid villi protrude into superior sagittal sinus

– Pia mater

• Delicate, vascularized connective tissue that clings tightly to brain

3. Cerebrospinal Fluid (CSF)

• Composition

– Watery solution formed from blood plasma

• Less protein and different ion concentrations than plasma

– Constant volume maintained through regular production and loss

• Normal volume ~ 150 ml; replaced every 8 hours

• Functions

– Gives buoyancy to CNS structures

• Reduces weight by 97%

– Protects CNS from blows and other trauma

– Nourishes brain and carries chemical signals

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Figure 12.24a Formation, location, and circulation of CSF

Superiorsagittal sinus

Choroid plexus

InterventricularforamenThird ventricle

Cerebral aqueductLateral apertureFourth ventricleMedian aperture

Central canalof spinal cord

(a) CSF circulation

1 The choroid plexus of each Ventricle produces CSF. 2 CSF flows through the ventriclesand into the subarachnoid space via the median and lateral apertures.

3 CSF flows through the subarachnoid space.

4 CSF is absorbed into the duralvenous sinuses via the arachnoid villi.

Arachnoid villus

Subarachnoid spaceArachnoid mater

Meningeal dura mater

Periosteal dura mater

Right lateral ventricle(deep to cut)

Choroid plexusof fourth ventricle

1

4

2

3

Lateral ventricles -> third ventricle via interventricular foramen -> Third ventricle -> fourth ventricle via cerebral aqueduct-> apertures to subarachnoid

4. Blood Brain Barrier• Helps maintain stable environment for brain

• Separates neurons from some bloodborne substances

• Selective barrier

– nutrients move by facilitated diffusion

– Metabolic wastes, proteins, toxins, most drugs, small nonessential amino acids, K+ all stopped at barrier

– Allows any fat-soluble substances to pass, including alcohol, nicotine, and anesthetics

• Composition

– Continuous endothelium of capillary walls

– Thick basal lamina around capillaries

– Feet of astrocytes - Provide signal to endothelium for formation of tight junctions

Cerebral Hemispheres

• Surface markings– Ridges (gyri), shallow grooves (sulci), and deep grooves (fissures)

– Longitudinal fissure

• Separates two hemispheres

– Transverse cerebral fissure

• Separates cerebrum and cerebellum

• Five lobes – divided by sulci– Frontal

– Parietal

– Temporal – lateral sulcus separates temporal and parietal lobes

– Occipital

– Insula – deep to temporal lobe

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Figure 12.4b Lobes, sulci, and fissures of the cerebral hemispheres.

Left cerebralhemisphere

Transversecerebralfissure

Cerebellum

Brain stem

Left lateral view

Figure 12.4c Lobes, sulci, and fissures of the cerebral hemispheres.

Frontal lobe

Postcentralgyrus

Parietal lobe

Centralsulcus

Precentralgyrus

Parieto-occipital sulcus(on medial surfaceof hemisphere)

Lateral sulcus

Temporal lobe

Occipital lobe

Transversecerebral fissure

Pons

Spinal cordFissure(a deepsulcus)

Gyrus

Cortex (gray matter)

Sulcus

White matter

Lobes and sulci of the cerebrum

Medulla oblongata

Cerebellum

Frontal lobeCentralsulcus

Gyri of insula

Temporal lobe(pulled down)

Location of the insula lobe

Figure 12.4d Lobes, sulci, and fissures of the cerebral hemispheres.

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Figure 12.4a Lobes, sulci, and fissures of the cerebral hemispheres.

Anterior

Longitudinalfissure

Frontal lobe

Cerebral veinsand arteriescovered byarachnoidmater

Left cerebralhemisphere

Parietal lobe

Right cerebralhemisphere

Occipitallobe

Superior view

Posterior

Cerebral Cortex

• Thin (2–4 mm) superficial layer of gray matter

– Billions of neurons and associated neuroglia

• 40% mass of brain

• Location of conscious mind:

– Awareness

– Sensory perception

– Voluntary motor initiation

– Language

– Memory storage

– Understanding

– Motivation and decisionmaking

4 General Considerations of Cerebral Cortex

1. Three types of functional areas

– Motor areas—control voluntary movement

– Sensory areas—conscious awareness of sensation

– Association areas—integrate diverse information

2. Each hemisphere concerned with contralateral side of body

3. Lateralization of cortical function in hemispheres

– Sides process info separately while sharing

4. Conscious behavior involves entire cortex in some way

– Cortical domains perform specific functions with much input from other areas

– Memory and association occur throughout cerebral cortex

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Figure 12.6a Functional and structural areas of the cerebral cortex

