PHS 3507 - Nervous Tissue (Chapter 12)

Nervous System Overview

• The nervous system detects changes in the environment that affect the body

• It works together with the endocrine system to help the body respond to these changes

• The nervous system controls:

  • Behavior

  • Memories

  • Movement

• Nervous tissue is excitable, meaning it can respond to stimulation and produce electrical signals

  • These electrical signals are called nerve impulses (action potentials)

3 Steps of the Nervous System

• 1. Sensory function = Detect

  • Detects internal and external stimuli (changes in/outside the body)

• 2. Interpretation = Analyze

  • The nervous system interprets/analyzes the information as detected

• 3. Motor response = React

  • The nervous system causes the body to respond or react

Nervous System Overview

Sensory Neuron → Interneuron → Motor Neuron

• Interneuron connects the sensory with motor neuron

Nervous System Organization

• The nervous system contains over 100 billion neurons

• It has 10-50 times more support cells, neuroglia (gilal cells)

• These cells are organized into 2 main divisions:

  • Central Nervous System (CNS)

  • Peripheral Nervous System (PNS)

Neurons vs. Neuroglia

• Neurons = the “thinking” cells

  • They recieve, process, and transmit information

  • They do this by controlling the flow of electrical charge across their cell membrane

  • Basically, each neuron does a small version of what the whole nervous system does

• Neuroglia (glial cells) = support cells

  • They provide support and nutrition for the brain

  • They do not process/manipulate information like neurons

  • They maintain the internal environment so neurons can work properly

Divisions of the Nervous System

• Central Nervous System (CNS)

  • Consists of the brain and spinal cord

• Peripheral Nervous System (PNS)

  • Consists of all nervous tissue outside of the brain and spinal cord

    • Nerves

    • Ganglia

    • Enteric plexuses

      • Sensory receptors

Further Division of the Nervous System

• Most signals that tell muscles to contract and glands to secrete start in the CNS

• The PNS is further divided into 3 parts:

  • Somatic Nervous System (SNS)

  • Autonomic Nervous System (ANS)

  • Enteric Nervous System (ENS)

Somatic Nervous System (SNS)

• Somatic Sensory (Afferent) Neurons

  • Carry information from sensory receptors in the head, body wall, and limbs toward the CNS

• Somatic Motor (Efferent) Neurons

  • Carry nerve impulses away from the CNS into the skeletal muscles of the body

  • Controls voluntary movement

• Interneurons

  • Conduct nerve impulses between the afferent and efferent neurons within the CNS

Autonomic Nervous System (ANS)

• Sensory Neurons

  • Carry information from autonomic sensory receptors, mainly in the visceral organs like the stomach or lungs towards CNS

• Motor Neurons

  • Carry nerve impulses from the CNS to the smooth muscle, cardiac muscle (heart), and glands

  • These are under involuntary control

• The motor part of the ANS has 2 branches that usually have opposite actions:

  • Sympathetic division

  • Parasympathetic division

Enteric Nervous System (ENS)

• The ENS is called the “brain of the gut”

  • The ENS was once considered part of the ANS

• It involuntarily controls GI propulsion, acid secretion, and hormone secretion

• It contains over 100 million neurons

  • These neurons are located in enteric plexuses (networks of neurons) that extend through most of the GI tract

SNS, ANS, ENS

Ganglia

• Ganglia are small groups of neuron cell bodies located outside the brain and spinal cord

  • Closely associated with cranial & spinal nerves

• There are different types of ganglia depending on the neurons they contain:

  • Somatic ganglia → somatic neurons

  • Autonomic ganglia → autonomic neurons

  • Enteric ganglia → enteric neurons

Neurons and Neuroglia

• Neurons and neuroglia work together in different parts of the nervous system

• Neurons

  • Are the functional units of the nervous system → they do the actual receiving, processing, and sending of information

  • Form complex networks in the brain and spinal cord that allow the CNS to control different regions of the body

• Neuroglia (Glial Cells)

  • Are smaller than neurons, but greatly outnumber them

  • Act like the “glue” of the nervous sytem

  • They support and maintain neuronal networks, helping neurons work properly

Neurons

• Several different types of neurons, most have:

  • Cell body, axon, dendrites, and axon terminals

Neurons Overview

• Neurons gather information at dendrites and process it in the dendritic tree and cell body

