Lecture #20

Nervous System

• Nervous system– Swift, brief responses to stimuli

• Endocrine system– Adjusts metabolic operations– Directs long-term changes

Two organ systems coordinate and direct activities of a body

Anatomical Classification of the Nervous System

• Central Nervous System– Brain and spinal cord

• Peripheral Nervous System– All neural tissue outside CNS

• Neurons

Cells in Nervous Tissue

• Neuroglia

• about half the volume of cells in the CNS• smaller than neurons• 5 to 50 times more numerous• do NOT generate electrical impulses• divide by mitosis• two types in the PNS

– Schwann cells– Satellite cells

• four types in the CNS– Astrocytes– Oligodendrocytes– Microglia– Ependymal cells

Neuroglia (Glia)

• Largest of glial cells• Star shaped with many processes projecting from the cell body• Help form and maintain blood-brain barrier• Provide structural support for neurons• Regulate ion concentrations for generation of nerve impulses/action potentials• Regulate nutrient concentrations for neuron survival• Take up excess neurotransmitters• Repair nervous tissue

Astrocytes

Oligodendrocytes

• Most common glial cell type• fewer processes than

astrocytes• round or oval cell body• forms myelin sheath around

the axons of neurons in CNS• form a supportive network

around CNS neurons• analogous to Schwann cells

of PNS

Microglia

• Small cells found near blood vessels• Phagocytic role - clear away dead cells• protect CNS from disease through phagocytosis of microbes• migrate to areas of injury where they clear away debris of injured cells - may also kill healthy cells

Ependymal Cells

• epithelial cells that line the cerebral cavities (ventricles) & central canal

• produce & circulate the cerebrospinal fluid (CSF) found in these chambers• form a structure with capillaries called a choroid plexus

• CSF = colourless liquid that protects the brain and SC against chemical & physical injuries, carries oxygen, glucose and other

necessary chemicals from the blood to neurons and neuroglia

CNS PNSVENTRICLECilia

Neuron Astrocyte

Oligodendrocyte

Capillary Ependymal cell

Schwann cell

Microglial cell

• Flat cells surrounding PNS axons• Support neurons in the PNS

PNS: Satellite Cells

PNS: Schwann Cells

• each cell produces part of the myelin sheath surrounding an axon in the PNS

• outmost layer of the sheath = neurilemma• contributes to regeneration of PNS axons• regions of no myelin = Nodes of Ranvier

Neurilemma

The Neuron

-comprised of: 1. cell body or soma2. dendrites3. an axon

-neurofilaments – cytoskeleton of the neuron-Nissl bodies – endoplasmic reticulum-perikaryon – region outside of the nucleus

2. Dendrites (little trees)- the receiving or input portion of the neuron-short, tapering and highly branched-surfaces specialized for contact with other neurons-bind the chemicals of neuronal communication = neurotransmitters-help trigger the nerve impulse in neurons = action potential

Neurons

3. Axons• long, thin cylindrical process of the neuron• conduct the action potential away from cell

body toward another neuron• joins the cell body at a cone-shaped elevation

= axon hillock• axon hillock becomes the axon • between the axon hillock and the axon =

trigger zone – site where the action potential arises

• cytoplasm = axoplasm• plasma membrane = axolemma• axon and collaterals end in fine processes

called axon terminals• swollen tips called synaptic end bulbs contain

vesicles filled with neurotransmitters• NTs are released when the action potential

enters the end bulb

Synapses• synapse points of communication between two neurons• two types

– 1. chemical – two neurons separated by a synaptic cleft• requires the release of chemicals called neurotransmitters from the pre-synaptic

neuron and the binding of this neurotransmitter by the post-synaptic neuron• majority of chemical synapses formed between end bulb and dendrites• some can form between end bulbs and cell body

Nucleus

DendritesStimulus

Axon hillock

Cellbody

Presynapticcell

Signaldirection

Axon

Synapse

Neurotransmitter

Synaptic terminals

Postsynaptic cell

Synapticterminals

2. electrical – direct connection between pre- and post-synaptic neuron

- connected via gap junctionsfaster of the two synapses

Functional Classification of Neurons• Sensory (afferent) neurons - PNS

– transport sensory information from skin, muscles, joints, sense organs & viscera to CNS

