Chapter 8: The Nervous System

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Functions of the nervous system

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Functions of the nervous system

1. sensory input : sensory receptors respond to external & internal stimuli. Ex: t sensors in skin- to brain & spinal cord

2. integration : interpretation of sensory input. Ex: from t receptors- to hypothalamus. (controls body temp.)

3. motor output : response by muscles, glands, and organs. Ex: cold t- hypothalamus- shivering (skeletal m. Contraction) - produce heat- warm body.

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Divisions of the Nervous System

1. Central Nervous System (CNS) - midline.

-brain

-spinal cord

2. Peripheral Nervous System (PNS) - project out from CNS, includes all the cranial & spinal nerves.

-Afferent (sensory) division: includes somatic and visceral divisions.

-Efferent (motor) division: includes somatic and autonomic divisions.

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Organization of the Nervous System

<p></p>
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Afferent nerves

Come from 2 sources:

1. Somatic sensory nerves come from the surface of the body, skeletal muscles, tendons & joints.

2. Visceral sensory nerves come from internal organs.

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Efferent nerves

Have 2 destinations :

1. Somatic efferent nerves innervate skeletal muscles.

2. Autonomic efferent nerves supply organs under involuntary control

-Ex: heart, digestive organs.

-There are 2 divisions of the autonomic nervous system: sympathetic & parasympathetic nervous systems.

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Nervous tissue Cells

1. Neurons (nerve cells): transmit impulses

2. Neuroglia: support & nourish neurons.

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Neuron structure

-Dendrite: receive signal from sensory receptors and other neurons. Shorter, many branches.

-Cell body: contains the nucleus and other organelles.

-Axon: conduct nerve signals away from cell body.

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Parts of Axon

-Axon terminal: enlarged end of Axon

-Nerve: bundle of parallel axons in PNS

-Tract: bundle of parallel axons in CNS.

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Cell body of neuron

<p></p>
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Cell body

<p></p>
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Dendrite

<p></p>
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Dendrites

<p></p>
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Axon

<p></p>
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Neuron structure cont'd.

-Axon may be covered by a myelin sheath (lipid coating)

-formed by Schwann cells or neurolemmocytes in PNS.

-formed by oligodendrocytes in CNS

-provides insulation.

-increases speed of impulse conduction.

-Nodes of Ranvier: gaps in myelin sheath.

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Nervous tissue

<p></p>
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Schwann cells

<p></p>
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Axon

<p></p>
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Schwann cells

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Nodes of Ranvier

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Nodes of Ranvier

<p></p>
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Types of Neurons

1. sensory neurons

2. motor neurons

3. interneurons

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Sensory neurons

-carry nerve impulses from sensory receipts to the CNS.

-almost all are unipolar- have one extension coming off of the cell body; it splits into 2 branches- one comes to the periphery & another goes to the CNS.

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Interneurons

"Association neurons"

-all are in CNS

-the vast majority of neurons in the body are interneurons.

-typically multipolar- have many dendrites and a single axon.

-convey nerve impulses between various parts of the CNS (between sensory & motor neurons, form brain to cord & vice versa)

-form complex pathways for thinking, memory, and language.

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Motor neurons

-carry nerve impulses from CNS to muscles, organs, or glands.

-all are multipolar neurons.

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Neuron Anatomy

<p></p>
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Nerve signal conduction

-neurons release neurotransmitters to communicate electrical signals between cells.

-axons carry electrical signals, called action potentials.

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Resting potential

Potential energy of a neuron.

-The cell membrane is polarized.

-positively charged outside the cell due to sodium ions (Na+).

-negatively charge inside due to potassium ions (K+) and large (can't leave the cell) negatively charged proteins.

-At rest, the membrane is impermeable to Na+ But permeable to K+.

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Resting potential cont'd.

-resting potential = 70mV!!

When the neuronal membrane is at rest, the resting potential is negative due to the accumulation of more sodium ions outside the cell than potassium ions inside the cell.

-the sodium/potassium pump moved ions to maintain resting potential. (Actively transports 3Na+ out of the neuron and 2K+ in).

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Action potential and membrane potential

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Action potential

-process of conduction of nerve signals.

-occurs in axons.

-begins with a stimulus, which activated voltage-regulated sodium gates (Na+ channels in the cell membrane)

-these gates are closed when the cell is at rest; the stimulus opens them & sodium ions rush into the cell (depolarization); the membrane becomes negative outside and positive inside.

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Action potential cont'd.

-if enough Na+ enters the cell, the cell potentially may reach the cell's threshold.