• Then they transmit the information down their axon to the axon terminals

Dendrites

• Dendrites mean “little trees” because they look like small branches

• They are the receiving end of the neuron—they receive signals/information from other cells

• They are short, highly branched structures that conduct impulses toward the cell body

  • Lots of branches allow them to receive many signals

• Dendrites also contain organelles

Cell Body

• The cell body contains a nucleus surrounded by cytoplasm

• Like other cells, neurons contain organelles such as:

  • Lysosomes

  • Mitochondria

  • Golgi complexes

  • Rough ER (RER) → protein production

• In neurons, the rough ER is called the Nissl bodies

  • Nissl bodies give the neuron a striped “tiger appearance”

• Neurons have no mitotic apparatus

  • This means mature neurons generally cannot undergo mitosis (cell division)

Axons

• Axons carries nerve impulses away from the cell body towards:

  • Another neuron

  • An effector cell (muscle or gland)

• Three important areas near the beginning of the axon

  • Axon hillock: where the axon connects to the cell body

  • Initial segment: the beginning part of the axon

  • Trigger zone: the junction between the axon hillock and inital segment

Axon Terminals

• The axon and its branches (collaterals) eventually split into many small endings called axon terminals (telodendria)

• Telodendria can be highly branched, similar to dendrites

• They interact with the dendrites of the “downstream” neurons

  • Downstream neuron = the next neuron that will receive the information

• The tops of some axon terminals become enlarged into bulb-shaped structures called synaptic end bulbs

Synapse

• A synapse is the place where communication happens between:

  • Two neurons, or

  • A neuron and an effector cell

• Presynaptic Cell

  • The cell sending the signal

• Postsynaptic Cell

  • The cell receiving the signal

• Synaptic Cleft

  • The tiny gap between the presynaptic and postsynaptic cells

Synaptic Vesicles and Neurotransmitters

• Synaptic end bulbs and other swollen areas (varicosities) on the axon terminals of the presynaptic neuron contain many synaptic vesicles

  • Synaptic vesicles = tiny membrane-covered sacs that store neurotransmitters

    • Neurotransmitters = chemicals that help send signals to the postsynaptic cell

      • Many neurons can contain 2 or even 3 different types of neurotransmitters

      • Each neurotransmitter can have a different effect on the postsynaptic cell

Action Potentials at a Synapse

• An action potential (electrical signal) travels down the presynaptic neuron

  • But the action potential cannot physically cross the synaptic cleft because there is a gap between the cells

• So the neuron uses neurotransmitters (chemical messengers) to carry the message across the gap

  • The neurotransmitter binds to the postsynaptic cell

  • This can cause a new action potential to be generated in the postsynaptic cell

Axonal Transport

• Materials are made or recycled in the neuron cell body

  • Some of these materials are needed in the axon and axon terminals

• Axonal transport moves these materials between the cell body and axon terminals
  
  

• Neurons have 2 transport systems that move these materials

  • Slow axonal transport

    • Moves axoplasm (material inside the axon)

    • Moves in one direction only:

      • Cell body → axon terminals

  • Fast axonal transport

    • Moves material in both directions:

      • Cell body → axon terminals

      • Axon terminals → cell body

Axonal Transport Cont.

• Slow axonal transport

  • Supplies new axoplasm (cytoplasm in axons) to developing or regenerating axons and replenishes axoplasm in growing and mature axons

• Fast axonal transport

  • Anterograde (foward)

    • Cell body → axon terminals

    • Moves organelles and synaptic vesicles (new supplies)

  • Retrograde (backward)

    • Axon terminal → cell body

    • Move membrane vesicles and other cellular materials back to the cell body (old supplies)

      • Once back, these materials are degraded/broken down or recycled

Fast Retrograde Transport

• Substances that enter a neuron at the axon terminals can be carried back toward the cell body

  • This is called fast retrograde transport

• It can carry:

  • Helpful substances

    • Trophic chemicals (Nerve Growth Factor; NGF)

  • Harmful substances

    • Tetanus toxin

    • Rabies virus

    • Polio virus

• These harmful substances can use the neuron’s retrograde transport to travel toward the CNS

• A deep cut or puncture wound in the head or neck can be serious than a similar injury in the leg

