Physiology 2130 Unit 2: Excitable Cells and Synaptic Transmission

excitable cell

cell use its resting membrane potential (RMP) to generate action poteitnal

action potential = electrochemical impulse

excitable cell VS non-excitable cell

• non-excitable cell = X generate action potentials

• EX → excitable cells = neurons, muscles, some endocrine

• EX → rest of body non excitable

What is an action potential?

• rapid electrical signal generated when an excitable cell depolarizes beyond threshold

• all or nothing response

How do excitable cells communicate through action potentials?

• communicate by generating and propagating action potentials along the neuron

• occur w depolarization events in cell if enough depolarization occurs & excitable cells fire to comm. w adjacent cells

What is depolarization?

• process by which ions move in and out of the cell

• GOAL = inside of the cell INC positive relative to the resting membrane potential

• occur cell constant but only when beyond -55mV, AP trigger

what is ion movement is controlled by?

• controlled by membrane proteins such as channels and pumps on membrane

• chemically/ligand gated & voltage gated channels help movement of ions in & out of cell

What are the 6 main components of an action potential?

1. stimulus & RMP

   • trigger VV depolarization events in excitable cell → inside cell INC +

   • RMP ~ −70 mV

   • K⁺ leak channels leaky & X fully closed at rest

   • voltage gated Na channel & chem gated K channel closed

2. Threshold

   • stimulus reaches approximately −55 mV

   • AP triggered

3. Depolarization

   • Voltage-gated Na⁺ channels open

     • Na⁺ enters cell → inside INC (+) → INC K move out of cell to counteract

     • membrane potential -70mV to -30mV

   • leaky K channel open

   • chem gated K channel closed → need chem bind to open

4. Repolarization

   • Voltage-gated K⁺ channels open

   • K⁺ leaves cell

   • voltage gated Na⁺ channels close

5. Hyperpolarization

   • cell INC (-) than RMP = DEC chance AP occur

   • Relative refractory period occurs → harder to do AP

   • chem gated K+ channel open → INC K+ leave cell

   • voltage gated Na+ channel closed → bc open voltage goated K+ channel

   • all channels move K+ out of cell

6. Return to resting membrane potential & resting state

example of Na+ & K+ channel AP

1. local potential depolarizes trigger zone’s axolemma to threshold of -55mV

2. voltage gated Na+ channel active → Na+ enters, axon section depolarizes

3. Na+ channel inactive & voltage gated K+ channel actives

   1. Na+ X enter

   2. K+ excite axon → repolarize

4. Na+ channel returns resting state, repolarize cont.

5. axolemma can hyperpolarize before K+ channel becomes resting state

   1. then return RMP

During hyperpolarization, channels are more selectively permeable to K than leak channels. True / False

True

What happens to the channels during the resting potential and what are they more permeable to

The leak channels open at rest, 20-25 times more permeable to K+ than to N+

What happens to the channels during the depolarization

The channels are selectively permeable to N+ than to K+

Failed initiations

The depolarization events below threshold

Why are voltage-gated Na+ and K+ channels called "voltage-gated"?

b/c it is a change in voltage that triggers their opening

what forms the absolute refractory period of the AP?

• Depolarization and repolarization phases

  • Na channel inactive & X reopen until membrane repolarized enough

• During this time, no AP can be elicited → ensure 1 direction AP travel

An AP can be generated during the relative refractory period T/F?

T, but a larger intensity stimulus would be required to produce an AP because the membrane is hyperpolarized

• refractory period = cell is more negative, reaching approximately -90 mV

• now more difficult to reach the threshold of -55M = INC stimulus needed to reach the threshold bc of how (-) cell is

Because of the absolute refractory period during which time the Na+ voltage-gated channels are closed, two APs cannot be fired one on top of the other. True/ False?

True

how do the channels change during a AP?

• Na+ channels active = INC in membrane potential & start of AP

• K+ channels help membrane repolarize

Why does the closing of the potassium channels cause the inside of the membrane to become more positive?

• closing of potassium channels slows the outward flow of K⁺

• cause the inside of the membrane to become less negative (or slightly more positive) before it fully returns to the resting potential

neurons

• excitable cells

• comm. w AP

structure

1. Soma (cell body)

2. Dendrites

3. Axon

4. Axon terminals

5. Myelin sheath

6. Schwann cells

7. Nodes of Ranvier

Soma

(cell body)

has nucleus & most organelles

Dendrites

• branch-like projections from soma

• get signals & info from other neurons → soma

• direct AP → soma

Axon

projections of cell body

AP AWAY from soma

Axon terminals

ends of axon

Release neurotransmitters to communicate with next cell

Myelin sheath

Fatty acid & protein insulating layer surrounding the axon

speeds signal transmission

Schwann cells

Produce myelin and support neuron survival in the PNS

cell surround axon

Nodes of Ranvier

-Gaps in the myelin sheath rich in ion channels that aid rapid AP propagation

-Unmyelinated axon membrane

What is the direction in which an action potential propagates?

