THE CENTRAL NERVOUS SYSTEM

THE BRAIN

EMBRYONIC DEVELOPMENT

• At three weeks’ gestation, the ectoderm forms the neural plate, which invaginates, forming the neural groove, flanked on either side by neural folds

• By the fourth week of pregnancy, the neural groove fuses, giving rise to the neural tube, which rapidly differentiates into the CNS

• The neural tube develops constrictions that divide the three primary brain vesicles:– Prosencephalon (forebrain)– Mesencephalon (midbrain)– Rhombencephalon (hindbrain)

NEURAL TUBE

BRAIN DEVELOPMENT

Effect of Space Restriction on

Brain Development

REGIONS AND ORGANIZATION

• The basic pattern of the CNS consists of a central cavity surrounded by a gray matter core, external to which is white matter

• In the brain, the cerebrum and cerebellum have an outer gray matter layer, which is reduced to scattered gray matter nuclei in the spinal cord

ARRANGEMENT OF

GRAY and WHITE MATTER

VENTRICLES

• The ventricles of the brain are continuous with one another, and with the central canal of the spinal cord.– They are lined with ependymal cells, and are filled

with cerebrospinal fluid• The paired lateral ventricles lie deep within each cerebral

hemisphere, and are separated by the septum pellucidum• The third ventricle lies within the diencephalon, and

communities with the lateral ventricles via two interventricular foramina

• The fourth ventricle lies in the hindbrain and communicates with the third ventricle via the cerebral aqueduct

BRAIN VENTRICLES

CEREBRAL HEMISPHERES• The cerebral hemispheres form the superior part of the brain, and

are characterized by ridges and grooves (convolutions) called gyri (elevated ridges of tissue) and sulci (hollow grooves) – Deeper grooves called Fissures separate large regions of the brain

• The cerebral hemispheres are separated along the midline by the longitudinal fissure, and are separated from the cerebellum along the transverse cerebral fissure

• The five lobes of the brain separated by specific sulci (all but the last named for the cranial bone that overlie them) are: frontal, parietal, temporal, occipital, and insula ( buried deep within the lateral sulcus: equilibrium)

• The cerebral cortex is the location of the conscious mind, allowing us to communicate, remember, and understand

CEREBRAL HEMISPHERES

• The two hemispheres are largely symmetrical in structure but not entirely equal in function

• There is a lateralization (specialization) of cortical function– NO function area of the cortex acts alone and

conscious behavior involves the entire cortex in one way or another

LOBE FISSURES

BRAIN CONVOLUTIONS

NEUROIMAGING

NEUROIMAGING

• Shows that specific motor and sensory functions are localized in discrete cortical areas called DOMAINS

• Many higher mental functions, such as memory and language, appear to have overlapping domains and are spread over very large areas of the cortex

NEUROIMAGING

• PET scans:– Positron emission

tomography– Positron: a particle

having the same mass as a negative electron but possessing a positive charge

– Shows maximal metabolic activity

NEUROIMAGING

• MRI scans:– Magnetic resonance

imaging– Reveals blood flow

CEREBRAL HEMISPHERES

• The cerebral cortex has several motor areas located in the frontal lobes, which control voluntary movement– The primary motor cortex allows conscious control of

skilled voluntary movement of skeletal muscles– The premotor cortex is the region controlling learned

motor skills– Broca’s area is a motor speech area that controls

muscles involved in speech production– The frontal eye field controls eye movement

CEREBRAL CORTEX

CEREBRAL CORTEX

• Primary motor area: conscious control of skilled voluntary movement of skeletal muscles

• Premotor cortex: region controlling learned motor behavior (typing, playing musical instrument)

• Frontal eye field: eye movement

CEREBRAL CORTEX• Prefrontal cortex:

– Most complicated cortical region– Involved with intellect, complex

learning abilities (cognition), recall, and personality

– Production of abstract ideas, judgment, reasoning, persistence, long-term planning, concern for others, and conscience

– In children matures slowly and is heavily dependent on positive and negative feedback

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

– Plays a role intuitive judgments and mood

– Tremendous elaboration of this region sets humans apart from other animals

– Language comprehension and word analysis

CEREBRAL CORTEX

• Somatic sensation: receives information from the general (somatic) sensory receptors in the skin and skeletal muscle and integrates the different sensory inputs (temperature, pressure, etc.)

