Central Nervous System (CNS)

•      CNS – composed of the brain and spinal cord

•      Cephalization

–  Elaboration of the anterior portion of the CNS

–  Increase in number of neurons in the head

–  Highest level is reached in the human brain

The Brain

•      Composed of wrinkled, pinkish gray tissue

•      Surface anatomy includes cerebral hemispheres, cerebellum, and brain stem

Embryonic Development

•      During the first 26 days of development:

–  Ectoderm thickens along dorsal midline to form the neural plate

–  The neural plate invaginates, forming a groove flanked by neural folds

–  The neural groove fuses dorsally and forms the neural tube

Primary Brain Vesicles

•      The anterior end of the neural tube expands and constricts to form the three primary brain vesicles

–  Prosencephalon – the forebrain

–  Mesencephalon – the midbrain

–  Rhombencephalon – hindbrain

Secondary Brain Vesicles

•      In week 5 of embryonic development, secondary brain vesicles form

–  Telencephalon and diencephalon arise from the forebrain

–  Mesencephalon remains undivided

–  Metencephalon and myelencephalon arise from the hindbrain

Adult Brain Structures

•      Fates of the secondary brain vesicles:

–  Telencephalon – cerebrum:  cortex, white matter, and basal nuclei

–  Diencephalon – thalamus, hypothalamus, and epithalamus

–  Mesencephalon – brain stem: midbrain

–  Metencephalon – brain stem: pons

–  Myelencephalon – brain stem: medulla oblongata

Adult Neural Canal Regions

•      Adult structures derived from the neural canal

–  Telencephalon – lateral ventricles

–  Diencephalon – third ventricle

–  Mesencephalon – cerebral aqueduct

–  Metencephalon and myelencephalon – fourth ventricle

Basic Pattern of the Central Nervous System

•      Spinal Cord

–  Central cavity surrounded by a gray matter core

–  External to which is white matter composed of myelinated fiber tracts

•      Brain

–  Similar to spinal cord but with additional areas of gray matter

–  Cerebellum has gray matter in nuclei

–  Cerebrum has nuclei and additional gray matter in the cortex

Ventricles of the Brain

•      Arise from expansion of the lumen of the neural tube

•      The ventricles are:

–  The paired C-shaped lateral ventricles

–  The third ventricle found in the diencephalon

–  The fourth ventricle found in the hindbrain dorsal to the pons

Cerebral Hemispheres

•      Form the superior part of the brain and make up 83% of its mass

•      Contain ridges (gyri) and shallow grooves (sulci)

•      Contain deep grooves called fissures

•      Are separated by the longitudinal fissure

•      Have three basic regions: cortex, white matter, and basal nuclei

Major Lobes, Gyri, and Sulci of the Cerebral Hemisphere

•      Deep sulci divide the hemispheres into five lobes:

–  Frontal, parietal, temporal, occipital, and insula

•      Central sulcus – separates the frontal and parietal lobes

•      Parieto-occipital sulcus – separates the parietal and occipital lobes

•      Lateral sulcus – separates the parietal and temporal lobes

•      The precentral and postcentral gyri border the central sulcus

Cerebral Cortex

•      The cortex – superficial gray matter; accounts for 40% of the mass of the brain

•      It enables sensation, communication, memory, understanding, and voluntary movements

•      Each hemisphere acts contralaterally (controls the opposite side of the body)

•      Hemispheres are not equal in function

•      No functional area acts alone; conscious behavior involves the entire cortex

Functional Areas of the Cerebral Cortex

•      The three types of functional areas are:

–  Motor areas – control voluntary movement

–  Sensory areas – conscious awareness of sensation

–  Association areas – integrate diverse information

Cerebral Cortex: Motor Areas

•      Primary (somatic) motor cortex

•      Premotor cortex

•      Broca’s area

•      Frontal eye field

Primary Motor Cortex

•      Located in the precentral gyrus

•      Composed of pyramidal cells whose axons make up the corticospinal tracts

•      Allows conscious control of precise, skilled, voluntary movements

•      Motor homunculus – caricature of relative amounts of cortical tissue devoted to each motor function