  • There is a shorter distance to the brain, so a harmful substance traveling through neurons can reach the brain faster than a similar injury in the leg

Classifying Neurons

• Neurons can have many different sizes and shapes

  • Some neurons are extremely long → almost as long as a person is tall, extending toward the lowest part of the brain

• Dendrites branch differently depending on the type of neuron and where it is located in the nervous system

• Some neurons have:

  • Very short axons

  • No axons at all

• Neurons can be classified based on 2 things:

  • Structure → What the neuron looks like

  • Function → What the neuron does

Structural Classification

• Structural classification = classifying neurons based on how many processes extend from the cell body

  • Processes = the axons and dendrites coming off the cell body

Multipolar Neurons

• Multipolar neurons have:

  • Many dendrites

  • One axon

• They are found throughout the brain and spinal cord

• The vast majority of neurons in the human body are multipolar

Bipolar neurons

• Bipolar neurons have:

  • One dendrite

  • One axon

• They help carry information for the special senses:

  • Sight, Smell, Hearing, Balance

• Because of this bipolar neurons are found in:

  • Retina of the eye (sight)

  • Inner ear (hearing and balance)

  • Olfactory area (smell)

Unipolar (Pseudounipolar) Neurons

• Unipolar neurons have one process extending from the cell body

  • That one process splits into branches that function in receiving and carrying sensory information

• They are commonly sensory neurons

  • They carry information about:

    • Touch

    • Stretching

• This information comes from the extremities → arms, hands, legs, and feet

Functional Classification of Neurons

• Functional classification = classifying neurons based on what they do

  • It is based on:

    • Whether their effect is excitatory or inhibitory

    • Which direction the action potential (AP) travels compared with the CNS

• 1. Sensory (Afferent) Neurons

  • Carry action potential into the CNS through cranial or spinal nerves

  • Most sensory neurons are unipolar

• 2. Motor (Efferent) Neurons

  • Carry action potential away from the CNS

  • Carry signals to effectors in the periphery (muscles, glands) through cranial or spinal nerves

  • Most motor neurons are multipolar

Interneurons (Association Neurons)

• Interneurons are mainly located inside the CNS (brain and spinal cord)

• They are found between sensory and motor neurons

  • Their job is to integrate (process) sensory information coming from sensory neurons

  • After processing the information, they activate the appropriate motor neuron to produce a response

• Most interneurons are multipolar in structure

Neurogila

• Neuroglia (glial cells) are support cells of the nervous system

  • Unlike neurons, they do not generate or conduct nerve impulses

• They support neurons by:

  • Forming the Blood-Brain Barrier (BBB)

  • Forming the myelin sheath → nerve insulation

  • Making cerebrospinal fluid (CSF) → circulates around the brain and spinal cord

  • Participating in phagocytosis

Types of Neuroglia

• 4 types of neuroglia in the CNS

  • Astrocytes → support neurons in the CNS

    • Maintain the chemical environment (Ca²⁺ & K⁺)

  • Oligodendrocytes → produce myelin in CNS

  • Microglia → participate in phagocytosis

  • Ependymal cells → form and circulate CSF

• 2 types of neuroglia in the PNS

  • Satellite cell → support neurons in the PNS

    • Schwann cell —> produce myelin in PNS

Myelination

• Myelination = the process of forming a myelin sheath around an axon

  • Myelin sheath acts like insulation around the axon

  • Allows nerve impulses (action potentials) to travel faster

• Different cells make myelin depending on the location

  • CNS → Oligodendrocytes

  • PNS → Schwann cells

Nodes of Raniver

• Nodes of ranvier = small gaps between sections of the myelin sheath where the axon is exposed

• In the PNS, each Schwann cell wraps around one segment of axon between two nodes of ranvier

  • Each myelinated segment is about 1 mm long and can have up to 100 layers of myelin

• The amount of myelin increases from birth to maturity

  • More myelin allows nerve impulses to travel much faster

• Multiple Sclerosis involves the immune system attacking and destroying myelin, which interferes with normal nerve conduction

Neuron Regeneration

• Around birth, most neurons lose their ability to undergo mitosis

• This means if a neuron is injured, it usually cannot make daughter cells to replace itself

  • This is different from epithelial cells, which can divide and replace damaged cells

