Ch. 11 The Nervous System

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What are Neurons?

Excitable cells that transmit electrical signals.

What are Supporting Cells?

Cells that surround and wrap neurons.

What is a Nerve Cell?

Structural units of the nervous system. Composed of a body, axon and dendrites. Long lived, amitotic, and have a high metabolic rate.

What is the Nerve Cell Body?

Perikaryon or Soma. Contains the nucleus and nucleolus, has well developed nissl-bodies (rough er), contains an axon hillock.

What is an Axon Hillock?

Cone shaped area from which axons arise.

What are the processes of the nervous system?

Axons and dendrites. Armlike extentions from the Soma.

What are processes called in the CNS?

Tracts.

What are processes called in the PNS?

Nerves.

What are Dendrites?

Short, tapering, and diffusing branced processes. They are the receptive, or input regions of the neuron.

What are Axons?

Slender processes of uniform, diameter arising from the hillock. Generate and transmit actions potentials. Secrete neurotransmitters from axonal terminals.

What are long axons called?

Nerve fibers.

What is the Axonal Terminal?

Branched terminus of an axon.

What is the Myelin Sheath?

Whittish, fatty (protein-lipoid), segmented sheath around most long axons. Protects axons. Electrically insulates fibers from one another. Increases speed of nerve impulse transmissions. Formed by Schwann cells.

What are Action Potentials?

Nerve Impulses. Electrical impulses that are carried along the lengths of the axons. Always the same regardless of the stimulus.

What are Passive, or Leakage Channels?

Type of plasma membrane ion channel that is always open.

What is a Chemically Gated Channel?

Type of plasma membrane ion channel that opens with a binding of a specific neurotransmitter.

What is a Voltage Gated Channel?

Type of plasma membrane ion channel that opens and closes in response to membrane potential.

What is a Mechanically Gated Channel?

Type of plasma membrane ion channel that opens and closes in response to physical deformation of receptors.

How does a Gated Channel Work?

EX: Na-/K+ Gated Channel. Closed when a neurotransmitter is not bound to the extracellular receptor. Na- cannot enter the cell and K+ cannot exit the cell. Open when a neurotransmitter is attached to the receptor. Na- enters the cell and K+ exits the cell.

How does a Voltage Gated Channel Work?

EX: Na- Channel. Closed when intracellular environment is negative. Na- cannot enter the cell. Open when the intracellular environment is positive. Na- can enter the cell.

What is Resting Membrane Potential?

The potential difference (-70mV) across the membrane of a resting neuron.

What is Depolarization?

The inside of the membrane becomes less negative.

What is Hyperpolerization?

The inside of the membrane becomes more negative then the resting potential.

What is Repolarization?

The membrane returns to its resting membrane potential.

What is the Absolute Refractory Period?

Time from the opening of the Na+ activation gates until the closing of inactivation gates. Prevents the neuron from generating an action potential. Ensures that each action potential is separate. Enforces one way transmission of nerve impulses.

What is the Relative Refractory Period?

The interval following the absolute refractory period when sodium gates are closed, potassium gates are open, repolarization is occuring. The threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events.

What is EPSP?

Excitatory Postsynaptic Potentials.

What is IPSP?

Inhibitory Postsynaptic Potentials.

What are Neurotransmitters?

Chemicals used for neuronal communication with the body and the brain. Over 50 have been indentified.

What are the main kinds of Neurotransmitters?

Acetylcholine, Biogenic Amines, Amino Acids, Peptides, Novel Messengers.

What is Acetylcoline?

First neurotransmitter identified and best understood. Released at the neuromuscular junction. Synthesized and enclosed in the synaptic vesicles. Degraded by the enzyme acetylcholinesterase (ACheE). Released by all neurons that stimulate skeletal muscle and some neurons of the ANS.

What are Biogenic Amines?

Catecholamine - dopamine, norepinephrine (NE), and epinephrine. Indolamines - seratonin and histamine. Broadly distributed in the brain. Play roles in emotional behaviors and our biological clock.

What are Amino Acids?

Include: GABA - Gamma aminobutyric acid, Glycine, Aspartate, Glutamate. Is found only in the CNS.