-if threshold is reached, large numbers of voltage-regulated sodium gates will open, & the cell potential will rise abruptly.

-this sharp depolarization continues until the fell potential is +35mV; at this point, the voltage-regulated sodium gates close and voltage-regulated potassium gates open.

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Repolarization

Cell potential becomes negative again!

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Action potential cont'd.

-after the action potential is complete, the Na+/K+ pump moves Na+ out & K+ into the cell (reach resting potential) to get ready for another action potential.

-action potentials are conducted, or propagated, down the axon away from the cell body!

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Resting potential & action potential in an unmyelinated axon

<p></p>
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nerve impulse

<p></p>
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Conduction of Action potentials

-as the axon depolarizes, Na+ will diffuse into the next section of the axon, causing depolarization to threshold at that location.

-continues down the axon, followed by repolarization.

-it is an all-or-none event; action potentials don't vary in size - the intensity of message is determined by the number of action potentials, not their size.

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Refractory period

-period of time after an action potential, during which an Axon cannot conduct another action potential.

-the refraction period is the time needed for

Na+/K+ pump to restore the original Na+ and K+ concentrations.

-ensures one-way direction of an impulse.

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Action potential in unmyelinated axon

<p></p>
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Conduction of action potentials cont'd.

In unmyelinated axons:

-slow (~1 meter/second)

- each section of Axon must be stimulated.

In myelinated axons:

-An AP at one node of ranvier can "jump" over myelinated portion of Axon to the next node.

-called saltatory conduction.

-much faster (>100 meters/second)

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Conduction of an action potential

<p></p>
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Transmission across a synapse.

-Axon terminal: small swelling at tips of branched end of Axon.

-Synapse:

-region of close proximity between 2 neurons.

-presynaptic membrane: membrane of first neuron.

-postsynaptic membrane: membrane of the next neuron.

-synaptic cleft: small gap between the presynaptic and the postsynaptic neuron.

-the neuromuscular junction is a type of synapse.

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Synapse

<p></p>
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synaptic structure and function

<p></p>
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Synaptic structure & function

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Transmission across synapse cont'd.

-Neurotransmitters: molecules stored in synaptic vesicles in the axon terminal that transmit a nerve impulse across a synapse.

-nerve impulse reaches Axon terminal

-calcium channels are opened and Ca2+ enters the terminal.

-causes synaptic vesicles to fuse with the presynaptic membrane and release the neurotransmitter by exocytosis into the synaptic cleft.

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Transmission across synapse cont'd.

-neurotransmitter then diffuses across the synaptic cleft to the postsynaptic membrane & binds to its specific receptor on ligand-regulates gates; they then open.

-the gates are closed when the cell is at rest; the NT acts as a ligand that opens the gates.

-removal is the ligand (the NT) closes the gates.

-depending on the NT, the postsynaptic membrane will be exited or inhibited.

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Neurotransmitter molecules

-at least 100 neurotransmitters have been identified.

-2 well known neurotransmitters : Acetylcholine (ACh); Norepinephrine (NE) = adrenaline.

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Chemical synapse

<p></p>
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Receptors linked to a channel protein

<p></p>
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Neurotransmitter molecules cont'd.

After a neurotransmitter has initiated a response, it is removed from the synaptic cleft to prevent continuous stimulation.

-enzymes may inactivate the neurotransmitter (Ex: ACh inactivated by acetylcholinesterase -AChE)

-it may be reabsorbed by presynaptic membrane.

Many drugs block or enhance the effect of neurotransmitters in the body.

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Central Nervous System (CNS)

-consists of the brain and spinal cord

-gray matrer: contains cell bodies & unmyelinated fbers.

-white matter: contains myelinated axons; myelin appears white.

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Meninges

-protective membranes of the CNS.

1. Dura Mater

2. Arachnoid matter

3. Pia matter

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Dura mater

Outer menix composed of 2 layers of tough, fibrous connective tissue.

-epidural space: fat-filled space between the

Dura mater & the skull or vertebrae.

-rural venous sinuses: spaces between the dura

Mater layers containing venous blood.

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Arachnoid Mater

Middle menix composed of spider web like connective tissue.

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Pia Mater

Deepest menix that adheres to the brain & spinal cord. Capillaries from pia Mater called choroid plexus (ependymal cells) secrete cerebrospinal fluid.

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Meninges

<p></p>
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Meninges

<p></p>
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Cerebrospinal fluid (CSF)

Produced from blood by choroid plexus into the ventricles (protective cushion, nutrition).

