BIO2305
The Central Nervous System
Nervous System
Functions
Sensory input – monitoring stimuli occurring inside & outside the body
Integration – interpretation of sensory input
Motor output – response to stimuli by activating effector organs
Organization of the Nervous System
Central nervous system (CNS)
Brain and spinal cord
Integration and command center
Peripheral nervous system (PNS)
Paired spinal & cranial nerves
Outside the CNS
Carries messages to and from brain and spinal cord
Terminology review
Nerve fiber - a general term for any neuronal process
Nerve - a bundle of many nerve fibers along the same path in
the PNS surrounded by a connective tissue layer
Ganglia - a cluster of nerve cell bodies in the PNS
Nucleus - a mass of cell bodies and dendrites in the CNS
Tract - a bundle of nerve fibers along the same path in the CNS
without a connective tissue layer
White matter - aggregations of myelinated processes from many neurons
Gray matter - contains either neurons cell bodies, dendrites, and axon terminals, or unmyelinated
axons and neuroglia
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Organization of the Nervous System
PNS: Afferent & Efferent Divisions
Afferent (sensory) division – transmits impulses from receptors to the CNS.
Somatic afferent fibers – carry sensory impulses from skin, skeletal muscles, and joints
Visceral afferent fibers – transmit sensory impulses from visceral organs
Motor (efferent) division – transmits impulses from the CNS to effector organs. Two subdivisions:
Somatic motor fibers – provides conscious control of skeletal muscles
Visceral motor fibers (aka Autonomic Nervous System) – regulates smooth muscle, cardiac
muscle, and glands
Afferent (Sensory)
Somatic Sensory
General somatic senses – receptors in skin, skeletal muscle, and joints
Touch, Pain, Vibration, Pressure, Temperature, Proprioceptive senses
Proprioception detects stretch in muscle, stress in tendons
Position and movement of body in space
Special somatic senses – receptors in eyes, ears, nose, tongue
Hearings, Balance, Vision, Smell, Taste
Visceral Sensory
General visceral senses – receptors in digestive, urinary, and reproductive organs
Stretch, Pain, Temperature, Nausea, Hunger
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Efferent (Motor)
Somatic Motor
General somatic motor
Contraction of skeletal muscles
Under voluntary control
Often called “voluntary nervous system”
Visceral Motor (Autonomic Nervous System)
General visceral motor
Regulates the contraction of smooth and cardiac muscle
and glands
Controls function of visceral organs
Often called the “involuntary nervous system”
Sympathetic Division – “Fight or Flight”
Parasympathetic Division – “Rest and Digest”
Central Nervous System:
Brain
Spinal cord
Neuron Classification
Functional:
Sensory (afferent) — transmit impulses toward the CNS
Motor (efferent) — carry impulses away from the CNS
Interneurons (association neurons) — shuttle signals through CNS pathways; Responsible
for integrating afferent information and formulating an efferent response to include higher
cognitive functions
CNS Support
Neuroglia (glial cells)
Non-excitable support cells of the CNS
Cerebrospinal fluid (CSF)
A salty solution that is continuously secreted into and circulating throughout the ventricles and
cavities of the brain and spinal cord
Blood brain barrier (BBB)
Tight junctions in the brain capillaries that prevent free exchange of many small and lipidsoluble substances between the blood and the CSF
Neuroglia (glial cells)
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Neuroglia
Neuroglia
Neuroglia – specialized nervous tissue cells that are smaller and more numerous than neurons. They
serve to carry out support functions such as vascularization, phagocytosis, and myelination.