What are Peptides?

Include: Substance P - mediator of pain signals. Beta Endorphin, dynorphin, and enkephalins. Act as a natural opiate; reduce pain perception. Binds to the same receptors as opiates and morphine. Gut-brain peptides - Somatostain, and cholectokinin.

What is the All Or None Phenomenon?

Action potentials either happen completely or not at all.

What is the role of the Sodium Potassium Pump?

Ionic redistribution back to resting conditions is restored by this. Restores at hyperpolerization.

Nervous System Functions

Sensory input - monitoring stimuli
Integration - interpretation of sensory input
Motor output - response to stimuli

Central nervous system (CNS)

Brain and spinal cord
Integration and command center

Peripheral nervous system (PNS)

Paired spinal and cranial nerves
Carries messages to and from the spinal cord and brain

PNS Functional Divisions

Sensory (afferent) division
Motor (efferent) division

Sensory (afferent) division

In the PNS, sensory afferent fibers carry impulses from skin, skeletal muscles, and joints to the brain (aka CNS)

Motor (efferent) division

In the PNS, transmits impulses from the CNS to effector organs

Motor Division: Two Main Parts

Somatic nervous system
Autonomic nervous system (ANS)

Somatic nervous system

Conscious control of skeletal muscles

Autonomic nervous system (ANS)

Regulates smooth muscle, cardiac muscle, and glands

Autonomic nervous system (ANS) divisions

sympathetic and parasympathetic

Neurons

excitable cells that transmit electrical signals
structural unit of the nervous system

Supporting cells

cells that surround and wrap neurons

Neuroglia

supporting cells of the nervous system
also called neuralgia or glial cells

Neuralgia functions

Provide a supportive scaffolding for neurons
Segregate and insulate neurons
Guide young neurons to the proper connections
Promote health and growth

Astrocytes

most abundant, versatile, and highly branched glial cells
cling to one or more neurons and their synaptic endings, and cover capillaries

Astrocytes functions

Support and brace neurons
Anchor neurons to their nutrient supplies
Guide migration of young neurons
Control the chemical environment

Microglia

small, ovoid cells with spiny processes

Microglia functions

Phagocytes that monitor the health of neurons

Ependymal cells

ciliated cells that range in shape from squamous to columnar
line the central cavities of the brain and spinal column

Ependymal cells functions

filters for CSF

Oligodendrocytes

branched cells that wrap CNS nerve fibers
help regulate how neuron fires

Schwann cells

aka neurolemmocytes
surround fibers of the PNS
help regulate how neuron fires

neuron characteristics

body, axon & dendrites
long, lived, high metabolic rate
do not undergo mitosis

Neuron plasma membrane functions

electrical signaling
cell-to-cell signaling during development

Nerve Cell Body (Perikaryon or Soma)

Contains the nucleus and a nucleolus
Is the major biosynthetic center
Is the focal point for the outgrowth of neuronal processes
Has no centrioles (hence its amitotic nature)
Has well-developed Nissl bodies (rough ER)
Contains an axon hillock - cone-shaped area from which axons arise

neuron processes

Armlike extensions from the soma

CNS neuron processes

tracts

PNS neuron processes

nerves

two types of neuron processes

axons and dendrites

Dendrites of Motor Neurons

'messenger' cells that receive stimulus
anchor the cell body
relay signal to cell body

Axons

slender processes that arise from hillock
one unbranched per neuron

axon collaterals

rare branching from the axon

axon functions

generate & transmit action potentials
secrete neurotransmittters from the axon terminals

myelin sheath

whitish, fatty, segmented sheath around most long axons
composed of 80% fat & 20% protein

myelin sheath functions

Protect the axon
Electrically insulate fibers from one another
Increase the speed of nerve impulse transmission

myelin sheath

Formed by Schwann cells in the PNS
The Schwann cell:
-Envelopes an axon in a trough
-Encloses the axon with its plasma membrane
-Has concentric layers of membrane that make up the myelin sheath

Nodes of Ranvier (Neurofibral Nodes)

