Bio 211- Anatomy and Physiology I

Today’s topics •Nervous system

Organization of the nervous system

In the human body, we have 2 systems that allow organs and tissues communicate with each other and coordinate the maintenance of homeostasis

1. Endocrine system – hormones, growth factors, etc…. Transmitted through the blood stream (lots more next semester in Bio 212)

2. NERVOUS SYSTEM – receives info, processes info, and causes action using nerves, brain, and spinal cord• Acquires information – sense receptors in tissues and organs, special

senses (vision, hearing, smell, etc…) and sends that info to the brain/spinal cord

• Processes information – brain and spinal cord determines what action (if any) should be taken

• Initiates action – brain and spinal cord send messages that cause muscles, organs, or tissues to carry out some action (muscle contraction, hormone release, etc…)

Organization of the nervous system

The nervous system is divided into two ANATOMICAL divisions:1. Central nervous system (CNS) – brain and spinal cord

2. Peripheral nervous system (PNS) – all the nerves and ganglia not part of the CNS• NERVES – bundle of neuronal axons wrapped up

in connective tissue• GANGLION (ganglia) – a swelling or enlargement

of a nerve where the cell bodies (soma) are located

Bundled axons from nerve #1

Dendrites and cell bodies of neurons from nerve #2

Bundled axons from nerve #2

Ganglion

Organization of the nervous system

•In addition to the ANATOMICAL division of the nervous system (CNS/PNS), the peripheral nervous system is further broken down into FUNCTIONAL divisions•2 Main divisions are :

1. Sensory – receives information and transmits it to the CNS (afferent neurons)2. Motor – sends signals from the CNS to muscles, organs, etc… (efferent neurons)

Somatic – refers to skin, muscles, joints, bones

Visceral – refers to internal organs like heart, lungs, liver, stomach, etc…

Subdivisions

1. Visceral sensory – receives info from internal organs transmits it to CNS

2. Somatic sensory – receives info from bones, joints, skin, muscles transmits it to CNS

3. Visceral motor – carries signals from CNS to internal organs• This is done unconsciously (AUTONOMIC

NERVOUS SYSTEM)

4. Somatic motor – carries signals from CNS to muscles, skin• Contraction of skeletal muscle

Organization of the nervous system

Organization of the nervous system

Autonomic nervous system

•This system is related to the visceral sensory and motor divisions•Information received from visceral sensory division is processed and signals are sent to organs via visceral motor division•Very important for regulating the functions of many organ systems (cardiovascular, respiratory, digestive, etc…)

•SYMPATHETIC division – stimulates the “fight or flight” response

Prepares the body for activity – increases heart rate, blood pressure, respiratory rate, blood glucose levels, dilates pupils

•PARASYMPATHETIC division – generally slows down many body functions (“rest and digest” response)

Decreases heart rate, decreases respiratory rate, increases activity of the digestive system

Organization of the CNS

The CNS is divided into 2 anatomical structures : the brain and spinal cord

Brain•3 main parts:

1. Cerebrum – constitutes most of the upper portion of the brain. Divided into 2 HEMISPHERES. Folds of brain tissue are called GYRI

(gyrus), grooves in the tissue are called SULCI (sulcus)

2. Cerebellum – Lies inferior to the cerebrum in the posterior cranial fossa

3. Brainstem – Most inferior part of the brain Midbrain, pons, medulla oblongata

Rostral = anteriorCaudal = posterior

•Cerebrum is divided into right and left cerebral hemispheres – separated by LONGITUDINAL FISSURE•Lobes of the cerebrum: roughly correspond to areas beneath the bones of the skull of the same name

1. Frontal – voluntary motor function, memory, mood, emotion, planning

2. Parietal – sensory reception, taste, some visual processing 3. Temporal – hearing, smell, learning, memory, some vision

processing, emotion 4. Occipital – primary area for vision

•Pattern of gyri and sulci will vary from person to person, but a few should be easily identified:

Central sulcus – separates frontal lobe from parietal lobe Lateral sulcus – separates temporal lobe from frontal and

parietal lobes above it

Parieto-occipital sulcus – separates parietal lobe from occipital lobePrecentral gyrus – gyrus located anterior to central sulcus – important for coordinating motor controlPostcentral gyrus – gyrus located posterior to central sulcus – important for processing sensory information