There are four types found in the CNS:
Astrocytes – star shaped with many processes, participate in the metabolism of
neurotransmitters, maintain proper K+ balance, help form blood brain barrier, and provide a link
between neurons and blood vessels
Oligodendrocytes – fewer processes and are smaller that astrocytes, most common type in
the CNS, they are involved in myelination
Microglia – small cells derived from monocytes that function as macrophages and carry out
phagocytosis
Ependymal – columnar to squamous epithelial cells often ciliated that form a lining in the
ventricles (fluid filled cavities in the brain)
CNS Protection
Hair, skin, cranium, vertebral column
Meninges (dura mater, arachnoid mater, pia mater)
Cerebrospinal fluid (CSF)
Blood brain barrier
Meninges
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Cerebrospinal Fluid (CSF)
Shock-absorbing medium
Provides an optimum and stable environment for generating APs
Provides a medium for the exchange of nutrients and wastes between blood and nervous tissue
Formed by selective transport across ependymal cells
Volume 125-150 mL and is replaced > 3 times/day, flow maintained by 10 mmHg pressure gradient
Choroid plexus
Choroid plexus – a network of blood capillaries in the walls of the ventricle
The capillaries are covered by ependymal cells that produces the CSF
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Blood Brain Barrier
Blood Brain Barrier – a functional barrier between the interstitial fluid and the blood
Structure:
Tight junctions promoted by astrocytes whose foot processes envelop brain capillaries &
sinuses
Function:
Limits permeability for most molecules
Except: gases and small, lipid-soluble molecules:
O2, CO2, alcohol, steroids, H2O
Protects brain: hormones & circulating chemicals
Protects CNS from chemical fluctuations
Prevents entry of harmful substances
Prevents entry of molecules that could act as neurotransmitters
Cerebral Circulation
Cerebral circulation – the movement of blood through the network of blood vessels supplying the
brain
Brain receives 15% of blood pumped by heart
Brain responsible for about half of body’s glucose consumption
Membrane transporters move glucose from plasma into the brain interstitial fluid
Membrane Transport Restrictions
Materials that diffuse through the lipid bilayer cannot be restricted: O2 CO2, alcohol, steroids, H2O
Materials that require carrier proteins for transport are restricted: glucose, amino acids and ions
Some materials cannot pass the barrier: potentially harmful substances, some hormones/drugs
Some areas of the brain do not have a blood-brain barrier (part of hypothalamus) because of their role
in monitoring blood homeostasis
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Brain Organization
Trillions of interneurons fill the brain
Up to 200,000 synapses each
Levels of complexity
Cerebral cortex
Basal nuclei
Thalamus
Hypothalamus
Cerebellum
Brain stem
Cerebrum
Cerebrum
Highly developed
Makes up about 80% of total brain weight (largest portion of brain)
Inner core houses basal nuclei
Outer surface is highly convoluted cerebral cortex
Highest, most complex integrating area of the brain
Plays key role in most sophisticated neural functions
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Cerebral Cortex
Three specializations:
Sensory areas - sensory input translated into perception
Motor areas - direct skeletal muscle movement
Association areas - integrate information from sensory and motor areas, can direct voluntary
behaviors
Cerebral Cortex
Each half of cortex divided into four major lobes:
Occipital lobe – carries out initial processing of visual input
Temporal lobe – initial reception of sound sensation, taste, smell
Parietal lobe – somatosensory processing
Frontal lobe responsible for
Motor processing
Voluntary motor activity
Speaking ability
Elaboration of thought
Homunculus
Homunculus - Latin for "little human“ – proportioned to highlight the
amount of cortical tissue of cerebrum dedicated to sensory and motor
signals that correspond with different parts of the body
Used in various fields of study to reflect the relative space that human
body parts occupy on the somatosensory cortex (sensory homunculus)
and the motor cortex (motor homunculus)
Sensory and motor areas for the hands and face are especially large
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Primary Somatosensory Cortex
Located in the postcentral gyrus on parietal lobe
Receives information from the skin and skeletal muscles
Exhibits spatial discrimination
Somatosensory tracts: from soma (body)  cortex
Somatosensory homunculus – caricature of relative amounts of cortical tissue devoted to each sensory
function
Primary Motor Cortex
Located in the precentral gyrus on the posterior portion of the frontal lobe
Composed of pyramidal cells whose axons make up the corticospinal tracts
Corticospinal tracts : from cortex  spinal cord
Allows conscious control of precise, skilled, voluntary movements
Motor homunculus – caricature of relative amounts of cortical tissue devoted to each motor function
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Language
Wernicke’s area
Concerned with language comprehension
Responsible for formulating coherent patterns of speech
Broca’s area
governs speaking ability
Functional Areas of the Cortex
Primary visual cortex – medial surface of occipital lobe receives input from the thalamus (lateral
geniculate nuclei) concerning shape, color, and movement
Primary auditory cortex – in superior part of the temporal lobe, interprets characteristics of sound,
hearing
Primary gustatory area – at base of post central gyrus in the parietal lobe receives impulses for taste
Primary olfactory area – medial aspect of temporal lobe - receives impulses for smell
Supplemental motor area – preparatory role in programming complex sequences of movement
Premotor cortex – orients the body and arms toward a specific target, deals with learned motor
activities of a complex and sequential nature.
