Nervous System Flashcards

Nervous System Pt. 1

Outline

• Central nervous system

• Peripheral nervous system

• Divisions and subdivisions of the PNS

• Basic structure of a neuron

• Structural and functional classes of neurons

• Neuroglial cells of CNS

• Neuroglial cell of the PNS

• Myelin sheath

• White and gray matter

Overview of Nervous System

• Coordinates cellular functions in 3 basic steps:

  • Senses information and sends messages to the central nervous system (CNS).

    • Changes within the body or changes in the external environment.

  • CNS receives and processes the information and determines appropriate response.

  • CNS issues commands to skeletal muscles and other organs to carry out response.

Nervous System Anatomical Subdivisions: CNS

• Central Nervous System (CNS)

  • Composed of brain and spinal cord.

  • Enclosed in the skull and vertebral column.

  • Carries out processing (integrating) functions of the nervous system.

Nervous System Anatomical Subdivisions: PNS

• Peripheral Nervous System (PNS)

  • Composed of nerves leading to and from the CNS.

  • Provides pathways of sensory input and motor output to structures that carry out its commands.

  • Functionally divided into two divisions:

    • Sensory

    • Motor

PNS: Sensory Division

• Carries signals from various receptors to the CNS.

  • Receptors in sense organs or sensory nerve endings.

• Provides information to CNS about environmental changes (stimuli) inside and outside the body.

PNS: Motor Division

• Carries signals from the CNS to structures.

  • Example: signals muscles to contract or glands to secrete hormones.

  • Ultimately carrying out response to stimuli.

• Cells and organs that respond to these signals are called effectors.

• Motor division has 2 subdivisions:

  • Somatic motor division

  • Autonomic nervous system (ANS)

Motor Division: Somatic Motor Division

• Carries signals to skeletal muscles.

• Produces voluntary movements.

• Movements under conscious control.

Motor Division: ANS

• Carries signals to cardiac muscle, smooth muscle, and glands.

• Produces involuntary response.

• Not under conscious control.

  • Ex) don’t have to tell your body to breathe

• ANS is subdivided into:

  • Sympathetic division

    • Prepares the body for action.

    • Think “fight or flight”.

    • Activated by fear or when we need extra energy for exercise.

  • Parasympathetic division

    • Involved in calming the body.

    • Think “rest and digest”.

Neurons

• Carry out the main functions of the nervous system: communication.

• ~1 trillion neurons in the nervous system.

• Usually consists of a sort of globular cell body where the nucleus is located and two or more extensions that reach out to other cells.

Neurons Basic Structure: Neurosoma

• Highly variable in shape.

• Neurosoma (soma or cell body).

  • Contains the nucleus = control center of the neuron.

  • Contain organelles as well.

    • Mitochondria

    • Lysosomes

    • Golgi complex

    • Rough endoplasmic reticulum

  • NO centrioles though.

  • Neurons do not undergo mitosis.

  • Neurons that die are not replaceable.

Neurons Basic Structure: Dendrites

• Thick arms arising from neurosoma and branching further.

• Look like branches on a leafless tree.

• “receiving end” of a neuron – receives input from neighboring neurons.

• Anywhere from 1 to thousands of dendrites.

• More dendrites = more information it can receive to incorporate into its response.

• Dendritic spines

  • Tiny protrusions form dendrites that increase connectivity between neurons.

  • Dynamic processes that change their density and structure in response to stimuli.

  • This “plasticity” is related to learning, memory, and cognitive function.

Neuron Basic Structure: Axon

• Axon hillock on one side of neurosoma gives rise to an axon.

• Output pathway for signals that it sends to other cells.

• “sending end”.

• Neurons have no more than one.

• Cylindrical structure – branches at ends into bulbs called axon terminals.

• Junction between the axon terminal and the cell it’s sending signal to = synapse.

Putting it into Perspective

• Neurosoma – 5-135micron in diameter

  • Scaled up: size of a tennis ball

• Dendrites –

  • Scaled up: fill a 30-person classroom

• Axon – 1-20micron in diameter up to 5ft long

  • Scaled up: mile-long garden hose

Stuctural and Functional Classes of Neurons

• Again, highly variable.

• Vary in shape depending on function and location.

• Can be classified both structurally and functionally.