Motor areas

Primary motor cortex

Premotor cortex

Frontaleye field

Broca's area(outlined by dashes)

Working memoryfor spatial tasks

Executive area fortask management

Working memory for object-recall tasks

Solving complex,multitask problems

Prefrontal cortex

Lateral view, left cerebral hemisphere

Sensory areas and relatedassociation areas

Primary somatosensorycortexSomatosensoryassociation cortex

Gustatory cortex(in insula)

Somatic sensation

Taste

Wernicke's area(outlined by dashes)

Primary visualcortex

Visualassociation area

Auditoryassociation area

Primary auditory cortex

Vision

Hearing

Central sulcus

Primary motorcortex

Motor associationcortex

Primary sensorycortex

Sensoryassociation cortex

Multimodal associationcortex

Figure 12.6b Functional and structural areas of the cerebral cortex

Corpuscallosum

Frontal eye field

Prefrontalcortex

Processes emotionsrelated to personaland social interactions

Orbitofrontalcortex

Olfactory bulb

Olfactory tract

FornixTemporallobe

Primaryolfactorycortex

Uncus

Calcarinesulcus

Parahippocampalgyrus

Parietal lobe

Somatosensoryassociation cortex

Parieto-occipitalsulcus

Occipitallobe

Visual associationarea

Primaryvisual cortex

Primary somatosensorycortex

Central sulcusPrimarymotor cortex

Cingulategyrus

Premotorcortex

Parasagittal view, right cerebral hemisphere

Primary motorcortex

Motor associationcortex

Primary sensorycortex

Sensoryassociation cortex

Multimodal associationcortex

Motor Areas of Cerebral Cortex

• Plan and control voluntary movement

• Located in frontal lobe

– Primary (somatic) motor cortex• precentral gyrus

– Premotor cortex

• anterior to primary MC

– Broca's area

• usually only in the left hemisphere

– Frontal eye field

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Primary Motor Cortex

• Large pyramidal cells of precentral gyri

• Long axons pyramidal (corticospinal) tractsof spinal cord

• Allows conscious control of precise, skilled, skeletal muscle movements

• Motor homunculi - upside-down caricatures represent contralateral motor innervation of body regions

Figure 12.7 Body maps in the primary motor cortex and somatosensory cortex of the cerebrum.Posterior

Motor SensoryAnterior

Primary motor

cortex

(precentral gyrus)

Primary somato-

sensory cortex

(postcentral gyrus)

Motor map in

precentral gyrus

Sensory map in

postcentral gyrus

Swallowing

Tongue

Jaw

Toes

Genitals

Foo

tK

nee

Hip

Tru

nk

Nec

k

Intra-abdominal

But cortex and motor unit cannot be precisely mapped

Anterior Association Area (Prefrontal Cortex)

The multimodal association areas collect and utilize sensory information and do the highest level of integration

• Most complicated cortical region

• Involved with intellect, cognition, recall, and personality

• Contains working memory needed for abstract ideas, judgment, reasoning, persistence, and planning

• Development depends on feedback from social environment

A portion of brain that is particularly well developed in humans

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Sensory Areas of Cerebral Cortex

• Conscious awareness of sensation

• Occur in parietal, insular, temporal, and occipital lobes

Figure 12.7 Body maps in the primary motor cortex and somatosensory cortex of the cerebrum.

Posterior

Motor SensoryAnterior

Primary motor

cortex

(precentral gyrus)

Primary somato-

sensory cortex

(postcentral gyrus)

Motor map in

precentral gyrus

Sensory map in

postcentral gyrus

Swallowing

Tongue

Jaw

Toes

Genitals

Foo

tK

nee

Hip

Tru

nk

Nec

k

Intra-abdominal

Primary Somatosensory Cortex

• In postcentral gyri of parietal lobe

• Receives general sensory information from skin, and proprioceptors of skeletal muscle, joints, and tendons

• Capable of spatial discrimination: identification of body region being stimulated

• Somatosensory homunculusupside-down caricatures represent contralateral sensory input from body regions

