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Central Nervous System

(CNS)

Physiology 152 – Allied Health

What is the CNS?CNS Consists of

Brain

Spinal Cord

CNS

Receives input from Sensory Neurons

Directs activity of Motor Neurons

Associates motor response(s) with sensory stimuli through Association Neurons to maintain homeostasis

CNS in VertebratesCNS of all vertebrates are at least capable of very basic

forms of learning and memory

Human CNS capabilities allow for:Behavioral modification based on experiencePerceptionsLearning and MemoryEmotionsSelf-awarenessDirect motor activity

Embryonic Development of the CNS

Figure: http://faculty.washington.edu/chudler/dev.html


(for student reference)

Ectoderm is the initial embryonic tissue that develops into the nervous system, the epidermis of skin as well as other structures (slide 4)

At 2 weeks of embryonic development the neural plate is formed (slide- 4)

A few days after the neural plate is formed, a groove is formed in the neural plate; creating the neural groove (slide 4)

The part of the ectoderm where the fusion of the two ends of the neural groove occurs creates a separate structure called the neural crest (slide 4)

Eventually, the neural tube will become the CNS and the neural crest will develop into the ganglia of the peripheral nervous system (PNS) (slide 4)

By middle of week 4, since gestation, 3 distinct swellings are evident on the anterior end of the neural tube which will form the brain (forebrain, midbrain, and hindbrain) (slide 6)

During the 5th week, a five-regioned brain develops and specific structures begin to form (slide 6)


Figure: http://faculty.irsc.edu/FACULTY/TFischer/AP1/AP%201%20resources.htm

Overview: Brain Development Sequence

Figure: http://faculty.washington.edu/chudler/dev.html

CNS CompositionGray matter

Consists of neuron cell bodies and dendrites Present in the surface layer (cortex) of the brain Present deeper within the brain (subcortical) in aggregations

called nuclei

White matter Consists of axon tracts Underlie cortex and surround nuclei Myelin Sheath give white matter the white color

Adult brain Contains ~100 billion neurons Weighs approximately 1.5 kg Receives 20% of total per minute body blood flow

CerebrumThe only structure of the

telencephalon

Accounts for 80% of brain’s mass

Primarily responsible for higher mental functions

Consists of right and left hemispheres (connected by corpus callosum)

Figure: Alan Hoffring/National Cancer Institute

Cerebral CortexIs the outer layer of the

cerebrum

Composed of 2-4 mm of gray matter and underlying white matter

Characterized by many convolutions

Gyri are the elevated fold of the convolutions

Sulci are the depressed grooves of the convolutions

Cerebral CortexEach cerebral

hemisphere is subdivided by deep sulci or fissures

Cerebral subdivisions:

Frontal lobe

Parietal lobe

Temporal lobe

Occipital lobe

Insula (insular lobe)

Cerebral CortexFrontal lobe

Anterior portion of each cerebral hemisphere

Central Sulcus (fissure) separates frontal lobe from parietal lobe

Functions: personality, higher intellectual processes, decision making

Cerebral Cortex Parietal lobe

Positioned superior to the occipital lobe in both hemispheres

Parieto-occipital sulcus (fissure) separates parietal lobe from the occipital lobe

Lateral sulcus or Sylvian fissure separates the parietal lobe from the temporal lobe

Functions: understanding speech, formulating words, interpretation of textures and shapes

Cerebral CortexOccipital lobe

Is the rearmost and posterior part of the cerebral hemispheres

Functions: Focusing the eye, correlation of visual images, conscious perception of vision

Cerebral CortexTemporal lobe

Bordered by parietal lobe, frontal lobe, and occipital lobe in both hemispheres

Located beneath the lateral sulcus (Sylvian fissure)

Functions: processing of sensory inputs, language comprehension, memory

Cerebral CortexInsula

Situated deep within the lateral sulcus

Functions: Memory, integration of cerebral activities

Motor and Sensory Areas

Figure: http://med-review.blogspot.com/

Precentral gyrus is the motor area of the brain

Postcentral gyrus is the sensory area of the brain

Homunculus map is a representation of the contralateral sensory/motor side of the body

Motor and Sensory Areas

The size of the mapped body region in the precentral (motor) gyrus is reflective of the number of the motor innervations. For i.e. the largest areas in the motor cortex represent body regions with largest number of motor innervations.