CNS

The brain and spinal cord of the CNS contain tissues known as gray matter and white matter• Gray matter primarily consists of neuronal dendrites, soma, and synapses• White matter primarily consists of bundles of axons (TRACTS) connecting one part of the

brain to another

•Outer layer of gray matter of cerebrum is known as the CEREBRAL CORTEX This is part of the cerebrum that carries out higher level brain functions that

separate humans from lower primates Many areas of the cerebral cortex have specialized functions

Cerebral Lateralization

•Although the 2 hemispheres of the cerebrum appear similar, in reality they have very different, specialized tasks

•Additionally, some tracts of the spinal cord crossover (DECUSSATE), one side of the brain will control the opposite side of the body’s functions

Sensory information from left side is processed by the right side of the brain

Motor information controlling the left side of the body originates from the right side of brain

Cerebellum

•Second largest part of the brain•Like the cerebrum, the cerebellum:

Right and left hemispheresConsists of an outer layer of gray matter and an

inner layer of white matter (much less white matter than cerebrum called ARBOR VITAE

•Important for processing many types of sensory signals received from many parts of the body

Damage leads to impaired motor coordination, interpreting some visual and auditory information, and timekeeping (can’t judge time)

•Makes up only ~10% of the brain volume but contains over 50% of the neurons!

•Recent research demonstrates additional roles in emotion, behavior, speech, sensory processing

Internal structures of the brain

Thalamus

Hypothalamus

Pituitary gland

Corpus callosum

MidbrainPons

Medulla oblongata

•Corpus callosum – thick bundle of nerve fibers linking hemispheres of cerebrum•Thalamus – a relay center in the brain that receives information and directs it to the different areas of the cerebrum•Hypothalamus – major control center of the autonomic nervous system and endocrine systems. Very important for relaying messages to the pituitary gland.•Pituitary gland – the “master gland” of the body. Receives info from the hypothalamus and secretes hormones that control the actions of many other glands in the body.

“BRAINSTEM”

•Midbrain – connects the cerebrum and cerebellum, pons, and medulla oblongata, helps coordinate some visual and auditory reflexes. •Pons – conducts signals between cerebrum and cerebellum. Sends sensory messages to thalamus. •Medulla oblongata – conducts motor signals from cerebrum to spinal cord and sensory signals from spinal cord to all other parts of the brain. Contains regions which control cardiovascular and respiratory systems as well as reflexes like coughing, sneezing

•Many cranial nerves originate within the brainstem

• Reticular Formation – network of nuclei scattered throughout brainstem. Important for motor control, sleep/consciousness, cardiovascular function, pain perception

Meninges•The brain is surrounded by three layers of connective tissue called MENINGES•Along with cerebrospinal fluid (CSF), meninges play an important role in protecting the brain•DURA MATER – thickest, most outer layer of meninges.

o In some parts the Dura Mater is divided into 2 layers: periosteal and meningeal. The Meningeal layer is attached to the arachnoid mater and forms barriers between parts of the brain:

FALX CEREBRI – separates right and left hemispheres of cerebrum TENTORIUM CEREBELLI – separates cerebellum from cerebrum

•ARACHNOID MATER – middle layer of meninges that is important for cushioning of the brain. Tiny extensions give the layer a “spider-like” appearance. CSF is found in the subarachnoid spaces between the arachnoid mater and the pia mater•PIA MATER – innermost, thinnest layer of meninges. Very thin layer of tissue that follows the contours of the gyri and sulci of the brain.

Cerebrospinal fluid and brain ventricles

•Cerebrospinal fluid is a specialized fluid that is found within (ventricles) and surrounding the brain (subarachnoid space)•Produced by choroid plexuses within the ventricles, ependymal cells lining the ventricles and within the subarachnoid space•Functions of CSF include :

1. Buoyancy – brain is suspended in CSF. Allows brain to reside in cranial cavity without its own weight damaging it.

2. Protection – Acts as a shock absorber when the head receives a blow3. Chemical Stability – CSF is eventually reabsorbed into the bloodstream allowing metabolic

wastes to be easily removed from the nervous system• CSF is produced and circulated within the brain via 4 ventricles (2 large lateral ventricle, third

ventricle, fourth ventricle) and eventually reaches the subarachnoid space surrounding the brain

•In the subarachnoid space, CSF is reabsorbed into the bloodstream by ARACHNOID VILLI – small extensions of the arachnoid mater

Bio 211- Anatomy and Physiology I

Today’s topics •Nervous system

Spinal Cord

The spinal cord comprises the inferior half of the CNS

Functions of the spinal cord:1. Conduction – acts as an “information highway” that allows communication

between the brain and the rest of the body2. Locomotion – although the brain controls the initiation, speed, direction of

walking, the spinal cord is responsible for generating the repetitive motor signals that coordinate walking (repeatedly putting one foot in front of the other without consciously thinking about it)