Somatosensory Association Cortex

• Posterior to primary somatosensory cortex

• Integrates sensory input from primary somatosensory cortex for understanding of object

• Determines size, texture, and relationship of parts of objects being felt

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• Primary visual cortex

– Extreme posterior tip of occipital lobe

– Receives visual information from retinas

– Map of retina’s sensory located here

• Visual association area

– Surrounds primary visual cortex

– Uses past visual experiences to interpret visual stimuli

– Complex processing involves entire posterior half of cerebral hemispheres

• Primary auditory cortex

– Superior margin of temporal lobes

– Interprets information from inner ear as pitch, loudness, and location

• Auditory association area

– Located posterior to primary auditory cortex

– Stores memories of sounds and permits perception of sound stimulus

• Vestibular Cortex

– Posterior part of insula and adjacent parietal cortex

– Responsible for conscious awareness of balance (position of head in space)

Primary olfactory cortex

– Medial aspect of temporal lobes

– Part of primitive rhinencephalon, along with olfactory bulbs and tracts

• Linked to limbic system

– Region of conscious awareness of odors

Visceral senses cortex

– Posterior to gustatory cortex

– Conscious perception of visceral sensations, e.g., upset stomach or full bladder

Gustatory cortex

– In insula just deep to temporal lobe

– Involved in perception of taste

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Posterior Association Area

• Large region in temporal, parietal, and occipital lobes

• Plays role in recognizing patterns and faces and localizing us in space

• Involved in understanding written and spoken language (Wernicke's area)

Lateralization of Cortical Function

• Left hemisphere

– Best at language, math, and logic

• Right hemisphere

– Best at Visual-spatial skills, intuition, emotion, and artistic and musical skills

• Hemispheres communicate almost instantaneously via fiber tracts and integrate the separate processing into one

• Hemispheres almost identical

• Lateralization - division of labor between hemispheres

• Cerebral dominance - hemisphere dominant for language (left hemisphere - 90% people)

SuperiorLongitudinal fissure

Lateral ventricle

Basal nuclei• Caudate• Putamen• Globus

pallidus

Thirdventricle

Pons

Medulla oblongata

Association fibers

(within hemisphere)

• Corpus callosum

Projection fibers(cerebral cortexto lower area)

• Internal capsule

White matter

Decussation(cross-over)of pyramids

Thalamus

• Corona radiata

Gray matter

Commissural fibers

(between hemispheres)

Frontal section

Cerebral White Matter • Myelinated fibers and tracts

• Communication between cerebral areas, and between cortex and lower CNS

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Caudate

nucleus

Putamen

Striatum Thalamus

Tail of caudatenucleus

Basal Nuclei (Ganglia)

Functions thought to be

– Influence muscle movements

– Regulate intensity of slow or stereotyped movements

– Filter out incorrect/inappropriate responses

– Inhibit antagonistic/unnecessary movements

– Role in cognition and emotion

Cerebral hemisphere

Septum pellucidum

Interthalamicadhesion(intermediatemass of thalamus)

InterventricularforamenAnteriorcommissure

Hypothalamus

Optic chiasma

Pituitary glandMammillary bodyPons

Medulla

oblongata

Spinal cord

Corpus callosum

Fornix

Choroid plexus

Thalamus

(encloses third ventricle)

PosteriorcommissurePineal gland

Epithalamus

CorporaquadrigeminaCerebralaqueduct

Midbrain

Arbor vitae (of cerebellum)Fourth ventricleChoroid plexusCerebellum

Diencephalon • Three paired structures

• Encloses third ventricle

Thalamus

• Dominates diencephalon (80% )

• Grouping of functionally specialized nuclei making up the superolateral walls of third ventricle

• Gateway to cerebral cortex

• Sorts, edits, and relays ascending input

– Impulses from hypothalamus for regulation of emotion and visceral function

– Impulses from cerebellum and basal nuclei to help direct motor cortices

– Impulses for memory or sensory integration

• Mediates sensation, motor activities, cortical arousal, learning, and memory

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Hypothalamus• Like thalamus, it consists of nuclei forming inferolateral walls of third

ventricle

• Infundibulum— neurologic and vascular connection to pituitary gland

• Controls autonomic nervous system directly, via brainstem, using hormones

– (e.g., blood pressure, rate and force of heartbeat, digestive tract motility, pupil size -generally all the ventral cavity organs and skin, involuntary muscle contraction)

• Physical responses to emotions (limbic system)

– Perception of pleasure, fear, and rage, and in biological rhythms and drives

– Survive and reproduce?