• Instead, damaged neurons may sometimes repair/regenerate damaged parts

• PNS

  • Some nerve regeneration can occur

  • Schwann cells play an important role in helping damaged nerve fibers regenerate

• CNS

  • Regeneration essentially doesn’t occur at all

  • Astrocytes tend to form scar tissue around the damaged area instead of helping the nerve regenerate

Neurolemma

• The neurolemma (sheath of Schwann) is the outer layer of a Schwann cell

  • It contains the Schwann’s cell’s nucleus and cytoplasm

  • It surrounds the myelin sheath

• When an axon in the PNS is injured:

  • The neurolemma helps the axon regenerate

  • It forms a regeneration tube

    • This tube guides and stimulates the damaged axon to regrow in the correct direction

Neuron Regeneration & Demylination

• For a neuron to regenerate after injury, it must:

  • It must be in the PNS

  • Its cell body must still be intact/alive

  • It must have functional Schwann cells with a neurolemma around the myelinated axon

    • The neurolemma helps guide axon regrowth

• Demyelination

  • Demyelination = loss or destruction of the myelin sheath around axons

  • It can result from:

    • Disease, radiation therapy, chemotherapy

  • Even one episode of demyelination can cause the affected nerves to deteriorate

Gray and White Matter

• White Matter

  • Made of mostly groups of myelinated axons from many neurons

  • It looks white because myelin contains a lot of lipid (fat)

• Gray Matter

  • Made mostly of neuron cell bodies and dendrites

  • Looks more gray because these areas lack large amount of myelin

Electrical Signals in Neurons

• Like muscle cells, neurons are electrically excitable

  • They can respond to stimulation by changing their electrical charge

• Neurons use 2 types of electrical signals:

  • Graded potentials

    • Used for short-distance communication

  • Action potential

    • Used for long-distance communication

Resting Membrane Potential (RMP)

• Neurons need a resting membrane potential (RMP) before they can produce electrical signals

• RMP = there is a difference in electrical charge across the neuron’s membrane

  • Similar to a battery having a difference in electrical charge between its two ends

• The RMP is created by:

  • Ion gradients → different amount of ions (like Na⁺ and K⁺) inside vs. outside the neuron

  • Ion channels → allow ions to move across the membrane

• Some ion channels open or close in response to specific stimuli

• The lipid bilayer does not allow ions to easily pass directly through it because it acts as an insulator

  • Therefore, ions such as Na⁺ and K⁺ must travel through ion channels to cross the membrane

Ion Channels & Negative Charge

• Ion channels are found in the plasma membrane of all cells

• They are especially important in neurons because neurons use ion movement to create electrical signals

  • Cells use a lot of energy to maintain a difference in electrical charge across their membrane

  • At rest, the inside of a neuron is more negative than the outside

Electrochemical Gradient

• When an ion channel opens, specific ions can move across the cell membrane according to their electrochemical gradient

  • Electrochemical gradient = 2 forces working together

• Chemical (concentration) gradient

  • Ions move from high concentration → low concentration

• Electrical gradient

  • Positive ions (cations) are attracted towards negative areas

  • Negative ions (anions) are attracted toward positive areas

    • Opposite charges attract

Types of Ion Channels

• Active (gated) channels open in response to a specific stimulus

  • Ligand-gated channels

    • Open when a chemical (ligand), such as a neurotransmitter, binds to them

    • Mainly found at synapses

  
  

  • Voltage-gated channels

    • Open/close when the electrical potential across the membrane changes

    • Mainly found along the axon

    • Important for action potentials

  
  

  • Mechanically-gated channels

    • Open in response to physical/mechanical deformation, such as pressure on a receptor

• Leakage Channels

  • Leakage channels are also gated, but not active

  • They open and close randomly which allow ions to “leak” across the membrane

Resting Membrane Potential (RMP)

• RMP is measured when the neuron is resting → it is not currently sending a nerve impulse

• At rest:

  • A small amount of negative charge builds up in cytosol along the inside of membrane

  • An equal amount of positive charge builds up in the extracellular fluid along the outside surface of the membrane

  • This difference in charge across the membrane creates the resting membrane potential

  • The rest of the cytosol is electrically neutral

Why the RMP is Negative

• At rest, the inside of the neuron is slightly negative compared with the outside

• This happens mainly because:

  • More K⁺ leaks out of the neuron than Na⁺ leaks in

  • This is because there are more K⁺ leakage channels than Na⁺ leakage channels

  • There are also large negatively charged proteins that stay trapped inside the cytosol

• Why do we need the Na⁺/K⁺ pump?