Association areas – tracts that connect motor and sensory areas and large parts of the cortex
Prefrontal association cortex – planning voluntary activity, decision making, creativity, personality
traits
Parietal-temporal-occipital association cortex – pools and integrates somatic, auditory, visual
sensations for complex perceptual processing, also involved with language
Limbic association cortex – motivation and emotion involved with memory
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Brain Function: Cerebral Lateralization
Each lobe has special functions
Cerebral Cortex
Integration of cortical regions – schematic linking of various regions of the cortex
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Basal Nuclei
Involved in the control of movement
Act by modifying ongoing activity in motor pathways
Primary functions:
Regulates muscle tone throughout the body
Selecting and maintaining purposeful motor activity while
suppressing useless or unwanted patterns of movement
Helping monitor and coordinate slow, sustained
contractions, especially those related to posture and
support
Controls large automatic movement
Diencephalon: Thalamus, Hypothalamus, Pituitary
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Thalamus
Final relay point for ascending sensory information
All sensory information relays through thalamus except olfaction
Coordinates the activities of the cerebral cortex and basal nuclei
Domain-specific information processing
Directs attention to stimulus of interest:
Medial geniculate nuclei – hearing
Lateral geniculate nuclei – vision
Ventral posterior nuclei – taste and somatic sensations (pain, pressure)
Anterior nucleus – emotions and memory
Hypothalamus
Receives indirect inputs from all sensory systems
Controls sympathetic and parasympathetic nervous
systems
Sends both neural and hormonal outputs to pituitary
Coordinates activities of the endocrine and nervous
systems
Induces emotions and behavioral drives
Coordinates voluntary and autonomic functions
Regulates body temperature
Coordinates circadian cycles
4Fs: feeding, fighting, fleeing, and reproductive
behavior
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Limbic System
Cingulated gyrus (Cingulum = “belt” in Latin)
Coordinates sensory input with emotions
Emotional responses to pain
Basic, inborn behavioral patterns related to survival and perpetuation of the species
Regulates aggressive behavior
Amygdala
Involved in emotional responses, hormonal secretions, and memory
Hippocampus
Consolidation of short-term memory to long-term memory
Sends memories out to the appropriate part of the cerebral hemisphere for long-term storage
and retrieving them when necessary
Plays important role in motivation and learning
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Cerebellum
Basic functions: coordination, balance, motor learning, etc.
Vestibulocerebellum – balance and control of eye movement
Spinocerebellum – enhances muscle tone and coordinates skilled voluntary movement – important
in synchronization and timing
Receives input concerning desired action from motor cortex
Receives feedback concerning actual action from proprioceptors, vestibular apparatus, eyes
Compares inputs and sends adjustments or corrective signals to motor tracts
Planning and initiation of voluntary activity by providing input to the cortical motor areas also involved in
procedural memories
Located posterior to medulla and pons
Brain Stem: Midbrain, Pons & Medulla
Brain stem – link between spinal cord and higher brain levels
Brain stem is composed of:
Midbrain
Pons
Medulla Oblongata
Cranial Nerves: 11 of 12 originate from brainstem
Olfactory nerve (CN I) originates from forebrain
Reticular Formation throughout core of brainstem
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Cranial Nerves
Midbrain
Controls eye movement
Relays signals for auditory and visual reflexes
Contains substantia nigra made of dopaminergic neurons responsible for eye movement, reward
seeking, and addiction
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Pons
Contains nuclei that help control respiration
Nuclei and tracts link the cerebellum with the cerebrum, brain stem, and spinal cord
Sensory and motor nuclei for four cranial nerves
Contains ascending, descending, and transverse tracts
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Medulla oblongata
Contains relay stations and reflex centers
Cardiovascular and respiratory rhythmicity centers
Cardiovascular center – regulates rate and force of heartbeat and
vasoconstriction/dilation
Respiratory center – regulates basic breathing rhythm
Medullary Pyramids – 90% of nerve tracts crossover at pyramids
Reticular formation begins in the medulla oblongata and extends superiorly into the pons and
midbrain of the brainstem
Medulla oblongata
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Reticular Activating System (RAS)
Reticulating (branching) network of neurons in brain stem (reticular formation)
Arousal, sleep, pain, & muscle tone
Arouses and activates cerebral cortex
Blocking ascending pathways between RAS and cerebral cortex creates state of unconsciousness
Controls overall degree of cortical alertness or level of consciousness:
maximum alertness
wakefulness
sleep
coma
ElectroEncephaloGram – EEG
Records electrical activity within cerebral cortex from EPSP, IPSPs
Used for
Diagnose cerebral dysfunction
Brain death
Sleep Patterns
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Consciousness – Sleep
Functions of Sleep