• Motor (efferent) neurons - PNS– send motor nerve impulses to

muscles & glands• Interneurons (association)

neurons- CNS– connect sensory to motor neurons– 90% of neurons in the body

Motor neuron

Dendrites

Axon

Cellbody

Sensory neuron

Organization of vertebrate nervous systems

• brain provides the processing/integrative functions• the spinal cord conducts information to and from the brain

and body• spinal cord and brain develop from the dorsal hollow nerve

cord• front end of the nerve cord expands to become the brain

– embryologic development of the brain results in the formation of a:• forebrain – gives rise to the cerebrum & the diencephalon• midbrain – gives rise to the midbrain• hindbrain - gives rise to the pons, medulla & cerebellum

• the remaining nerve cord becomes the spinal cord

Divisions of the nervous system

Efferent neuronsAfferent neurons

Central NervousSystem

(information processing)

Peripheral NervousSystem

Sensoryreceptors

Internaland external

stimuli

Autonomicnervous system

Motorsystem

Control ofskeletal muscle

Sympatheticdivision

Parasympatheticdivision

Entericdivision

Control of smooth muscles,cardiac muscles, glands

The peripheral nervous system

• the somatic nervous system controls voluntary motor impulses to skeletal muscle– also receives sensory input from muscles, joints, tendons and

skin• the autonomic nervous system has sympathetic,

parasympathetic, and enteric divisions• the sympathetic division regulates arousal and the

“fight-or-flight” response• the parasympathetic division has antagonistic effects on

sympathetic target organs and promotes calming and a return to “rest and digest” functions

• The enteric division controls activity of the digestive tract, pancreas, and gallbladder

The ANS

Parasympathetic division

Action on target organs:Constricts pupil

of eye

Stimulates salivarygland secretion

Constrictsbronchi in lungs

Slows heart

Stimulates activityof stomach and

intestines

Stimulates activityof pancreas

Stimulatesgallbladder

Promotes emptyingof bladder

Promotes erectionof genitalia

Cervical

Thoracic

Lumbar

SynapseSacral

Sympatheticganglia

Sympathetic division

Action on target organs:

Dilates pupil of eye

Accelerates heart

Inhibits salivarygland secretion

Relaxes bronchiin lungs

Inhibits activity ofstomach and intestines

Inhibits activityof pancreas

Stimulates glucoserelease from liver;inhibits gallbladder

Stimulatesadrenal medulla

Inhibits emptyingof bladder

Promotes ejaculationand vaginal contractions

Functional divisions of the peripheral nervous system

• Afferent– Sensory information from receptors into CNS

• Efferent – Motor commands from the brain to muscles and

glands– Somatic division• Voluntary control over skeletal muscle

– Autonomic division• Involuntary regulation of smooth and cardiac muscle, glands

• fish:– CNS consisting of a brain and spinal cord– sensory receptors widely distributed over the body

• touch and temperature• also are specialized receptors for smell, vision, hearing, equilibrium and balance• e.g. external nares – snouts of fishes – lead to olfactory receptors• receptors for equilibrium, balance and hearing are located in the inner ear• lateral-line system – along each side of the fish and branching over the head

– responsive to pressure, vibration– how the fish “hears”

• amphibians:– brain is similar to other vertebrates– brain develops from three embryologic divisions: forebrain (smell,

autonomic control centers), midbrain (vision) & hindbrain (heart rate and respiration)

– sensory receptors over the skin• bare nerve endings for heat, cold and pain• lateral-line system just like fishes – response to vibrations• chemoreceptors in the nasal epithelium, linings of the mouth and tongue

and over the skin– vision becomes an important sense

• hunt via sight• number of adaptations have taken place to create the terrestrial eye• eyes become located on the front of the head – not the sides• this provides binocular vision for improved depth perception• lower eyelid called the nictating membrane is moveable and cleans the

eye surface• orbital glands to wash and lubricate the eye• lens is large and round – set back from the cornea and is surrounded by a

fold of epithelium called the iris• focusing requires refraction of light – provided by the cornea and

changing the position of the lens• to focus on close objects – the lens is moved forward• retina contains photoreceptors called rods and cones