Dendrites → Soma → Axon → Axon terminals

DSAAT

What is saltatory conduction and why is it advantageous?

Saltatory conduction occurs in myelinated neurons, where the action potential jumps from one Node of Ranvier to the next instead of traveling continuously along every section of membrane.

Advantages of saltatory conduction

-Increases transmission speed by 10-15 times compared with unmyelinated neurons

-Improves efficiency

-Allows rapid communication over long distances

-Conserves energy because fewer ions cross the membrane

What is the all-or-nothing principle of an action potential?

• if membrane depolarization reaches threshold (~−55 mV) → an action potential occurs

• if threshold is not reached → no action potential occurs

• action potentials always same amplitude

What determines the direction of the propagation of an action potential?

The direction is determined by the refractory periods, especially the absolute refractory period.

What happens during the absolute refractory period?

• Voltage-gated Na⁺ channels become inactive

• Another action potential cannot immediately occur in the area that just fired

• Because the membrane behind the AP cannot fire again immediately, the signal moves forward only, preventing backward propagation.

Another AP cannot be elicited while the previous one is in the absolute refractory period. Why?

Because the ion channels are inactive during this time.

The relative refractory period (or hyperpolarization phase) makes the membrane more negative relative to the resting potential. T/F?

T, As a consequence, it is harder to reach threshold. The depolarization of the membrane will ONLY move in one direction

The AP only travels in one direction due to the absolute refractory period in only myelinated neurons. T/F?

False, both myelinated and unmyelinated neurons.

propagation of AP

• propagate 1 direction in neuron

• when neurotransmitter released from presynaptic neuron → bine to ion channel in post synaptic cell & depolarize

What are glial cells?

Glial cells (neuroglia) are support cells of the nervous system that provide the environment necessary for neurons to function properly.

. Glial cells make up approximately 90% of the brain.

Unlike neurons, they do not primarily transmit electrical signals

T

Glial roles include:

Support

Protection

Nutrient delivery

Insulation (myelin production)

Maintenance of neuronal environment

What are different types of neurons present in the brain?

Bipolar neurons

Unipolar neurons

Multipolar neurons

Bipolar neurons

-One axon and one dendrite

-Found mainly in specialized sensory structures such as the retina

Unipolar neurons

-Single process extending from the cell body

-Primarily sensory neurons in the peripheral nervous system

Multipolar neurons

-One axon with many dendrites

-Most common neuron type in the central nervous system

-They connect the CNS with the effector organs.

What are 6 examples of glial cells?

Astrocytes

Schwann cells

Oligodendrocytes

Ependymal cells

Microglia

Satellite cells

Astrocytes (Astrocytes are the most abundant cells in the brain.)

-Physical and nutritional support

-Transport nutrients

-Hold neurons in place

-Remove debris

-Digest dead neurons

-Regulate extracellular environment

-Promote synaptic connections

-Participate in injury response

Schwann cells

-Produce myelin in the PNS

-Support neuron survival

-Aid nerve regeneration

Oligodendrocytes

Produce myelin in CNS

One cell can myelinate several axons

Ependymal cells

-Produce and circulate cerebrospinal fluid (CSF)

-Regulate ion and glucose movement

-Help distribute hormones and signal molecules associated with the CNS.

Microglia

-Immune defence cells

-Remove damaged tissue and pathogens

Satellite cells

-Support neurons in the PNS

-Provide nutrients and structural support

How do different pathologies impact the nervous system?

-One major pathology discussed is Multiple Sclerosis (MS).

-MS is an autoimmune disease in which the immune system attacks the myelin sheath surrounding neurons.

Multiple Sclerosis (MS) effects

Myelin damage slows or blocks action potential transmission

Communication between neurons becomes impaired

Muscles may fail to receive signals

Can lead to weakness or paralysis

The CNS is made of the brain and spinal cord, while

the PNS is made of the nerves that go from the CNS to muscles and organs, like the heart, liver and stomach.

The PNS can be divided into the somatomotor (going to skeletal muscles to power voluntary movement) and

and autonomic (going to other organs that are automatically controlled by the brain and are not under voluntary control) nervous systems.

