1 Chapter 12 The Central Nervous System Lecture 19 Marieb’s Human Anatomy and Physiology Marieb Hoehn

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Chapter 12

The Central Nervous System

Lecture 19

Marieb’s HumanAnatomy and

Physiology

Marieb Hoehn

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Divisions of the Nervous System

CNS PNS

You are here

Brain – Embryology & Overview

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Table & Figure From: Marieb & Hoehn, Human Anatomy & Physiology, 9th ed., Pearson, 2013

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Overview of the Brain

Functions• regulates visceral activities• coordinates muscular movements• interprets sensations• determines perception• stores memory• carries out reasoning• makes decisions• determines personality

Major Parts

• diencephalon• thalamus• hypothalamus

• brain stem• midbrain (mesencephalon)• pons• medulla oblongata

• cerebellum

• cerebrum (two hemispheres)

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Protection of the Brain

• The brain is protected– Mechanically by

• The skull bones

• The meninges

• The cerebrospinal (CSF) fluid

– Biochemically by the blood-brain barrier• Capillaries interconnected by tight junctions

• Astrocytes/ependymal cells control permeability of general capillaries/choroid capillaries

• May be obstacle to delivery of drugs

• May become more permeable during stress

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Meninges of the Brain

- dura mater – outer, tough (anchoring dural folds)

- arachnoid mater – web-like

- pia mater – inner, delicate

- Subdural space – like interstitial fluid

- Subarachnoid space – CSF

*Singular of meninges is meninx

Figure From: Marieb & Hoehn, Human Anatomy & Physiology, 9th ed., Pearson, 2013

Dural Folds

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Falx Cerebri – within longitudinal fissure; separates cerebral hemispheres

Tentorium Cerebelli – above cerebellum; separates occipital lobe from cerebellum


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Ventricles of the Brain

• interconnected cavities• within cerebral hemispheres and brain stem• continuous with central canal of spinal cord• filled with cerebrospinal fluid (CSF)

• lateral ventricles (1, 2)• third ventricle (3)• fourth ventricle (4)• cerebral aqueduct

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Cerebrospinal Fluid• secreted by choroid plexus of ventricles (~500 ml/day)

• circulates in ventricles, central canal of spinal cord, and subarachnoid space

• completely surrounds brain and spinal cord

• clear liquid (more Na+ and Cl-, but less K+, Ca2+, glucose, and protein than plasma)

• nutritive and protective

• helps maintain stable ion concentrations in CNS

Flow of CSF

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(Luscka)

(Magendie)

(Monro)


Know

Overview of the Cerebrum of the Brain

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-Over 85% of brain mass, with about 14 billion multipolar neurons in cortex- Lobes named for overlying bones. (See sulci above for divisions)


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Functions of Cerebrum• interpretation• initiating voluntary movements• storing memory• retrieving memory• reasoning• center for intelligence and personality

The cerebrum can be divided into several functional areas:

- Motor (frontal cortex) - Sensory (parietal, occipital, and temporal cortex)- Association (all lobes)

Points to keep in mind: - Each cerebral hemisphere receives information from, and sends information to, the opposite side of the body - Although symmetrical, the cerebral hemispheres are not entirely equal in function

Functions of Parts of Brain

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Part of Brain Major FunctionMotor areas Primary motor cortex (Precentral gyrus) Voluntary control of skeletal muscles

Broca’s area (motor speech area) Controls muscles needed for speech

Frontal eye field Controls muscles needed for eye movementSensory areas Cutaneous Sensory Area (postcentral gyrus) Receives somatic sensations

Visual area (occipital lobe) Receives visual sensations Auditory area (temporal lobe) Receives auditory sensationsAssociation areas (all lobes) Analyze and interpret sensory experiences; coordinate motor responses

memory, reasoning, verbalization, judgment, emotions Basal nuclei Subconscious control certain muscular activities, e.g., learned movement patterns (a nucleus is a

collection of neuron cell bodies in the CNS); putamen, globus pallidus, caudateLimbic system controls emotions , produces feelings, interprets sensory impulses, facilitates memory storage and

retrieval (learning!)Diencephalon Thalamus gateway for sensory impulses heading to cerebral cortex, receives all sensory impulses (except

smell) Hypothalamus Vital functions associated with homeostasisBrainstem Midbrain Major connecting center between spinal cord and brain and parts of brainstem; contains corpora

quadrigemina (visual and auditory reflexes) Pons Helps regulate rate and depth of breathing, relays nerve impulses to and from medulla oblongata

and cerebellum Medulla Oblongata Contains cardiac, vasomotor, and respiratory control centers, contains various nonvital reflex

control centers (coughing, sneezing, vomiting)Reticular formation (system) Filters incoming sensory information; habituation , modulates pain, arouses cerebral cortex into

state of wakefulness (reticular activating system)

Cerebellum Subconscious coordination of skeletal muscle activity, maintains posture

Brain – Sensory and Motor Areas

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*Somatosensory = Somesthetic

(Gnostic)

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Cerebral Cortex Motor/Sensory Homunculi

17Notice the relative amount of cortical tissue devoted to each sensory function.

