Nervous System

Autonomic Nervous System (involuntary)

Affects smooth muscles, cardiac muscle, glands

Sympathetic Division- Activated when body is stressed (flight or fight response)

speeds heartconstricts blood vesselssignals for secretions from sweat and adrenal glandsslows digestion

Parasympathetic Division- Activated under normal conditions

slows heartrelaxes blood vesselsincreases digestion

Somatic Nervous System(voluntary)

• Carries impulses from sensory receptors in skin and sensory organs of the head

• Carries impulses to motor neurons in skeletal muscles

Nerve Tissue2 cell types:

Neuroglia: provide structural frameworkinsulate axonsperform phagocytosiscirculate cerebrospinal fluid

Neurons:conduct impulses

Types of Glial CellsNameAstrocytes

Ependymal Cells

Microglia

OligodendrocytesMultiple Sclerosis affects these and impairs impulses

Schwann cells

LocationBrain and spinal cord

Line cavities inbrain and spinalcord

Brain and spinal cord

Brain and spinal cord

Large nerves of PNS

Functionanchors neurons to capillariesforms blood-brain barriercontrols ion flow around neuronsaids in impulse conduction

Help form and circulate cerebrospinal fluid

Phagocytize invading microorganisms and dead nerve tissue

Form myelin sheathInsulate CNS axons

Insulate large PNS axonsPossibly assist in regeneration

Neuron Anatomy

Neurons have a wide variety of shapes and sizesGeneralized Structure:

Dendrites: short, branched- receive impulses- transmit impulses towards cell body

Cell Body: middle section- holds organelles typical of other cells

Nissl bodies: similar to rough ERNeurofibrils: similar to microtublulesNo spindle fibers = NO MITOSIS

Axon: nerve fiber, very long (up to 1m)- wrapped in myelin sheath- transmit impulses away from cell body

Made from tight layers of Schwann cellslittle cytoplasmcell membrane of high lipid concentration

high insulation

outer layer of myelin sheath contains cytoplasm and nuclei of Schwann cells = neurilemma

Gaps between Schwann cells = nodes of Ranvier

Fibers that may be enclosed by Schwann cells but lack myelin sheath

Fibers that are enclosed by tightly wrapped Schwann cells to form the myelin sheath

Neuron typesStructural Differences:

Multipolar Neurons:many dendritesone axoncarry impulses from CNS to skeletal muscles

Bipolar Neurons:one dendriteone axonfound in special sensory areas (eyes, ears,nose)

Unipolar Neurons:single nerve fiber extends from cell body, then branches into one axon and one dendritecarry impulses from skin receptors to spinal cord

Neuron typesFunctional Differences:

Sensory Neurons (afferent neurons):unipolar carry impulses from periphery to CNS

Association Neurons (interneurons):multipolarlocated within CNSrelay impulses from one region of CNS to anotherconduct impulses from sensory neurons to motor

neuronsMotor Neurons (efferent neurons):

multipolarcarry impulses from CNS to responders (muscles,

glands)

Nerve ImpulsesResting Membrane Potential

- separation of charge between extracellular and intracellular environments- due to unequal distribution of ions across the cell membranes- created by ion channels

integral proteins of membrane guard openings

Resting Membrane Potential

High concentration of sodium ions outside cell

High concentration of potassium ions inside cell

High concentration of chlorine ions inside cell

+

-

Net positive charge outside cell

Net negative charge inside cell

+

Sodium channel Potassium channel

Polarized State

Action PotentialNet positive charge outside cell

Net negative charge inside cell

Sodium channel Potassium channel

Depolarized State

Stimulus must reach a threshold

Sodium ion channels open

Sodium ions flow into the cell

Briefly reverse electrical charges in cell and outside cell membrane Net positive

charge inside cell

Net negative charge outside cell

Action PotentialNet positive charge outside cell

Net negative charge inside cell

Sodium channel Potassium channel

Repolarization

Sodium ion channels close

Potassium ion channels open

Potassium ions flow out of the cell =

net movement of + ions out of cell

Restores resting potential

Action Potential

Sodium-Potassium Pump switches locations of sodium and potassium ion concentrations

Impulse is further transmitted down the neuron as depolarized areas stimulate adjoining areas to depolarize (positive feedback)

Impulse only travels in one direction because preceding area is busy repolarizing.

