Chapter 11: Nervous System and Nervous Tissue Fundamentals

Functions of the Nervous System: Sensory Input

Monitors changes that occur inside and outside the body

• Collecting information and sending the information to different parts of the body

Functions of the Nervous System: Integration

Processing and interpretation of input information —> the nervous system “decides” what response to make

• The “thinking” part of the nervous system (mostly brain, but sometimes spinal cord)

Interprets what receptor means and then figures out what it needs to do to resolve the stimuli

Functions of the Nervous System: Motor Output

Response is carried out

• Carried out by effector organ

The Central Nervous System

Composed of the brain and spinal cord

• Responsible for interpreting sensory input and deciding motor output

Peripheral Nervous System

Composed of nerves that extend from the CNS to the rest of the body

• Everywhere from the top of the head to the feet

Allows information to be sent between the CNS and the rest of the body

• Regulates and maintains homeostasis

Neuroglia (Glial Cells): Astrocytes

Projections from membrane surface connect to and wrap around neurons, nerve endings, and surrounding blood capillaries

• “Star cells”, basically have arms that attach and wrap around neuron and capillaries

Provides nutrient supply for neuron cells

• Blood supply and nutrients go through astrocyte —> and then the astrocyte attaches to neuron to be delivered

Allows migration of young neurons

• Very important for brain development in utero

Clean up area surrounding neurons

• Leaked K+ ions, neurotransmitter

Neuroglia (Glial Cells): Microglial Cells

Contact nearby neuron cells to monitor neuron health

Migrate toward injured neurons and transform into macrophage and phagocytize the neuron

• Keeps neuron system intact, neurons that are damaged interrupt other neurons or stop neurons from functioning

Neuroglia (Glial Cells): Ependymal Cells

Most ependymal cells have cilia (moves substances and propels cell)

• Lines central cavities of CNS to circulate cerebrospinal fluid (CSF) within cavities

  • CSF very important for protecting brain structures (liquid cusion)

  • Cilia is used to make sure the fluid is being well circulated

Neuroglia (Glial Cells): Satellite Cells

Support and protect neuron cell in PNS, lines the neuron and make sure it is developed correctly and nutrient uptake

• Most associated with astrocytes

Neuroglia (Glial Cells): Oligodendrocytes (CNS) and Schwann Cells (PNS)

Wrap around nerve fibers in CNS and PNS

• Creates an insulating covering called a myelin sheath for neurons

Structure of a Neuron: Cell Body

Portion of cell containing the nucleus

• Synthesizes proteins and neurotransmitters and plasma membrane can receive information

• Most of these are found in the CNS and are protected by bone

• Clusters of these in CNS are called nuclei, in PNS are called ganglia

Structure of a Neuron: Dendrites

Main receptive region of neuron

• Provides increases surface area for incoming signals and convey incoming messages toward the cell body

• Can be multiple

Structure of a Neuron: Axon

Single, long “nerve fiber” extending from the cell body

The conducting region of the neuron

• Generates and transmits nerve impulses away from the cell body

Axon branches at the end to form terminal branches and axon terminals

• Neurotransmitters released at axon terminal to pass the impulse to the next neuron

Myelin Sheath

Protects and electrically insulates long and/or large nerve fibers to increase speed at which impulses are transmitted

• Speeds up the rate of transmission

• Found only on axon portion of the neuron

Myelination in the PNS (Schwann cells)

• Multiple Schwann cells on the axon form the myelin sheath

• Myelin Sheath Gaps

  • Region of axon that is “exposed” due to absence of Schwann cell covering

Classification of Neurons: Sensory (afferent) Neurons

Afferent neurons transmit signals from the body to the CNS

• Receptive endings of this neuron type can function as actual sensory structure, or as associated with large sensory receptors

Classification of Neurons: Motor (efferent) neuron

These neurons transmit motor response from CNS to the body

• Impulses travel to effector organs (muscle or glands)

Usually muscle tissue or glands (controls activity to make appropriate change in sensory variable)

Classification of Neurons: Interneuron

Found in between sensory and motor neurons

• Pass signals through CNS pathways where integration occurs

• Can connect to other interneurons —> allows communication

  • Helps brain make the right decision in motor output

Gated Ion Channels

Have a gate that must be opened before ions can move

Chemically Gated

• Only open when a certain chemical (neurotransmitter) binds to protein

Voltage Gated

• Open and close in response to changing membrane potentials

Mechanically Gated

• Open in response to physical deformation of receptor

• Sensory input

Depolarization

When the inside of the cell becomes more positively charged compared to the resting membrane potential

• Sodium (+) ions will move to the ICF and then overpower the (-) charged proteins, less of a difference in charge, less potential

Excitation of a neuron

• Makes the neuron more likely to send a message

Hyper-Polarization

When the inside of the cell becomes more negatively charged compared to the resting membrane potential

• It makes the ICF super negative, there is greater difference in charge, so there is more potential

Inhibits a neuron

• Less likely to send a neuron (helps silence a neuron)

Graded Potentials

Graded—> magnitude varies directly with stimulus strength

• Strong stimulus = strong grades potential

Graded potentials only occur over short distances

• Current dies off quickly

Can be depolarizing or hyper-polarizing

These are necessary to initiate a nerve impulse (along or near the dendrites, generally it is a message that is being received by neuron)

Action Potentials (Nerve Impulses)

Can only be produced by neurons and muscle cells

All action potentials have a consistent strength

• Do not decay with distance

Action potentials occur long distances and are only depolarizing 

• Membrane potential changes from -70mV to +30mV every single time

Generating Action Potentials

An action potential is the ACTUAL message that the nervous system is receiving

Generating an AP involves changing the permeability of the plasma membrane to ions via the opening of voltage-gated ion channels in membrane in response to changing membrane potentials

