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Biol 067: Section 13 - Nervous System A. Overview of the nervous system: 1. 2 parts of the Nervous System: 2. How CNS and PNS are divided and interconnected: Central nervous system Sensory nerves - carry sensory info to brain and spinal cord somatic sensory nerves - FROM skin, muslces, joints, special senses visceral sensory nerves - FROM body organs Motor nerves - from CNS to effectors Somatic motor nerves - (controls movement) TO skin, skeletal muscles, tendons -voluntary Autonomic motor nerves - (controls body function) To smooth muscle, cardiac muscle, organs, glands - involuntary Parasympathetic division "rest and digest" "normal state" Sympathetic division "Fight or flight" Brain Spinal cord Peripheral Nervous system – messages via spinal and cranial nerves Nervous system Central Nervous System (CNS) Peripheral Nervous System (PNS)

Peripheral Nervous system messages via spinal and …viubiology067.weebly.com/.../8/0/1/8/...13_nervous_system_student_c… · Autonomic motor nerves ... Brain Spinal cord Peripheral

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Biol 067: Section 13 - Nervous System

A. Overview of the nervous system:

1. 2 parts of the Nervous System:

2. How CNS and PNS are divided and interconnected:

Central nervous system

Sensory nerves -

carry sensory info to brain and spinal cord

somatic sensory nerves - FROM skin, muslces, joints, special

senses

visceral sensory nerves - FROM

body organs

Motor nerves -

from CNS to effectors

Somatic motor nerves -

(controls movement)

TO skin, skeletal muscles, tendons

-voluntary

Autonomic motor nerves -(controls body

function)

To smooth muscle, cardiac muscle, organs,

glands -involuntary

Parasympathetic division

"rest and digest"

"normal state"

Sympathetic division

"Fight or flight"

Brain Spinal

cord

Peripheral Nervous system – messages via spinal and cranial nerves

Nervous system

Central Nervous

System (CNS)

Peripheral Nervous

System (PNS)

B. Neurons and how they work:

Nervous tissue: made up of 2 types of cells

1) Neurons – transmit nerve impulse

2) Neuroglia cells – support and service neurons

1. Types of neurons: classified by function

i. Sensory neuron -takes info from sensory receptor (detects changes in the environment)

to CNS

ii. Motor neuron – takes info away from CNS to an effector (i.e. muscle fiber, gland, etc)

iii. Interneuron – convey info between neurons – sum up info before passing it along to

motor neurons inside CNS

2. Structure:

1. dendrites – receive signals from sensory receptors or other neurons and send signals

towards cell body from axon

2. cell body – contains nucleus and organelles

3. axon – conducts nerve impuls along its length

3. Myelin sheaths

Cover long axons

Protective layer – contains myelin (a lipid substance) that insulates axon

Has gaps – nodes of Ranvier – used in neuron transmission

In Peripheral Nervous system (PNS) – act as insulator – sheath formed by Schwann cells

(a type of neuroglia)

In CNS – different kind of neuroglia but still contains myelin

Myelin sheath in long axons not short

In Central nervous system (CNS) – white matter –white because of myelin/ grey matter

– no myelin

MS (multiple sclerosis) and leukodystrophies (degeneration of white matter in brain)

caused by loss of myelin –( generally caused by defect in genes involved in growth and

maintenance of myelin, and/or environmental – not really understood)

C. Nerve impulse

Is the way a neuron transmits info

2 states: 1. resting potential

2. action potential

Can be measured with an oscilloscope by measuring voltage difference between inside

and outside of axon.

1. Resting potential: =potential energy of neuron

Is when an Axon is not conducting an impulse

-65 to -70millivolts mV inside of membrane is negative compared to outside of axon

Charge difference is related to ion concentration difference across axon membrane.

There is relatively more large negatively charged ions on the inside of the nerve cell

during resting potential than outside the nerve cell.

Even though it is called ‘resting’ the nerve cell is busy keeping the potassium at a

relatively even concentration –it drifts out of potassium channel but then gets attracted

back in by the negative ions on the inside – and the Na+ ions are being pumped to the

outside through the Na+potassium pump.