Sodium gates remain closed despite stimuli. Refractory Period

Saltatory conduction:

Speed of impulse in unmyelinated nerve fiber = 10m/sec

Myelin blocks continuous flow of ions

Impulse jumps across myelin sheath from one node of Ranvier to next.

cut distance = decreased time

Speed = 130 m/sec

All – or –None Response

Action potential does not occur unless stimulus is strong enough (threshold)

Action potential always occurs at its maximum strength

Summation

Series of subthreshold stimuli are applied quickly and have a cumulative effect that can lead to an action potential

Cell to Cell Transmission

Synapse = junction between cells

Synaptic End Bulb: rounded end of presynaptic neuron’s axoncontains synaptic vesicles (hold neurotransmitters)

Synaptic cleft = gap between synaptic end bulb and postsynaptic neuron

Cell to Cell Transmission

1.) Nerve impulse arrives at synaptic end bulb2.) Signals calcium channels to open

Calcium flows into presynaptic cell3.) Ions cause vesicles to fuse with plasma membrane and

release neurotransmitters by exocytosis4.) Neurotransmitters diffuse across synaptic cleft5.) Contact membrane of postsynaptic neuron6.) Results in excitation or inhibition of postsynaptic neuron7.) Result is limited to a fraction of a second8.) Enzymes inactivate or transport neurotransmitters away

Excitatory TransmissionNeurotransmitters increase membrane permeability to sodium ions

Facilitation:each molecule of neurotransmitter

received by postsynaptic neuron allows more sodium ions into cell = partial polarization

Inhibitory Transmission

Neurotransmitters increase membrane permeability to potassium ions

Hyperpolarization:More and more potassium ions flow

out of the cell while chlorine ions flow in.Creates an even larger positive charge outside the cell and larger negative charge inside the cell.

Common Neurotransmitters

Acetylcholine – released in spinal cord and at neuromuscular junctions

Norepinephrine DopamineSerotonin

Affect sleep, mood, motor functions, and pleasure recognition

EndorphinsEnkephalins

Natural pain killersIn spinal cord and brain

Cell to Cell Transmission

Postsynaptic dendrites may receive thousands of signals from thousands of presynaptic neurons at the same time

Cells may receive conflicting signals at the same time

Overall effect is determined by the sum of the total incoming signals

CNS – Spinal Cord

Extends from base of brain, through foramen magnum, down vertebral canal to the 1st or 2nd lumbar vertebra

CNS – Spinal Cord

Spinal cord is protected by 3 layers:

Vertebral Column

Cerebrospinal Fluid (CSF)

Meninges (3)

Meninges

Dura Matertough outer layer

Arachnoidthick, cobweb like layer of collagen fibers

Subarachnoid Spacefilled with CSF

Pia Materattached to spinal cord’s outer surface; thin, delicate

Epidural space filled with fat and areolar C.T.found between dura mater and vertebral

column

Spinal Cord Anatomy31 segments

- 1 pair of spinal nerves branches from each- spinal nerves relay info between spinal cord and

peripheral body

Spinal Cord Anatomy

Cervical and Lumbosacral Enlargements- thickened areas of spinal cord-serve upper and lower appendages

Conus medullaris: tapered end of spinal cordCauda equina: collection of spinal nerves that continue beyond conus medullarisFilium Terminale: extension of pia mater continuing beyond the spinal cord, other meninges, to the back of the coccyx

Anterior Median Fissure:

Posterior Median Fissure::

Grooves that partially divide spinal cord into right and left portions

Spinal Cord Cross SectionGray Matter: - central section- unmyelinated

Front

Posterior Horn

Anterior Horn

Contain terminal endings of sensory neurons

Contain cell bodies of motor neurons

Lateral Horn

Found only in thoracic and first two lumbar segments

Sensory neuron cell bodies lie in clusters (ganglia) outside spinal cord

Gray Commissure-surrounds central canal

- filled with CSF

Spinal Cord Cross SectionWhite Matter: - Outer section- myelinated

Columns = bundles of myelinated fibers that represent major nerve pathways extending up and down

the spinal cord (Nerve Tracts)

Lateral Column

Anterior Column

Posterior Column

Spinal Cord Functions

Conduction Pathways:Ascending Tracts – carry sensory info to brainDescending Tracts – carry motor info away from brain

Spinal Cord FunctionsReflex Centers:

simplest pathways an impulse can takeinvolve few neuronsimpulse does not travel to higher levels of brain =

NO THINKING

Reflex Arc:

Receptor

Sensory Neuron

Association Neuron

Motor Neuron

Effector

In CNSQuickly process info and send it to the appropriate motor neurons

Receptors have the ability to generate action potentials based on changes in their environments

Association neurons also link to other parts of the nervous system so that you can further process info after the crisis has passed.

Common Reflex TypesWithdrawal Reflex

protectiveminimizes injuries due to rapid response

Patellar Reflexinvolves only sensory and motor neuronsused to diagnose nervous disorders

Somatic Reflexes: - Effectors are skeletal muscles - Often cause large muscle movements

Visceral Reflexes: - effectors are smooth and cardiac muscles - Responses include heart and breathing rate changes, vomiting, sneezing, and coughing

CNS – Brain3 major regions:

Forebrain: largest section; contains cerebrum and diencephalonMidbrain: smallest sectionHindbrain: contains pons, medulla oblongata, cerebellum

Midbrain + pons + medulla oblongata = brain stem

CNS – Brain

Brain is protected by 3 layers:

Cranium

Cerebrospinal Fluid (CSF)

Meninges (3)