• hearing also becomes well developed in the amphibians– auditory system transmits both vibrations and sound– ears of frogs and toads consists of a tympanic membrane, a middle ear and an inner ear

• vocalization– sound production apparatus – larynx and vocal cords– mainly a reproductive function of the male frog– salamanders do not vocalize– advertisement calls to announce territory– breeding calls– female responds with reciprocation calls to indicate receptiveness– distress calls – loud enough to scare the predator

• reptile:– brain is similar to other vertebrates– increased cerebral size vs. amphibians to accommodate the increased

sense of smell– increased cerebellum– hearing mainly for the detection of vibration (can have a lateral line system)– vision is the dominant sense in most reptiles

• optic lobe is larger in reptiles vs. amphibians• snakes – focus by moving the lens forward • all other reptiles focus by rounding the lens by the action of ciliary

muscles surrounding the lens • some reptiles possess a median or parietal eye – for distinguishing light

and dark– olfactory senses are better developed in reptiles than in amphibians

• development of a partial secondary palate to increase the surface area for olfactory epithelium

• may also possess blind-ending pouches = Jacobson’s (vomeronasal) organs– snakes – use forked tongues to bring in air particles into the

mouth – travel to the Jacobson’s organs for “smelling”– heat sensitive pits in vipers – “sixth sense”

• pits located between the eyes and nostrils• minute temp differences are seen as infrared rays• used in prey location

Major Regions of the Mammalian Brain

Neuronal Organization: CNS

•Two kinds of neural tissue found in both brain and spinal cord:1. Gray matter =

neuroglial cells + unmyelinated axons, and dendrites/cell bodies of neurons

-when it forms the outer layer of the cerebrum = cerebral cortex

-Gray matter also found as nuclei deep in the brain = clusters of neuronal cell bodies in the CNS-Collections of nuclei can form a center (higher brain function)

Neuronal Organization: CNS

• 2. White matter = myelinated axons • Cell bodies found in gray matter• White matter tracts = bundles of

axons– For the conduction of nerve

impulses– Brain – three types of tracts

(commisural, association, projection)

– Spinal cord - Two types: sensory and motor tracts (ascending and descending)

male vs. female brain:http://www.youtube.com/watch?v=L29KmQxEA3E

• Cerebrum = largest portion-left and right cerebral hemispheres divided by the longitudinal

fissure-connected by an accumulation of white matter - the corpus callosum-folded into ridges and grooves: grooves = sulci

-sulci divide the cerebrum into lobes-ridges = gyri (gyrus)

Cerebrum

-outermost layer of the cerebrum, contains billions of gray matter neurons – less than 5mm thick!!

-white matter tracts extend out and run to other gray matter areas (either another gyrus or a nucleus)

-the cortex of each gyri contains neurons for the specific processing of sensation, voluntary movement, speech, all thought processes

-gyri can be classified as: primary or association areas

-primary areas are for the initial processing of raw sensory information or motor commands - e.g. primary visual, auditory & gustatory areas

-also contains gyri that are called association areas for integration and analysis of specific sensory info & help in making of “decisions”

e.g. somatosensory, visual, auditory, language and common integrative areas

The Cerebral Cortex

The Cerebral Cortex

• awareness of surroundings, language, cognition, memory and consciousness

• cognition:– job of the neocortex– first 6 layers of the cerebral cortex– the more convoluted the

neocortex (i.e more gyri and sulci) – the higher the cognitive function????

