The Nervous System: Structure, function, and cellular components

What is the Central nervous system (CNS) composed of?

Composed of the brain and spinal cord

What protects the CNS?

Protected by skull and vertebral column

What is the CNS responsible for?

Processes and integrates all neural information

What is the CNS’s function?

Central command center for bodily functions

What is the Peripheral nervous system (PNS) composed of?

All nerves and neural components outside the CNS

What does the PNS connect?

Connects the CNS to the rest of the body

What does the PNS include?

Sensory and motor pathways

What is the PNS divided into?

Divided into somatic and autonomic divisisions

Define afferent neurons (it’s function, where they are located, and what they are also known as)

Information flows from the periphery to the CNS

Carry sensory information (touch, pain, temperature)

Cell bodies located in dorsal root ganglia

Also known as sensory neurons

Define efferent neurons (it’s function, where they are located, and what they are also known as)

Information flows from the CNS to the periphery

Carry motor commands to muscles and glands

Cell bodies located in ventral horn of spinal cord

Also called motor neurons pathways

Image to review

How neurons are in the human brain?

100 billion

Each neuron is connected to how many others?

Each neuron connects with approximately 1,000 other neurons

How are neurons organized?

Neurons are organized into circuits and networks that process specific information

What are the cellular components that form the foundation of all neural activity, enabling everything from basic reflexes to complex cognitive functions like memory, learning, and consciousness?

Scale, connectivity and organization

What does the soma (cell body) contain?

Contains the cell nucleus

What is the site of protein synthesis?

Soma (cell body)

What does the soma (Cell body) produce?

Produces neurotransmitters and hormones

What is the soma (cell body) also known as?

Metabolic center of the neuron

What are dendrites?

Branching extensions from the cell body

What does the dendrites receive?

Receives synaptic input (Synaptic input refers to the signals a neuron receives from other neurons at synapses)

What does the dendrites do?

Conduct signals toward soma and increase surface area for connections

Describe the location of the axon and where does it begin?

It is a long projection from the soma

It begins at the axon hillock

What does the axon do?

Conducts action potentials and make synaptic contacts

Image to review

How can neurons be classified?

By function, morphology, and neurotransmitters

What are some examples of neurons being classified by it’s function?

Sensory (afferent) neurons

Motor (efferent) neurons

Interneurons (local circuit)

What are some examples of neurons being classified by it’s morphology?

Morphology are it’s shape BTW

Multipolar neurons
Bipolar neurons
Pseudounipolar neurons

What are some examples of neurons being classified by it’s neurotransmitters?

Cholinergic neurons
GABAergic neurons
Dopaminergic neurons
Glutamatergic neurons

What is the most abundant type of neuron in the CNS?

Multipolar neuron

What does a multipolar neuron look like?

Multiple dendrites and single axon

Where is the multipolar neuron found and what are some examples?

They are found on brain and spinal cord

Examples: motor neurons, pyramidal cells

Where are the pseudounipolar neurons found?

Found primarily in spinal ganglia

What does a pseudounipolar neurons look like?

It is one process that divides into two, one going to the periphery and the other to the CNS

No dendrites. The axon serves both functions

What does the pseudounipolar neurons do?

Relay sensory information to CNS

Where are the bipolar neurons found?

Found in retina and olfactory epithelium

What does the bipolar neuron look like?

Single dendrite and single axon

What does the bipolar neuron do?

Specialized for sensory transduction (they are responsible for directly converting a stimulus into a neural signal)

Linear transmission of information (the signal flows in a straight way)

What are glial cells also called?

The supporting cast

How are neurons compared to glial cells?

While neurons are the primary signaling cells of the nervous system, glial cells outnumber neurons by approximately 10:1 and perform essential supportive functions

What do glial cells do?

Support and protect neurons

Provide the stem cell pool within the nervous system

Maintain homeostasis of the neural environment

Mediate immune responses to inflammation and injury

Participate in synaptic signaling and modulation

Guide neuronal development and migration

What are astroglia cells also called?

The neural caretakers

What are the types of astroglia cells?

Fibrous astrocytes - found in white matter

Protoplasmic astrocytes - found in gray matter

Müller cells - specialized astroglia in retina

What do astroglia cells do?

Provide physical and metabolic support to neurons

Regulate blood flow to active neural regions

Supply nutrients and remove waste products

What are the homeostatic functions for astroglia cells?

Maintain ion balance at synapses

Recycle neurotransmitters after synaptic transmission

Form essential component of blood-brain barrier

What do Myelinating Glial Cells form?

