Central Nervous System

(CNS)

Peripheral Nervous System

PNS

ganglia & nerves

connects the CNS with the rest of the body

a) afferent division

b) efferent division

Afferent division 

of the

PNS

(bringing in)

-somatic sensory nerve cells

-visceral sensory nerve cells

Efferent division

of the

PNS

(outgoing)

-somatic(voluntary) nervous system

-autonomic(involuntary) nervous system

                                -sympathetic

                                       -parasympathetic

[image]

THE BRAIN

The BRAIN

-occupies the cranial cavity

-cerebrum

-brainstem

-cerebellum

Cerebrum

consists of right and left hemispheres

(divided into lobes)

connected by the corpus collosum

Brainstem

attaches the brain to spinal cord

-midbrain

-pons

-medulla oblongata

Midbrain

prominent bundles of fibers from/to

the hemispheres

PONS

connections between cerebrum

and cerebellum for 

motor efficiency

Medulla Oblongata

cardiac, vasomotor and

respiratory centers

Cerebellum

integrates sensory info about

body posture

and

modulates cerebral control over

skeletal muscles

The Spinal Cord

the continuation of the CNS outside of 

the skull.

-contains white matter

and grey matter

White matter

contains mylinated nerve fibers that

connect the brain with

the rest of the body

Grey Matter

[image]

consists of unmyelinated fibers

and nerve cell bodies.

(the commissure communicates

the right side with the left)

Meninges

-dura mater

-arachnoid

-pia mater

(cushion that protects brain)

Dura Mater

fibrous CT

-the outermost layer forms

venous sinuses.

-trauma may cause subdural hematoma

Arachnoid

weblike

no blood vessels

but cerebrospinal fluid

instead

Pia Mater

very thin and vascular

(makes brain shiny)

Meningitis

an inflammation commonly 

involving the arachnoid

and the pia mater

Somatic (PNS)

nerve supply to the muscular system

and skin

(soma=body wall)

involved CONSCIOUS activity

Autonomic (PNS)

nerve supply to the viscera (soft organs)

involves UNCONSCIOUS actions

relating to organs and glands

Sympathetic division 

of the ANS of PNS

(thoracolumbar)

preganglionic cell located in T1-L2

the ganglion is paravertebral and the

postganglionic neuron has a long axon

(hyped up) Ach → NE

Parasympathetic division

of the ANS of PNS

(craneosacral)

preganglionic fiber in brain or sacral

the ganglion is near or inside the

target organ.

the postganglionic neuron is short

(yawn, cold, tired)

ACh → ACh

Neurotransmitters

of the

PNS

sympathetic division of the ANS:

-pregang. neurons release Ach(cholinergic)

-postgang. release norepinephrine (adrenergic)

parasympathetic division of the ANS:

-pregang. neurons release Ach

-postgang. release Ach

The adrenal gland recieves cholinergic(ACH) sympathic neurons and release epinephrine and norepinephrine into bloodstream

Somatic efferent neurons are all CHOLINERGIC(ACh)

The primary AFFERENT neurotransmitter remains UNKNOWN

(proposed: ATP, serotonin,histamine,prostoglandin..)

The Neuron

neurons are the nerve cells that

receive, process and forward

impulses.

Neuroglia

all other nerve cells besides the neurons

that are only supportive.

Dendrites

are cell projections that

receive and bring

impulses to the cell body

Axon

is the fiber that carries impulses

AWAY

from the cell body

Neurofibrils

in the cell body and its

projections guide the 

transport of materials

Chromatophilic

granules

indicate the presence of 

neurotransmitter

material

Cell membrane

Membrane Potential

the difference in electrical charge between the inside and outside of the cell. represents stored electrical energy that can be put to work.

(membrane potential)

Excitable Cell membranes 

have the capability of..

..changing their permeability so they

can allow ions in or out of the cell

in response to stimulation

Mechanism in membrane potential

membrane proteins larger than the

thickness of the cell membrane have one 

end within the cytoplasm and

another outside the cell,

thus acting as channels that can be

open or obstructed (GATED)

Consequence in membrane potential

polarization

Polarization

the ion distribution on the cytoplasmic

side can differ from that on the

outside of the cell

Action Potential

(significance of membrane pot.)

membrane stimulation alters 

permeability and polarization and may

cause a nerve impulse, called an

action potential.

Results in the transfer of info.