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Hemispheric (Cerebral) Lateralization

Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001

Categorical hemisphere

Representational hemisphere

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Basal Nuclei (formerly basal ganglia)• nuclei are masses of gray matter in CNS

• deep within cerebral hemispheres

• three nuclei: caudate nucleus and putamen, (together called the striatum), and the globus pallidus

• subconscious control certain muscular activities, e.g., learned movement patterns

1. Receive input from entire cerebral cortex.

2. Relay motor impulses originating in the substantia nigra, along with their own output, through the thalamus to the motor cortex to influence muscle movement.

Brain – Cerebral White Matter

20Figure From: Marieb & Hoehn, Human Anatomy & Physiology, 9th ed., Pearson, 2013

Three types of myelinated tracts form cerebral white matter:

1. Association – same hemisphere

2. Commisural – between corresponding gyri in opposite hemispheres

3. Projection – Ascending and descending tracts

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Limbic SystemConsists of

• portions of frontal lobe• portions of temporal lobe• hypothalamus• thalamus• basal nuclei• other deep nuclei• associated with sense of smell (less significant)

The motivational system

Figure from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

Functions• controls emotions• produces feelings• interprets sensory impulses• facilitates memory storage and retrieval (learning!)

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Memory• A “Memory” is the persistence of knowledge that can be

accessed (we hope!) at a later time.• Memories are not stored in individual “memory cells” or

neurons; they are stored as pathways called engrams, or memory traces that use strengthened or altered synapses.

• Immediate memory lasts a few seconds, e.g., remembering the earliest part of a sentence to make sense of it.

• Short-term memory (STM) lasts a few seconds to a few hours– Working memory is a form of this (repeating a phone number over

to yourself just long enough to dial it – and then forget it!)– Limited to a few ‘bits’ of information (about 7-9). So, ‘chunk up’!

• Long-term memory (LTM) can last a lifetime– Can hold much more information that STM– Declarative (events and facts)– Procedural (motor skills)

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Diencephalon• between cerebral hemispheres and brainstem• surrounds third ventricle

• thalamus• hypothalamus• epithalamus

• optic tracts• optic chiasm• infundibulum• posterior pituitary• mammillary bodies• pineal gland

(Tectum)

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Diencephalon - Thalamus

Relay for vision

Relay for hearing

Ventral nuclei

- Posterior; relay for taste

- Anterior and Lateral; voluntary motor

- Forms wall of third ventricle

- Crude interpretation center for pain, touch, pressure, temperature

The ‘gateway’ to the cerebral cortex. Major relay for sensory information coming into the cerebral cortex, roles in cortical arousal, learning, and memory


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Diencephalon - Hypothalamus

- Heart rate and blood pressure- Body temperature- Stimulation of the pituitary (links nervous and endocrine) - Water balance (ADH)- SM contraction (OT)- Feeding/satiety centers- Movement/secretions of glands and intestines- Sleep and wakefulness- Rage/aggression- Psychosomatic illness

Hypothalamus - maintains homeostasis by regulating visceral activities (see list below for examples…)


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Brain Stem

Three Parts1. Midbrain2. Pons3. Medulla Oblongata

(Tectum)

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Midbrain• between diencephalon and pons

• contains bundles of fibers that join lower parts of brainstem and spinal cord with higher part of brain

• cerebral aqueduct

• cerebral peduncles – bundles of nerve fibers

• contains red nucleus (rubro-) and substantia nigra

• corpora quadrigemina – centers for visual and auditory reflexes

(Tectum)


Major connecting center between spinal cord and brain and parts of brainstem

Origins of: CN III, IV

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Pons

• rounded bulge on underside of brainstem

• between medulla oblongata and midbrain

• helps regulate rate and depth of breathing

• relays nerve impulses to and from

1. medulla oblongata and brainstem via longitudinal tracts

2. cerebellum via transverse tracts

Origins of CN V, VI, VII, VIII

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Medulla Oblongata• enlarged continuation of spinal cord running through foramen magnum of skull