3. Reflexes – involved in the processing and execution of motor responses due to certain stimuli (knee and elbow reflexes are good examples)

Spinal cord anatomy

•Consists of a long cylinder of nervous tissue extending from foramen magnum to the first lumbar vertebrae (only occupies upper 2/3 of vertebral column)•Spinal cord gives rise to 31 pairs of spinal nerves that exit through intervertebral foramen and travel to all muscles, tissues of the body

•Distal to the end of the spinal cord (around L1 vertebrae), spinal nerves continue to travel down the vertebral column and exit intervertebral foramina of lumbar and sacral vertebrae – bundles of spinal nerves is called CAUDA EQUINA (looks like horse’s tail)

•Spinal cord is covered by same three layers of connective tissue as brain (dura, arachnoid, pia mater)

Spinal cord anatomy

•Also composed of gray and white matter (orientation is opposite of brain)

•White matter consists of large numbers of myelinated axons bundled together called COLUMNS

ASCENDING and DESCENDING TRACTS are found within the columns

•Gray matter consists of soma, dendrites….site of synapses

•Spinal cord is covered by same three layers of connective tissue as brain (dura, arachnoid, pia mater)

Spinal tracts•Ascending tracts – bundles of axons carrying sensory information TO the brain

Typically involves 3 neurons – 1st order neuron carries info from the tissue, muscle, or organ to the spinal cord or brainstem, 2nd order neurons carry info from 1st order neurons to the thalamus which sorts the information and sends it on to other parts of the brain via 3rd order neurons

•Descending tracts – bundles of axons carrying motor information FROM the brain Typically involve 2 neurons – UPPER MOTOR NEURON begins in the cerebral cortex or brainstem and

carries motor information to a LOWER MOTOR NEURON (in brainstem or spinal cord) which carries info to target muscle or tissue

•Decussation – some spinal tracts will crossover (decussate) as they travel to/from the brain.

•All tracts are BILATERAL, meaning there is one on each side of the spinal cord•IPSILATERAL tracts originate and terminate on same side of body/brain•CONTRALATERAL tracts decussate

Spinal nerves and ganglia

•NERVE = Bundles of neuronal axons held together by connective tissue•Individual nerve fibers are surrounded by ENDONEURIUM and bundled into larger units called FASCICLES•FASCICLES are surrounded by PERINEURIUM•Several fascicles are bundled together and surrounded by EPINEURIUM, thereby creating a nerve

Most nerves in the body are considered MIXED NERVES since they will contain a mix of:

Motor and sensory neurons Myelinated and unmyelinated neurons

•MOTOR NERVES carry only motor nerve fibers •SENSORY NERVES carry only sensory nerve fibers - very uncommon. Cranial nerves I, II, VIII are examples•GANGLIA are swellings along a nerve where there is a concentration of synapses between axons of presynaptic neurons and dendrites and neuronal cell bodies (soma) of postsynaptic neurons

GANGLIA are swellings along a nerve where there is a concentration of synapses between axons of presynaptic neurons and dendrites and neuronal cell bodies (soma) of postsynaptic neurons

Spinal nerves

•Dorsal and ventral roots combine to form the spinal nerve near the spinal cord

Dorsal roots are carrying SENSORY information back to the CNS

Ventral roots are carrying MOTOR information away from the CNS

•After leaving the intervertebral foramen the nerve divides into dorsal, ventral, and communicating RAMI

Contain both motor and sensory nerve fibers True spinal nerve is only about an inch long

•DORSAL RAMUS innervates local muscles and joints of the spine and also the skin of the back•VENTRAL RAMUS innervates ventral and lateral skin and muscles of the trunk and also will give rise to nerves of the limbs•COMMUNICATING RAMUS allows for flow of nerve fibers to/from CNS and sympathetic chain (part of autonomic nervous system)