The Oligodendrocytes (CNS) and Schwann Cells (PNS) both form the myelin sheath

Define oligodendrocytes (where it’s found, function, and fun fact)

Found exclusively in central nervous system

Create myelin sheaths around CNS axons

Less capable of regeneration than Schwann cells

Define schwann cells (where it’s found, function, and fun fact)

Found exclusively in peripheral nervous system

Act as phagocytes to clear debris after injury

Regulate neurotransmitter levels at neuromuscular junction

What is the functional significance of myelin?

Myelin increases the speed of action potential conduction through saltatory conduction (when it jumps from one gap to the next) allowing for rapid neural communication across long distance

Demylenating conditions like multiple sclerosis highlight how important these glial cells are

What are microglia also called as?

The neural immune system

Microglia are the resident immune cells of the central nervous system

What do the microglia cells do?

Constant surveillance of neural environment

Phagocytosis (eliminating) of cellular debris and pathogens

Release inflammatory molecules (cytokines, chemokines)

Synaptic pruning during development (synaptic pruning= eliminating weak or unused synaptic connections between neurons)

Modulation of neuronal circuits (aka, changing the strength, timing, or overall activity of a neural circuit to alter its function or the information it processes)

What happens when microglial activity are not regulated?

It can cause microglia cells to become dysfunctional and contributing to the onset and worsening of neurodegenerative diseases like Alzheimer's, Parkinson's, and ALS

What are the two type of specialized glial cells?

Ependymal cells and polydendrocytes

What does the ependymal cells do?

Line the ventricles of the brain

Create barrier between CSF and neural tissue

Contribute to choroid plexus structure

Ciliated cells help circulate CSF

May retain neural stem cell properties

What does the polydendrocytes do?

Function as neural stem/progenitor cells

Generate both neurons and glial cells

Respond to neural injury

Why are ependymal cells and polydendrocytes important?

They perform unique functions in specific regions of the nervous system, highlighting the complexity and specialization of neural support systems

What are synapses also known as?

Communication junctions

Synapses are specialized junctions where neurons communicate with other cells. Information processing in the nervous system depends on these precise connections

Define axodendritic

The most common type of synapses

Axon terminals will contact with dendrites of another neuron

The dendritic tree will receive thousands of axodendritic synaptic inputs.

Thousands of these inputs allow for temporal and spatial summation. If enough signal adds up, the neuron will dire an action potential (temporal summation= signals coming close together in time pile up, spatial summation = signals coming from different spots on the dendrites combine)

Define axosomatic

Axon terminals contact directly on cell soma.

Less common but powerful, especially near axon hillock, where action potentials originate

Define axoaxonic

Axon terminals contact another axon.

These can powerfully modulate neurotransmitter release at the terminal, providing presynaptic control

Define blood-brain barrier (BBB)

The blood-brain barrier (BBB) is a highly selective semipermeable border that separates the circulating blood from the brain and extracellular fluid in the central nervous system

What are the key components to the blood-brain barrier?

Endothelial cells with tight junctions

Basement membrane

Astrocytic end-fee

What is the function of the blood-brain barrier?

Protects brain from pathogens and toxins

Regulates ion balance for optimal neural function

Controls selective transport of nutrients

Prevents most medications from reaching brain tissue

Define ions

Charged particles that create electrical potentials across cell
membranes

Define homeostasis

Homeostasis maintains stable internal conditions despite external changes.

Define membrane potential

Difference in electrical charge across a membrane. Resting potential is the stable voltage when a neuron is inactive

Define polarization

Normal state

Define depolarization

Membrane potential becomes LESS negative (moving closer to zero)

Action Potential more likely to occur

Define hyperpolarization

Membrane potential becomes MORE negative

Action potential less likely to occur

Define action potential

Electrical signal that travels along a neuron. Propagation is the movement of this signal along the axon

Define leak channels

Passive channels that allow specific ions to diffuse across the membrane continuously

Define mechanically-gated

Sensitive to physical forces like pressure, stretch, or vibration

Define ligand-gated

Open or close in response to specific neurotransmitters or chemical signals.

Define voltage-gated

Respond to changes in membrane potential; critical for action potential generation.

Define thermally-gated

Activated by temperature changes; important for sensory perception

What does membrane potential represent?

The membrane potential represents the electrical charge difference across a neuron's membrane. This potential changes during neural activity

Define resting potential

Homeostatic state

Maintained at approximately -70mV with the inside negative and outside positive.

ATP-powered ion pumps maintain this differential

Define local potential

Initial changes from sensory input or synaptic activity.

May be excitatory or inhibitory.

Define action potential and what takes place

The propagating wave of depolarization that transmits information along the axon.

Occurs when threshold is reached.

