The Nervous The Nervous SystemSystem

Chapter 9Chapter 9

Maintaining Maintaining Homeostasis…Homeostasis…

Your body has physiological and Your body has physiological and biochemical mechanisms in biochemical mechanisms in place to help it maintain a stable place to help it maintain a stable internal environment. internal environment. Internal body temperature: 37ºCInternal body temperature: 37ºCBlood pH : 7.4Blood pH : 7.4Concentration of glucose in the Concentration of glucose in the blood: blood: 100mg/mL100mg/mLBlood pressure : 120/70 mm HgBlood pressure : 120/70 mm Hg

The Nervous System serves as a The Nervous System serves as a high-speed communication high-speed communication system that receives and system that receives and interprets information about interprets information about changes in your external and changes in your external and internal environment. This internal environment. This information helps it direct your information helps it direct your body into making the changes body into making the changes needed to maintain homeostasis.needed to maintain homeostasis.

Structure of the Nervous Structure of the Nervous SystemSystem

Two Main divisions:Two Main divisions: Central Nervous System (CNS)Central Nervous System (CNS) Peripheral Nervous System (PNS)Peripheral Nervous System (PNS)

The Central Nervous The Central Nervous System (CNS)System (CNS)

Contains the nerves of the brain and Contains the nerves of the brain and spinal cordspinal cord

It acts as a coordinating centre for It acts as a coordinating centre for incoming and outgoing information.incoming and outgoing information.

Think of this like processor in your Think of this like processor in your computer, it controls everything.computer, it controls everything.

The Peripheral Nervous The Peripheral Nervous System (PNS)System (PNS)

12 Cranial nerves12 Cranial nerves 31 Spinal nerves 31 Spinal nerves These nerves carry information from These nerves carry information from

the organs of the body to the CNSthe organs of the body to the CNS PNS is further divided into Somatic PNS is further divided into Somatic

and Autonomic nerves.and Autonomic nerves.

What does this look What does this look like ?like ?

Somatic Nervous SystemSomatic Nervous System

Contains Somatic nervesContains Somatic nerves Control skeletal muscles, bones, skin Control skeletal muscles, bones, skin

and sensory organsand sensory organs The SNS is viewed as The SNS is viewed as voluntaryvoluntary

control of your body.control of your body. Helps keep the body in touch with it’s Helps keep the body in touch with it’s

surroundings.surroundings. Therefore: broken down further into Therefore: broken down further into

sensory neuronssensory neurons and and motor motor neuronsneurons..

Autonomic Nervous Autonomic Nervous SystemSystem

Contains Autonomic NervesContains Autonomic Nerves Contain special Contain special motor nervesmotor nerves that that

control the internal organs of the bodycontrol the internal organs of the body Just like the SNS it too is further Just like the SNS it too is further

broken down into sympathetic nervous broken down into sympathetic nervous system and the parasympathetic system and the parasympathetic nervous system. nervous system.

Viewed as Viewed as involuntary controlinvoluntary control (heartbeat, digestion, respiration)(heartbeat, digestion, respiration)

Acts as an “on-off” switch in your bodyActs as an “on-off” switch in your body

ANS Cont’dANS Cont’d In other words: they act opposite each otherIn other words: they act opposite each other- Sympathetic NS- Sympathetic NS

-“fight or flight” reaction-“fight or flight” reaction- prepares the body for danger (stress)- prepares the body for danger (stress)

- Parasympathetic NS- Parasympathetic NS- counters the effects of the - counters the effects of the

Sympathetic NSSympathetic NS- directs activities of internal organs - directs activities of internal organs

when when body is at restbody is at rest- relaxes- relaxes

Nervous System Breakdown…..

What is a Neuron ?

A neuron is an electrically excitable A neuron is an electrically excitable cell that processes and transmits cell that processes and transmits information by electrical and information by electrical and chemical signaling. chemical signaling.

But First….Cells in the But First….Cells in the Nervous SystemNervous System

Nervous system contains 2 types of cells:

•Glial cells – nonconducting cells which are important for structural support (Like glue).

•Neurons – Functional units of the nervous system (the nerve itself)

NeuronsNeurons

Neurons are subdivided into 3 Neurons are subdivided into 3 categories:categories:

1)1) SensorySensory (afferent) – sense & relay info (afferent) – sense & relay info (stimulus) to the CNS (afferent means (stimulus) to the CNS (afferent means towards)towards)

2)2) InterneuronsInterneurons – link neurons within the – link neurons within the bodybody

3)3) Motor neuronsMotor neurons (efferent) – relay info to (efferent) – relay info to the effectors (ex. of effectors are muscles, the effectors (ex. of effectors are muscles, organs and glands). organs and glands).