The size of the mapped body region in the postcentral (sensory) gyrus is reflective of the density of receptors. For i.e. the largest areas in the sensory cortex represent body regions with highest densities of receptors.

Language and Aphasiology

In the last century, understanding of brain function has been governed by study methodologies such as aphasiology

Aphasiology is the study of language impairment through brain damage


Aphasiology research helped define two loosely defined brain regions in the left cortical hemisphere:

Broca’s area

Function: speech production

Wernicke’s area

Function: understanding written and spoken language


Broca and Wernicke’s areas are connected by a fiber tract called arcuate fasciculus.

It appears that the words form and are conceptualized in the Wernicke’s area and are communicated via Broca’s area

Cerebral Lateralization

Each cerebral cortex controls movement of the contralateral (opposite) side of the body

Somatesthetic sensation from each side of the body projects to the contraletral postcentral gyrus

Right and left cerebral hemispheres communicate via a large fiber tract called the corpus callosum

Cerebral Lateralization

Experiments have shown:

Right hemisphere has limited verbal ability

Right hemisphere is better at recognizing faces

Analytical and language abilities are characteristics of the left hemisphere

Visuospatial ability is a characteristic of the right hemisphere

Basal NucleiAre masses of gray

matter situated deep within the white matter of the cerebrum

Prominent structure is the corpus striatum (composed of caudate nucleus and putamen)

Controls voluntary movement

Figure: Brain E-Books brainmind.net

ThalamusSituated between

cerebral cortex and midbrain

Functions as a relay station to the cerebral cortex for sensory (except smell) signals

Is involved in regulation of sleep and consciousness

Figure: Brain E-Books brainmind.net

Limbic System

Amygdala

Involved in memory and emotional processing

Is part of the medial temporal lobe and is involved in consolidating short-term memory into long-term memory.

Note: When the consolidation process is over, long-term memory is independent of the medial temporal lobe

Septum

Involved in pleasure

Limbic System

Cingulate Cortex

Involved in pain and visceral response

Hippocampus

Is a critical part of the memory system

Is part of the medial temporal lobe and is involved in consolidating short-term memory into long-term memory

Fornix

Carries signals from hippocampus to hypothalamus

Hypothalamus

Involved in:

Reward processing

Punishment processing

Regulates:

Pituitary gland

Blood pressure

Hunger

Thirst

Metabolism

Heart rate

Emotion and MotivationBrain regions of paramount importance in emotion and

motivation:

Hypothalamus

Limbic system

Experiments have shown the hypothalamus and limbic system to be involved in the following feelings and behaviors:

Aggression

Stimulation of amygdala produces rage and aggression

Lesions of amygdala results in docility (docile: easily handled/taught)

Emotion and Motivation Experiments have shown the hypothalamus and limbic system to

be involved in the following feelings and behaviors (con’t):

Fear

Produced by stimulation of amygdala and hypothalamus

Removal of limbic system results in absence of fear

Feeding

Hypothalamus contains both feeding center and satiety center

Stimulation of the feeding center will result in overeating

Stimulation of the satiety center results in stop of feeding behavior

Emotion and MotivationExperiments have shown the hypothalamus and limbic

system to be involved in the following feelings and behaviors (con’t):

Sex

Hypothalamus and limbic system are involved in sexual drive and sexual behaviors

Goal-directed behavior (reward and punishment system)

Stimulation of certain parts of the hypothalamus can be more rewarding than food or sex in motivating behavior.