Anions

negative charged ions

(phosphate, sulfate,proteins)

cannot diffuse through the cell membrane

Active Transport

the sodium potassium pump works 

constantly to move

sodium out of the cell

and potassium into the cell

Diffusion

slower than active transport

there is a passive leakage of 

sodium into the cell(slow) and

potassium out of the cell (faster)

resulting in a net loss of positive

charge (K+) by the cytoplasm

Concentration gradient

more Na+ outside the cell

more K+ in the cytoplasm

maintained by the sodium potassium pump

Resting membrane potential

a state of electrical polarization

which is developed and maintained

by a living, resting neuron

across its plasma membrane

Resting membrane

the resting potential is about

-70millivolts

Membrane depolarization

stimulation → sodium channels now open

sodium enters cell at the point of

stimulation.

the cytoplasm becomes less negative

maybe even positive

Repolarization

slower potassium channels

then open,

and the loss of K+ re-establishes the polarization

Hyperpolarization

if the cytoplasm becomes more

negative: inhibition

Peripheral Nerve

the average nerve consists of numerous bundles of 

nerve fibers, which are axons and dendrites with very little else in

common other than their anatomical destination

-epineurium

-perineurium

-endoneurium

Epineurium

the superficial collagenous fascia

surrounding the nerve

Perineureum

intermediate fascia

surrounding the fiber fascicles

Endoneurium

deep fascia surrounding

individual nerve fibers

The compound

action potential

1. the stimulation of a large nerve

2. results in action potentials 

corresponding to different types of fibers

ex: motor sensory myelinated and unmyle.

3.thus a signal can travel at different

velocities

Alpha

somatic 

sensory

and motor

30mph

Beta

touch and pressure

15 mph

Gamma

muscular spindles

10mph

Delta

pain and temp.

5 mph

"B"

 preganglionic ANS

1mph

"C"

postganglionic ANS

.25MPH

The axon terminal

several fine extensions

near the axons end, each with a 

specialized ending called a

synaptic knob(terminal button)

Collaterals

branches the axon may give off

although it starts as a single fiber

Synapse

the transfer of info from a 

presynaptic neuron to a post synaptic

neuron across a synaptic cleft

Facilitation

The depolarization to threshold of a

postsynatic neuron...

depends on the balance between excitatory

and inhibitory input

Below threshold excitation makes

the next depolarization more likely...

thus placing the neuron in a state of 

facilitation

Stimulation of "A"

causes:

postsynaptic potentials in 1 and 2

facilitation of 3 and 4

Stimulation of "A" and "B"..

Convergence

[image]

Many presynaptic neurons

synapse with one

post synaptic neuron

allows subthreshold info to add up 

to threshold levels

Divergence

ONE presynaptic neuron

synapses with MANY

post synaptic neurons

allows the amplification of an impulse

The generator potential

a graded potential that can be affected by

the strength of the stimulus and the rate

of removal or application of the stimulus.

can increase or decrease the magnitude of the post synaptic impulse

Consecutive generator potentials

have...

Spacial summation

more neurons (nerve fibers)

are recruited

Temporal summation

more impulses are transmitted

per unit-time.

Modulation of info transfer

allowed by changes that can take place at the synapse:

-one neuron can communicate with one or more neurons

-communication can be excitatory or inhibitory or occur with some or all of the other neurons

-nerve impulses involve hundreds of synapses as they traverse pools of neurons

Nerve fibers depolarize on...

an "all or none" basis

Nerve impulses can be..

Excitatory post synaptic potentials

sodium opens

the neurotransmitter opens NA+ channels and causes a depolarization

EXCITATION: a postsynaptic potential either takes place or becomes more likely

Inhibitory postsynaptic potential

(blocked) potassium opens

the neurotransmitter opens K+ channels and causes a hyperpolarization.

INHIBITION: a postsynaptic potential is less likely to occur

Classification of Neurons

-bipolar

-multipolar

-unipolar

BIPOLAR

-two processes

one dendrite and one axon

ex: in retina

UNIPOLAR

one process occurs

for both

dendrite and axon

MULTIPOLAR

many dendrites

and 

one axon

Functionally, neurons can be..