• Gustatory cortex: taste• General interpretation area:

– Found in one hemisphere only (usually left)

– Receives input from all incoming signals into a single thought or understanding of the situation

CEREBRAL CORTEX

• Visual association area: recognizes a flower or a person’s face

• Auditory association area: memories of sounds

CEREBRAL CORTEX

LANGUAGE AREAS:LEFT HEMISPHERE • Broca’s area:

– Motor speech area that controls muscles (tongue, lips, throat) involved in speech production

– Considered to be present in only one hemisphere (usually the left)

– Becomes active as we prepare to speak and even when we think about (plan) many voluntary motor activities other than speech

• Wernicke’s area:– Language comprehension and articulation– Believed to be the area responsible for

understanding written and spoken language– Involved in sounding out unfamiliar words

• Prefrontal cortex: language comprehension and word analysis

• Lateral and Ventral parts of temporal lobe: coordinate auditory and visual aspects of language when reading

CORRESPONDING AREARIGHT HEMISPHERE

• Non-language dominance• Involved in body language and non-verbal

emotional (affective) components of language rather than speech mechanics

• Allows the lift and tone of our voice and our gestures to express our emotions when we speak

• Permits us to comprehend the emotional content of what we hear ( a soft response to your question conveys quite a different meaning than a sharp reply)

LATERALIZATION

• We use both cerebral hemispheres for almost every activity, and the hemispheres appear nearly identical– BUT, there is division of labor, and each

hemisphere has unique abilities not shared by its partner (LATERALIZATION)

• Although one cerebral hemisphere or the other “dominates” each task, the term cerebral dominance designates the hemisphere that is dominant for language

LATERALIZATION

• Right Hemisphere:– 10% of people– Non-language dominant– Visual-spatial skills, intuition, emotion, artistic

and musical skills, poetic, creative– Most left-handed– More often males

LATERALIZATION

• Left Hemisphere:– 90% of people– Greater control over language abilities, math

and logic– Most right handed

LATERALIZATION

• BILATERAL:– Ambidextrous– Could be cerebral confusion: Is it your turn or

mine?– Learning disabilities (dyslexia, etc.)

CEREBRAL CORTEX

CEREBRAL HEMISPHERES

• There are several sensory areas of the cerebral cortex that occur in the parietal, temporal, and occipital lobes– The primary somatosensory cortex allows spatial discrimination

and the ability to detect the location of stimulation– The somatosensory association cortex integrates sensory

information and produces an understanding of the stimulus being felt

– The primary visual cortex and visual association area allow reception and interpretation of visual stimuli

– The primary auditory cortex and auditory association area allow detection of the properties and contextual recognition of sound

– The olfactory cortex allows detection of odors– The gustatory cortex allows perception of taste stimuli– The vestibular cortex is responsible for conscious awareness of

balance

Motor and Sensory Areas of the

Cerebral Cortex

CEREBRAL CORTEX

• Do not confuse the sensory and motor areas of the cortex with sensory and motor neurons: All neurons in the cortex are interneurons

Motor and Sensory Areas of the

Cerebral Cortex• Red: Primary (somatic)

motor cortex– Located in the precentral

gyrus of the frontal lobe of each hemisphere

• Central sulcus: groove between Red/Blue

• Blue: Primary somatosensory cortex– Located on the postcentral

gyrus of the parietal lobe, just posterior to the premotor cortex

Motor and Sensory Areas of the

Cerebral Cortex

• The body is typically represented upside down: the head at the inferolateral part of the precentral gyrus, and the toes at the superomedial end

Motor and Sensory Areas of the

Cerebral Cortex• PRIMARY MOTOR CORTEX

– The motor innervation of the body is contralateral (opposite)

– The left primary motor gyrus controls muscles on the right side of the body, and vice versa

– Misleading: a given muscle is controlled by multiple spots on the cortex and that individual cortical motor neurons actually send impulses to more than one muscle

• In other words: individual motor neurons control muscles that work together in a synergistic way (so that one does not over react)