Therefore inside more negative than outside of nerve cell at resting potential (due to

large negative ions that stay inside the cell)

***Unequal distribution due to sodium – potassium pump = membrane protein that actively transports Na+ out and K+ into axon thru separate channels

Conctrn of

K+ inside

higher then

outside –

larger

negative

ions keep

inside

relatively

more neg.

then

outside

Conctrn

of Na+

greater

on

outside

2. Action Potential (pg 345 of text shows graphic)

When axon is conducting an impulse

Rapid change in polarity or charge across membrane

Nerve impulse consists of electrochemical change across membrane

a) Due to message from chemoreceptor, gates of sodium channels open first and Na+

moves into axon

Membrane potential changes from -65 mV to +40mV = depolorization

Change inside of axon from – to +

This is because now the inside is more positive due to all the positive Na ions coming in.

Almost immediately after depolarization, the Na channels close and the K+ channels open…

However, before action potential can be reached, must depolarize enough to cross

threshold of ~-40 mV – once crossed it will continue all the way through action

potential - If it doesn’t reach threshold impulse will not go anywhere.

b) Positive repel positive – so the K+ ions now get pushed out the open gates of potassium

channel, K+ flows to outside of axon

=40mV -65 mV = repolorization

Change in action potential

Inside of axon resumes negative charge as positive K+ ions exit – now the inside is

relatively negative again due to the relative number of the larger negative ions

relative to the remaining positive ions (which is the Na+ this time)

After the impulse has passed the nerve cell gets busy returning to resting potential

because the Na K pump moves K+ back to inside and Na+ to outside - then its ready

for another stimulus

E. Propogation of action potential

As it travels down axon = successive depolarization and repolarization of axon occurs

Refractory period, Na+ gates unable to open as soon as repolarization occurs (while

Na+/K+ pump is putting it back to original state)

Ensures action potential moves forward (not backward) and towards its axon branches

Ion exchange occurs at Nodes of Ranvier in myelinated sheaths

Causes action potential to travel faster = saltatory conduction

Impulse jumps from node to node

F. Transmission across a synapse:

Axon – axon branch – axon bulb (axon terminal) on end

Axon bulb lies close to dendrite of cell body of next neuron

Space=synaptic cleft

Region of close proximity = synapse

action potential

Transmission of action potential across synapse accomplished by neurotransmitters

which are molecules stored in synaptic vesicles

Steps in transmission

1) Nerve impulse travels along axon to axon terminal

2) As nervous impulse reaches bulbs, gated channels open and Ca2+ enters bulb – as

concentration of Ca2+ increases, causes synaptic vesicles to merge with pre-synaptic

membrane (NB diagram)

3) Neurotransmitters released into cleft, diffuse across and bind with receptor proteins on

post synaptic membrane -

4) Cause post synaptic neuron excitation (causes Na+ to diffuse into post synaptic neuron)

or inhibition (causes K+ to diffuse out of post synaptic neuron) depending on

neurotransmitter – once response is initiated, neurotransmitter removed from cleft by

a) Post synaptic membrane enzyme that inactivates the NT or/

b) Pre synaptic membrane reabsorbs the NT possibly for recycling

This is required so constant stimulation or inhibition of post synaptic membrane doesn’t

occur

F. Synaptic integration

1000 – 10,000 synapses/neuron is common therefore must have a method to integrate signal =

summing up all the signals received

If neuron receives more excitory signals has depolarizing effect – axon will transmit a nerve

impulse after reaching threshold signal – either by 1 axon sending a rapid # of signals or many

signals from different neurons

Or if receives more inhibitory signals – has a hyperpolarizing effect – can stop axon from firing

takes it further from an action potential

G. Divisions of the Nervous System

1. Central Nervous System:

Definition: lies in midline of body =spinal cord and brain - Sensory info received,

voluntary motor impulses initiated here

Both protected by bone – vertebrae (spinal cord), skull (brain)

Both wrapped in protective membrane known as meninges

Space filled with cerebrospinal fluid to cushion and protect CNS parts

0

+2

2

4

7

8

Time (milli seconds)

threshold

resting

potential

excitatory signal

inhibitory signal

integration

a) Spinal Cord:

1) Structure of spinal cord – fig 13.7

Located at base of brain and into vertebral canal

Vertebra joined so spinal cord passes thru middle

Spinal nerves pass thru lateral openings between vertebra

Spinal cord contains:

Central canal -contains cerebrospinal fluid

Grey matter – central H shaped, contains cell bodies and short non-myelinated

fibers – contains parts of sensory and motor neurons and interneurons

White matter – around grey matter, contains myelinated axons in bundles called

tracts

Tracts

- Ascending – take info to brain, located dorsally

- Descending – take info from brain, located ventrally

- Both cross just after they enter and exit brain therefore left side –

controls rt side and vice versa

2) Function of spinal cord

Provides a means of communication between brain and peripheral nerves that

leave the cord.