In fact, glial cells make up 90% of the brain. Their role is

to provide the necessaary environment for the neurons to function properly.

Contrast and compare the central and peripheral nervous systems

Central Nervous System (CNS)

-Consists of:Brain /Spinal cord

Main function:

-Integrates and processes information

-Coordinates responses and body functions

Peripheral Nervous System (PNS)

-Consists of nerves connecting CNS to the rest of the body

-Carries signals between organs and the CNS

Peripheral Nervous System (PNS)

-Consists of nerves connecting the CNS to the rest of the body

-Carries signals between organs and the CNS

Divided into:

Somatomotor system

-Controls voluntary movement of skeletal muscle

-Autonomic nervous system

Controls involuntary organs such as the heart and digestive system

Compare the central and peripheral nervous system

Comparison: Both systems communicate through neurons and action potentials, but the CNS mainly processes information while the PNS transmits it.

Somatomotor system

Autonomic nervous system

-Controls voluntary movement of skeletal muscle

-Controls involuntary organs such as the heart and digestive system

Frontal lobe

Primary motor cortex-

Premotor cortex

Prefrontal cortex

Skeletal muscle movement and planning

Temporal lobe

Hearing

Auditory processing

Smell

Short-term memory

Parietal lobe

Somatosensory information

Touch and sensory integration

Occipital lobe

Vision

Visual processing

Cerebellum

Coordination of movement

Balance

Sensory integration

As the structure with the largest number of neurons in the brain, the cerebellum receives input from somatic receptors, receptors for equilibrium, balance and motor neurons from the higher centers of the brain.

Brain stem

controls ,Heart rate,Respiration,Swallowing

Receives sensory input as it travels from the spinal cord, and integrates sensory information before sending it to the cortex

Corpus callosum

Connects right and left hemispheres

Integrates motor and sensory information

Thalamus

Processes sensory information before sending it to cortex

receives sensory input as it travels from the spinal cord, and integrates sensory information before sending it to the cortex

Hypothalamus

Regulates endocrine functions:

-Temperature

-Hunger

-Thirst

-Hormones

Midbrain

-Eye movement

-Visual and auditory reflexes

Pons

-Relay station between the cerebellum and the cerebral cortex

-Relay center between cerebellum and cortex

-Helps regulate breathing

the pons also coordinates and controls breathing.

Medulla

Breathing

Blood pressure

Swallowing

Controls involuontary functions such as breathing, swallowing and hear rate.

Diencephalon

This structure consists of two major areas: the thalamus and the hypothalamus.

Integrates sensory information through the thalamus and regulates endocrine function through the hypothalamus

What are the two main types of brain cells?

Neurons

-Specialized cells that process and transmit information through action potentials

Glial cells (neuroglia)

-Support and maintain neurons

-Make up approximately 90% of the brain

What are some pathologies that can impact cell-to-cell communication in the brain?

Multiple Sclerosis (MS)

-MS is an autoimmune disease in which the body's immune system attacks myelin sheaths surrounding axons.

Effects:

-Damaged myelin slows or blocks action potential conduction

-Reduced communication between neurons

-Muscle weakness

-Sensory problems

-Possible paralysis

Because myelin is required for fast saltatory conduction, damage significantly impairs communication throughout the nervous system. T/F?

T

Difference btn Left/ Right Hemisphere

The left hemisphere sends signals to activate muscles on the right side of the body, while sensory information from the right side of the body travels to the left hemisphere (and vice-versa).

dips or valleys on the brain/ bumps on the brain

sulci/ gyri)

They increase the surface area of the brain and are so prominent that they serve as landmarks to divide the cerebral hemispheres into lobes

The brain stem is made up of

midbrain, pons and medulla oblongata.

The medulla is continuous to the spinal cord. The brain stem incorporates nine cranial nerves.

The brain stem incorporates nine cranial nerves.

The pituitary gland

The pituitary gland primarily regulates other endocrine organs.

The anterior pituitary is derived from epithelial tissue of the pharynx, while the posterior pituitary derives from neural tissue of the hypothalamus.

The hormones secreted by the pituitary are involved in stress response, lactation, growth, development and reproduction.

The function of the pituitary is regulated by the hypothalamus.

Hormones

Hormones are chemicals that cells use to communicate with each other "long-distance", through the blood stream.

They send information related to growth, stress, development and homeostasis regulation from higher integration centers to effector organs, like the skin, muscles and many other tissues.

The premotor cortex (motor association area) works with the prefrontal cortex to integrate movement information with other sensory inputs to generate perception (or interpretation) of stimuli. T/F?

T