Clear, colorlessCirculates within and around spinal cord and brainCushions, nourishes, removes metabolic wastesMost is found in ventricles

Choroid Plexus(capillaries within lateral ventricles) (astrocytes)

+ neuroglia = blood brain barrier

Selectively filters blood plasma

Continuous production of CSF creates pressureCirculation resultsReabsorption occurs in arachnoid by arachnoid villi

Empty into superior sagittal sinus (vein)

Ventricles- Cavities within brain- Continuous with central canal of spinal cord and subarachnoid space

Lateral Ventriclesone in each cerebral hemisphereconnect to third ventricle by foramen of Monro

(2)

Third Ventriclemiddle of deincephalonconnect to fourth ventricle by cerebral aqueduct

Fourth Ventriclebetween cerebellum and medulla oblongata

Cerebrum

Largest structure of the brain“higher brain”

responsible for various complex thinking, learning and memorization functions

receives and interprets sensationsinitiates responses

Cerebrum – Structural Characteristics

Convolutions:wrinklesoccur as embryo’s brain rapidly develops

Gyri: ridges

Sulci: shallow grooves

Fissures: deep grooves

Longitudinal Fissure:divides right and left cerebral hemispheres across brain’s midline

Transverse Fissure:divides lower margin of the cerebrum from the cerebellum

Cerebrum - 4 Functional Divisions

Frontal Lobe:Primary Motor Area

contols specific muscles of groups of musclesPremotor Area

controls coordinated, precise movements of skeletal muscles that are usually

learnedBroca’s Area

controls speech muscles

Cerebrum - 4 Functional Divisions

Parietal Lobe:General Sensory Area

Mainly receives sensations from skin; pinpoints locations

Somesthetic Association Areareceives impulses from thalamus and general

sensory areainterpretes nature of sensationsstores memories of past sensory experiences

Primary Gustatory Areainterprets taste

Cerebrum - 4 Functional Divisions

Occipital Lobe:Primary Visual Area

Visual Association Area

Interpret and analyze images

Cerebrum - 4 Functional Divisions

Temporal Lobe:Primary Auditory Area

receives impulses from ears and interprets the nature of sound

Auditory Association Areatranslates sounds of speech into thought

Primary Olfactory Areainterprets smell

Gnostic Area (usually only the left temporal lobe)integrates all sensory incoming signals into

conscious thought or understandingactivates other parts of cerebrum to cause

proper responses

CNS - Brain

Central Sulcusseparates frontal and parietal lobes

Lateral Sulcusseparates frontal and temporal lobes

Cross section revels external layer (2mm) of gray matter

Source of integrative functionsBillions of cell bodies and synapses

CNS - Brain

Lies underneath gray matter

Extends in 3 directions:- hemisphere to hemisphere = corpus callosum- one region in a hemisphere to another- one hemisphere to other brain parts

Interrupted by basal ganglia/nuclei- Masses of gray matter embedded in white matter

CNS - Brain

Diencephalon:

Found below corpus callosumComposed of gray matter

ThalamusHypothalamus

Pineal GlandPituitary Gland

CNS – BrainDiencephalon

Principle relay station for: - Sensory impulses traveling to the cerebral cortex- Involuntary motor impulses traveling outward

Recognizes primitive (survival) sensations quickly

Pineal Gland is attached to its posterior

CNS – BrainDiencephalon

Controls involuntary body activities that effect homeostasis Controls ANS

Regulates visceral activitiesBody’s thermostatRegulates food and water intakeMaintains sleeping patternsStimulates or inhibits pituitary glandAssociated with emotion (along with the rest of the limbic system

(thalamus, cerebral cortex, basal ganglia, other nuclei)

Midbrain2 portions:

ANTERIOR POSTERIOR

Cerebral Peduncles- bundles of myelinated fibers- connects motor pathways of cerebrum and cerebellum

Corpora Quadrigemina- contains reflex centers for rapid eye, head and trunk movements

Pons

- Regulates breathing rhythm- Relays sensory impulses from peripheral nerves to the cerebral cortex

Medulla Oblongata

Cardiac Center- regulates heart rate

Vasomotor Center- regulates blood pressure

Respiratory Center- controls the depth and rhythm of breathing- involved in consciousness

Pyramids- locations where descending motor fibers cross to

opposite sides

CerebellumVermis

- connects 2 hemispheres Arbor Vitae

- tree-like pattern of white matter

Thin shell of gray matterConvolutions = foldsFolia = ridgesSulci = grooves

PNSGanglia

- Clusters of neuron cell bodies in PNS

Nerves:parallel bundles of nerve fibers (axons) enclosed in 3 sheaths of C.T.

Epineurium: tough, fiberous outer covering of nerve

Fascicles: groups of nerve fibers wrapped together in smaller bundles wrapped in perineurium

Endoneurium: innermost wrapping, surrounds each fiber separately

3 nerve types, dependent on type of fibers it contains:

- Sensory (afferent)- Motor (efferent)- Mixed