  • Na⁺ is constantly leaking in

  • K⁺ is constantly leaking out

  • If this continued without being corrected, the ion gradients would gradually disappear and the RMP would eventually be lost

• The Na⁺/K⁺ ATPase (sodium-potassium pump) fixes this:

  • Pumps Na⁺ back out as it leaks in

  • Pumps K⁺ back in as it leaks out

  • This maintains the ion gradients needed for the RMP

Resting Membrane Potential (RMP) Cont.

• A typical neuron has an RMP of about -70 mV

  • The negative sign means the inside of the neuron is more negative than the outside

Polarization & Graded Potentials

• A neuron with a resting membrane potential (RMP) is called polarized

  • Polarized = there is a difference in charge across the membrane

  • Inside is more negative than outside

• When polarized, the neuron is “primed” or ready to potentially produce an action potential

• But an action potential doesn’t happen immediately

  • 1. Graded potentials happen first

  • 2. They depolarize the neuron → make the inside less negative

  • 3. If the neuron reaches threshold, an action potential starts

• Where do graded potentials come from?

  • A stimulus opens:

    • Ligand-gated channels → opened by chemicals/neurotransmitters

    • Mechanically gated channels → opened by pressure/stretch

  • Ions flow through these channels → producing a small, localized electrical current

    • This is a graded potential

  • It spreads to nearby parts of the membrane for only a short distance

  • It gets weaker as it travels and dies out within a few millimeters

Depolarizing vs. Hyperpolarizing Graded Potentials

• Start at the neuron’s RMP, usually around -70 mV

• Depolarizing graded potential

  • Inside becomes less negative

  • Example: -70 mV → -60 mV

  • Moves the neuron closer to firing an action potential

• Hyperpolarizing graded potential

  • Inside becomes more negative

  • Example: -70 mV → -80 mV

  • Moves the neuron farther away from firing an action potential

Graded Potentials

• Graded potentials can have different names depending on:

  • The type of stimulus

  • Where in the neuron they occur

• They have variable amplitudes (strengths)

  • Some graded potentials are small

  • Others are larger

• Multiple graded potentials can interact:

  • Summate (add together) → make a larger graded potential

  • Cancel each other out → make a smaller graded potential

• Graded potentials occur mainly in the dendrites and cell body

  • They do not travel down the axon

Action Potential (AP)

• Unlike a graded potential, an action potential (AP) travels down the entire axon to the axon terminals

• During an action potential:

  • The membrane potential reverses → inside temporarily becomes positive

  • Then it returns to its resting membrane potential (RMP)

• Threshold

  • The stimulus must be strong enough to reach threshold

    • Threshold (liminal) stimulus reached → a full-strength action potential occurs and travels down the axon

    • Subthreshold (subliminal) stimulus → not strong enough → no action potential

Action Potential Phases

• An AP has two main phases:

  • Depolarizing phase

  • Repolarizing phase

Beginning of an Action Potential

• The neuron starts at its RMP of about -70 mV

• A depolarizing graded potential makes the inside less negative

  • If it reaches threshold (about -55 mV) → an action potential begins

• Then:

  • Voltage-gated Na⁺ channel open

  • Na⁺ rapidly rushes into the neuron

    • Since Na⁺ is positive, the inside becomes more and more positive

    • This is called the depolarization phase of the action potential

Depolarization → Repolarization

• Depolarization

  • Only about 20,000 Na⁺ ions enter each small area of the membrane

  • Even this small amount causes a large change in membrane potential

  • The membrane potential can reach about +30 mV

  • So the inside becomes positive

• Repolarization

  • K⁺ channels open

  • K⁺ rushes out of the neuron

  • Since K⁺ is leaving, the inside becomes more negative again

  • The membrane potential moves back toward the RMP of about -70 mV

After-Hyperpolarization

• During repolarization, voltage-gated K⁺ channels are open, so K⁺ moves out of the neuron

  • K⁺ flow out of the neuron, making the inside more negative

• Sometimes the K⁺ channels stay open longer than needed

  • Because they are still open, more K⁺ continues to leave

  • This makes the membrane potential more negative than the normal RMP, reaching about -90 mV

