Central Nervous System Brain and Spinal Cord Learn and Understand - - PowerPoint PPT Presentation

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

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

Septum pellucidum Inferior horn Lateral aperture Lateral ventricle Anterior horn Interventricular foramen Third ventricle Cerebral aqueduct Fourth ventricle Central canal Posterior horn Inferior horn Median aperture Lateral aperture 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)

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

Skin of scalp Periosteum Bone of skull Dura mater

• Periosteal layer
• Meningeal layer

Arachnoid mater Pia mater Arachnoid villus Blood vessel Falx cerebri (in longitudinal fissure only) Superior sagittal sinus Subdural space Subarachnoid space

• 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

Figure 12.24a Formation, location, and circulation of CSF

Superior sagittal sinus Choroid plexus Interventricular foramen Third ventricle Cerebral aqueduct Lateral aperture Fourth ventricle Median aperture Central canal

• f spinal cord

(a) CSF circulation 1 The choroid plexus of each Ventricle produces CSF. 2 CSF flows through the ventricles and into the subarachnoid space via the median and lateral apertures. 3 CSF flows through the subarachnoid space. 4 CSF is absorbed into the dural venous sinuses via the arachnoid villi. Arachnoid villus Subarachnoid space Arachnoid mater Meningeal dura mater Periosteal dura mater Right lateral ventricle (deep to cut) Choroid plexus

• f 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

• f 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

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

Left cerebral hemisphere Transverse cerebral fissure Cerebellum Brain stem Left lateral view

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

Frontal lobe Postcentral gyrus Parietal lobe Central sulcus Precentral gyrus Parieto-occipital sulcus (on medial surface

• f hemisphere)

Lateral sulcus Temporal lobe Occipital lobe Transverse cerebral fissure Pons Spinal cord Fissure (a deep sulcus) Gyrus Cortex (gray matter) Sulcus White matter Lobes and sulci of the cerebrum Medulla oblongata Cerebellum

Frontal lobe Central sulcus Gyri of insula Temporal lobe (pulled down) Location of the insula lobe

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

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

Anterior Longitudinal fissure Frontal lobe Cerebral veins and arteries covered by arachnoid mater Left cerebral hemisphere Parietal lobe Right cerebral hemisphere Occipital lobe 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

Figure 12.6a Functional and structural areas of the cerebral cortex

Motor areas Primary motor cortex Premotor cortex Frontal eye field Broca's area (outlined by dashes) Working memory for spatial tasks Executive area for task management Working memory for

• bject-recall tasks

Solving complex, multitask problems Prefrontal cortex Lateral view, left cerebral hemisphere Sensory areas and related association areas Primary somatosensory cortex Somatosensory association cortex Gustatory cortex (in insula) Somatic sensation Taste Wernicke's area (outlined by dashes) Primary visual cortex Visual association area Auditory association area Primary auditory cortex Vision Hearing Central sulcus Primary motor cortex Motor association cortex Primary sensory cortex Sensory association cortex Multimodal association cortex

Figure 12.6b Functional and structural areas of the cerebral cortex

Corpus callosum Frontal eye field Prefrontal cortex Processes emotions related to personal and social interactions Orbitofrontal cortex Olfactory bulb Olfactory tract Fornix Temporal lobe Primary

• lfactory

cortex Uncus Calcarine sulcus Parahippocampal gyrus Parietal lobe Somatosensory association cortex Parieto-occipital sulcus Occipital lobe Visual association area Primary visual cortex Primary somatosensory cortex Central sulcus Primary motor cortex Cingulate gyrus Premotor cortex Parasagittal view, right cerebral hemisphere Primary motor cortex Motor association cortex Primary sensory cortex Sensory association cortex Multimodal association cortex

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

Primary Motor Cortex

• Large pyramidal cells of precentral gyri
• Long axons  pyramidal (corticospinal) tracts
• f 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 Sensory Anterior Primary motor cortex (precentral gyrus) Primary somato- sensory cortex (postcentral gyrus) Motor map in precentral gyrus Sen Sensor sory y ma map in in post stcentral l gyr yrus Swallowing Tongue Jaw Toes Genitals Foot Knee Hip Trunk Neck 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