Premotor Cortex

•      Located anterior to the precentral gyrus

•      Controls learned, repetitious, or patterned motor skills

•      Coordinates simultaneous or sequential actions 

•      Involved in the planning of movements

Broca’s Area

•      Broca’s area

–  Located anterior to the inferior region of the premotor area

–  Present in one hemisphere (usually the left)

–  A motor speech area that directs muscles of the tongue

–  Is active as one prepares to speak

Frontal Eye Field

•      Frontal eye field

–  Located anterior to the premotor cortex and superior to Broca’s area

–  Controls voluntary eye movement

Sensory Areas

•      Primary somatosensory cortex

•      Somatosensory association cortex

•      Visual and auditory areas

•      Olfactory, gustatory, and vestibular cortices

PrImary Somatosensory Cortex

•      Located in the postcentral gyrus, this area:

–  Receives information from the skin and skeletal muscles

–  Exhibits spatial discrimination

•      Somatosensory homunculus – caricature of relative amounts of cortical tissue devoted to each sensory function

Somatosensory Association Cortex

•      Located posterior to the primary somatosensory cortex

•      Integrates sensory information

•      Forms comprehensive understanding of the stimulus

•      Determines size, texture, and relationship of parts

Visual Areas

•      Primary visual (striate) cortex

–  Seen on the extreme posterior tip of the occipital lobe

–  Most of it is buried in the calcarine sulcus

–  Receives visual information from the retinas

•      Visual association area

–  Surrounds the primary visual cortex

–  Interprets visual stimuli (e.g., color, form, and movement)

Auditory Areas

•      Primary auditory cortex

–  Located at the superior margin of the temporal lobe

–  Receives information related to pitch, rhythm, and loudness

•      Auditory association area

–  Located posterior to the primary auditory cortex

–  Stores memories of sounds and permits perception of sounds

–  Wernicke’s area

Association Areas

•      Prefrontal cortex

•      Language areas

•      General (common) interpretation area

•      Visceral association area

Prefrontal Cortex

•      Located in the anterior portion of the frontal lobe

•      Involved with intellect, cognition, recall, and personality

•      Necessary for judgment, reasoning, persistence, and conscience

•      Closely linked to the limbic system (emotional part of the brain)

•      Located in a large area surrounding the left (or language-dominant) lateral sulcus

•      Major parts and functions:

–  Wernicke’s area – involved in sounding out unfamiliar words

–  Broca’s area – speech preparation and production

–  Lateral prefrontal cortex – language comprehension and word analysis

–  Lateral and ventral temporal lobe – coordinate auditory and visual aspects of language

General (Common) Interpretation Area

•      Ill-defined region including parts of the temporal, parietal, and occipital lobes

•      Found in one hemisphere, usually the left

•      Integrates incoming signals into a single thought

•      Involved in processing spatial relationships

Visceral Association Area

•      Located in the cortex of the insula

•      Involved in conscious perception of visceral sensations

Lateralization of Cortical Function

•      Lateralization – each hemisphere has abilities not shared with its partner

•      Cerebral dominance – designates the hemisphere dominant for language

•      Left hemisphere – controls language, math, and logic

•      Right hemisphere – controls visual-spatial skills, emotion, and artistic skills

Cerebral White Matter

•      Consists of deep myelinated fibers and their tracts

•      It is responsible for communication between:

–  The cerebral cortex and lower CNS center, and areas of the cerebrum

•      Types include:

–  Commissures – connect corresponding gray areas of the two hemispheres

–  Association fibers – connect different parts of the same hemisphere

–  Projection fibers – enter the hemispheres from lower brain or cord centers

Basal Nuclei

•      Masses of gray matter found deep within the cortical white matter

•      The corpus striatum is composed of three parts

–  Caudate nucleus

–  Lentiform nucleus – composed of the putamen and the globus pallidus

–  Fibers of internal capsule running between and through caudate and lentiform nuclei

Functions of Basal Nuclei

•      Though somewhat elusive, the following are thought to be functions of basal nuclei

–  Influence muscular activity

–  Regulate attention and cognition

–  Regulate intensity of slow or stereotyped movements

–  Inhibit antagonistic and unnecessary movement

Diencephalon

•      Central core of the forebrain

•      Consists of three paired structures – thalamus, hypothalamus, and epithalamus

•      Encloses the third ventricle

Thalamus

•      Paired, egg-shaped masses that form the superolateral walls of the third ventricle

•      Connected at the midline by the intermediate mass

•      Contains four groups of nuclei – anterior, ventral, dorsal, and posterior

•      Nuclei project and receive fibers from the cerebral cortex

Thalamic Function

•      Afferent impulses from all senses converge and synapse in the thalamus

•      Impulses of similar function are sorted out, edited, and relayed as a group

•      All inputs ascending to the cerebral cortex pass through the thalamus

•      Plays a key role in mediating sensation, motor activities, cortical arousal, learning, and memory

Hypothalamus

•      Located below the thalamus, it caps the brainstem and forms the inferolateral walls of the third ventricle

•      Mammillary bodies

–  Small, paired nuclei bulging anteriorly from the hypothalamus

–  Relay station for olfactory pathways

•      Infundibulum – stalk of the hypothalamus; connects to the pituitary gland

–  Main visceral control center of the body

Hypothalamic Nuclei

Hypothalamic Function

•      Regulates blood pressure, rate and force of heartbeat, digestive tract motility, rate and depth of breathing, and many other visceral activities

•      Is involved with perception of pleasure, fear, and rage

•      Controls mechanisms needed to maintain normal body temperature

•      Regulates feelings of hunger and satiety

•      Regulates sleep and the sleep cycle

Endocrine Functions of the Hypothalamus

•      Releasing hormones control secretion of hormones by the anterior pituitary

•      The supraoptic and paraventricular nuclei produce ADH and oxytocin

Epithalamus

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

•      Pineal gland – extends from the posterior border and secretes melatonin

–  Melatonin – a hormone involved with sleep regulation, sleep-wake cycles, and mood

•      Choroid plexus – a structure that secretes cerebral spinal fluid (CSF)

Brain Stem

•      Consists of three regions – midbrain, pons, and medulla oblongata

•      Similar to spinal cord but contains embedded nuclei

•      Controls automatic behaviors necessary for survival

•      Provides the pathway for tracts between higher and lower brain centers

•      Associated with 10 of the 12 pairs of cranial nerves

Midbrain

•      Located between the diencephalon and the pons

•      Midbrain structures include:

–  Cerebral peduncles – two bulging structures that contain descending pyramidal motor tracts

–  Cerebral aqueduct – hollow tube that connects the third and fourth ventricles

–  Various nuclei

Midbrain Nuclei

•      Nuclei that control cranial nerves III (oculomotor) and IV (trochlear)

•      Corpora quadrigemina – four domelike protrusions of the dorsal midbrain

•      Superior colliculi – visual reflex centers

•      Inferior colliculi – auditory relay centers

•      Substantia nigra – functionally linked to basal nuclei

•      Red nucleus – largest nucleus of the reticular formation; red nuclei are relay nuclei for some descending motor pathways

Pons

•      Bulging brainstem region between the midbrain and the medulla oblongata

•      Forms part of the anterior wall of the fourth ventricle

•      Fibers of the pons:

–  Connect higher brain centers and the spinal cord

–  Relay impulses between the motor cortex and the cerebellum

•      Origin of cranial nerves V (trigeminal), VI (abducens), and VII (facial)