    • This phase is called after-hyperpolarization

  • When the voltage-gated K⁺ channels finally close, the membrane potential returns to the normal RMP of about -70 mV

All-or-None Principle

• If the neuron does not reach threshold → no action potential

• If a neuron reaches threshold → a full action potential occurs

  • A stronger stimulus does not make a bigger action potential

Absolute Refractory Period

• After an action potential starts, there is a short period called the absolute refractory period

  • During this time, the neuron cannot start another action potential, even if the stimulus is extremely strong

  • This happens because they voltage Na⁺ channels are activated and then inactivated

  • The inactivated Na⁺ channels must reset back to their resting state before another action potential can occur

Relative Refractory Period

• The relative refractory period happens after the absolute refractory period

• During this time, the neuron can produce another action potential, but it requires a stronger-than-normal stimulus

• This happens because:

  • Na⁺ channels have reset → so another AP is still possible

  • But voltage-gated K⁺ channels are still open

  • K⁺ is still leaving the neuron, the inside if more negative than normal

    • Therefore, it’s harder to reach threshold

Action Potential Propagation

• The action potential (AP) starts at the trigger zone near the axon hillock

  • It then propagates (travels) down the axon

  • The electrical current spreads forward and triggers the next part of the neuron

• There are 2 ways this happens:

  • Continuous conduction → unmyelinated axons

    • The AP is regenerated along each neighboring section of the axon

    • This is slower

  • Saltatory conduction → myelinated axons

    • Myelin covers most of the axon

    • The AP is regenerated mainly at the nodes of Raniver

    • It appears to jump from node to node

    • This is much faster

What Affects Action Potentials

• What affects how fast an action potential travels?

  • Axon diameter

  • Amount of myelin

  • Temperature

  • Nodes of Ranvier

• What tells the brain how strong a stimulus is?

  • Frequency code → how fast/often neurons fire

  • Recruitment → how many neurons fire

Fiber Types

• Axons are classified into A, B, and C fibers based on their size, speed, and myelination

• A Fibers

  • Largest and fastest → 130 m/sec

  • Myelinated

  • Carry and touch pressure sensations

  • Many motor neurons are also A fibers

• B Fibers

  • Medium size and speed → 15 m/sec

  • Myelinated

  • Include:

    • Visceral sensory neurons

    • Autonomic preganglionic neurons

• C Fibers

  • Smallest and slowest → about 2 m/sec

  • Unmyelinated

  • Include:

    • Sensory neurons

    • Autonomic motor neurons

Signal Transmission at a Synapse

• Signal transmission goes one way:

  • Presynaptic neuron (sender) → Postsynaptic neuron (receiver)

• Steps:

  • 1. Action potential reaches the synaptic end bulb of the presynaptic neuron

  • 2. Voltage-gated Ca²⁺ channels open

  • 3. Ca²⁺ enters the presynaptic neuron

  • 4. Ca²⁺ triggers the release of neurotransmitters

  • 5. Neurotransmitters travel across the synaptic cleft

  • 6. Neurotransmitters bind to ligand-gated receptors on the postsynaptic cell

  • 7. This produces graded potentials in the postsynaptic cell

Electrical → Chemical → Electrical

• As a chemical synapse, the signal changes forms:

• Presynaptic neuron: electrical → chemical

  • Action potential arrives = electrical signal

  • Causes neurotransmitter release = chemical signal

• Postsynaptic neuron: chemical → electrical

  • Neurotransmitter binds to receptors

  • This creates a postsynaptic potential (graded potential) = electrical signal

• Synaptic Delay

  • These steps take a tiny amount of time

  • This creates a synaptic delay of about 0.5 milliseconds

Excitatory vs. Inhibitory Neurotransmitters

• Both excitatory and inhibitory neurotransmitters effects occur in the CNS and PNS

  • Excitatory → makes the postsynaptic cell more likely to fire an action potential

  • Inhibitory → makes the postsynaptic cell less likely to fire an action potential

• Example: Acetylcholine

  • ACh is commonly released by neurons in the PNS and by some neurons in the CNS

    • At the neuromuscular junction (NMJ) → ACh is excitatory → helps skeletal muscles contract