Spinal nerves, and all rami contain both sensory and motor neurons Mixed nerves

Spinal nerve structure overview

Organization of Spinal Nerves

Neurons

3 Classes of neurons: 1. Sensory (Afferent) – specialized neurons

that are capable of detecting touch, pressure, heat, light, etc…• These neurons originate in skin,

muscles, organs and end in the CNS• Bring info TO the CNS

2. Interneurons – located exclusively in the CNS• These neurons are the link “between”

sensory neurons and motor neurons• Carry out the processing of

information and decision making3. Motor (Efferent) – neurons that are capable

of triggering an action in a muscle, gland, or organ• These neurons start in the CNS and

end at a muscle or organ

Neuron anatomy

1. Soma – cell body2. Dendrites – extensions that RECEIVES signals from

other cells 3. Axon – long fiber that CARRIES signals to another 4. Synatic knob – enlargement at end of axon that

contains neurotransmitter 5. Myelin – insulating covering of the axon6. Schwann/oligodendrocyte – glial cells that

support the neurons

Myelin and support cells

The axons of many of the neurons in the PNS and CNS are coated with an insulating layer of MYELIN• Like the plastic around an electrical wire• Increases the speed that neurons can transmit signals

•Myelin actually consists of layers of the plasma membrane of Schwann/oligodendrite cells wrapped around the axon

Schwann cells surround axons in the PNS Oligodendrite cells surround axons in the CNS

•The layers of plasma membrane (myelin) are so tightly wrapped that there is no cytoplasm in between the layers

•Since myelin is essentially plasma membrane it is roughly 80% lipid (fat) and 20% proteins

•The process of myelination takes place during infancy and early childhood

•Adequate consumption of fat is VERY important for normal brain development during early years (especially infancy)

Support cells of the nervous system

Support cells of the nervous system are collectively referred to as GLIAL cells

Glial cells of the CNS:1. Oligodendrocytes – myelin producing cells of the CNS2. Ependymal cells – cuboid-shaped cells (not epithelial) with cilia that line the

internal cavities of the brain and spinal cord• Primary job is to produce and circulate CEREBROSPINAL FLUID

3. Microglia – tiny macrophages that phagocytose bacteria, viruses, and dead nervous tissue• Help protect the neuron from infection and damage

4. Astrocytes – glial cells that play a supportive role in the CNSa) Physical and nutritional support of neuronsb) Produce growth factors that enhance neuron growthc) Absorb excess neurotransmitter and K+ ionsd) Turn into scar tissue when nervous tissue is damagede) Have little extensions (PERIVASCULAR FEET) that surround blood vessels

creating a BLOOD BRAIN BARRIER – prevents certain substances/toxins in blood from reaching the nervous tissue (VERY IMPORTANT)

Support cells of the nervous system

Glial cells of the PNS1. Schwann cells – myelin producing cells of the PNS

Also assist in the repair of damaged neuronal axons2. Satellite cells – thought to assist in the support and nourishment of PNS neurons

Bio 211- Anatomy and Physiology I

Today’s topics •Nervous system

OUTSIDE (+)

INSIDE (-)

LigandNa+

Na+

Ligand-gated -vs- Voltage gated ion channels

Ligand

Na+

Na+

•Ligand gated ion channels are ion channels that require a ligand in order for them to open and cause depolarization

Na+ channels at the motor end plate of a neuromuscular junction are this type When a neurotransmitter binds to the channel it opens up letting Na+ enter the cell Usually only found in specific parts of the cell membrane (i.e., dendrites, etc…)

•Voltage gated ion channels are ion channels that require a change in membrane voltage to open and cause depolarization

These channels sense changes in membrane voltage (a nearby depolarization) that triggers them to open up and let Na+ enter the cell

These channels are found all throughout the majority of a neuron These channels are responsible for propagating the action potential down an axon

Electrophysiology of neurons

Electrophysiology refers to the study of how the flow and location of charged ions across a cell’s membrane affect its function and activity • Like muscle cells, neurons are ELECTRICALLY EXCITABLE

•Remember that the inside of a polarized cell (like a neuron) is NEGATIVELY CHARGED and the outside is POSITIVELY CHARGED

Na+/K+ ATPase helps maintain the charge difference – pumps more (+) ions out than in (3 Na+ out / 2 K+ in)

Na+/K+ATPase uses about 70% of the ATP in the nervous system

Large (-) ions, proteins, DNA, etc… are also trapped inside (semi-permeable membrane)

Very similar principles as in muscle cells!!