Reflex arc centre (talked about later)

Integration of incoming info from many sensory neurons before motor impulse

sent out.

b) Brain Structure and Function

Handout– be able to id main parts of brain

Ventricles: brain has 4 ventricles = interconnecting cavities that produce and act as a

reservoir for cerebrospinal fluid.(2x lateral, third, fourth,)

Main parts of brain:

1) Cerebrum

2) diencephalon

3) Cerebellum

4) Brain stem

1) Cerebrum

Largest part of the brain in humans

Left and rt cerebral hemispheres

Highest centre to receive info- commands voluntary responses

Higher thought processes – learning, memory, speech

Cerebral cortex (grey matter of cerebrum)

Thin convoluted outer layer of grey matter

Sensation, voluntary movement, consciousness

Cerebral hemispheres (handout – be able to id main parts)

Cerebrum is divided into left and right hemispheres by longitudinal fissure

Each hemisphere divided into lobes by sulci (grooves)

i. Parietal lobe – back, top

ii. Temporal lobe – lies below frontal and parietal lobe – (sides by ears)

iii. Occipital lobe – very back

iv. Frontal lobe – front

i. Parietal lobe – back

a) Primary somatosensory area

Sensory info from skin and skeletal muscles

b) Primary taste area – taste sensations

c) Somatosensory association area – integrates sensory info from skin and muscles

ii. Temporal lobe – lies below frontal and parietal lobe

Primary auditory area – receives info from ears

Auditory association area – integrates sensory info

Speech area – called Wernicke’s area – helps us understand written and spoken

word and send to Broca’s area

iii. Occipital lobe – dorsal to parietal lobe

Primary visual area - receives info from eyes

And association area –associates new and previously received visual info

iv. frontal lobe: front of cerebral cortex

a) pre motor area – organizes motor functions for skilled motor activities/ sends to

primary motor area

b) primary motor area - voluntary muscle commands begin here, sends to cerebellum

c) Broca’s area – motor speech area – sends to 1o motor area

d) Prefrontal area – reasoning and plan actions, receives info from other association

areas

(Association area = any part of the cerebral cortex involved in the integration of

information – and where memory is stored)

White matter of cerebrum

Rest of cerebrum composed of white matter

Consists of long myelinated fibers organized into tracts

Ascending from lower brain centres , sends info to primary somatosensory area

Descending communicates with lower brain centres

Tracts inside cerebrum take info between different sensory, motor, and other

association areas

Corpus callosum – contains tracts that join 2 cerebral hemispheres – like a bridge

between 2 hemispheres

2) Diencephalon

Region that encircles 3rd ventricle

Contains thalamus and hypothalamus and pineal gland

Thalamus = integrates sensory info (except smell) and relays to cerebrum – involved in memory

and emotion

Hypothalamus = homeostasis - integration centre for autonomic system – regulates hunger,

sleep, thirst, temperature, water balance and controls pituitary gland (serves as link btwn

nervous system and endocrine system)

Pineal gland = secretes hormone – (not sure of its role) – gland pokes out from under

hypothalamus

3) Cerebellum

Posterior to brainstem, separated by 4th ventricle

Has 2 portions surface = grey matter inside = white matter

Integrates sensory and motor information

i.e. posture, balance, skeletal muscles and coordination

receives info from sensory (eyes, ears, joints etc) as to where body is presently

positioned then receives motor output from cerebral cortex about where it should be

then sends message to skeletal muscles to correct

learning new motor skills

4) Brain Stem Contains:

a) Midbrain – also has reflex centre for visual, auditory and tactile senses

-Relay stn for tracts going to and from spinal cord and cerebrum or spinal cord and

cerebellum

b) Pons (bridge)

-This contains bundles of axons travelling between cerebellum and CNS

-Helps with breathing rate and head movement

c) Medulla oblongata – lies between spinal cord and pons

-Contains vital centres which regulate heart beat, breathing, blood pressure

-Contains reflex centres for vomiting, coughing, sneezing hiccupping and swallowing etc.