      • At some other synapses → ACh can be inhibitory

Amino Acid Neurotransmitters

• Many amino acids can act as neurotransmitters in the brain

  • Glutamate → mainly excitatory

    • Released by nearly all excitatory neurons in the brain

    • Makes the next neuron more likely to fire

  • GABA → inhibitory

    • Used at about 1/3 of brain synapes

    • Makes the next neuron less likely to fire

  • Valium (diazepam) enhances GABA’s inhibitory effects

    • This decreases neuronal activity → causes sedation

Modifying Neurotransmitter Effects

• The effects of neurotransmitters can be changed at different steps:

  • Synthesis → the body can make more or less neurotransmitter

  • Release → neurotransmitter release can be increased or blocked

  • Removal → neurotransmitter can be removed faster or prevented from being removed

  • Receptors → receptors can be activated or block

• Agonist vs. Antagonist

  • Agonist → activates/stimulates a receptor

    • Increases or mimics the normal effect

  • Antagonist → blocks a receptor

    • Decreases/prevents the normal effect

EPSPs vs. IPSPs

• A neurotransmitter can cause 2 types of graded potentials in the postsynaptic neuron:

  • EPSP = Excitatory Postsynaptic Potential

    • Causes depolarization

    • Makes the inside less negative

    • Moves the neuron closer to threshold

    • Makes the neuro more likely to fire an action potential

    • Usually, one EPSP alone is not enough to reach a threshold

  • IPSP = Inhibitory Postsynaptic Potential

    • Causes hyperpolarization

    • Makes the inside more negative

    • Moves the neuron farther from threshold

    • Makes the neuron less likely to fire an action potential

Spatial vs. Temporal Summation

• Summation = graded potentials (EPSPs and IPSPs) added together

• Spatial summation = where

  • Postsynaptic potentials come from different locations/neurons around the same time

  • Their effects combine

• Temporal summation = when

  • Postsynaptic potentials arrive every close together in time

  • Usually, the same input fires repeatedly before the previous graded potential disappears

  • Their effects add together

• What happens after summation?

  • The neuron adds up all EPSPs and IPSPs:

    • ESPS → toward the threshold

    • IPSPs → away from threshold

  • Net effect determine whether action potential is started

Neurotransmitter Removal

• After a neurotransmitter sends its message, it must be removed from the synaptic cleft

  • If it stayed there, it would keep affecting the postsynaptic cell continuously

  • This could keep stimulating or inhibiting a neuron, muscle, or gland when it shouldn’t

• There are 3 ways neurotransmitters are removed:

  • Diffusion → neurotransmitters moves away from the synaptic cleft

  • Enzymatic degradation → an enzyme breaks it down

  • Reuptake → cells take the neurotransmitters back up

• Example: Acetylcholine (ACh)

  • ACh is released into the synaptic cleft

  • After it sends its signal, the enzyme acetylcholinesterase (AChE) breaks it down

How Neurons Process Information

• A neuron receives many ESPSs and IPSPs from other neurons

  • These signals are added together (summated)

  • Their net effect is evaluated at the trigger zone

Integration

• Integration = when a postsynaptic neuron combines all EPSPs and IPSPs it receives and decides how to respond

  • Reaches threshold → action potential fires

  • Doesn’t reach threshold → no action potential

  • This happens over and over between many neurons, especially interneurons in higher brain regions, including the thalamus and cerebral cortex

Neural Circuits

• A neuronal network = a group of interconnected neurons that communicate and process information together

  • One network can contain thousands or even millions of neurons

• There are different ways neurons can be connected, called neural circuits

  • Diverging

  • Converging

  • Reverberating

  • Parallel after-discharge

Neural Circuits

• Diverging circuit = few → many

  • A small number of neurons stimulate a much larger number of neurons

• Converging circuit = many → few

  • The opposite of diverging

  • Many neurons send information toward a smaller number of neurons

• Reverberating circuit = loop

  • Signals travel through a circuit and then feed back through it repeatedly

  • Used for repetitive/ongoing activites such as:

    • Breathing, coordinated muscle activity, waking up, short-term memory

• Parallel after-discharge = one → many paths → one

  • One presynaptic neuron stimulates multiple neurons/pathways

  • Those pathways eventually synapse with the same postsynaptic neuron

  • Used for precise activities, such as mathematical calculations