Electrical stimulation of neurons

•UNLIKE skeletal muscle cells which are stimulated by ACh, neurons can be stimulated by numerous neurotransmitters, chemicals, heat, light, among others•LIKE skeletal muscle cells, an action potential only originates from a specific part of a neuron

Typically the dendrite of a neuron (like motor end plate in S.M.) – contains ligand-gated Na+ channels

•Depolarization at this site is known as a LOCAL POTENTIAL•Local potential spreads from dendrites to soma and if the stimulation is strong enough an action potential is generated that travels down the axon

•Local potentials are initiated by the opening of ligand-gated Na+ channels that depolarize the membrane of the dendrite (like we saw with ACh in muscle cells)

•Local potentials: 1. Variable strength (more/less ligand → more/less Na+

flow)2. Reversible – removal of stimulus stops local potential

Na+ channels close, K+ channels open → repolarization3. Excitatory OR INHIBITORY – some ligands can cause a

neuron to become HYPERPOLARIZED – prevents action potential

Ligand-gated Na+ channels

Very high concentration of

Voltage-gated Na+ channels

Voltage-gated Na+ channels

Very few Na+ channels of either type

1. Neurotransmitter binds to ligand-gated Na+ channels on dendrite2. Na+ rushes in and depolarizes membrane near dendrite (LOCAL POTENTIAL)3. Na+ diffuses away from dendrites towards TRIGGER ZONE4. If concentration of Na+ is high enough, V.G.-Na+ channels in the trigger zone open up5. Opening of V.G.-Na+ channels causes depolarization of membrane near trigger zone (ACTION POTENTIAL)6. Triggers wave of action potentials that proceed down length of axon

LOCAL POTENTIALS are

found here

Action potentials

In contrast to LOCAL POTENTIALS, which:(1) can be of variable strength(2) decrease in strength away from point of initiation(3) can be reversible,

ACTION POTENTIALS are:1. Are “all-or-none” – as long as the local potential reaches or exceeds

threshold, the neuron will fire an action potential at maximum voltage

2. Action potentials have the same strength at all points along the axon (no decrease in strength)

3. Action potentials are irreversible – once the local potential reaches threshold, the action potential will proceed to completion (end of the axon)• Once it starts, it can’t be stopped

Signal conduction

In order for a nerve to communicate with another nerve, muscle, or tissue, its signal must travel from the soma to the end of the axon• A NERVE SIGNAL is not really the same thing as an

ACTION POTENTIAL A nerve signal refers to the wave of many

action potentials that travel down the axon An action potential is more of a local

occurrence – i.e., the LOCAL depolarization and repolarization of the membrane

• Waves of action potentials only travel IN ONE DIRECTION

The area behind the action potential is in a REFRACTORY PERIOD – action potential can’t be triggered (cell hasn’t returned to its resting polarized state)

Signal transduction in myelinated nerve fibers

•Myelin sheath of axon is not continuous•Small areas of the axon remain unmyelinated – Called NODES OF RANVIER•Areas that ARE myelinated are referred to as INTERNODES

•Remember that myelination increases the speed of nerve signals – but how?

•V.G.-ion channels are FAR more concentrated at the Nodes of Ranvier than the internodes

Makes sense since the myelin covering would prevent extracellular Na+ from entering the cell anyway

Therefore, depolarization occurs MUCH more easily at Nodes of Ranvier than at the internodes

Signal transduction in myelinated nerve fibers

•When an action potential is generated at one Node, Na+ rushes into the cell depolarization•That Na+ diffuses along the axon (past the internode) and triggers the opening of V.G.- Na+ channels downstream at the next Node•So…action potentials “jump” from one Node to the next Node – SALTATORY CONDUCTION

Spinal nerve structure overview

Action potentials -2

Action potential -1

Nervous system animations(right click on each and select “open hyperlink”)

Bio 211- Anatomy and Physiology I

Today’s topics •Nervous system

Neuronal synapses

•The PRE-SYNAPTIC NEURON is located before the synapse and releases the neurotransmitter the is received by the POST-SYNAPTIC NEURON after the synapse

•The axon of a pre-synaptic neuron can synapse with another neuron through its:

1. Dendrites2. Soma3. Axon

Structure of a synapse

Pre-synaptic neuron

Post-synaptic neuron

Synaptic cleft

Synaptic bulb of pre-synaptic neuron (containing neurotransmitter)

Like we saw with the neuromuscular junction, the axon of a pre-synaptic neuron doesn’t physically contact the post-synaptic neuron• The 2 neurons are separated by a SYNAPTIC CLEFT

•When a nerve impulse reaches the synaptic bulb of the pre-synaptic neuron, it triggers the release of NEUROTRANSMITTERS•The neurotransmitters travel across the synaptic cleft and bind to specific receptors on the surface (dendrite, soma, or axon) of the post-synaptic neuron•The receptors on the surface of the post-synaptic neuron are LIGAND-GATED Na+ CHANNELS that will open up and allow Na+ ions to rush in → depolarization of the post-synaptic neuron and the initiation of an action potential