•      Contains nuclei of the reticular formation

Medulla Oblongata

•      Most inferior part of the brain stem

•      Along with the pons, forms the ventral wall of the fourth ventricle

•      Contains a choroid plexus on the ventral wall of the fourth ventricle

•      Pyramids – two longitudinal ridges formed by corticospinal tracts

•      Decussation of the pyramids – crossover points of the corticospinal tracts

Medulla Nuclei

•      Inferior olivary nuclei – gray matter that relays sensory information

•      Cranial nerves X, XI, and XII are associated with the medulla

•      Vestibular nuclear complex – synapses that mediate and maintain equilibrium

•      Ascending sensory tract nuclei, including nucleus cuneatus and nucleus gracilis

•      Cardiovascular control center – adjusts force and rate of heart contraction

•      Respiratory centers – control rate and depth of breathing

The Cerebellum

•      Located dorsal to the pons and medulla

•      Protrudes under the occipital lobes of the cerebrum

•      Makes up 11% of the brain’s mass

•      Provides precise timing and appropriate patterns of skeletal muscle contraction

•      Cerebellar activity occurs subconsciously

Anatomy of the Cerebellum

•      Two bilaterally symmetrical hemispheres connected medially by the vermis

•      Folia – transversely oriented gyri

•      Each hemisphere has three lobes – anterior, posterior, and flocculonodular

•      Neural arrangement – gray matter cortex, internal white matter, scattered nuclei

•      Arbor vitae – distinctive treelike pattern of the cerebellar white matter

Cerebellar Peduncles

•      Three paired fiber tracts that connect the cerebellum to the brain stem

•      All fibers in the cerebellum are ipsilateral

•      Superior peduncles connect the cerebellum to the midbrain

•      Middle peduncles connect the pons to the cerebellum

•      Inferior peduncles connect the medulla to the cerebellum

Cerebellar Processing

•      Cerebellum receives impulses of the intent to initiate voluntary muscle contraction

•      Proprioceptors and visual signals “inform” the cerebellum of the body’s condition

•      Cerebellar cortex calculates the best way to perform a movement

•      A “blueprint” of coordinated movement is sent to the cerebral motor cortex

Cerebellar Cognitive Function

•      Plays a role in language and problem solving

•      Recognizes and predicts sequences of events

Functional Brain System

•      Networks of neurons working together and spanning wide areas of the brain

•      The two systems are:

–  Limbic system

–  Reticular formation

Limbic System

•      Structures located on the medial aspects of cerebral hemispheres and diencephalon

•      Includes the rhinencephalon, amygdala, hypothalamus, and anterior nucleus of the thalamus

•      Parts especially important in emotions:

–   Amygdala – deals with anger, danger, and fear responses

–   Cingulate gyrus – plays a role in expressing emotions via gestures, and resolves mental conflict

•      Puts emotional responses to odors – e.g., skunks smell bad

Limbic System: Emotion and Cognition

•      The limbic system interacts with the prefrontal lobes, therefore:

–  One can react emotionally to conscious understandings

–  One is consciously aware of emotion in one’s life

•      Hippocampal structures – convert new information into long-term memories

Reticular Formation

•      Composed of three broad columns along the length of the brain stem

–  Raphe nuclei

–  Medial (large cell) group

–  Lateral (small cell) group

•      Has far-flung axonal connections with hypothalamus, thalamus, cerebellum, and spinal cord

Reticular Formation: RAS and Motor Function

•      RAS – reticular activating system

–  Sends impulses to the cerebral cortex to keep it conscious and alert

–  Filters out repetitive and weak stimuli

•      Motor function

–  Helps control coarse motor movements

–  Autonomic centers regulate visceral motor
functions – e.g., vasomotor, cardiac, and respiratory centers