Sensory Areas of Cerebral Cortex

• Conscious awareness of sensation
• Occur in parietal, insular, temporal, and
• ccipital lobes

Figure 12.7 Body maps in the primary motor cortex and somatosensory cortex of the cerebrum. Posterior Motor Sensory Anterior Primary motor cortex (precentral gyrus) Primary somato- sensory cortex (postcentral gyrus) Motor map in precentral gyrus Sen Sensor sory y ma map in in post stcentral l gyr yrus Swallowing Tongue Jaw Toes Genitals Foot Knee Hip Trunk Neck 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 homunculus

upside-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

• bject
• Determines size, texture, and relationship of

parts of objects being felt

• 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

• r full bladder

Gustatory cortex

– In insula just deep to temporal lobe – Involved in perception of taste

Posterior Association Area

• Large region in temporal, parietal, and
• ccipital 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)

Superior Longitudinal fissure Lateral ventricle Basal nuclei

• Caudate
• Putamen
• Globus

pallidus Third ventricle Pons Medulla oblongata Association fibers (within hemisphere)

• Corpus callosum

Projection fibers (cerebral cortex to lower area)

• Internal

capsule White matter Decussation (cross-over)

• f 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

Caudate

nucleus

Putamen

Striatum

Thalamus Tail of caudate nucleus

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 Interthalamic adhesion (intermediate mass of thalamus) Interventricular foramen Anterior commissure Hypothalamus Optic chiasma Pituitary gland Mammillary body Pons Medulla

• blongata

Spinal cord Corpus callosum Fornix Choroid plexus Thalamus (encloses third ventricle) Posterior commissure Pineal gland Epithalamus Corpora quadrigemina Cerebral aqueduct Midbrain Arbor vitae (of cerebellum) Fourth ventricle Choroid plexus Cerebellum

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

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, actions

necessary for survival

• Contains fiber tracts connecting higher and lower neural

centers

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

Figure 12.10b Midsagittal section of the brain.

Lateral ventricle (covered by septum pellucidum) Third ventricle Anterior commissure Hypothalamus Corpus callosum Fornix Thalamus Posterior commissure Pineal gland Corpora quadrigemina Cerebral aqueduct 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 of cranial nerve I) Optic nerve (II) Optic chiasma Optic tract Mammillary body Midbrain Pons Temporal lobe Medulla

• blongata

Cerebellum Spinal cord

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

Thalamus Medulla

• blongata

Diencephalon Brain stem View (b) View (a) View (c) Diencephalon Mammillary body Oculomotor nerve (III) Trochlear nerve (IV) Middle cerebellar peduncle Abducens nerve (VI) Vestibulocochlear nerve (VIII) Pyramid Ventral root of first cervical nerve Decussation of pyramids Optic chiasma Optic nerve (II) Optic tract Crus cerebri of cerebral peduncles (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

Thalamus Medulla

• blongata

Diencephalon Brain stem View (b) View (a) View (c) Hypothalamus Midbrain Pons Diencephalon Thalamus Midbrain

• Superior

colliculus

• Inferior

colliculus

• Trochlear nerve (IV)
• Superior cerebellar peduncle

Corpora quadrigemina

• f 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 of fourth ventricle Facial nerve (VII) Choroid plexus (fourth ventricle) Dorsal median sulcus Dorsal root of first 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.

Tectum

Periaqueductal gray matter Oculomotor nucleus (III) Dorsal Medial lemniscus Red nucleus Substantia nigra Fibers of pyramidal tract Superior colliculus Cerebral aqueduct Reticular formation Crus cerebri of cerebral peduncle Ventral Midbrain

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

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

Hypoglossal nucleus (XII) Dorsal motor nucleus

• f vagus (X)

Inferior cerebellar peduncle Lateral nuclear group Medial nuclear group Raphe nucleus Medial lemniscus Fourth ventricle Solitary nucleus Vestibular nuclei (VIII) Cochlear nuclei (VIII) Nucleus ambiguus Inferior

• livary

nucleus Pyramid Choroid plexus Reticular formation 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 vitae Cerebellar cortex Pons Fourth ventricle Medulla

• blongata

Posterior lobe Flocculonodular lobe Choroid plexus

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.