Reticular formation

Network of nuclei which are masses of grey matter and fibers which extend length of

brainstem

Receives sensory signals – pass up to higher centres

Receives motor signals – passes down to spinal cord

2. Peripheral Nervous System

a) Location and Structure:

PNS lies outside the CNS (brain and spinal cord)

Contains nerves (bundles of axons)

Nerve=bundle of nerve fibers (axons)/Single nerve fiber=axon

2 types of nerves: cranial - arise from the brain/ spinal arise from spinal cord

Sensory fibers/nerves– send info to CNS

Motor nerves – info from CNS

Cell bodies are in CNS or in ganglia (ganglia=collection of cell bodies in PNS

b) Types of Nerves:

1) Cranial nerves – 12 pairs, attached to brain

Some sensory,

Some motor (don’t need to know function of each one)

some mixed – contain both sensory and motor fibers

Most cranial nerves inervate head, neck, and face - but vagus nerve (which

begins in medulla oblongata) goes to most organs as well as pharynx and larynx

2) Spinal nerves

31 paired nerves

Emerge from spinal cord by 2 roots or branches

Both roots join to form spinal nerve that leaves CNS – becomes a mixed nerve = both

sensory and motor bundled together in one nerve.

Conduct impulse away from cord to effectors

c) Divisions of the PNS:

The peripheral nervous system has 2 divisions:

1) Somatic nervous system

2) Autonomic nervous system

1) Somatic Nervous system

Nerves that take sensory impulses to CNS from sensory receptors and motor commands

away.

Serve the skeletal muscles, skin, and tendons

Voluntary response = brain

Involuntary response = reflex = spinal cord or brain

Reflex Arc – short circuit response thru sensoryinterneuronsmotor nerves in spinal

cord – integrate info from sensory neurons and relay signals to motor neurons – allows

response rapidly to sensory stimulus

Some info also reaches brain (say ‘Ow” later)

2) Autonomic nervous system

Fig 13.17? good review of sympathetic and parasympathetic structure and function – see

contrary effects – give example

Regulates cardiac and smooth muscle, organs, and glands

Has 2 divisions: Sympathetic and parasympathetic

Both:

Act automatically and are usually involuntary

Innervate all internal organ

They use 2 neurons and 1 ganglion for each impulse

1st neuron – cell body in CNS and preganglionic fiber that enters the ganglion

2nd neuron – cell body in ganglion and postganglionic fiber that leaves the ganglion

Reflex actions of the autonomic system regulate things such as blood pressure and

breathing rate and are important to maintain homeostasis

Autonomic motor nerves vs somatic motor nerves

1. Sympathetic division – fight or flight

Used in flight or fight situations

Preganglionic fibers from middle of spinal cord (thoracic-lumbar region)

Pre-ganglionic fiber is short, terminate in ganglion close to spinal cord

Postganglionic fiber is long (contacts organ)

Causes increased heartbeat, increased ventilation, decreased digestion

Usually neurotransmitter is Norepinephrine (acts like adrenaline)

2. Parasympathetic division – rest and digest

Includes some cranial nerves (i.e., vagus nerve etc.) and sacral nerves (arise from the

sacral {bottom} part of the spinal cord)

therefore this division may be referred to as craniosacral portion of the autonomic

nervous system

preganglionic fiber is long

ganglia lies near or in organ

postganglionic fiber is short

this division promotes normal steady state – relaxed

promotes digestion, decreased heartbeat

uses neurotransmitter called acetylcholine

Summary of somatic and autonomic motor nerves:

Autonomic motor pathways

Somatic motor pathway

Sympathetic motor Parasympathetic motor

Control Vol/invol invol involuntary # of neurons/message

1 2 (preganglionic shorter than post)

2 (preganglionic longer than post)

Loctn of motor fiber Most cranial nerves and all spinal nerves

Thoracic-lumbar spinal nerves

Cranial and sacral spinal nerves

Neurotransmitter acetylcholine Norepinephrine (NE) Acetylcholine (Ach) effectors Skeletal muscle,

skin, tendons Smooth & cardiac muscle, glands, & organs

Smooth & cardiac muscle, glands, & organs