(efferent means away from) (efferent means away from)

Examples of Sensory and Examples of Sensory and Motor NervesMotor Nerves

Remember…Remember… Sensory neurons (ex. You touching Sensory neurons (ex. You touching

something hot) send a signal TOWARD something hot) send a signal TOWARD (afferent) your spinal cord(afferent) your spinal cord

In the spinal cord the signal meets In the spinal cord the signal meets interneurons which relay information interneurons which relay information to…to…

Motor neurons carry that signal AWAY Motor neurons carry that signal AWAY (efferent) from your spinal cord, causing (efferent) from your spinal cord, causing the effectors (in this case muscle in your the effectors (in this case muscle in your hand to contract and pull away). hand to contract and pull away).

What are Effectors ?What are Effectors ?

An An effector effector neuron is a gland neuron is a gland or muscle (motor or muscle (motor neuron) that neuron) that causes a change causes a change or something to or something to happenhappen

They bring about They bring about a change a change

Structure of a NeuronStructure of a Neuron

All neurons contain: dendrites, cell bodies All neurons contain: dendrites, cell bodies and axons.and axons.

DendritesDendrites: receive information or : receive information or impulses/stimuli (tree-like structure)impulses/stimuli (tree-like structure)

Cell bodyCell body: contains a nucleus as well as : contains a nucleus as well as organelles like all cells. Regular cell organelles like all cells. Regular cell processes occur here.processes occur here.

AxonAxon: an extension of cytoplasm that : an extension of cytoplasm that projects nerve impulses from the cell body projects nerve impulses from the cell body (usually long and thin)(usually long and thin)

Structure of a Neuron Structure of a Neuron Cont’dCont’d

Many axons are covered by a Many axons are covered by a myelin myelin sheathsheath

Myelin is composed of a fatty acid and Myelin is composed of a fatty acid and proteinprotein

Formed by Formed by Schwann cellsSchwann cells (special glial (special glial cells) cells)

Myelin insulates the axon and prevents Myelin insulates the axon and prevents loss of charged ions from the nerve cell.loss of charged ions from the nerve cell.

MyelinatedMyelinated

Between each Between each myelin sheathmyelin sheath is an area is an area called the called the nodes of Ranviernodes of Ranvier

Nerve impulses jump from one node to Nerve impulses jump from one node to anotheranother

Therefore, these nodes of Ranvier speed Therefore, these nodes of Ranvier speed up the movement of nerve impulses up the movement of nerve impulses

Did You Know ?Did You Know ?

More than 100 More than 100 axons could be axons could be placed inside the placed inside the shaft of a single shaft of a single human hairhuman hair

Nerves are made Nerves are made of many neurons of many neurons held togetherheld together

Myelin and Multiple Myelin and Multiple SclerosisSclerosis

Myelin sheaths Myelin sheaths (on the axons in (on the axons in the CNS) become the CNS) become damageddamaged

This leads to This leads to demyelinization demyelinization and scarring and scarring

What do you What do you think happens ? think happens ?

The SynapseThe Synapse The synapse is the The synapse is the

junction between junction between the terminal the terminal branches of a branches of a neuron and another neuron and another cell.cell.

Impulses are Impulses are transmitted across transmitted across this gapthis gap

An axon may An axon may synapse with 1000 synapse with 1000 other neuronsother neurons

Nerve Action – Nerve Action – Synaptic Synaptic TransmissionTransmission

A nerve A nerve impulseimpulse is both an electricalis both an electrical and a and a chemicalchemical event event

Electrical synapseElectrical synapse: occurs at “gap : occurs at “gap junctions” junctions”

Chemical synapse:Chemical synapse: Occurs Occurs between between twotwo

neighbouring cells where chemicalneighbouring cells where chemical

messengers relay the message to the messengers relay the message to the

neighbouring cell.neighbouring cell.

Nerve Action – Synaptic Nerve Action – Synaptic TransmissionTransmission

We have many We have many more chemical more chemical synapsessynapses

Not as fast as electrical Not as fast as electrical But are more specificBut are more specific Ex. Ex. Electrical synapseElectrical synapse occur in occur in

situations where you require the situations where you require the fastest possible response (defensive fastest possible response (defensive reflexes)reflexes)

Chemical Synapses Chemical Synapses

A nerve at rest has a relative A nerve at rest has a relative negative charge inside and a positive negative charge inside and a positive charge outsidecharge outside K ions are concentrated inside the K ions are concentrated inside the

neuron and Na ions are concentrated neuron and Na ions are concentrated outside the neuronoutside the neuron

Chemical Synapse Cont’dChemical Synapse Cont’d

This This neg. chargeneg. charge is is maintained by maintained by sodium-potassium pumpssodium-potassium pumps which which control the movement of Na and K ions control the movement of Na and K ions through the cell membrane. (ATP through the cell membrane. (ATP supplies the energy to fuel the pumps).supplies the energy to fuel the pumps).