Motor(efferent)

Sensory(afferent)

or

Interneurons (association neurons)

MOTOR(EFFERENT)

neurons

from CNS to effector

Multipolar

Sensory(afferent)

neurons

from receptor to CNS

unipolar or bipolar

Interneurons

(association neurons)

capability to inhibit

Multipolar

Communication between neurons across commisure

MOTOR

innervation

CNS→motor nerves→periphery

the stimulus for voluntary contraction:

1.      originates in neurons of the cerebral cortex

-may be transmitted to centers in brain stem

-or may descend via the spinal cord

2. Synapses with motor neuron of ventral horn

3. motor impulse exits via spinal nerve (to muscle)

Sensory

innervation

Periphery→Sensory nerve→CNS

Sensory Stimuli:

1.originates in specialized nerve endings of the periphery

2. sensory fiber reaches spinal cord via a spinal nerve

3. dendrite enters doral root ganglion

4. axon leaves doral root ganglion and enters spinal cord

5. synapses with (inter) neuron of dorsal horn

6. impulse ascends to brain through white matter tract of spinal cord

The spinal REFLEX

ARC

the sensory neuron synapses with a neuron of the gray matter of the spinal cord at basically the same level at which it enters.

receptors reach effectors

WITHOUT BRAIN INVOLVEMENT

there is no concious thought in a reflex

the sensory impulses DO NOT enter an ascending tract of the spinal cord to reach a higher center of the CNS

In the spinal reflex arc, a 

particular stimulus always..

elicits the same specific response

(note clinical applicability)

Components of the spinal

reflex

-receptor

-afferent neuron

-efferent neuron

-effector

Monosynaptic Reflex

when the afferent neuron synapses with a motor neuron

Myotactic reflex

monosynaptic (no interneurons involved)

important in maintaining posture against gravity.

-a muscle is stretched

-muscle spindles register stretch and generate a nerve impulse

-afferent neuron: through dorsal root ganglion, into spinal cord then

-activates the motor neuron of the muscle in question

ex: knee jerk(hitting knee stretches tendon, muscles contract)

Withdrawal reflex

polysynaptic (interneurons involved)

most CNS reflexes are polysynaptic.

-sensory receptor is activated

-afferent neuron carries the impulse to the spinal cord

-impulse reaches interneuron

-one activates a motor neuron(causes withdrawal of the limb)

-another reaches CNS centers (causing awareness)

integrative spinal reflex

there are 2 types:

1) ipsilateral flexion

2) contralateral extension

Ipsilateral Flexion

ipsi = the same

 starts with receptor activation

♦motor neuron to flexor

♦inhibitory neuron to extensor muscle

♦interneurons send commissural fibers to opposite

side of the spinal cord.

ex: stepping on tac. The leg and foot that step on the tac react with an ipsi reflex.

Contralateral extension

contra = opposite

starts with commissural interneurons

♦ motor neuron to contralateral extensor muscle (excitatory)

♦ inhibitory neuron to contralateral flexor muscle (inhibitory)

Sensory receptors

if activated by the proper stimulus, they generate nerve

impulses in sensory fibers.

types:

-exteroceptors

-interoceptors

-mechanoreceptors

-thermoreceptors

-chemoreceptors

-photoreceptors

exteroceptors

exter = outside

-receive stimuli from body surface

(touch pressure, pain, temperature)

Interoceptors

inter = inside

-receive stimuli from internal environment of the body

(pressure, pain, chemical changes)

♦visceroceptors

♦propioceptors

Viceroceptors

(part of interoceptors)

vicer = soft organ

monitor blood and viscerae

(pressure, blood gases pH)

Proprioceptors

(part of interoceptors)

think soma

receive information on position of body parts

ex: know where your toes are pointing

(muscle spindles and golgi tendon organs)

Mechanoreceptors

mechan = mechanical, or movement

somatic.

respond to physical deformation

Thermoreceptors

thermo= tempurature

skin, mucosal membranes.

respond to temperature

Chemoreceptors

chemo= chemical

pH, gases, taste.

Respond to chemical reaction

Photoreceptors

photo= light

Retina. 

Transduce light energy into nerve impulse

Muscle Spindle

encapsulated stretch receptor located

within skeletal muscle

the spindle constantly monitors changes in muscle length

Intrafusal fibers

modified skeletal muscle fibers in

a muscle spindle

Motor nerve fibers

_________ intrafusal fibers.

innervate

Sensory nerve fibers sensitive to

stretch wrap around the 

___________ fibers.

intrafusal fibers

Golgi tendon organ

sensory fibers in the tendon monitor

tension.

they assist in maintaining posture and inhibit muscle contraction if tension becomes excessive.

Why does the muscle spindle constantly 

monitor changes in muscle length?

the intrafusal fibers maintain a degree of 

tension at all times, which is close to the 

threshold for the sensory nerve fibers.

Any further stretch of the perimeter of the intrafusal fiber (contraction of the muscle) activates the sensory fibers.