Gaps in the myelin sheath between adjacent Schwann cells
They are the sites where axon collaterals can emerge

Unmyelinated Axons

grey matter neurons
Schwann cells are present but myelin sheath is not b/c they do not coil around axons

white matter

in the PNS, dense collections of myelinated fibers
'faster' than gray matter

gray matter

in the CNS, mostly soma and unmyelinated fibers
'slower' than white matter

action potentials

aka nerve impulses
Electrical impulses carried along the length of axons
Always the same regardless of stimulus
the underlying functional feature of the nervous system

Electrical Current

Reflects the flow of ions rather than electrons

potential on either side of membranes when:

The number of ions is different across the membrane
The membrane provides a resistance to ion flow

Types of plasma membrane ion channels

passive or leakage channels
chemically gated channels
voltage-gated channels
mechanically gated channels

passive, or leakage, channels

always open

chemically gated channels

open with binding of a specific neurotransmitter

voltage-gated channels

open and close in response to membrane potential

mechanically gated channels

open and close in response to physical deformation of receptors

Operation of a Gated Channel

Example: Na+-K+ pump
Closed when a neurotransmitter is not bound to the extracellular receptor
Na+ cannot enter the cell and K+ cannot exit the cell
Open when a neurotransmitter is attached to the receptor
Na+ enters the cell and K+ exits the cell

Operation of a Voltage-Gated Channel

Example: Na+ channel
Closed when the intracellular environment is negative
Na+ cannot enter the cell
Open when the intracellular environment is positive
Na+ can enter the cell

When gated channels are open:

Ions move quickly across the membrane
Movement is along their electrochemical gradients
An electrical current is created
Voltage changes across the membrane

Electrochemical Gradient

electrical and chemical gradients taken together
Ions flow along their chemical gradient when they move from an area of high concentration to an area of low concentration
Ions flow along their electrical gradient when they move toward an area of opposite charge

Resting Membrane Potential (Vr)

potential difference (-70 mV) across the membrane of a resting neuron

Membrane Potentials: Signals

Used to integrate, send, and receive information

Membrane potential changes are produced by:

Changes in membrane permeability to ions
Alterations of ion concentrations across the membrane

Types of signals

graded potentials and action potentials

3 events that cause
Changes in Membrane Potential

depolarization
repolarization
hyperpolarization

Depolarization

the inside of the membrane becomes less negative

Repolarization

the membrane returns to its resting membrane potential

Hyperpolarization

the inside of the membrane becomes more negative than the resting potential

Graded Potentials

Magnitude varies directly with the strength of the stimulus
Short-lived, local changes in membrane potential
Decrease in intensity with distance
Sufficiently strong graded potentials can initiate action potentials
Current is quickly dissipated due to the leaky plasma membrane
Only travel over short distances

Action Potentials (APs)

'neuron firing' +35mV
Action potentials are only generated by muscle cells and neurons
They do not decrease in strength over distance
They are the principal means of neural communication

Action Potential: Resting State

Na+ and K+ channels are closed
Leakage accounts for small movements of Na+ and K+
Each Na+ channel has two voltage-regulated gates

Action Potential: Depolarization Phase

Na+ gates are opened; K+ gates are closed
Cell becomes more positive

Depolarization Phase Threshold

a critical level of depolarization (-55 to -50 mV)
At this point, the process becomes self-generating

Action Potential: Repolarization Phase

sodium gates close, voltage-sensitive K+ gates open
K+ exits the cell and cell becomes more negative, as usual

Action Potential: Hyperpolarization

Potassium gates remain open, causing an excessive efflux of K+
The neuron is insensitive to stimulus and depolarization during this time

Action Potential:
Role of the Sodium-Potassium Pump

Repolarization
-Restores the resting electrical conditions of the neuron
-Does not restore the resting ionic conditions

Phases of the Action Potential

1 - resting state
2 - depolarization phase
3 - repolarization phase
4 - hyperpolarization