Stimulation of other parts of the hypothalamus stimulate a punishment system

Memory

Amnesia (loss of memory) found to result from:

Damage to the temporal lobe

Damage to the hippocampus

Damage to the caudate nucleus (i.e. Huntington’s disease)

Damage to the dorsomedial thalamus (i.e. alcoholoics suffering from Koraskoff’s syndrome with thiamine/vitamin B1 deficiency)

Modern-day techniques for studying brain function

Magnetoencephlography (MEG)

Measures brain activity through transmitted magnetic fields produced by electrical currents in the brain

Electroencephlography (EEG)

Measures brain activity through direct measurement of electrical activity of the brain

Able to discern REM sleep (Rapid-eye movement sleep stage when dreams occur)


Four types of EEG patterns:

Alpha waves

Best recorded from parietal and occipital regions

Person is awake and relaxed with eyes closed

Frequency: 10-12 cycles/second

Child < 8 years old; Frequency : 4-7 cycles/second

Beta waves

Are strongest from the frontal lobes, especially the area near precentral gyrus



Four types of EEG patterns (Con’t):

Theta waves

Emitted from the temporal and occipital lobes


Delta waves

Emitted in a general pattern from the cerebral cortex



Positron Emission Tomography (PET)

Is a functional neuroimaging technique that uses injection of radioisotopes in the bloodstream to measure brain activity

Functional Magnetic Resonance Imaging (fMRI)

Is a functional neuroimaging technique that measures brain activity by measuring associated changes in blood flow to any respective part of the brain or the brain as a whole


Recap

Midbrain: Regions and Functions

Corpora quadrigemina Involved in visual reflexRelay center for auditory information

Cerebral pedunclesPair of structures composed of ascending and descending

fiber tracts

Red nucleusArea of gray matter deep in the midbrainMaintains connections with cerebrum and cerebellum Involved in motor coordination

Midbrain: Regions and Functions

Substantia nigraPart of the Nigrostriatal systemNigrostriatal system projects from the substantia nigra to the

corpus striatum of the basal nucleiNigrostriatal system is required of motor coordinationDegeneration of nigrostriatal fibers result in Parkinson’s

Disease

Dopaminergic pathwaysMesolimbic dopamine

systemReward pathwayAxons use dopamine

as a neurotransmitter (dopaminergic axons)

Axons leave the midbrain to the nucleus accumbens and prefrontal cortex

Dopaminergic pathwaysNigrostriatal

dopamine systemMotor control

pathwayAxons use dopamine

as a neurotransmitter (dopaminergic axons)

Axons leave the substantia nigra of the midbrain and synapse in the corpus striatum

Recap

Hindbrain: Regions, Subregions, and Functions

MetencephelonPons

Surface fibers connect to the cerebellum

Deeper fibers are part of motor and sensory tracts passing from medulla oblongata

Holds nuclei associated with cranial nerves

Holds nuclei involved in breathing regulation

Figure: http://www.knowabouthealth.com/normal-brain-communication-found-in-people-with-agenesis-of-the-corpus-callosum/8844/


Metencephelon (con’t)Cerebellum

Second largest structure of the brain

Contains outer gray and inner white matter (like cerebrum)

Holds fibers that pass from cerebellum, pass through the red nucleus to the thalamus, and then to motor areas of the cerebral cortex


Metencephelon (con’t)Cerebellum (con’t)

Holds fiber tracts that connect the cerebellum with the pons, medulla oblongata, and spinal cord

Recieves input from proprioceptors (joint,tendon, muscel receptors)

Heavily involved in coordination of movement


Myelencephalon Composed of only the

Medulla Oblongata Medulla Oblongata

Continuous between the pons and the spinal cord

All ascending and descending fiber tracts between spinal cord and brain pass through the medulla

Contains the pyramids where decussation of fibers occur

Contains nuclei involved in motor control, regulation of breathing and cardiovascular response