Brain Waves

•      Normal brain function involves continuous electrical activity

•      An electroencephalogram (EEG) records this activity

•      Patterns of neuronal electrical activity recorded are called brain waves

•      Each person’s brain waves are unique

•      Continuous train of peaks and troughs

•      Wave frequency is expressed in Hertz (Hz)

Types of Brain Waves

•      Alpha waves – regular and rhythmic, low-amplitude, slow, synchronous waves indicating an “idling” brain

•      Beta waves – rhythmic, more irregular waves occurring during the awake and mentally alert state

•      Theta waves – more irregular than alpha waves; common in children but abnormal in adults

•      Delta waves – high-amplitude waves seen in deep sleep and when reticular activating system is damped

Brain Waves: State of the Brain

•      Brain waves change with age, sensory stimuli, brain disease, and the chemical state of the body

•      EEGs can be used to diagnose and localize brain lesions, tumors, infarcts, infections, abscesses, and epileptic lesions

•      A flat EEG (no electrical activity) is clinical evidence of death

Epilepsy

•      A victim of epilepsy may lose consciousness, fall stiffly, and have uncontrollable jerking, characteristic of epileptic seizure

•      Epilepsy is not associated with, nor does it cause, intellectual impairments

•      Epilepsy occurs in 1% of the population

Epileptic Seizures

•      Absence seizures, or petit mal – mild seizures seen in young children where the expression goes blank

•      Grand mal seizures – victim loses consciousness, bones are often broken due to intense convulsions, loss of bowel and bladder control, and severe biting of the tongue

Control of Epilepsy

•      Epilepsy can usually be controlled with anticonvulsive drugs

•      Valproic acid, a nonsedating drug, enhances GABA and is a drug of choice

•      Vagus nerve stimulators can be implanted under the skin of the chest and can keep electrical activity of the brain from becoming chaotic

Consciousness

•      Encompasses perception of sensation, voluntary initiation and control of movement, and capabilities associated with higher mental processing

•      Involves simultaneous activity of large areas of the cerebral cortex

•      Is superimposed on other types of neural activity

•      Is holistic and totally interconnected

•      Clinical consciousness is defined on a continuum that grades levels of behavior – alertness, drowsiness, stupor, coma

Types of Sleep

•      There are two major types of sleep:

–  Non-rapid eye movement (NREM)

–  Rapid eye movement (REM)

•      One passes through four stages of NREM during the first 30-45 minutes of sleep

•      REM sleep occurs after the fourth NREM stage has been achieved

Types and Stages of Sleep: NREM

•      NREM stages include:

–   Stage 1 – eyes are closed and relaxation begins; the EEG shows alpha waves; one can be easily aroused

–   Stage 2 – EEG pattern is irregular with sleep spindles (high-voltage wave bursts); arousal is more difficult

–   Stage 3 – sleep deepens; theta and delta waves appear; vital signs decline; dreaming is common

–   Stage 4 – EEG pattern is dominated by delta waves; skeletal muscles are relaxed; arousal is difficult

•      Characteristics of REM sleep

–  EEG pattern reverts through the NREM stages to the stage 1 pattern

–  Vital signs increase

–  Skeletal muscles (except ocular muscles) are inhibited

–  Most dreaming takes place

Sleep Patterns

•      Alternating cycles of sleep and wakefulness reflect a natural circadian rhythm

•      Although RAS activity declines in sleep, sleep is more than turning off RAS

•      The brain is actively guided into sleep

•      The suprachiasmatic and preoptic nuclei of the hypothalamus regulate the sleep cycle

•      A typical sleep pattern alternates between REM and NREM sleep

Importance of Sleep

•      Slow-wave sleep is presumed to be the restorative stage

•      Those deprived of REM sleep become moody and depressed

•      REM sleep may be a reverse learning process where superfluous information is purged from the brain

•      Daily sleep requirements decline with age

Sleep Disorders

•      Narcolepsy – lapsing abruptly into sleep from the awake state

•      Insomnia – chronic inability to obtain the amount or quality of sleep needed