• Structurally: based on the number of processes that arise from the neurosoma

  • Multipolar

  • Bipolar

  • Unipolar

• Functionally:

  • Sensory (afferent) neurons

  • Interneurons

  • Motor (efferent) neurons

Structural Classes: Multipolar

• One axon

• Multiple dendrites

• Most common type

Structural Classes: Bipolar

• One axon

• One dendrite

• Includes neurons associated with sense organs

  • Neurons in the nose for olfaction (smell)

  • Neurons in the retina for vision

  • Sensory neurons of the ear

Structural Classes: Unipolar

• Only one process leading away from the neurosoma

• Branches into a T

  • One arm of T receives input via dendrites

  • Other arm is the axon

• Dendrites receive signals from sources such as skin and joints

• Involved in senses of touch and pain

• Axon leads to the spinal cord

Functional Classes: Sensory (afferent) Neurons

• Specialized in detecting stimuli and sending information about it to the CNS

• Originate in structures such as the

  • Eyes

  • Ears

  • Skin

  • Joints

• Carry information about sound, light, touch, pain, etc to the CNS

• Afferent refers to a signal traveling toward the CNS

  • Think A for Arrival

• Usually unipolar or bipolar

Functional Classes: Interneurons

• Perform integrative functions

• Process, store, and retrieve information and “make decisions” about how the body should respond to stimuli

• Only found in the CNS

• Most abundant of all of the neurons

• Usually multipolar

Functional Classes: Motor (efferent) Neurons

• Specialized to carry outgoing signals from the CNS to the cells and organs that carry out commands

• Begin in CNS and extend to muscle fibers and gland cells

• Efferent refers to carrying signals away from the CNS

  • Think E for exit

• Usually multipolar

Neuroglia

• Outnumber neurons at least 10 to 1

• We know less about neuroglia than neurons

• Range of functions are still actively being researched

• Support neurons and play a central in healthy functioning of NS

• 4 kinds of glial cells in the CNS

  • Oligodendrocytes

  • Ependymal cells

  • Microglia

  • Astrocytes

• 2 kinds of glial cells in the PNS

  • Satellite cells

  • Schwann cells

Neuroglial Cells of the CNS

• Oligodendrocytes

  • Resemble and octopus – multiple arm-like processes

  • Arms surround nearby axons and form a layer of insulation called the myelin sheath

• Ependymal cells

  • Line the internal fluid-filled cavities of the brain and spinal cord

  • Produce cerebrospinal fluid (CSF)

  • Have cilia on their surface which help circulate CSF

• Microglia

  • Small phagocytic cells that wander the CNS and destroy pathogens, debris, or foreign matter

  • Become concentrated in areas damaged by infection, trauma or stroke

Neuroglial Cells of the CNS

• Astrocytes

  • Most abundant glial cells

  • Constitute over 90% of brain tissue in some areas

  • Many diverse roles

    • Form structural framework of nervous tissue

    • Extensions called perivascular feet contact blood vessels and maintain the blood-brain barrier (separates CNS from the rest of the body)

    • Supports energetic demands of neurons by adjusting local blood flow

    • Secrete growth factors that promote neuron growth and synapse formation

    • Maintain extracellular environment (ex: regulate K+ levels)

    • Convert glucose to lactate to supply energy to neurons

    • Form scar tissue in damaged regions of the CNS

Neuroglial Cells of the PNS

• Satellite cells

  • Surround cell bodies of peripheral neurons

  • Insulates them electrically and regulates their chemical environment

• Schwann cells

  • Wrap around axons of peripheral neurons

  • Forms a sleeve called the neurilemma

  • Deposits multiple layers of its membrane between the neurilemma and the axon

  • Forms the myelin sheath of peripheral nerve axons

  • Also play a critical role in repairing damaged axons

• Oligodendrocytes do not play a role in regenerating axons of neurons in CNS

  • Thus damaged neurons in CNS are lost forever – far less likely to become damaged through – enclosed in bone

Myelin Sheath

• Important in signal conduction by the axon of a neuron

• Formed by oligodendrocytes in CNS

  • Arms reach out and form myelin sheath of many neurons

• Formed by Schwann cells in PNS

  • Forms myelin sheath of only one neuron

• Sheath consists of layers of plasma membranes that encircle the axons

• Think of it like an insulating blanket – ensures rapid, efficient signal conduction

• Similar to the rubber or plastic that covers an electrical cord

Myelin Sheath

• Myelin sheath is segmented

• Gaps between segments are called nodes of Ranvier

• Myelin-covered segments called internodes

• Not all axons are myelinated

• Myelinated axons conduct signals much faster than unmyelinated axons

  • Unmyelinated ex: neurons that cause digestive secretions

    • Not urgent – so the speed of unmyelinated is good enough

  • Myelinated ex: sensory neurons that respond to pain

    • Need quick reaction to retract from the source of pain

Forms of Nervous Tissue

• 2 forms of nervous tissue

  • White matter

  • Gray matter

• Differ functionally and by location

White Matter

• Formed by bundles of nerve axons called tracts

• Travel up and down the spinal cord, between different regions of the brain, and between the brain and spinal cord

• Many of the axons here are myelinated – giving it a white glistening appearance

• Carry signals from place to place – no actual processing

• Forms outer surface tissue in the spinal cord

• Is the deepest tissue in the brain

Gray Matter

• Where neurosomas, dendrites, and synapses are located

• Relatively little myelin here, so the tissue looks more dull in color

• Hense gray matter

• The information processing part of the CNS

• Is the deepest tissue in the spinal cord

• Forms outer surface tissue in the brain

Nervous System Pt. 2

Outline

• Resting membrane potential (RMP)

• Maintaining RMP

• Nerve Signaling

• Local potential

• Action potential

• Conducting a nerve signal

  • Myelinated

  • Unmyelinated

• Transmitting a nerve signal

Resting Membrane Potential

• Electrical signals of nerves are created by movement of Na+ and K+.