Neurotransmitters

Although some neurons in our body can be stimulated by light, heat, mechanical force (eyes, ears, stretch receptors), most neurons communicate using chemical neurotransmitters

4 Classes of chemical neurotransmitters

1. Acetylcholine – used by motor neurons to communicate with muscle fibers

2. Amino acids – Some neurons store and release individual amino acids to communicate (glycine, glutamate, γ-amino butyrate, aspartate, etc…)

3. Monoamines – Basically amino acids that have their carboxylic acid groups removed• Epinephrine (adrenaline), histamine, dopamine are all monoamines

4. Neuropeptides – very short proteins • Neuropeptides are synthesized in the SOMA and then transported down

the axon to the synaptic bulb where they are stored until needed

See Table 12.3 for a summary of the different neurotransmitters

Neurotransmitter release and action

So far we’ve talked about how neurons STIMULATE other neurons or cells, however, some neurotransmitters can have an INHIBITORY effect and prevent the stimulation of those target neurons or other cells

Excitatory Synapse

1. Arrival of a nerve signal to the pre-synaptic bulb triggers the opening of voltage gated Ca2+ channels

2. Entry of Ca2+ triggers the exocytosis of ACh from the bulb

3. ACh travels across the synaptic cleft and binds to ligand-gated ion channels on the post-synaptic cell

4. ACh causes ion channels to open up allowing Na+ to rush into the cell causing depolarization and initiation of a LOCAL POTENTIAL

5. If the stimulation is strong enough (i.e., enough Na+ enters the cell) the Na+ will begin to diffuse away and depolarize other parts of the membrane – this triggers the opening of VOLTAGE GATED ion channels throughout the rest of the neuron• Ligand-gated channels trigger the initial local action

potential, but voltage gated channels are required to trigger action potentials throughout the rest of the cell (i.e., down the axon)

Inhibitory Synapses

In the case of an inhibitory synapse, the end result is INHIBITION of post-synaptic neuron• GABA-ergic synapses are a good example of this (GABA = γ-amino butyrate, an amino acid

neurotransmitter)• Post-synaptic neurons of contain LIGAND GATED ion channels that bind GABA (or molecules

similar to GABA)

OUTSIDE (+)

INSIDE (-)

GABA

GABA

Cl-

Cl-

GABA

K+

K+

•GABA can trigger the opening of Cl- or K+ channels Entry of Cl- or exit of K+ makes the interior of the cell MORE NEGATIVE (HYPERPOLARIZED), and

more difficult or impossible to stimulate (depolarize)•Many drugs (anesthetics, alcohol, barbiturates) activate these synapses causing depression of the CNS

Overdose is lethal – brain can’t regulate heartbeat, breathing, etc…

Synapse Actions

Regulation of nerve signals

Although the ability of a neuron to activate (or inhibit) other neurons and cells is important, it is equally important that we are able to STOP that signal• Inability to turn off the signal leads to constant stimulation (or inhibition)• Ability to regulate nerve signals is critical to maintaining homeostasis

Once a neurotransmitter has bound to its receptor on the post-synaptic neuron and triggered an action potential, we need to a way to remove it so the neuron doesn’t keep firing

1. Diffusion – some neurotransmitters simply diffuse away from the synapse and are taken up by astrocytes

2. Reuptake – some neurotransmitters (amino acids, monoamines) are taken back up by the presynaptic neuron via ENDOCYTOSIS

3. Degradation/metabolism – some neurotransmitters (ACh) are broken down by enzymes – byproducts can’t stimulate neurons ACh is broken down by acetylcholinesterase at the neuromuscular junction

Neuromodulators are specialized molecules that can modify a neuron’s ability to communicate1. Can increase/decrease receptors on post-synaptic neurons – more or less sensitive2. Can increase/decrease production of neurotransmitters – more or less able to stimulate3. Can increase/decrease metabolism of neurotransmitters – alters length of stimulation

Regulation of nerve signals

Up until now we’ve focused on how ONE neuron stimulates ONE other neuron, in reality the nervous system is much more complex• A given neuron may receive and send signals to dozens of other neurons• It is the COMBINED signals of many neurons that will tell a post-synaptic neuron whether or not

to fire an action potential This is referred to as NEURAL INTEGRATION – basically primitive information processing