Cervical enlargement Dura and arachnoid mater Conus medullaris Cauda equina Filum terminale Sacral spinal nerves Lumbar spinal nerves Thoracic spinal nerves Cervical spinal nerves The spinal cord and its nerve roots, with the bony vertebral arches removed. The dura mater and arachnoid mater are cut open and reflected laterally. Lumbar enlargement

Terminus of medulla

• blongata
• f brain

Spinal nerve rootlets Dorsal median sulcus

• f spinal cord

Cranial dura mater Sectioned pedicles of cervical vertebrae Cervical spinal cord.

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

Spinal cord Denticulate ligament Arachnoid mater Vertebral arch Denticulate ligament Dorsal median sulcus Dorsal root Spinal dura mater Thoracic spinal cord, showing denticulate ligaments. Figure 12.26c Gross structure of the spinal cord, dorsal view.

Spinal cord First lumbar vertebral arch (cut across) Spinous process of second lumbar vertebra Cauda equina Conus medullaris Filum terminale Inferior end of spinal cord, showing conus medullaris, cauda equina, and filum terminale. Figure 12.26d Gross structure of the spinal cord, dorsal view.

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 Subarachnoid space (contains CSF) Pia mater Arachnoid mater Spinal meninges Bone of vertebra Dorsal root ganglion Body

• f vertebra

Dura mater Cross section of spinal cord and vertebra

Figure 12.28a Anatomy of the spinal cord.

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

Dorsal median sulcus Gray commissure Dorsal horn Ventral horn Lateral horn Gray matter Central canal Ventral median fissure Pia mater Arachnoid mater Spinal dura mater White columns Dorsal funiculus Ventral funiculus Lateral funiculus Dorsal root ganglion Spinal nerve Dorsal root (fans out into dorsal rootlets) Ventral root (derived from several ventral rootlets) The spinal cord and its meningeal coverings

Figure 12.28b Anatomy of the spinal cord.

Dorsal horn (interneurons) Dorsal root (sensory) Dorsal root ganglion Somatic sensory neuron Visceral sensory neuron Visceral motor neuron Somatic motor neuron Spinal nerve Ventral 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

SS VS 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

• utputs)

– 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

Dorsal white column Fasciculus gracilis Fasciculus cuneatus Dorsal spinocerebellar tract Ventral spinocerebellar tract Lateral spinothalamic tract Ventral spinothalamic tract Ventral white commissure Lateral reticulospinal tract Lateral corticospinal tract Rubrospinal tract Medial reticulospinal tract Ventral corticospinal tract Vestibulospinal tract Tectospinal tract Descending tracts Ascending tracts

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

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

Dorsal spinocerebellar tract (axons of second-order neurons) Medial lemniscus (tract) (axons of second-order neurons) Nucleus gracilis Nucleus cuneatus Medulla oblongata Fasciculus cuneatus (axon of first-order sensory neuron) Joint stretch receptor (proprioceptor) Axon of first-order neuron Muscle spindle (proprioceptor) Fasciculus gracilis (axon of first-order sensory neuron) Lumbar spinal cord Touch receptor Spinocerebellar pathway Dorsal column–medial lemniscal pathway Cervical spinal cord

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

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

Primary somatosensory cortex Axons of third-order neurons Thalamus Cerebrum Midbrain Cerebellum Pons Spinocerebellar pathway Dorsal column–medial lemniscal pathway

Spinothalamic Pathways

• Lateral and ventral spinothalamic tracts
• Transmit pain, temperature, coarse touch, and

pressure impulses within lateral spinothalamic tract

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-order neurons Temperature receptors Spinothalamic pathway Lateral spinothalamic tract (axons of second-order neurons)

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

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

Primary somatosensory cortex Axons of third-order neurons 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

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

Cerebral peduncle Pyramidal cells (upper motor neurons) Primary motor cortex Internal capsule Cerebrum Midbrain Cerebellum Pons Pyramidal (lateral and ventral corticospinal) pathways

Ventral corticospinal tract Pyramids Decussation

• f pyramids

Lateral corticospinal tract 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

• bjects in visual field

Indirect (Multineuronal) System

• Reticulospinal and vestibulospinal tracts—

maintain balance

• Rubrospinal tracts—control flexor muscles
• Superior colliculi and tectospinal tracts

mediate head movements in response to visual stimuli