• Regulates

– body temperature – sweating/shivering

– hunger and satiety in response to nutrient blood levels or hormones

– water balance and thirst – cells here chemically monitor blood, attempt to control blood concentration

– sleep-wake cycles – uses visual information

• Controls endocrine system

Epithalamus

• Most dorsal portion of diencephalon; forms roof of third ventricle

• Pineal gland (body)—extends from posterior border and secretes melatonin

– Melatonin—a hormone that makes you sleepy

• Along with hypothalamus, lack of sensory stimuli and low light levels may trigger desire to sleep

Brain Stem

• Three regions

– Midbrain

– Pons

– Medulla oblongata

• Similar in structure to spinal cord but contains nuclei embedded in white matter

• Controls automatic, often heavily repeated, behaviors necessary for survival

• Contains fiber tracts connecting higher and lower neural centers

• Nuclei associated with 10 of the 12 pairs of cranial nerves

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Figure 12.10b Midsagittal section of the brain.

Lateral ventricle(covered by septumpellucidum)

Third ventricle

Anterior commissure

Hypothalamus

Corpus callosum

Fornix

Thalamus

PosteriorcommissurePineal gland

Corporaquadrigemina

Cerebralaqueduct

Midbrain

Arbor vitae

Fourth ventricle

Cerebellum

Medulla oblongata

Pons

Optic chiasma

Epithalamus

Mammillary body

Figure 12.12 Inferior view of the brain, showing the three parts of the brain stem: midbrain, pons, and medulla oblongata.

Frontal lobe

Olfactory bulb(synapse point ofcranial nerve I)

Optic nerve (II)

Optic chiasma

Optic tract

Mammillary body

Midbrain

Pons

Temporallobe

Medulla

oblongata

Cerebellum

Spinal cord

Figure 12.13a Three views of the brain stem (green) and the diencephalon (purple).

Thalamus

Medulla oblongata

Diencephalon

Brain stem

View (b)

View (a) View (c)

Diencephalon

Mammillary body

Oculomotor nerve (III)

Trochlear nerve (IV)

Middle cerebellarpeduncleAbducensnerve (VI)Vestibulocochlearnerve (VIII)

PyramidVentral root of firstcervical nerveDecussation ofpyramids

Optic chiasmaOptic nerve (II)Optic tract

Crus cerebri of cerebralpeduncles (midbrain)

Trigeminal nerve (V)Pons

Facial nerve (VII)

Glossopharyngeal nerve (IX)

Hypoglossal nerve (XII)

Vagus nerve (X)

Accessory nerve (XI)

Spinal cord

Ventral view

Hypothalamus

Midbrain

Pons

• Thalamus• Hypothalamus

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Thalamus

Medulla oblongata

Diencephalon

Brain stem

View (b)

View (a) View (c)

Hypothalamus

Midbrain

Pons

Diencephalon

Thalamus

Midbrain

• Superiorcolliculus

• Inferiorcolliculus

• Trochlear nerve (IV)• Superior cerebellar peduncle

Corporaquadrigeminaof tectum

Pons• Middle cerebellar peduncle

Medulla oblongata

• Inferior cerebellar peduncle

• Vestibulocochlear nerve (VIII)

• Glossopharyngeal nerve (IX)• Vagus nerve (X)• Accessory nerve (XI)

Pineal gland

Floor offourth ventricle

Facial nerve (VII)

Choroid plexus(fourth ventricle)Dorsal median sulcus

Dorsal root offirst cervical nerve

Dorsal view

Figure 12.13c Three views of the brain stem (green) and the diencephalon (purple).

Midbrain Anatomy and Nuclei

Centrally-located

• serves as a pathway

– Projection fibers of pyramidal neurons located in cerebral peduncles

– Link to cerebellum

• Serves as a relay and reflex center

– two cranial nerve nuclei here related to movement of eyes

• Reflexive motor impulses cause eyes to follow objects

– Reflexive responses of the head when startled by sound

– Modify passing motor signals from motor cortex

• Supress unintended movement

• Pathway for corrective signals of cerebellum to cerebrum

Figure 12.14a Cross sections through different regions of the brain stem.