The The membrane ismembrane is now said to be now said to be polarizedpolarized (due to (due to unequal distribution unequal distribution of positively charged ions inside and of positively charged ions inside and outside the nerve celloutside the nerve cell))

Movement of the Movement of the Action Action PotentialPotential

So What Happens ?So What Happens ? An external An external stimulus changes the stimulus changes the

ability of the membrane to keep out ability of the membrane to keep out the Nathe Na++ ions & they rush in ions & they rush in

In other words sodium channels open In other words sodium channels open allowing sodium to rush inallowing sodium to rush in

Voltage change across the membraneVoltage change across the membrane This is known as an This is known as an action potentialaction potential

This rapid inflow of Na causes a This rapid inflow of Na causes a charge reversal – charge reversal – depolarizationdepolarization

This change in charge stimulates This change in charge stimulates adjacent points in the nerve cell & a adjacent points in the nerve cell & a wave of impulse passes along the wave of impulse passes along the nerve cellnerve cell

An impulse ends when K ions rush to An impulse ends when K ions rush to the outside and the outside and repolarizerepolarize the the membranemembrane

A nerve impulse can travel at the A nerve impulse can travel at the rate of 100m/secrate of 100m/sec

A A nerve cell needs time to nerve cell needs time to “recuperate”“recuperate” or become or become repolarizedrepolarized (return to normal) (return to normal)

This is called the This is called the refractory periodrefractory period and usually lasts 1 – 10 msand usually lasts 1 – 10 ms

Saltatory conductionSaltatory conduction is when the is when the impulse jumps from one node of impulse jumps from one node of Ranvier to the nextRanvier to the next – so this is faster – so this is faster in myelinated neuronsin myelinated neurons

It shortens the distance the action It shortens the distance the action potential has to travelpotential has to travel

Saltatory Conduction Visual Aid

So then what ?So then what ?

Our Our action potential is traveling action potential is traveling down the axondown the axon

It It reaches the Synaptic terminalreaches the Synaptic terminal The impulse The impulse causescauses chemicals called chemicals called

neurotransmittersneurotransmitters to be released to be released from the vesicles in the synaptic from the vesicles in the synaptic knobs or (presynaptic membrane).knobs or (presynaptic membrane).

The chemicals travel across the The chemicals travel across the synaptic cleft (gap) and cause an synaptic cleft (gap) and cause an impulse to be started along the impulse to be started along the postsynaptic membranepostsynaptic membrane

Synaptic Transmission Video

NeurotransmitterNeurotransmitter

AcetylcholineAcetylcholine – Found in many – Found in many presynaptic terminalspresynaptic terminals

Acts as an Acts as an excitatoryexcitatory neurotransmitterneurotransmitter on many on many postsynaptic terminalspostsynaptic terminals

Causes NaCauses Na+ + channels to open and…channels to open and… Tell me what happens…Start with Tell me what happens…Start with

creates a depolarizationcreates a depolarization

What happens to left over What happens to left over neurotransmitter ?neurotransmitter ?

It simply cannot remain in the It simply cannot remain in the synaptic cleft or Nasynaptic cleft or Na++ channels will channels will stay openstay open

CholinesteraseCholinesterase – present, enzyme – present, enzyme which breaks down acetylcholinewhich breaks down acetylcholine

This causes sodium channels to close This causes sodium channels to close and the nerve cell returns to resting and the nerve cell returns to resting membrane potentialmembrane potential

Threshold LevelThreshold Level

Important to note:Important to note: A stimulus must be above a certain level to A stimulus must be above a certain level to

produce a responseproduce a response (Approx -50mV) (Approx -50mV) The The all or none response - all or none response - neurons neurons

either fire or not at all ( e.g. like a trigger either fire or not at all ( e.g. like a trigger of a gun)of a gun)

Q: What causes a greater intensity of Q: What causes a greater intensity of response?response?

A: Not a stronger stimulus, but more A: Not a stronger stimulus, but more impulses (known as impulses (known as summation)summation)..

ThresholdThreshold

Action Potential – How a Action Potential – How a stimulus moves down a stimulus moves down a

neuronneuron

Action Potential Animation

Understanding…Understanding…

On pg. 426 – do questions 3,4,5,7,8,9 On pg. 426 – do questions 3,4,5,7,8,9 and 13and 13

Helpful if you read pages 419-424Helpful if you read pages 419-424