• Think of a neuron like a battery – positive end and negative end

  • Positive outside of the cell

  • Negative inside

• Charge difference in unstimulated neuron is called the resting membrane potential (RMP)

Resting Membrance Potential

• Resting membrane potential is caused by differences in concentrations of ions inside and outside of the cell

• Outside of cell

  • Na+ ions 12x more concentrated outside of the membrane

• Inside of cell

  • K+ is 38x more concentrated inside of the cell

  • More negative anions (such as phosphates, proteins, and nucleic acids)

• The Interior of the cell is more negatively charged than the outside

• Unequal distribution of ions = cell membrane is polarized

Nerve Signaling

• Nerve signaling is based on ion channels in the plasma membrane

• 2 primary kinds

  • Ligand-gated channels

    • Opens or closes in response to a ligand binding to it

  • Voltage-gated channels

    • Opens and closes in response to changes in membrane potential

Local Potential

• Step 1: excited the neuron

  • Usually happens at dendrite

  • Usually via a ligand binding a ligand-gated channel

• The channel opens and allows Na+ to flow into the neuron

• Makes the inside of the neuron less negative

  • Normally RMP = -70mV

  • Na+ raises voltage closer to 0mV – this change is called depolarization

• Voltage change near the point of stimulation – local potential

• Change in membrane potential spreads – if strong enough will produce an action potential

  • A more dramatic shift in membrane voltage in the axon

Local Potential

• Local potential

  • 1. stimulation results in Na+ entering neuron – local potential spreads towards axon hillock

Action Potential: Reaching Threshold

• 1. local potential reaches axon hillock

• Lots of voltage-gated channels in this area for Na+ and K+

• If local potential reaches the minimum voltage called threshold, voltage-gated channels open and an action potential is produced

Action Potential: Depolarization

• 1. Na+ rushes into the neuron and causes a rapid rise in voltage

• Na+ starts to close around 0mV

• By the time they close, voltage has usually risen to about +35mV

Action Potential: Repolarization

• 1. Voltage-gated K+ channels open

• K+ flows out of neuron – inside of neuron become negative again

• This shift back to a negative membrane potential is called repolarization

• Slightly more K+ leaves than necessary – becomes slightly more negative than RMP – called hyperpolarization

Conducting a Nerve Signal

• Action potential in one area of the membrane causes a domino effect of action potentials down the axon – this is a nerve signal

• After action potential occurs, that area of the membrane cannot be stimulated for a short time

  • This is called a refractory period

• Thus a zone of refractory membrane trails behind the nerve signal

• Prevents nerve signal from reversing and going towards neurosoma

Conducting a Nerve Signal – Unmyelinated Neurons

• Unmyelinated axons have a high density of voltage-gated Na+ and K+ channels along the entire axon

• An action potential occurs at every point along the axon

• This is called continuous conduction

Conducting a Nerve Signal – Myelinated Neurons

• Very few voltage-gated channels in myelin-covered internodes

• Action potentials cannot occur here due to myelin and too few channels

• Nodes of Ranvier are highly concentrated in ion channels

• This is the only place in the myelinated axon that can generate an action potential

Conducting a Nerve Signal – Myelinated Neurons

• Energy transfer along internodes is faster than at nodes of Ranvier

• Appears as if the signal is jumping from node to node

• This is called saltatory conduction

• Nerve signals travel as fast as 120m/s in myelinated axons vs no more than 2m/s in unmyelinated ones

Transmitting a Nerve Signal to Next Cell

• What happens when a wave of action potentials reaches the axon terminal?

• This is where the chemical part of nervous communication comes in

• Cell nerve signal is coming from = Presynaptic neuron

• Cell to be stimulated = postsynaptic neuron

• The gap between the two = synaptic cleft

Transmitting a Nerve Signal to Next Cell

• The Axon terminal of the presynaptic neuron contains synaptic vesicles loaded with chemicals called neurotransmitters

• Arrival of nerve signal triggers synaptic vesicles to undergo exocytosis

• Releases neurotransmitters into the synaptic cleft

• Diffuse across the cleft and bind to receptors on the postsynaptic neuron

Transmitting a Nerve Signal to Next Cell

• Neurotransmitters are either broken down by enzymes or reabsorbed by presynaptic neuron

  • Ex) AChE

• More than 100 known neurotransmitters

  • Ex) ACh, norepinephrine, serotonin, dopamine, etc.

Parkinson’s Diseases

• Progressive loss of motor control beginning in a person’s 50-60’s

• Due to the degeneration of dopamine-releasing neurons

• Dopamine plays a roles in

  • Reward and motivation

  • Movement

  • Mental health

• Acts as an inhibitory neuron that prevents excessive activity – loss of these neurons causes involuntary muscle contractions