TectumPeriaqueductal gray matterOculomotor nucleus (III)

Dorsal

Medial lemniscusRed nucleusSubstantianigra

Fibers ofpyramidal tract

Superiorcolliculus

Cerebral aqueduct

Reticular formation

Crus cerebri of cerebral peduncle

Ventral

Midbrain

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Pons

• Consider position in brain

• Fibers of pons

– Connect higher brain centers and spinal cord

– Relay impulses between motor cortex and cerebellum

• Middle cerebellar peduncles communicate with cerebellum

• Pontine nuclei allow synapse point with cerebral motor neurons

• Pontine nuclei

– Origin of three cranial nerves related to facial muscles, eye movement, general senses of face and cavities

– Some nuclei of reticular formation (coordinated movement)

– Respiratory nuclei help maintain normal rhythm of breathing

Medulla Oblongata

• Consider position

– Ascending pathway for certain general skin/body senses

• Nucleus cuneatus and nucleus gracilis

– Relay joint and muscle conditions to cerebellum

• Olivary nuclei and inferior cerebellar peduncles

– Projection fibers including motor neurons passing through the pyramids

– Joins spinal cord at foramen magnum

Medulla oblongata

• Autonomic role and integration center

– Cranial nerve nuclei involved in

• Chewing and swallowing (hypoglossal and glossopharyngeal)

• Monitoring blood pressure and blood gases (glossopharyngeal and vagus)

• Monitoring head position and movement (vestibulocochlear)

• Monitoring condition of thoracic and abdominal organs (vagus)

– Autonomic nuclei

• Cardiac and vasomotor centers control blood pressure and blood flow

• Respiratory centers control rate and depth of breathing

• Centers vomiting, swallowing, coughing, sneezing, hiccupping

– Instructed by hypothalamus but acts reflexively

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Figure 12.14c Cross sections through different regions of the brain stem.

Hypoglossal nucleus (XII)Dorsal motor nucleus of vagus (X)Inferior cerebellar peduncleLateralnucleargroup

Medialnucleargroup

RaphenucleusMedial lemniscus

Fourth ventricle Solitary nucleus

Vestibular nuclei(VIII)

Cochlear nuclei(VIII)

Nucleus ambiguus

Inferior olivarynucleus

Pyramid

Choroidplexus

Ret

icu

lar

form

atio

n

Medulla oblongata

Cerebellum

• 11% of brain mass

• Input from cortex, brain stem and sensory receptors allows it to apply a learned movement to body’s current position

• Allows smooth, coordinated movements

Anatomy

• Cerebellar hemispheres connected by vermis

• Each hemisphere has three lobes

– Anterior, posterior, and flocculonodular

• Folia

• Arbor vitae

Figure 12.15a Cerebellum.Anterior lobe

Arbor vitaeCerebellar cortex

Pons

Fourth ventricle

Medulla oblongata

Posterior lobe

Flocculonodular lobe

Choroid plexus

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Cerebellar Processing of Motor Activity

• All fibers in cerebellum are ipsilateral

• Cerebellum receives impulses from cerebral cortex of intent to initiate voluntary muscle contraction

• Signals from proprioceptors and visual and equilibrium pathways continuously "inform" cerebellum of body's position and momentum

• Cerebellar cortex calculates the best way to smoothly coordinate muscle contraction

• "Blueprint" of coordinated movement sent to cerebral motor cortex and brain stem nuclei

• May compare actual with expected output and adjust accordingly

Spinal Cord: Gross Anatomy and Protection

• Location– Begins at the foramen magnum

– Ends at L1 or L2 vertebra

• Functions

– Provides two-way communication to and from brain

– Contains spinal reflex centers

• Protected by bone, meninges, and CSF

• Terminates in conus medullaris

• Dural and arachnoid membranes extend to sacrum, beyond end of cord at L1 or L2

– Epidural space

– CSF in subarachnoid space

– Filum terminale extends to coccyx

– Denticulate ligaments

Figure 12.26a Gross structure of the spinal cord, dorsal view.

Cervicalenlargement

Dura andarachnoidmater

ConusmedullarisCaudaequina

Filumterminale

Sacralspinal nerves

Lumbarspinal nerves

Thoracicspinal nerves

Cervicalspinalnerves

The spinal cord and its nerve roots, with the bonyvertebral arches removed. The dura mater and arachnoid mater are cut open and reflected laterally.