•      Sleep apnea – temporary cessation of breathing during sleep

Memory

•      Memory is the storage and retrieval of information

•      The three principles of memory are:

–  Storage – occurs in stages and is continually changing

–  Processing – accomplished by the hippocampus and surrounding structures

–  Memory traces – chemical or structural changes that encode memory

Stages of Memory

•      The two stages of memory are short-term memory and long-term memory

•      Short-term memory (STM, or working memory) – a fleeting memory of the events that continually happen

•      STM lasts seconds to hours and is limited to 7 or 8 pieces of information

•      Long-term memory (LTM) has limitless capacity

Transfer from STM to LTM

•      Factors that effect transfer of memory from STM to LTM include:

–  Emotional state – we learn best when we are alert, motivated, and aroused

–  Rehearsal – repeating or rehearsing material enhances memory

–  Association – associating new information with old memories in LTM enhances memory

–  Automatic memory – subconscious information stored in LTM

Categories of Memory

•      The two categories of memory are fact memory and skill memory

•      Fact (declarative) memory:

–  Entails learning explicit information

–  Is related to our conscious thoughts and our language ability

–  Is stored with the context in which it was learned

Skill Memory

•      Skill memory is less conscious than fact memory and involves motor activity

•      It is acquired through practice

•      Skill memories do not retain the context in which they were learned

Structures Involved in Fact Memory

•      Fact memory involves the following brain areas:

–  Hippocampus and the amygdala, both limbic system structures

–  Specific areas of the thalamus and hypothalamus of the diencephalon

–  Ventromedial prefrontal cortex and the basal forebrain

Structures Involved in Skill Memory

•      Skill memory involves:

–  Corpus striatum – mediates the automatic connections between a stimulus and a motor response

–  Portion of the brain receiving the stimulus

–  Premotor and motor cortex

Mechanisms of Memory

•      Neuronal RNA content is altered

•      Dendritic spines change shape

•      Extracellular proteins are deposited at synapses involved in LTM

•      Number and size of presynaptic terminals may increase

•      More neurotransmitter is released by presynaptic neurons

•      New hippocampal neurons appear

•      Long-term potentiation (LTP) is involved and is mediated by NMDA receptors

•      Synaptic events involve the binding of brain-derived neurotropic factor (BDNF)

•      BDNF is involved with Na⁺, Ca²⁺, and Mg²⁺ influence at synapses

Proposed Memory Circuits

Protection of the Brain

•      The brain is protected by bone, meninges, and cerebrospinal fluid

•      Harmful substances are shielded from the brain by the blood-brain barrier

Meninges

•      Three connective tissue membranes lie external to the CNS – dura mater, arachnoid mater, and pia mater

•      Functions of the meninges

–  Cover and protect the CNS

–  Protect blood vessels and enclose venous sinuses

–  Contain cerebrospinal fluid (CSF)

–  Form partitions within the skull

Dura Mater

•      Leathery, strong meninx composed of two fibrous connective tissue layers

•      The two layers separate in certain areas and form dural sinuses

•      Three dural septa extend inward and limit excessive movement of the brain

–  Falx cerebri – fold that dips into the longitudinal fissure

–  Falx cerebelli – runs along the vermis of the cerebellum

–  Tentorium cerebelli – horizontal dural fold extends into the transverse fissure

Arachnoid Mater

•      The middle meninx, which forms a loose brain covering

•      It is separated from the dura mater by the subdural space

•      Beneath the arachnoid is a wide subarachnoid space filled with CSF and large blood vessels

•      Arachnoid villi protrude superiorly and permit CSF to be absorbed into venous blood

Pia Mater

•      Deep meninx composed of delicate connective tissue that clings tightly to the brain

Cerebrospinal Fluid (CSF)

