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
Embryonic Development of the CNS
(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)
Embryonic Development of the CNS
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
Language and Aphasiology
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
Language and Aphasiology
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)
Modern-day techniques for studying brain function
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
Frequency: 13-25 cycles/second
Modern-day techniques for studying brain function
Four types of EEG patterns (Con’t):
Theta waves
Emitted from the temporal and occipital lobes
Frequency: 5-8 cycles/second
Delta waves
Emitted in a general pattern from the cerebral cortex
Frequency: 1-5 cycles/second
Modern-day techniques for studying brain function
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
Modern-day techniques for studying brain function
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/
Hindbrain: Regions, Subregions, and Functions
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
Hindbrain: Regions, Subregions, and Functions
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
Hindbrain: Regions, Subregions, and Functions
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