Lumbarenlargement

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Terminus ofmedullaoblongataof brain

Spinal nerverootlets

Dorsalmedian sulcusof spinal cord

Cranialdura mater

Sectionedpedicles ofcervicalvertebrae

Cervical spinal cord.

Figure 12.26b Gross structure of the spinal cord, dorsal view.

Spinal cord

Denticulateligament

Arachnoidmater

Vertebralarch

Denticulateligament

Dorsalmediansulcus

Dorsal root

Spinal duramater

Thoracic spinal cord, showingdenticulate ligaments.

Figure 12.26c Gross structure of the spinal cord, dorsal view.

Spinal cord

First lumbarvertebral arch(cut across)

Spinousprocess of second lumbarvertebra

Caudaequina

Conusmedullaris

Filumterminale

Inferior end of spinal cord, showingconus medullaris, cauda equina, andfilum terminale.

Figure 12.26d Gross structure of the spinal cord, dorsal view.

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Spinal Cord

• Spinal nerves (Part of PNS)

– 31 pairs

• Cervical and lumbosacral enlargements

– Nerves serving upper and lower limbs emerge here

• Cauda equina

– Collection of nerve roots at inferior end of vertebral canal

Epidural space(contains fat)

Subdural space

Subarachnoidspace(contains CSF)

Pia mater

Arachnoid mater Spinal meninges

Bone ofvertebra

Dorsal rootganglion

Bodyof vertebra

Dura mater

Cross section of spinal cord and vertebra

Figure 12.28a Anatomy of the spinal cord.

Dorsal roots – sensory input to cordDorsal root (spinal) ganglia—cell bodies of sensory neurons

Dorsal median sulcus

Gray commissureDorsal hornVentral hornLateral horn

Graymatter

Central canal

Ventral median fissure

Pia mater

Arachnoid mater

Spinal dura mater

Whitecolumns

Dorsal funiculus

Ventral funiculus

Lateral funiculus

Dorsal rootganglion

Spinal nerve

Dorsal root(fans out into dorsal rootlets)

Ventral root(derived from severalventral rootlets)

The spinal cord and its meningeal coverings

Figure 12.28b Anatomy of the spinal cord.

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Dorsal horn (interneurons)Dorsal root(sensory)

Dorsal rootganglion

Somatic sensory neuron

Visceral sensoryneuron

Visceral motorneuron

Somatic motor neuron

Spinal nerveVentral root

(motor)

Ventral horn(motor neurons)

Interneurons receiving input from somatic sensory neurons

Interneurons receiving input from visceral sensory neurons

Visceral motor (autonomic) neurons

Somatic motor neurons

SSVS

VM

SM

SS

VS

VM

SM

Figure 12.29 Organization of the gray matter of the spinal cord.

Dorsal horns - interneurons that receive somatic and visceral sensory input

Ventral horns - some interneurons; somatic motor neurons; axons exit cord via ventral roots

Lateral horns (only in thoracic and superior lumbar regions) - sympathetic motor neurons

White Matter • Myelinated and nonmyelinated nerve fibers allow communication

between parts of spinal cord, and spinal cord and brain

• Run in three directions

– Ascending – up to higher centers (sensory inputs)

– Descending – from brain to cord or lower cord levels (motor outputs)

– Transverse – from one side to other (commissural fibers)

• Divided into three white columns (funiculi) on each side

– Dorsal (posterior), lateral, and ventral (anterior)

• Each spinal tract composed of axons with similar destinations and functions

Dorsalwhitecolumn

Fasciculus gracilis

Fasciculus cuneatus

Dorsalspinocerebellar tract

Ventralspinocerebellartract

Lateral spinothalamictract

Ventral spinothalamictract

Ventral white commissure

Lateralreticulospinal tractLateralcorticospinaltractRubrospinal tract

Medialreticulospinal tract

Ventralcorticospinal tract

Vestibulospinal tract

Tectospinal tract

Descending tractsAscending tracts

Figure 12.30 Major ascending (sensory) and descending (motor) tracts of the spinal cord, cross-sectional view.