– may not be true– birds can be relatively smart

• do not have a convoluted neocortex

Human brain

Avian brain

Thalamus

Midbrain

Hindbrain CerebellumAvian brainto scale

Thalamus

Midbrain

Hindbrain

Cerebellum

Cerebrum (includingcerebral cortex)

Cerebrum(including pallium)

The Cerebral Cortex

• sensory processing:– numerous gyri whose cerebral cortex

processes sensory information– parietal lobe – somatosensory areas

& gustatory areas• primary somatosensory gyrus –

receives sensory information from the skin, muscles and joints

– temporal lobe – auditory areas– occipital lobe – visual areas– once sensory information is

processed – info gets sent to the frontal lobe where movements are planned

– one of the major areas for this – primary motor area in the frontal lobe

Frontal lobe Parietal lobe

Primarymotor cortex

Primarysomatosensorycortex

GenitaliaToes

Abdominalorgans

Tongue

JawLips

Face

EyeBrowNeck

Thumb

FingersHand

Wrist

ForearmElbow

ShoulderTrunk

Hip

Knee

TonguePharynx

JawGumsTeeth

Lips

Face

Nose

EyeThumb

FingersHand

ForearmElbow

Upper arm

Head

Neck

TrunkH

ipLeg

The Cerebral Cortex

• language and speech:– French physician Pierre Broca – involved

in mapping cognitive functions to specific areas of the cerebral cortex

– became interested in those patients who could understand language but could not speak

– identified an area of the left front lobe in the majority of people = Broca’s area

– damage to this area results in the inability to speak (aphasia) – but the ability to understand language• responsible for motor commands to the

muscles of the face• active when speaking

– in the left temporal lobe = Wernicke’s area (Karl Wernicke)• active when hearing language

– damage to this area – able to speak BUT unable to understand language

Hearingwords

Speakingwords

Seeingwords

Generatingwords

Max

Min

Lateralization• Broca’s areas and Wernicke’s area are in the left hemisphere in most

people– these people are mostly right-handed also– Broca’s area is active in 96% of right-handed people but only 76% of left-

handed people• language is principally the job of the left hemisphere

– along with math and logical thought• right hemisphere will have other distinct functions

– recognition of faces, patterns, spatial relationships and non-verbal thinking• differences in the function of the hemispheres is referred to as

lateralization• when the two hemispheres work together – do so through the

commissures – corpus callosum, anterior & posterior commissures

The Frontal Lobe

• motor commands• personality• decision making• damage to the frontal lobe can

affect all of these• but intellect and memory are left

intact• specific destruction of the frontal

lobe to affect personality = lobotomy

The basal ganglia: -made up of several gray matter nuclei found deep within the cerebrum1. receives sensory input from the cerebral cortex & provides output to the motor areas of the cortex 2. integrates motor commands3. regulates the initiation & termination of muscle movements4. also functions to anticipate body movements & controls subconscious contraction of skeletal muscle

• comprised of the:• 1. striatum

– caudate nucleus– putamen– nucleus accumbens

• 2. globus pallidus • 3. claustrum• 4. substantia nigra• 5. subthalmic nucleus

• Diencephalon– includes the hypothalamus,

thalamus, epithalamus and subthalamus

– thalamus: 80% of the diencephalon• paired oval shaped lobes of gray

matter organized into nuclei, interconnected with white matter tracts

• major relay station for most sensory impulses from the SC, brain stem into the cerebrum

• relays motor information from the cerebellum to the cerebrum (for coordination)

• crude perception of pain, heat and pressure (refined in cerebrum)

Diencephalon

Diencephalon

ThalamusPineal glandHypothalamus

Pituitary gland

Spinal cord

Brainstem

Midbrain

Pons

Medullaoblongata

Diencephalon• hypothalamus-emotions, autonomic functions, hormone production-made of numerous nuclei and tracts

3. regulates emotional and behavioral patterns – rage,aggression, pain and pleasure + sexual arousal4. regulates eating & drinking – hypothalamus contains a thirst center which responds to a rise in osmotic pressure in the ECF (dehydration)5. controls body temperature – monitors temp of bloodflowing through the hypothalamus

1. control of the ANS – integrates signals from the ANS (regulated smooth and cardiac muscle contraction) major regulator of visceral activities (heart rate, food movements, contraction of bladder)2. produces hormones & connects with pituitary to regulate its activity

•epithalamus = pineal gland – part of the endocrine system-secretes the hormone melatonin-increased secretion in dark-promote sleepiness and helps set the circadian rhythms of the body (awake/sleep period)