•      Watery solution similar in composition to blood plasma

•      Contains less protein and different ion concentrations than plasma

•      Forms a liquid cushion that gives buoyancy to the CNS organs

•      Prevents the brain from crushing under its own weight

•      Protects the CNS from blows and other trauma

•      Nourishes the brain and carries chemical signals throughout it

Choroid Plexuses

•      Clusters of capillaries that form tissue fluid filters, which hang from the roof of each ventricle

•      Have ion pumps that allow them to alter ion concentrations of the CSF

•      Help cleanse CSF by removing wastes

Blood-Brain Barrier

•      Protective mechanism that helps maintain a stable environment for the brain

•      Bloodborne substances are separated from neurons by:

–  Continuous endothelium of capillary walls

–  Relatively thick basal lamina

–  Bulbous feet of astrocytes

Blood-Brain Barrier: Functions

•      Selective barrier that allows nutrients to pass freely

•      Is ineffective against substances that can diffuse through plasma membranes

•      Absent in some areas (vomiting center and the hypothalamus), allowing these areas to monitor the chemical composition of the blood

•      Stress increases the ability of chemicals to pass through the blood-brain barrier

Cerebrovascular Accidents (Strokes)

•      Caused when blood circulation to the brain is blocked and brain tissue dies

•      Most commonly caused by blockage of a cerebral artery

•      Other causes include compression of the brain by hemorrhage or edema, and atherosclerosis

•      Transient ischemic attacks (TIAs) – temporary episodes of reversible cerebral ischemia

•      Tissue plasminogen activator (TPA) is the only approved treatment for stroke

Degenerative Brain Disorders

•      Alzheimer’s disease – a progressive degenerative disease of the brain that results in dementia

•      Parkinson’s disease – degeneration of the dopamine-releasing neurons of the substantia nigra

•      Huntington’s disease – a fatal hereditary disorder caused by accumulation of the protein huntingtin that leads to degeneration of the basal nuclei

Spinal Cord

•      CNS tissue is enclosed within the vertebral column from the foramen magnum to L₁

•      Provides two-way communication to and from the brain

•      Protected by bone, meninges, and CSF

•      Epidural space – space between the vertebrae and the dural sheath (dura mater) filled with fat and a network of veins

•      Conus medullaris – terminal portion of the spinal cord

•      Filum terminale – fibrous extension of the pia mater; anchors the spinal cord to the coccyx

•      Denticulate ligaments – delicate shelves of pia mater; attach the spinal cord to the vertebrae

•      Spinal nerves – 31 pairs attach to the cord by paired roots

•      Cervical and lumbar enlargements – sites where nerves serving the upper and lower limbs emerge

•      Cauda equina – collection of nerve roots at the inferior end of the vertebral canal

Cross-Sectional Anatomy of the Spinal Cord

•      Anterior median fissure – separates anterior funiculi

•      Posterior median sulcus – divides posterior funiculi

Gray Matter and Spinal Roots

•      Gray matter consists of soma, unmyelinated processes, and neuroglia

•      Gray commissure – connects masses of gray matter; encloses central canal

•      Posterior (dorsal) horns – interneurons

•      Anterior (ventral) horns – interneurons and somatic motor neurons

•      Lateral horns – contain sympathetic nerve fibers

Gray Matter and Spinal Roots

Gray Matter: Organization

•      Dorsal half – sensory roots and ganglia

•      Ventral half – motor roots

•      Dorsal and ventral roots fuse laterally to form spinal nerves

•      Four zones are evident within the gray matter – somatic sensory (SS), visceral sensory (VS), visceral motor (VM), and somatic motor (SM)

White Matter in the Spinal Cord

•      Fibers run in three directions – ascending, descending, and transversely

•      Divided into three funiculi (columns) – posterior, lateral, and anterior

•      Each funiculus contains several fiber tracks

–  Fiber tract names reveal their origin and destination

–  Fiber tracts are composed of axons with similar functions

White Matter: Pathway Generalizations

•      Pathways decussate

•      Most consist of two or three neurons

•      Most exhibit somatotopy (precise spatial relationships)