Propagation of an Action Potential

wave of impulse moving down the axon to the axon terminals

Action Potential Threshold

membrane is depolarized
membrane potential goes from resting state -70mV to -55mV

stimulus intensity

the CNS determines stimulus intensity by the frequency of impulse transmission

absolute refractory period

neuron cannot fire
time from the Na+ activation gates until the closing of inactivation gates

relative refractory period

Na+ gates are closed, K+ gates are open
repolarization is occurring
threshold level is elevated, allowing strong stimuli to increase the frequency of action potential events

axon diameter

the larger the diameter, the faster the impulse

myelin sheath presence

dramatically increases impulse speed

saltatory conduction

Current passes through a myelinated axon only at the nodes of Ranvier avoiding fats
Action potentials are triggered only at the nodes and jump from one node to the next
Much faster than conduction along unmyelinated axons
Na+ channels are concentrated at these nodes

Multiple Sclerosis

scar tissue in myelin sheath causes too long of a jump resulting in muscle control
spreads to diaghram & causes respiratory failure

Central Nervous System (CNS)

This nervous system consists of the brain and spinal cord.

Peripheral Nervous System (PNS)

The section of the nervous system lying outside the brain and spinal cord

Sensory or Afferent

Part of the (PNS) that carry impulses towards the (CNS)

Motor or Efferent

Part of the (PNS) that carry impulses away from the (CNS)

Somatic Nervous System

This nervous system conduct impulses from the (CNS) to skeletal muscles.

Autonomic Nervous System (ANS)

This nervous system regulate the activity of smooth muscles.

Glial cells

These neurons are associated closely with much smaller cells., Cells that provide basic support systems for neurons and perform a variety of maintenance functions

Astrocytes

Star shaped cells found throughout the CNS, cleaning up debris in the extracellular space and removing neurotransmitters from the synaptic cleft, connects neurons to nearby capilaries, components of the blood-brain barrier

Microglia

Smallest neuroglial cells; phagocytic cells that engulf cellular debris, waste products and pathogens. increase in number as a result of infection or injury

Ependymal Cell

A glial cell that lines membranes within the brain and spinal cord and helps form cerebrospinal fluid

Oligodendrocytes

Provides myelination in CNS, electrically insulates certain axons, & speeds up rate of electrical signals

Satellite cells

Surrounds the neuron cell bodies in ganglia of PNS little is known of their function (PNS)

Schwann cells

Supporting cells of the peripheral nervous system responsible for the formation of myelin.

Neurons

Cells specialized for transmitting nerve impulses.

Perikaryon

Cytoplasm surrounding the nucleus of a neuron

Dendrites

The branching extensions of a neuron that receive messages and conduct impulses toward the cell body

Axon

The extension of a neuron, ending in branching terminal fibers, through which messages pass to other neurons or to muscles or glands

Axon collaterals

A branch of an axon

Telodendria

Series of fine, terminal extensions branching from the axon tip.

Axolemma

The plasma membrane of the axon

Myelin Sheath

A layer of fatty tissue segmentally encasing the fibers of many neurons; enables vastly greater transmission speed of neural impulses as the impulse hops from one node to the next.

Neurilemma

Portion of the Schwann cell which includes the exposed part of its plasma membrane.

Nodes of Ranvier

Small gaps in the myelin sheath of medullated axons

White matter

Regions of the brain and spinal cord containing a dense collection of myelinated fibers.

Gray Matter

Contains mostly nerve cell bodies and unmyelinated fibers.

Multipolar Neurons

Neurons having three or more processes.

Bipolar Neurons

Neruons having two processes

Interneurons

Central nervous system neurons that internally communicate and intervene between the sensory inputs and motor outputs

Potential difference

The difference in electrical charge between two points in a circuit expressed in volts.

Depolarization

Sodium rushes into neuron through membrane, potassium ruses out; results in a change in charge

Hyperpolarization

The movement of the membrane potential of a cell away from rest potential in a more negative direction.

Resting State

Na+ and K+ channels are closed

Depolarizing phase

Increase in Na+ permeability and reversal of membrane potential.

Repolarizing phase

Decrease in Na+ permeability and an increase in K+ permeability

Sodium potassium pump

Helps establish a difference in charge across the membrane (membrane potential) moves ions and molecules against the concentration gradient (takes energy)

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