Remember that a neuron can have either a stimulatory or inhibitory effect on the post-synaptic neuron• Most neurons receive signals from BOTH TYPES (allows regulation)• If a neuron receives more stimulatory signals than inhibitory action potential fires• If a neuron receives more inhibitory signals than stimulatory no action potential

SUMMATION refers to the process by which nerve signals “add up” to stimulate another neuron

• TEMPORAL SUMMATION – ONE neuron sending signals fast enough to trigger action potential in another neuron (strong signal from one)

• SPATIAL SUMMATION – MANY neurons sending slower signals so that their combined effect is to trigger an action potential (weak signal from many)

Bio 211- Anatomy and Physiology I

Today’s topics •Nervous system

Reflexes

Reflexes are reactions to a particular stimulus

Properties of a reflex:1. Requires stimulation – they are not random or spontaneous, but require a stimulus2. Quick – very rapid, short-lived response3. Involuntary – we cannot control reflexes. Reflexes occur whether or not we are aware of the

coming stimulus. (think of the knee-jerk reflex)4. Predictable – reflexes occur in the same way each time

Reflexes typically fall into two categories: SOMATIC and VISCERAL •Somatic reflexes typically involve reactions to some sort of physical stimuli on skin, muscles, or tendons•Visceral reflexes involve altering the function of visceral organs in response to a stimulus (increasing heart rate in response to low blood pressure)

Reflex Arc1. Sensory receptors – receptors in skin, muscle, bone, internal organs that detect stretch, pressure,

temperature, etc…2. Afferent nerve fibers – nerve fibers carrying information TO the CNS3. Integrating center – afferent neurons synapse with interneurons in CNS. It is here that the decision is

made whether or not to send a signal via the efferent neuron4. Efferent neuron – nerve fibers carrying information AWAY FROM CNS5. Effector – skeletal muscle that will carry out the response

Somatic Reflex Arc

The autonomic nervous system

•The autonomic nervous system (ANS) is the motor system that regulates the actions of visceral organs like the heart, lungs, many glands, and smooth muscle, etc…•The ANS carries out its actions unconsciously

•ANS provides REGULATION of many organs and glands activities

Not necessarily needed for functionEx.: ANS can speed up or slow down heart

rate, breathing rate, gland secretion, etc… HOWEVER, these organs can still operate without ANS – just can’t control them!

Divisions of the ANS

The ANS consists of two divisions, the SYMPATHETIC DIVISION and PARASYMPATHETIC DIVISION• Most visceral organs will be innervated by both divisions allowing tight control of organ activity• The divisions differ in their anatomy and method of action• Within a given organ, the divisions may have contrasting OR cooperative effects

Sympathetic Division Parasympathetic Division

•Responsible for the “fight or flight” response Adapts body for strenuous activity Increases blood pressure, breathing rate,

blood glucose levels, blood flow to heart and skeletal muscle,

Decreases blood flow and function of “non-essential” tissues and organs like skin, digestive system, and waste elimination

•Important during times of stress, anger, fear, sexual arousal, etc…

•Responsible for the “rest and digest” response Has more of a calming effect on the body Reduces use of energy and prepares the body

for “maintenance” functions like digestion, waste elimination, etc…

• It is tempting to think of the sympathetic division as speeding things up and the parasympathetic division as slowing things down – Not necessarily always true

Digestive and waste elimination is a good example

Visceral reflexes

•The ANS regulates organ function using VISCERAL REFLEXES Sort of similar to somatic reflexes, but slower in response

time

•The visceral reflex involves:1. Sensory receptors that detect stretch, chemicals,

temperature, pH, etc…• Located inside or close to an organ

2. Afferent (sensory) neurons that carry information back to the CNS

3. Interneurons in the CNS that process this information and determine what action is needed

4. Efferent (motor) neurons that carry motor signals away from the CNS

5. Effector – the organ or tissue whose function is being regulated

Neural pathways used in the ANS

•The ANS utilizes both the central nervous system and the peripheral nervous system to carry out its functions

Control centers are found in various parts of the brain• Hypothalamus, midbrain, pons, medulla oblongata

ANS sensory and motor neurons travel in many cranial and spinal nerves• Ex.: CN VII (facial nerve) contains somatic motor neurons as well as sympathetic and

parasympathetic neurons (tear/saliva production)• Therefore, many cranial and spinal nerves really are MIXED

•ANS uses 2 neurons to get from spinal cord to target - PREGANGLIONIC and POSTGANGLIONIC neuron As opposed to somatic motor neurons which only use ONE neuron