Motor and Sensory Areas of the

Cerebral Cortex

• PRIMARY SOMATOSENSORY CORTEX– Receives information from

the general (somatic) sensory receptors located in the skin and from proprioceptors in skeletal muscles (locomotion, posture, and tone)

– Right hemisphere receives input from the left side of the body and vice versa

FIBER TRACTS

FIBER TRACTS

CEREBRAL HEMISPHERES

• Several association areas are not connected to any sensory cortices– The prefrontal cortex is involved with intellect, cognition, recall,

and personality, and is closely linked to the limbic system– The language areas involved in comprehension and articulation

include Wernicke’s area, Broca’s area, the lateral prefrontal cortex, and the lateral and ventral parts of the temporal lobe

– The general interpretation area receives input from all sensory areas, integrating signals into a single thought

– The visceral association area is involved in conscious visceral sensation

CEREBRAL CORTEX

CEREBRAL CORTEX

CEREBRAL HEMISPHERES

• There is lateralization of cortical functioning, in which each cerebral hemisphere has unique abilities not shared by the other half– One hemisphere (often the left) dominates language abilities,

math, and logic, and the other hemisphere (often the right) dominates visual-spatial skills, intuition, emotion, and artistic and musical skills

• Cerebral white matter is responsible for communication between cerebral areas and the cerebral cortex and lower CNS centers

• Basal nuclei consist of a group of subcortical nuclei, which play a role in motor control and regulating attention and cognition

BASAL NUCLEI

BASAL NUCLEI• The precise role of the basal

nuclei has been elusive because of their inaccessible location and because their functions overlap to some extent with those of the cerebellum

• Role in motor control is complex• Plays a role in regulating attention

and in cognition (reasoning/thinking)

• Important in starting, stopping, and monitoring movements executed by the cortex

– Inhibit unnecessary movements• Disorders result in either too much

or too little movement as exemplified by Huntington’s and Parkinson’s disease

BASAL NUCLEI

MIDSAGITTAL REGION(Diencephalon and Brain Stem)

DIENCEPHALON

• The diencephalon is a set of gray matter areas, and consist of the thalamus, hypothalamus, and epithalamus– The thalamus plays a key role in mediating sensation,

motor activities, cortical arousal, learning, and memory

– The hypothalamus is the control center of the body, regulating ANS activity such as emotional response, body temperature, food intake, sleep-wake cycles, and endocrine function

– The epithalamus includes the pineal gland, which secretes melatonin and regulates the sleep-wake cycle

DIENCEPHALON

VENTRAL BRAIN

BRAIN STEM

• The brain stem, consisting of the midbrain, pons, and medulla oblongata, produces rigidly programmed, automatic behaviors necessary for survival– The midbrain is comprised of the cerebral peduncles,

corpora quadrigemina, and substantia nigra– The pons contains fiber tracts that complete

conduction pathways between the brain and spinal cord

– The medulla oblongata is the location of several visceral motor nuclei controlling vital functions such as cardiac and respiratory rate

BRAIN STEM

BRAIN STEM• Just above the medulla-spinal

cord junction, most of the fibers cross over to the opposite side before continuing their descent into the spinal cord or ascent into the brain

• This crossover point is called the Decussation of the Pyramids (longitudinal ridges of the medulla)

– Formed by the large pyramidal tracts descending from the motor cortex

• Consequence of this crossover is that each cerebral hemisphere chiefly controls the voluntary movements of muscles on the opposite (contralateral) side of the body

BRAIN STEM

BRAIN STEM

BRAIN STEM NUCLEI

BRAIN STEM NUCLEI

CEREBELLUM• The cerebellum processes inputs from several structures and

coordinates skeletal muscle contraction to produce smooth movement– There are two cerebellar hemispheres consisting of three lobes each

• Anterior and posterior lobes coordinate body movements and the flocculonodular lobes adjust posture to maintain balance

– Three paired fiber tracts, the cerebellar peduncles, communicate between the cerebellum and the brain stem

• Cerebellar processing follows a functional scheme in which the frontal cortex communicates the intent to initiate voluntary movement to the cerebellum, the cerebellum collects input concerning balance and tension in muscles and ligaments, and the best way to coordinate muscle activity is relayed back to the cerebral cortex