Reticular FormationComplex network of

nuclei within: medulla, pons, midbrain, thalamus, and hypothalamus

Functions as a reticular activating system (RAS)Filters background

stimuliRegulates sleep-wake

cycleRegulates consciousness

and sleep

60 Minute episode on fMRIhttp://www.youtube.com/watch?v=8jc8URRxPIg

Spinal CordRuns in the

vertebral column

Made up of white and gray matter

Gray matter is centrally located and surrounded by white matter (unlike the brain)

Spinal CordGray matter forms

an H in the center of the spinal cord with two dorsal horns and two ventral horns

White matter is made up of ascending and descending fiber tracts

Fiber tracts are arranged in 6 columns called funiculi

Spinal Cord TractsTwo types: Ascending and Descending Tracts

Terminology:Ascending tract names start with a prefix spino- and

end with the brain region where the fibers first synapse (i.e. spinothalamic)

Descending tract names start with a prefix reflecting the brain region where the fibers start and ends with the suffix –spinal (i.e. corticospinal)

Spinal Cord TractsAscending Tracts

Fiber tracts that convey sensory information to the brain from receptors throughout the body

Two types of information Exteroceptive: Arise from external environment of the body (i.e.

pain, touch, temperature) Proprioceptive: Arise from the internal environment of the body

(i.e. muscles, joints)

Usually, sensory information from the right side of the body end up in the left hemisphere of the brain

Usually, sensory information from the left side of the body end up in the right hemisphere of the brain

Spinal Cord TractsAscending tracts (con’t)

Three types of neurons in ascending pathways1st order sensory neurons2nd order neurons cross to

the opposite side (decussate) and travel to higher level of CNS

3rd order neurons situated in the thalamus and extends to sensory regions of the cortex

Spinal Cord TractsDescending Tracts

Originate from the brainRelay motor activity from the brain to the rest of the bodyTwo groups of fiber tracts

Corticospinal/pyramidal tracts : Motor tracts that descend without synaptic interruption from cerebrum to spinal cord; Originate primarily from motor cortex

Extrapyramidal tracts: Motor tract projections that carry autonomic motor impulses (i.e. for speech and swallowing); Originate in the midbrain and brain stem

Spinal Cord TractsTwo groups of fiber

tracts Corticospinal/

pyramidal tracts : Motor tracts that descend without synaptic interruption from cerebrum to spinal cord; Originate primarily from motor cortex

Extrapyramidal tracts: Motor tract projections that carry autonomic motor impulses (i.e. for speech and swallowing); Originate in the midbrain and brain stem

Cranial and Spinal NervesCNS communicates with the body via nerves

that exit the CNS from:Brain (Cranial nerves)Spinal cord (Spinal nerves)

Cranial and Spinal nerves, and cell bodies outside the CNS make up the PNS

Cranial and Spinal NervesCranial Nerves

Count : 12 pairs2 pairs arise from neuron cell bodies in the forebrain10 pairs arise from the midbrain and hindbrainDesignated by Roman numerals, which reflect order of

position (numbered from front to the back of brain)Names indicate the structure innervated by these

nerves (i.e. Facial)Most cranial nerves are mixed nerves which indicates

that the nerve contains both sensory and motor fibers

Cranial and Spinal NervesSpinal Nerves

Count: 31 pairsGroups:

8 cervical12 thoracic5 lumbar5 sacral1 coccygeal

All are mixed Nerves

Spinal NervesFibers are

packaged together in the nerve and separate near the attachment of nerve to spinal cord

Two “roots” to each nerve:Dorsal root

(sensory)Ventral root

(motor)

Reflex ArcStimulation of sensory

receptors evokes action potentials that are conducted into the spinal cord by sensory neurons

A Sensory neuron synapses with an association neuron

Association neuron synapses with a somatic motor neuron

Somatic motor neuron then conducts impulses out of the spinal cord to the muscle and stimulates a reflex contraction

Textbook Reference

Human Physiology (6th edition) by Stuart Ira Fox

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