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Ascending Pathways

• First-order neuron

– Conducts impulses from cutaneous receptors and proprioceptors

– Synapses with second-order neuron

• Second-order neuron

– Interneuron

– Cell body in dorsal horn of spinal cord or medullary nuclei

– Axons extend to thalamus or cerebellum

• Third-order neuron

– Interneuron

– Cell body in thalamus

– Axon extends to somatosensory cortex

Ascending Pathways

• Three main pathways:

– Two transmit somatosensory information to sensory cortex via thalamus

• Dorsal column–medial lemniscal pathways

– Provide discriminatory touch and conscious proprioception

• Spinothalamic pathways

– Provide less-discriminatory touch and pain signals

– Spinocerebellar tracts terminate in the cerebellum

• Convey unconscious information about muscle or tendon stretch to cerebellum

– Used to coordinate muscle activity

Dorsalspinocerebellartract (axons ofsecond-orderneurons)

Medial lemniscus (tract)(axons of second-order neurons)Nucleus gracilisNucleus cuneatus

Medulla oblongata

Fasciculus cuneatus(axon of first-order sensory neuron)

Joint stretchreceptor(proprioceptor)Axon of

first-orderneuronMusclespindle(proprioceptor)

Fasciculus gracilis(axon of first-order sensory neuron)

Lumbar spinal cord

Touch receptor

Spinocerebellar pathway Dorsal column–medial lemniscalpathway

Cervical spinal cord

Figure 12.31a Pathways of selected ascending spinal cord tracts. (2 of 2)

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Figure 12.31a Pathways of selected ascending spinal cord tracts. (1 of 2)

Primarysomatosensorycortex

Axons of third-orderneurons

Thalamus

Cerebrum

Midbrain

Cerebellum

Pons

Spinocerebellar pathway Dorsal column–medial lemniscalpathway

Spinothalamic Pathways

• Lateral and ventral spinothalamic tracts

• Transmit pain, temperature, coarse touch, and pressure impulses within lateral spinothalamictract

Figure 12.31b Pathways of selected ascending spinal cord tracts. (2 of 2)

Medulla oblongata

Pain receptors

Cervical spinal cord

Lumbar spinal cord

Axons of first-orderneurons

Temperaturereceptors

Spinothalamic pathway

Lateralspinothalamictract (axons ofsecond-orderneurons)

Transmit pain, temperature, coarse touch, and pressure impulses within lateral spinothalamic tract

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Figure 12.31b Pathways of selected ascending spinal cord tracts. (1 of 2)

Primarysomatosensorycortex

Axons of third-orderneurons

Thalamus

Cerebrum

Midbrain

Cerebellum

Pons

Spinothalamic pathway

Descending Pathways and Tracts

• Deliver efferent impulses from brain to spinal cord

• Two groups

– Direct pathways—pyramidal tracts

– Indirect pathways—all others

• Motor pathways involve two neurons:

– Upper motor neurons

• Pyramidal cells in primary motor cortex

– Lower motor neurons

• Ventral horn motor neurons

• Innervate skeletal muscles

The Direct (Pyramidal) Pathways

• Impulses from pyramidal neurons in precentral gyri pass through pyramidal (corticospinal)l tracts

• Descend without synapsing

• Axons synapse with interneurons or ventral horn motor neurons

• Direct pathway regulates fast and fine (skilled) movements

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Figure 12.32a Three descending pathways by which the brain influences movement. (1 of 2)

Cerebralpeduncle

Pyramidal cells(upper motor neurons)

Primary motor cortex

Internal capsule

Cerebrum

Midbrain

Cerebellum

Pons

Pyramidal (lateral and ventral corticospinal) pathways

Ventralcorticospinaltract

Pyramids

Decussationof pyramids

Lateralcorticospinaltract

Skeletal muscle

Pyramidal (lateral and ventral corticospinal) pathways

Medulla oblongata

Cervical spinal cord

Lumbar spinal cord

Somatic motor neurons(lower motor neurons)

Figure 12.32a Three descending pathways by which the brain influences movement. (2 of 2)

Indirect (Multineuronal) System

• Complex and multisynaptic

• Includes brain stem motor nuclei, and all motor pathways except pyramidal pathways

• These pathways regulate

– Axial muscles maintaining balance and posture

– Muscles controlling coarse limb movements

– Head, neck, and eye movements that follow objects in visual field

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Indirect (Multineuronal) System

• Reticulospinal and vestibulospinal tracts—maintain balance

• Rubrospinal tracts—control flexor muscles

• Superior colliculi and tectospinal tractsmediate head movements in response to visual stimuli