CEREBELLUM

CEREBELLUM

FUNCTIONAL BRAIN SYSTEMS

• Functional brain systems consist of neurons that are distributes throughout the brain but work together– The limbic system is involved with emotions, and is

extensively connected throughout the brain, allowing it to integrate and respond to a wide variety of environmental stimuli

– The reticular formation extends through the brain stem, keeping the cortex alert via the reticular activating system, and dampening familiar, repetitive, or weak sensory inputs

LIMBIC SYSTEM

RETICULAR FORMATION

HIGHER MENTAL FUNCTIONS

BRAIN WAVE PATTERNS

• Normal brain functions results from continuous electrical activity of neurons, and can be recorded with an electroencephalogram, or EEG

• Patterns of electrical activity are called brain waves, and fall into four types: alpha, beta, theta, and delta waves

BRAIN WAVES

CONSCIOUSNESS

• Consciousness encompasses conscious perception of sensations, voluntary initiation and control of movement, and capabilities associated with higher mental processing

SLEEP AND

SLEEP-AWAKE CYCLES• Sleep is a state of partial unconsciousness from which a

person can be aroused, and has two major types that alternate through the sleep cycle– Non-rapid eye movement (NREM) sleep has four stages– Rapid eye movement (REM) sleep is when most dreaming

occurs

• Sleep patterns change throughout life, and are regulated by the hypothalamus

• NREM sleep is considered to be restorative, and REM sleep allows the brain to analyze events or eliminate meaningless information

MEMORY• Memory is the storage and retrieval of information

– Short-term memory, or working memory, allows the memorization of a few units of information for a short period of time

– Long-term memory allows the memorization of potentially limitless amounts of information for very long periods

– Transfer of information from short-term to long-term memory can be affected by a high emotional state, repetition, association of new information with old, or the automatic formation of memory while concentrating on something else

– Fact memory entails learning explicit information, is often stored with the learning context, and is related to the ability to manipulate symbols and language

– Skill memory usually involves motor skills, is often stored without details of the learning cortex, and is reinforced through performance

– Learning causes changes in neuronal RNA, dendritic branching, deposition of unique proteins at LTM synapses, increase of presynaptic terminals, increase of neurotransmitter, and development of new neurons in the hippocampus

MEMORY PROCESS

MEMORY CIRCUITS

PROTECTION OF THE BRAIN

MENINGES

• Meninges are three connective tissue membranes that cover and protect the CNS, protect blood vessels and enclose venous sinuses, contain cerebrospinal fluid, and partition the brain– The dura mater is the most durable, outermost

covering that extends inward in certain areas to limit movement of the brain within the cranium

– The arachnoid mater is the middle meninx that forms a loose brain covering

– The pia mater is the innermost layer that clings tightly to the brain

MENINGES

MENINGES

DURA MATER

CEREBROSPINAL FLUID

• Cerebrospinal (CSF) is the fluid found within the ventricles of the brain and surrounding the brain and spinal cord

• CSF gives buoyancy to the brain, protects the brain and spinal cord from impact damage, and is a delivery medium for nutrients and chemical signals

CEREBROSPINAL FLUID

CEREBROSPINAL FLUID

HYDROCEPHALUS

The blood-brain barrier is a protective mechanism that helps

maintain a protective environment for the brain

HOMEOSTATIC IMBALANCES OF THE BRAIN

• Traumatic head injuries can lead to brain injuries of varying severity: concussion, contusion, and subdural or subarachnoid hemorrhage

• Cerebrovascular accidents (CVAs), or strokes, occur when blood supply to the brain is blocked resulting in tissue death

• Alzheimer’s disease is a progressive degenerative disease that ultimately leads to dementia

• Parkinson’s disease results from deterioration of dopamine-secreting neurons of the substantia nigra, and leads to a loss in coordination of movement and a persistent tremor

• Huntington’s disease is a fatal hereditary disorder that results from deterioration of the basal nuclei and cerebral cortex