Activation Gate

• Voltage sensitive opens at depolarization

Inactivation Gate

• Blocks channel to prevent Na+ movement

Generating Action Potentials: Resting Membrane Potential (1)

Neuron is not active

• All voltage gated channels are closed at the resting rate (-70mV)

K+ leakage channels are still open

Generating Action Potentials: Depolarization (2)

Voltage gated Na+ channels open at the trigger point of the axon

Membrane will reach a threshold voltage (-55mV) as more Na+ enters the cells

• At this voltage, action potentials becomes self-generating

Effect

• Na+ freely enters the cell and membrane potential changes -70—>+30-mV

• Inside of the cell becomes less negative

• This is the actual message

Generating Action Potentials: Repolarization (3)

This is when the ax of Na+ action potential *ends*

At +30mV

• Voltage gated Na+ gate close

• Na+ permeability drops rapidly

  • Net influx of Na+ into cell stops completely

• Voltage gated K+ channels open

  • K+ leaves the cell —> restores (-) internal charge of cell

Generating Action Potentials: Hyper-polarization (4)

Excess K+ leaves cell

• Result —> inside the cell becomes more negative than resting membrane potential

• Na+ pumps works to re establish normal Na+ and K+ concentrations outside and inside the cell

  • 3 Na+ out, 2 K+ in

Action Potentials: Stimulus Strength

APs are independent of stimulus strength

Strong Stimuli

• Impulses are sent too many messages about 1 specific thing

Weak Stimuli

• Lesser number of action potentials about 1 specific thing

Action Potentials: Absolute Refractory Period

Begins with Na+ gated channels open, continues until Na+ channels rest to their original state

During this time, another AP cannot be generated in the area, no matter how strong the stimulus is

Importance

• Ensure each AP is a separate, all or non event

• Enforces one way transmission of the AP

Action Potentials: Relative Refractory Period

Occurs immediately after the absolute refractory period

Stimuli that are relatively weak cannot generate an AP, but an exceptionally strong stimulus can generate an AP

• This is b/c since the relative refractory period the mV is much lower than the resting membrane potential

Conduction Speed

The faster the speed —> the quicker it gets to the brain/spinal chord

Axon Diameter

• Larger axon = faster conduction

Degree of myelination

• More myelination = faster conduction

Types of conduction: Continuous conduction

Propagation in un-myelinated fibers

• Voltage gated ion channels are adjacent for the entire length of the axon

Stop/Go type conduction

Types of Conduction: Saltatory Conduction

Propagation in myelinated fibers

Voltage gated ion channels are only in myelin sheath gaps

• AP generated in myelin sheath gap

Traveling longer distances of the axon which makes the conduction faster

Action Potentials: Transmission of Signals

Signals are transmitted between neurons at a synapse

• Synapse are a junction between two neurons that send information from neuron to the next

Neurons are separated by synaptic cleft —> fluid filled spc

Transmission of Signals: Presynaptic vs Postsynaptic Neurons

Presynaptic Neurons

• Conduct impulses toward the synapse

• The neuron that is sending

Postsynaptic Neuron

• Conduct signal away from the synapse

• The neuron that is receiving the message

Process in Transmission of Action Potentials

1. Action potential arrives at axon terminal of presynaptic neuron

2. Voltage gated Ca²⁺ channels in terminal open

   1. Ca2+ enters the axon terminal of presynaptic neuron

3. Synaptic vesicles in axon terminal fuse with membrane in response to Ca2+ influx

   1. Neurotransmitter enters the synaptic cleft

4. Neurotransmitter crosses cleft

5. Neurotransmitter (chemical receptor) binds to receptors on the postsynaptic neuron membrane

   1. binding cause ion channels to open

      1. Ion flow generates a graded potential (hyper/depolarizing)

6. Neurotransmitter in synaptic cleft is disposed of

   1. Re-uptake of neurotransmitters by an astrocyte or by the pre-synaptic neuron

   2. Degradation of neurotransmitter by an enzyme

   3. Diffusion of neurotransmitter of of the synapse

Postsynaptic Potentials

A graded potential created at the postsynaptic neuron after binding to neurotransmitter

Neurotransmitter binding cause graded potentials that vary in strength according to:

• Amount of neurotransmitter released

• How long does the neurotransmitter stays in synaptic cleft

Effect of neurotransmitter can either be excitatory or inhibitory

Excitatory Postsynaptic Potential (EPSP)

Binding of neurotransmitter causes the membrane to depolarize

But

• A single EPSP cannot induce an AP

• Several EPSP will be added together to generate an AP

Inhibitory Postsynaptic Potential (IPSP)

Binding of neurotransmitter causes the membrane to hyperpolarize

• K+ channels or Cl- channels open, making inside of cell more negative

Makes the chances of generating an AP more difficult, which helps prevent neurons from being overstimulated

Neurotransmitters and their Effects: Acetylcholine

Released at neuromuscular junctions to stimulate muscle contraction and is released by many neurons in the autonomic nervous system

• Always stimulatory

Neurotransmitters and their Effects: Norepinephrine and Epinephrine

Stimulates fight or flight response by autonomic nervous system

Neurotransmitters and their Effects: Dopamine

Learning (via award), motor control (prevents excessive or nonfunctional movements, Parkinson’s have decreases amount of these neurons), cognition

Neurotransmitters and their Effects: Endorphins

Natural opiates —> pain relief and mood booster

Neurotransmitters and their Effects: Gamma-Aminobutyric Acid

Reduces neuron activity by causing hyperpolarization of neurons