Sympathetic division anatomy

•Preganglionic sympathetic nerve fibers exit the spinal cord with spinal nerves (T1-L2) and then will branch off from the spinal nerves and enter the sympathetic chain via COMMUNICATING RAMI

•Preganglionic fibers that enter the sympathetic chain can then:

1. Synapse with a postganglionic fiber (occurs in sympathetic chain) and then travel to the target organ

2. Pass right through the sympathetic chain and synapse with postganglionic fibers just beyond the sympathetic chain

Also known as THORACOLUMBAR DIVISION

Parasympathetic division anatomy

•Parasympathetic fibers exit the CNS via cranial nerves or the sacral region of the spinal cord

•Preganglionic fibers travel with cranial or sacral nerves and synapse with postganglionic fibers near or inside the target organ• CN 3,7,9 carry preganglionic parasympathetic

fibers that control saliva, tear production, swallowing, and pupil constriction

• CN 10 (vagus nerve) carries the vast majority (~90%) of preganglionic parasympathetic fibers that innervate the heart, lungs, and digestive organs

Also known as CRANIOSACRAL DIVISION

Neurotransmitters and receptors in the ANS

•Although the sympathetic and parasympathetic often have contrasting effects on a given organ (i.e., raising/lowering heart rate) each division may also have opposite effects on different organs

Ex.: The parasympathetic division decreases contraction of cardiac muscle but can increase contraction of smooth muscle

The effects of each division on a given organ is due to differences in neurotransmitters used and receptors on the target tissue

Neurotransmitters

•The ANS primarily uses two different neurotransmitters to communicate between pre- and postganglionic neurons as well as with their target organs or tissues

1. Acetylcholine2. Norepinephrine (noradrenaline)

•All Preganglionic neurons release acetylcholine as their neurotransmitter•Postganglionic fibers of the parasympathetic division also release ACh •Postganglionic fibers of the sympathetic division will release either norepinephrine (most common) or ACh (less common)

•Neurons that release ACh are known as CHOLINERGIC FIBERS•Neurons that release norepinephrine are known as ADRENERGIC FIBERS

Receptors

The main reason that a division of the ANS can have different effects on different organs is due to the differences in receptors on different tissues

Cholinergic receptors 2 types that both bind ACh,

• Nicotinic receptors – cells with nicotinic receptors are STIMULATED by AChThis type of receptor is found at neuromuscular junctions of skeletal muscle

• Muscarinic receptors – cells with muscarinic receptors could be either stimulated or inhibited by ACh

Receptors

Adrenergic receptors: 2 classes of receptors that both bind norepinephrine (NE)

• Alpha-adrenergic receptors - Typically binding of norepinephrine to alpha receptors is EXCITATORY

• Beta-adrenergic receptors - Typically binding of norepinephrine to beta receptors is INHIBITORY

• Ex.: Binding of NE to alpha receptors in blood vessels causes smooth muscle contraction (vasoconstriction), but binding of NE to beta receptors in the bronchioles of the lung causes smooth muscle relaxation (bronchodilation)

MANY, MANY pharmaceuticals are designed to specifically activate or inhibit different adrenergic receptors

Actually are very complex since there are multiple forms of each class of receptor

Single and Dual Innervation

•Most organs are innervated by BOTH sympathetic and parasympathetic nerve fibers•Many times the two divisions have opposing effects on an organ – ANTAGONISTIC EFFECTS

Sympathetic division increases heart rate, but parasympathetic division slows it down•Sometimes the two divisions have similar effects on an organ – COOPERATIVE EFFECTS

Both sympathetic and parasympathetic divisions stimulate saliva production in salivary glands

•HOWEVER, some organs are only innervated by one division of the ANS

In these cases control is achieved by regulating the frequency or strength of stimulation

Ex.: Contraction of smooth muscle in blood vessels is primarily controlled by the sympathetic division• Greater stimulation leads to contraction• Weaker stimulation allows relaxation

Summary of neurotransmitters and receptors in the ANS

1. ALL preganglionic neurons use ACh as their neurotransmitter• As a result, ALL postganglionic

neurons contain receptors for ACh (nicotinic)

2. With the exception of skeletal muscle, most tissues that are controlled by the parasympathetic division contain muscarinic receptors for ACh

3. Most tissues that are controlled by the sympathetic division contain adrenergic receptors that bind norepinephrine (NE)• Binding of NE to alpha-adrenergic

receptors generally is excitatory• Binding of NE to beta-adrenergic

receptors is generally inhibitory