THE SPINAL CORD

EMBRYONIC DEVELOPMENT

• The spinal cord develops from the caudal portion of the neural tube

• Axons from the alar plate form white matter, and expansion of both the alar and ventral plates gives rise to the central gray matter of the cord

• Neural crest cells form the dorsal root ganglia, and send axons to the dorsal aspect of the cord

EMBRYONIC SPINAL CORD

GROSS ANATOMY AND

PROTECTION• The spinal cord extends from the foramen magnum of

the skull to the level of the first or second lumbar vertebrae– It provides a two-way conduction pathway to and from the brain

and serves as a major reflex center• Fibrous extensions of the pia mater anchor the spinal

cord to the vertebral column and coccyx, preventing excessive movement of the cord

• The spinal cord has 31 pairs of spinal nerves along its length that define the segments of the cord

• There are cervical and lumbar enlargements for the nerves that serve the limbs, and a collection of nerve roots (caudal equine) that travel through the vertebral column to their intervertebral foramina

SPINAL CORD

SPINAL CORD

LUMBAR TAP

CROSS-SECTIONAL ANATOMY

• Two grooves partially divide the spinal cord into two halves: the anterior and posterior median fissures

• Two arms that extend posteriorly are dorsal horns, and the two arms that extend anteriorly are ventral horns

• In the thoracic and superior lumbar regions, there are also paired lateral horns that extend laterally between the dorsal and ventral horns

• Afferent fibers form peripheral receptors form the dorsal roots of the spinal cord

• The white matter of the spinal cord allows communication between the cord and brain

• All major spinal tracts are part of paired multineuron pathways that mostly cross from one side to the other, consist of a chain of two or three neurons, and exhibit somatotropy

SPINAL CORD

SPINAL CORD

Organization of the

Gray Matter of the Spinal Cord

CROSS-SECTIONAL ANATOMY

• Ascending pathways conduct sensory impulses upward through a chain of three neurons– Nonspecific ascending pathways receive input from many different types

of sensory receptors, and make multiple synapses in the brain– Specific ascending pathways mediate precise input from a single type of

sensory receptor– Spinocerebellar tracts convey information about muscle and tendon

stretch to the cerebellum• Descending pathways involve two neurons: upper motor neurons

and lower motor neurons– The direct, or pyramidal, system regulates fast, finely controlled, or

skilled movements– The indirect, or extrapyramidal, system regulates muscles that maintain

posture and balance, control coarse limb movements, and head, neck, and eye movements involved in tracking visual objects

ASCENDING/DESCENDING TRACTS

ASCENDING PATHWAY

ASCENDING PATHWAY

DESCENDING PATHWAY

DESCENDING PATHWAY

SPINAL CORD TRAUMA AND DISORDERS

• Any localized damage to the spinal cord or its roots leads to paralysis (loss of motor function) or paresthesias (loss of sensory function)

• Poliomyelitis results from destruction of anterior horn neurons by the polio virus

• Amyotrophic lateral sclerosis (ALS), or Lou Gehrig’s, disease is a neuromuscular condition that involves progressive destruction of anterior horn motor neurons and fibers of the pyramidal

LUMBAR MYELOMENINGOCELE

DIAGNOSTIC PROCEDURES FOR

ASSESSING CNS DYSTUNCTION

• Pneumoencephalography is used to diagnose hydrocephalus, and allows X-ray visualization of the ventricles of the brain

• A cerebral angiogram is used to assess the condition of cerebral arteries to the brain in individuals that have suffered a stroke or TIA

• CT scans and MRI scanning techniques allow visualization of most tumors, intracranial lesions, multiple sclerosis plaquwes, and areas of dead brain tissue

• PET scan can localize brain lesions that generate seizures and diagnose Alzheimer’s disease

DEVELOPMENTAL ASPECTS OF

THE CENTRAL NERVOUS SYSTEM

• The brain and spinal cord grow and mature throughout the prenatal period due to influence from several centers

• Gender-specific areas of the brain and spinal cord develop depending on the presence or absence of testosterone

• Lack of oxygen to the developing fetus may result in cerebral palsy, a neuromuscular disability in which voluntary muscles are poorly controlled or paralyzed as a result of brain damage

• Age brings some cognitive decline but losses are not significant until the seventh decade