Depolarizing wave in consistent direction

Describe how action potential step-by-step

At it’s resting state, the inside of neuron is negative (-70mV)

-Sodium (+) is mostly on the outside and potassium (+) mostly inside

-The gates for sodium and potassium gates are closed

Stimulus occur, which means depolarization begins

-Stimulus opens up some Na+ channels

-Na+ will rush in, making inside less negative

-If it reaches threshold, action potential will start (-55mV)

Rising phase (depolarization)

-More Na+ channels open

-Huge influx of Na+ makes inside positive

Falling phase (repolarization)

-Na+ channels closes

-K+ channels opens, K+ rushes out, making the inside negative again

Undershoot (hyperpolarization)

-K+ channels stay open a bit too long

-Inside becomes extra negative

Return to resting potential

-K+ channels close

-Sodium-potasium pump restores normal balance (3 Na+ pumped out and 2 K+ pumped in)

What are the negatively charged ions?

Chlorine and anions

What are the positively charged ions?

Sodium, potassium, and calcium

Describe how action potential is an all or none response

Once threshold is reached, full action potential always occurs

Action potentials function as binary signals - they either occur completely or not at all. This "all-or-none" principle ensures reliable signal transmission throughout the nervous system.

Define action potential

Action potentials are electrical impulses that travel along the neuron's membrane. They represent the fundamental mechanism of information transmission in the nervous system

When are some key components to action potential?

Changes in membrane permeability for different ions drive the
action potential

Na+ influx initiates depolarization

K+ efflux drives repolarization

Enables communication between neurons

What happens if there’s a sodium influx in action potential?

Influx means moving into the cell

Na+ channels open in response to reaching threshold potential.
Sodium ions rush into the cell, causing depolarization as the
membrane potential becomes more positive.

What happens if there’s a potassium efflux in action potential?

Efflux means moving out of the cells

Na+ channels inactivate quickly. K+ channels open more slowly,
allowing potassium to flow out of the cell, causing repolarization as
the membrane potential returns to negative.

What is restoration in action potential?

Na+/K+ ATPase pumps work to restore original ion concentrations,
moving Na+ out and K+ in, reestablishing the resting membrane
potential.

What are the phases of action potential?

A. Resting Membrane Potential

B. Rapid depolarization due to influx of Na+ channels

C. Rapid depolarization due to efflux of K+ channels

D. RMP restored-diffusion of ions

Define the all of none principle in action potential

If threshold is reached – Cell will fire

If depolarization reaches threshold (typically -55mV), a full
action potential is generated

If threshold is not reached, no action potential occurs

The magnitude of an action potential is constant regardless of
stimulus strength

Stronger stimuli do not produce larger action potentials - they
may produce more frequent ones

This binary nature ensures reliable signal transmission throughout the nervous system and prevents signal degradation over long distances.

Define absolute refractory period

During this phase, the neuron cannot generate another action potential regardless of stimulus strength.

No response

Typically lasts 1-2 milliseconds

Ensures unidirectional propagation (ensures that an action potential travels only one direction down the axon)

Define relative refractory period

During this phase, a stronger-than- normal stimulus may trigger another
action potential

Decreased response

Requires higher threshold for firing

Why is the refractory period important?

The refractory period is critical for proper neural function. It limits the maximum frequency of action potentials, controls information transmission rates, and prevents backflow of the signal

What are the types of neural conduction?

Unmyelinated and myelinated axons

Define unmyelinated axons in regards to action potential

In unmyelinated axons, action potentials propagate continuously along the entire membrane:

Na+ channels distributed throughout axon length

Signal travels as continuous wave

Slower conduction velocity

Common in autonomic nervous system

Define myelinated axons in regards to action potential

In myelinated axons, action potentials "jump" between nodes of Ranvier:

Na+ channels concentrated at nodes

Signal "jumps" between nodes (saltatory conduction)

Much faster conduction velocity

More energy efficient

The more myelin and larger the axon diameter, the faster the signal transmission

What are factors affecting signal speed and integration

Axon diameter, myelination, temporal summation, and spatial summation

How does axon diameter affect signal speed and integration

Larger diameter axons conduct signals faster due to decreased internal
resistance. This is why giant squid axons (up to 1mm diameter) were
crucial for early neurophysiology research

How does myelination affect signal speed and integration

Myelin sheaths created by Oligodendrocytes (CNS) or Schwann cells
(PNS) insulate axons and enable saltatory conduction, increasing
transmission speed up to 100 times

How does temporal summation affect signal speed and integration?

Integration of multiple signals arriving in rapid succession at a single
location. If multiple small signals arrive within milliseconds, their effects
add up