“Catch up” time – restore biochemical and physiological processes
Role of adenosine
Increased levels while awake
Inactivates RAS
Caffeine blocks adenosine receptors
Shift gears: long term structural and chemical changes required to consolidate memory and learning
Learning and Memory
Learning has two broad types
Associative – conditioning, linking two events together
Non-associative
Habituation
Sensitization
Memory has several types
Short-term and long-term
Reflexive and declarative
Memory
The storage of acquired knowledge for later recall
Memory trace – neural change responsible for retention or storage of knowledge
Short-term memory – lasts for seconds to hours
Long-term memory – retained for days to years
Consolidation – process of transferring and fixing short-term memory traces into long-term memory
stores
Working memory – temporarily holds and interrelates various pieces of information relevant to a
current mental task
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Long-Term vs Short-Term Memory
Short Term Memory
Seconds to hours
Limited capacity
Rapid retrieval
Synaptic alterations
Changes in ion channels
Presynaptic facilitation cAMP
Long term potentiation (LTP)
Basic Learning - Behavior
Habituation – decreased responsiveness to a repetitive
stimulus
1. Closure of Ca2+ channels
2. Reduced neurotransmitter release
3. Decrease EPSP
4. Fewer postsynaptic APs generated
Sensitization – increase responsiveness to a
noxious or intense stimulus
1. Release of serotonin from interneuron
2. Increases cAMP in presynaptic neuron
3. Blocks K+ channels and prolongs AP
4. Ca2+ channels are open longer
5. Increasing neurotransmitter output
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Long Term Potentiation (LTP)
LTP - Activity at a synapse brings about sustained changes in the quality or quantity of synaptic
connections
Glutamate (NT) from presynaptic neuron binds to both AMPA and NMDA receptors
AMPA receptor:
Glutamate opens a Na+ channel, and a Na+ influx depolarizes the cell
NMDA receptor:
Glutamate opens the Ca2+ channel gate
Depolarization by AMPA receptor causes ejection of Mg2+ ion from NMDA channel
Once NMDA channel is opened (by opening of Ca2+ gate and ejection of Mg2+), Ca2+ enters the cell,
and acts in 2nd messenger pathways
2nd messenger pathways cause the postsynaptic cell to become more sensitive to glutamate
(upregulating AMPA and NMDA receptors, increase in dendrites and overall surface area of
postsynaptic neuron)
Retrograde Signaling: Postsynaptic cell also releases an unknown paracrine (once thought to be
nitric oxide, but scientists are now unsure) that acts on the presynaptic cell to enhance glutamate
release
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Long Term Memory
Last days to years
Unlimited capacity
Permanent structural changes
Formation of new synapses between existing neurons
Increased dendritic surface area
Increase in neurotransmitter receptors
Changes in neurotransmitter synthesis
Long Term Memory
Consolidation is affected by:
Amount of rehearsal
Association of new & old data
Level of excitement/importance of information
CREB – regulatory proteins that activate genes important in long term memory storage
Memory Processing
Hippocampus (limbic system)
Short term memory and consolidation
Declarative memory (facts), specific objects
Requires conscious recall
Cerebellum – “how to” memories
Motor skills (procedural)
Subconscious recall
Prefrontal cortex – working memory
Processes new and retrieved information
Temporary storage
Problem solving, planning, organizing
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Spinal Cord Regions
Cervical
Thoracic
Lumbar
Sacral
Spinal Cord Organization
Gray matter: mostly cell bodies
Dendrites & terminals
Spinal reflex integrating center
White matter
Bundles of myelinated axons
Ascending tracts (sensory) – send action potentials superiorly to brain
Descending tracts (motor) – bring action potentials inferiorly from brain to body
Dorsal roots – allow action potentials to enter the spinal cord, providing sensory information
from the peripheral tissues
Ventral roots – allow action potentials to exit the spinal cord and reach peripheral tissues
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Spinal Cord Organization
Spinal Integration and Reflex Pathway
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Neural Reflexes
Stimulus
Sensory receptor
Sensory (afferent) neuron
CNS integration
Efferent (motor) neuron
Effector (target tissue)
Response (movement)
Feedback to CNS
Reflexes
Reflex - a fast, predictable, automatic response to changes in the environment that helps maintain
homeostasis.
Three types of reflexes: cranial reflexes, somatic reflexes, and visceral (autonomic) reflexes
Reflex arc - simplest pathway, involving five components:
1. Receptor – distal end of a sensory neuron (dendrite) that responds to a stimulus
2. Sensory neuron/afferent pathway – conducts impulse from receptor to axon terminal located
in the gray matter.
3. Integrating center in CNS – may be monosynaptic or polysynaptic involving other association
neurons
4. Motor neuron/efferent pathway – relays impulse from integrating center to part of the body
that will respond.
5. Effector – gland or muscle that responds to motor neuron impulse.
Monosynaptic and Polysynaptic Reflexes
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Patellar Tendon Reflex: Stretch & Reciprocal Inhibition
Cross Extensor Reflex/Withdrawal Reflex
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