Diencephalon

BRAIN STEM

•comprised of three structures: midbrain, pons & medulla

BRAIN STEM• Medulla oblongata

– continuation of the spinal cord– inferior part of the brain stem– relays sensory information and controls

automatic motor functions– white matter connects the white matter of

the spinal cord with the rest of the brain– contains several nuclei also - these nuclei

regulate autonomic functions - reflex centers for regulating heartbeat and BP (cardiovascular center), respiration (respiratory center), plus vomiting, coughing, sneezing, hiccuping and swallowing

• Pons= “bridge”- connection point from cerebrum to

cerebellum – via white matter tracts– nuclei help control both voluntary &

involuntary motor responses• e.g.Pneumotaxic and apneustic

nuclei – help regulate breathing with medulla

BRAIN STEM

• Midbrain (Mesencephalon)– relay station between the cerebrum

and the spinal cord and cerebellum– contains white matter tracts that

connect the midbrain to the cerebrum = cerebral peduncles• White matter tracts that conduct

impulses from the cerebrum to the pons and medulla into the spinal cord

• contains white matter tracts that connect the midbrain to the cerebellum = cerebellar peduncles

• one nucleus = substantia nigra – produces large amounts of dopamine - loss of these neurons = Parkinsons

BRAIN STEM

• Cerebellum– divided into hemispheres with

lobes - like the cerebrum• anterior and posterior lobes

– has a superficial layer of gray matter called the cerebellar cortex - like the brain

– deep to this gray matter are tracts of white matter (arbor vitae) and gray matter nuclei

– controls voluntary and involuntary motor activities• evaluates and coordinates

motor activities initiated by the cerebrum and corrects problems by sending info back to the cerebrum

• regulate posture & balance

Integrative Functions

• Arousal and Sleep• Memory and Learning• Emotions• Language and speech

Protection: The Cranial Meninges• Cranium is covered with protective

membranes = meninges– Cranial meninges are continuous with

spinal meninges– 3 layers: – 1. outer, fibrous dura mater – forms

sheets (falx) that separate the cerebrum and the cerebellum into the hemispheres and the cerebellum from the cerebrum

– in the cranial region - comprised of an outer endosteal layer and an inner meningeal layer

2. middle arachnoid mater3. inner, thin pia mater

Cranial Meninges– -there are spaces between these membranes– A. subarachnoid space: between the arachnoid and pia mater

» large veins run through the subarachnoid space» connecting membranes joining the pia and arachnoid mater also found

here (arachnoid trabeculae)• B. subdural space: between the arachnoid and the dura mater• C. epidural space – between the dura mater and the vertebral canal in the spinal

column

Protection: CSF• brain contains fluid-filled chambers =

Ventricles– 2 lateral ventricles, 1 third ventricle, 1 fourth

ventricle– connects to the central canal which runs into

the spinal canal– these chambers contain cerebrospinal fluid– made by specialized structures in the

ventricles – choroid plexus (contain ependymal cells)

– continually circulates - ventricles and central canal to subarachnoid space

– all CSF makes its way back to the subarachnoid space

Flow of CSF

draining points for CSF = arachnoid villi

Spinal Cord• length in adults = 16 to 18 inches• Cervical and lumbar

enlargements– cervical = C4 to T1, nerves to and

from upper limbs– lumbar = T9 to T12, nerves to and

from lower limbs• Tapers to conus medullaris• filium terminale arises from the

CM - extension of the pia mater that anchors the SC to the coccyx

• 31 segments each with– Dorsal root ganglia

• Sensory neuron cell bodies– Pair of dorsal roots– Pair of ventral roots

•Cervical •and lumbar enlargements

Histology of the Spinal Cord• Central gray matter– organized as gray horns

• Peripheral white matter– Myelinated and

unmyelinated axons running up and down the cord

– Organized as white matter tracts

Organization of White Matter• Organization of Gray Matter• 1. Posterior gray horns

– for incoming sensory information

• 2. Anterior gray horns– for outgoing somatic motor neurons

• 3. Lateral gray horns– for outgoing autonomic motor neurons

• Gray commissures– axons of interneurons crossing the cord

• Organization of White matter– Anterior, lateral and posterior white

columns– Contain tracts of myelinated neurons

• Ascending tracts relay sensory information up the spinal cord to brain

• Descending tracts carry motor information down the spinal cord