•      Pathways are paired (one on each side of the spinal cord or brain)

White Matter: Pathway Generalizations

Main Ascending Pathways

•      The central processes of fist-order neurons branch diffusely as they enter the spinal cord and medulla

•      Some branches take part in spinal cord reflexes

•      Others synapse with second-order neurons in the cord and medullary nuclei

•      Fibers from touch and pressure receptors form collateral synapses with interneurons in the dorsal horns

Three Ascending Pathways

•      The nonspecific and specific ascending pathways send impulses to the sensory cortex

–  These pathways are responsible for discriminative touch and conscious proprioception

•      The spinocerebellar tracts send impulses to the cerebellum and do not contribute to sensory perception

Nonspecific Ascending Pathway

•      Nonspecific pathway for pain, temperature, and crude touch within the lateral spinothalamic tract

Specific and Posterior Spinocerebellar Tracts

•      Specific ascending pathways within the fasciculus gracilis and fasciculus cuneatus tracts, and their continuation in the medial lemniscal tracts

•      The posterior spinocerebellar tract

Descending (Motor) Pathways

•      Descending tracts deliver efferent impulses from the brain to the spinal cord, and are divided into two groups

–  Direct pathways equivalent to the pyramidal tracts

–  Indirect pathways, essentially all others

•      Motor pathways involve two neurons (upper and lower)

The Direct (Pyramidal) System

•      Direct pathways originate with the pyramidal neurons in the precentral gyri

•      Impulses are sent through the corticospinal tracts and synapse in the anterior horn

•      Stimulation of anterior horn neurons activates skeletal muscles

•      Parts of the direct pathway, called corticobulbar tracts, innervate cranial nerve nuclei

•      The direct pathway regulates fast and fine (skilled) movements

Indirect (Extrapyramidal) System

•      Includes the brain stem, motor nuclei, and all motor pathways not part of the pyramidal system

•      This system includes the rubrospinal, vestibulospinal, reticulospinal, and tectospinal tracts

•      These motor pathways are complex and multisynaptic, and regulate:

–   Axial muscles that maintain balance and posture

–   Muscles controlling coarse movements of the proximal portions of limbs

–   Head, neck, and eye movement

Extrapyramidal (Multineuronal) Pathways

•      Reticulospinal tracts – maintain balance

•      Rubrospinal tracts – control flexor muscles

•      Superior colliculi and tectospinal tracts mediate head movements

Spinal Cord Trauma: Paralysis

•      Paralysis – loss of motor function

•      Flaccid paralysis – severe damage to the ventral root or anterior horn cells

–  Lower motor neurons are damaged and impulses do not reach muscles

–  There is no voluntary or involuntary control of muscles

•      Spastic paralysis – only upper motor neurons of the primary motor cortex are damaged

–  Spinal neurons remain intact and muscles are stimulated irregularly

–  There is no voluntary control of muscles

Spinal Cord Trauma: Transection

•      Cross sectioning of the spinal cord at any level results in total motor and sensory loss in regions inferior to the cut

•      Paraplegia – transection between T₁ and L₁

•      Quadriplegia – transection in the cervical region

Poliomyelitis

•      Destruction of the anterior horn motor neurons by the poliovirus

•      Early symptoms – fever, headache, muscle pain and weakness, and loss of somatic reflexes

•      Vaccines are available and can prevent infection

Amyotrophic Lateral Sclerosis (ALS)

•      Lou Gehrig’s disease – neuromuscular condition involving destruction of anterior horn motor neurons and fibers of the pyramidal tract

•      Symptoms – loss of the ability to speak, swallow, and breathe

•      Death occurs within five years

•      Linked to malfunctioning genes for glutamate transporter and/or superoxide dismutase