• conducts all ascending (olive) and descending (pyramids - decussation) impulses between brain and spinal cord

• contains cardiac, vasomotor, and respiratory control centers

• nucleus gracilis and nucleus cunneatus on dorsal side; sensory info, cross over, then send to thalamus

Origins (nuclei) of: CN IX, X, XI, and XII

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Reticular Formation• complex network of nerve fibers scattered throughout the brain stem

• extends into the diencephalon

• connects to centers of hypothalamus, basal nuclei, cerebellum, and cerebrum

• filters incoming sensory information; habituation

• modulates pain

• arouses cerebral cortex into state of wakefulness

Ascending portion is called the ‘reticular activating system’

(prefix = reticulo-)

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Cerebellum• integrates sensory information concerning position of body parts

• coordinates skeletal muscle activity

• helps to maintain posture

•May also be involved in several sensory, linguistic, emotional and non-motor functions

• virtually all fibers entering and leaving are ispsilateral


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Spinal Cord Structure• extends from the foramen magnum to 2nd lumbar vertebra

• cervical and lumbar enlargements

• cauda equina (horse’s tail) – thin nerve fibers that exit at different level than they arise (note that spinal cord does not extend into this area of the lumbar spine). Begins around L2 and extends to S5. Good area for lumbar puncture and collection of CSF.


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Meninges of the Spinal Cord

Space between the dura mater and the vertebral body is called the epidural space

Figures from: Saladin, Anatomy & Physiology, McGraw Hill, 2007

Denticulate ligaments – branches of pia mater connecting to the arachnoid

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Cross Section of Spinal Cord

• is a center for spinal reflexes• aids in locomotion• is a conduit for nerve impulses to and from the brain

The spinal cord…

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Organization of Spinal Gray Matter


You should know the major areas of gray matter of within the spinal cord:

Posterior = sensory

Lateral = visceral motor

Anterior = somatic motor

Spinal Cord and Nerve Roots

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Ventral root - axons of motor neurons whose cell bodies are in spinal cord

Dorsal root - axons of sensory neurons in the dorsal root ganglion

Dorsal root ganglion - cell bodies of sensory neurons


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Organization of Spinal White Matter


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Tracts of the Spinal Cord• Ascending tracts conduct sensory impulses to the brain• Descending tracts conduct motor impulses from the brain to motor neurons reaching muscles and glands

All the axons in a tract share a common origin and destination

Tracts are usually named for their place of origin (1st) and termination (2nd)

Most axons cross over during their travel. What will this mean clinically?

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Ascending Tracts• fasciculus cuneatus/gracilis - fine touch, pressure, body movement - cross (decussate) in medulla

• spinothalamic - crude pain, temperature, pressure, and touch - cross in spinal cord

• spinocerebellar - subconscious coordination of muscle movements (1st and 2nd order neurons) - ipsilateral

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Decussation

(crossing over)

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1st, 2nd, and 3rd Order Sensory Neurons

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2

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1st order neuron – from receptor to the spinal cord (cell bodies are located in the dorsal root ganglion)

2nd order neuron – from spinal cord to thalamus

3rd order neuron – from thalamus to sensory cerebral cortex - terminate in the cerebral cortex

Decussation

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Descending Tracts• corticospinal (direct, pyramidal) - voluntary movement of skeletal muscles - lateral cross in medulla - contralateral

• reticulospinal (indirect, extrapyramidal) - subconscious muscle tone, sweat glands - some lateral cross, anterior do not cross

• rubrospinal (indirect, extrapyramidal) - subconscious regulation of upper limb tone/movement - cross in brain (less important in humans)

Upper motor – begin in precentral gyrus of cortex

Lower

Upper MN – Cerebral cortex to spinal cord

Lower MN – Spinal cord to effector

Decussation

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Review• The brain is protected by the

– Skull bones– Meninges– CSF– Blood-brain barrier

• The meninges of the brain and spinal cord consist of the– Dura mater– Arachnoid (membrane)– Pia mater

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Review• Important motor areas of cerebral cortex

– Precentral gyrus (Primary motor area)

– Broca’s area

– Frontal eye field

• Important sensory areas of cerebral cortex– Postcentral gyrus (Primary cutaneous sensory)

– Visual area (occipital lobe)

– Auditory area (temporal lobe)

• The spinal cord is a– Center for spinal reflexes– Conduit for nerve impulses to and from the brain

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Review

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Review

Spinal cord contains nerve tracts

Ascending = sensory

Descending = motor

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1 Chapter 12 The Central Nervous System Lecture 19 Marieb’s Human Anatomy and Physiology Marieb Hoehn