-found in the:

-subarachnoid space: space between the

arachnoid Mater & pia Mater.

-ventricles: hollow, interconnecting cavities

Of the brain (choroid plexus).

-central canal: hollow, space of spinal cord.

-reabsorbed back into dural venous sinuses.

-blockages can occur that can cause hydrocephalus.

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CSF

<p></p>
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Ventricles of the brain

<p></p>
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Ventricles of the brain

<p></p>
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The spinal cord

Extends from foreman magnum to first lumbar vertebrae.

-protected by vertebral column.

-occupies vertebral canal, which contains the

spinal cord.

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Structure of spinal cord

-spinal nerves extend from spinal cord in between vertebrae.

-intervertebral disks: (fibrocartilage) separates each vertebrae.

-herniated disk: disk torn open; may press on

spinal nerves & cause pain & loss of function.

-central canal and subarachnoid space contain CSF.

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Structure of spinal cord cont'd.

-centrally located gray matter.

-H shaped

-contains interneurons & portions of sensory &

motor neurons.

-Spinal nerve:

-Posterior (dorsal) roots: contain sensory fibers

That are entering the grey matter of the spinal

cord.

-posterior root ganglion: enlarged area of

posterior root; contains cell bodies of

sensory fibers.

-Anterior (ventral) roots: contain motor fibers exiting the grey matter.

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Spinal cord

<p></p>
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Spinal cord

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Spinal cord

<p></p>
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Spinal nerve

<p></p>
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Dorsal root ganglion

<p></p>
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Ventral root

<p></p>
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White matter

-Posterior: white matter composed of ascending tracts carrying sensory information to the brain.

-Anterior: white matter composed of descending tracks carrying information from the brain.

-Tracts generally cross from one side of the spinal cord to the other.

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Spinal cord

Provides communication between brain & peripheral nerves.

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Reflex arcs

-sensory receptors respond to a stimulus.

-impulse travels over sensory neurons to the spinal cord.

-interneurons integrate data and relay a response by way of motor neurons.

-motor neurons cause effectors to respond.

<p>-sensory receptors respond to a stimulus.</p><p>-impulse travels over sensory neurons to the spinal cord.</p><p>-interneurons integrate data and relay a response by way of motor neurons.</p><p>-motor neurons cause effectors to respond.</p>
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Reflexes

-Automatic involuntary responses to change inside & outside the body.

-cranial reflexes involve the brain.

-spinal reflexes involve only the spinal cord.

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Reflex arc showing spinal reflex

<p></p>
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Reflexes cont'd.

-reflexes can be used to determine if the nervous system is reacting properly, can help avoid injury, and help maintain balance.

-2 examples of reflexes used to test the function of nervous system:

1. Knee-jerk reflex.

2. Ankle-jerk reflex.

These 2 reflexes are used to maintain balance.

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Brains 4 major structures

1. Cerebrum

2. Diencephalon

3. Cerebellum

4. Brain stem

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ventricles of the brain

-2 lateral ventricles (cerebrum)

-third ventricle (diencephalon)

-fourth ventricle (brain stem & cerebellum)

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4 major portions of the brain

<p></p>
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The human brain

<p></p>
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Ventricles of the brain

<p></p>
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Cerebrum

Largest portion of the brain.

-receives sensory input, carries out integration & initiates voluntary motor responses.

-coordinates the activities of the other parts of the brain.

-involved in higher thought processes.

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Cerebrum

<p></p>
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Cerebrum

<p></p>
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Cerebrum cont'd.

Divided into 2 cerebral hemispheres:

-longitudinal fissure: divides them.

-connected internally by the corpus callosum (white matter)

-Gyri: (ridges =convolutions) are separated by sulci (shallow grooves).

Five lobes:

1. Frontal lobe

2. Parietal lobes

3. Temporal lobes

4. Occipital lobe

5. Insula

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Cerebral hemispheres

<p></p>
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Longitudinal fissure

<p></p>
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Corpus callosum

<p></p>
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Convolutions (gyri)

<p></p>
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Frontal lobe

<p></p>
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Central sulcus

separates frontal and parietal lobes

<p>separates frontal and parietal lobes</p>
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Parietal lobe

<p></p>
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Temporal lobe

<p></p>
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Occipital lobe

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Insula

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Insula lobe

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lobes of cerebral hemisphere

<p></p>
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Cerebral cortex

-outer layer of cerebrum; made of gray matter.

-accounts for sensation, voluntary movement, information processing, consciousness.

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Cerebral cortex

<p></p>
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Cerebral cortex - gray matter

<p></p>
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