Neurons

Neuron Structure 🧠

A neuron is a type of cell that has all the normal cellular components, including the soma (cell body) and various extensions.

Soma (Cell Body)

The soma is the central part of the neuron where the cell's genetic material is located. It is responsible for maintaining the cell's basic functions.

Dendrites and Axons

• Dendrites: extensions that receive signals from other neurons and transmit them to the soma

• Axons: extensions that carry signals away from the soma to other neurons, muscles, or glands

The direction of signal transmission is the key difference between dendrites and axons:

Types of Neurons

Myelin Sheath 🧵

The myelin sheath is a fatty, insulating layer that surrounds the axon, allowing electrical impulses to transmit more quickly.

Myelin Structure

The myelin sheath is created by specialized cells:

• Oligodendrocytes in the central nervous system (CNS)

• Schwann cells in the peripheral nervous system (PNS)

The myelin sheath does not cover the entire axon, leaving gaps called nodes of Ranvier. These gaps allow the electrical impulse to "jump" from node to node, a process known as saltatory conduction.

Axon Structure 🌐

The axon is the extension of the neuron that carries signals away from the soma.

Axon Components

• Axon hillock: the initial segment of the axon, where the action potential is generated

• Axon terminus: the end of the axon, where the signal is transmitted to other neurons or cells

• Terminal buttons or terminal boutons: the small endings of the axon terminus that make connections with other cells

Resting and Action Potentials ⚡️

The resting potential is the state of the neuron when it is not transmitting a signal.

The sodium-potassium pump (Na+/K+ ATPase) maintains the resting potential by pumping sodium ions out of the cell and potassium ions into the cell.

The action potential is the electrical impulse that travels down the axon, allowing the neuron to transmit a signal.

The resting membrane potential is the difference in electrical potential between the inside and outside of a cell, caused by the ion imbalance across the cell membrane. This potential is typically around -70 millivolts.

Ion Imbalance and Concentration Gradients

The sodium-potassium pump helps maintain this ion imbalance by pumping 3 sodium ions out of the cell and 2 potassium ions into the cell, resulting in a net loss of one positive ion.

The Sodium-Potassium Pump

The sodium-potassium pump is an ATP-dependent pump that helps maintain the ion imbalance across the cell membrane. It pumps 3 sodium ions out of the cell and 2 potassium ions into the cell, resulting in a net loss of one positive ion.

Leak Channels

Leak channels are channels that allow ions to slowly leak out of the cell, helping to maintain the ion imbalance. They are important for maintaining the resting membrane potential.

Voltage-Gated Channels

Voltage-gated channels are channels that only open at a certain membrane potential, called the threshold. They are important for generating action potentials.

Resting Membrane Potential and Voltage-Gated Channels

Definitions

Why Do Cells Maintain a Resting Membrane Potential?

Cells maintain a resting membrane potential to:

• Utilize diffusion and electrical potential for various cellular processes

• Establish a concentration gradient and electrical gradient that can be used for action potentials and other cellular processes

• Provide a store of energy that can be used to do work

Examples of Resting Membrane Potential in Other Cells

• Muscle cells: Calcium gradients help create contraction

• Neurons: Resting membrane potential helps generate action potentials## Action Potentials 🌊

Key Terms

• Depolarization:

• Repolarization:

• Rehyperpolarization:

• Equilibrium Potential:

The Action Potential Process

The action potential process can be broken down into several stages:

Ion Movement

• Sodium Ions: Move into the cell during depolarization, making the inside of the cell more positive.

• Potassium Ions: Move out of the cell during repolarization, making the inside of the cell more negative.

Voltage-Gated Channels

• Sodium Voltage-Gated Channels: Fast, opening quickly to allow sodium ions to flow into the cell.

• Potassium Voltage-Gated Channels: Slow, opening later to allow potassium ions to flow out of the cell.

All-or-None Event

• Action Potential: An all-or-none event, meaning that if the threshold potential is reached, an action potential will occur, and if not, it will not occur.

• Significance: Understanding the action potential process is crucial for understanding various physiological processes, including signaling, motor potentials, and the effects of certain medications.

Clinical Significance

• Epileptology: Understanding the action potential process is important for studying seizures and seizure medications, which often target sodium channels.

• Pathophysiology: Knowing the underlying physiology of the action potential process is essential for understanding how diseases and medications affect the body.## 🌟 Action Potential Propagation 🌟

How Action Potentials Spread

The action potential is a rapid change in the membrane potential of a neuron, caused by the movement of ions across the cell membrane. This change in potential triggers the opening of voltage-gated sodium channels, allowing sodium ions to rush into the cell.

As the sodium ions enter the cell, the membrane potential becomes more positive, triggering the opening of more voltage-gated sodium channels. This creates a wave of depolarization that spreads along the length of the neuron.

Refractory Periods

The refractory period is a critical component of action potential propagation. It is the time during which the neuron is unable to fire another action potential.

Why Action Potentials Move in One Direction

The action potential moves in one direction because of the refractory period. Once an action potential has occurred, the sodium channels are inactivated, preventing the action potential from traveling backwards.

Equilibrium Potential

The equilibrium potential is the point at which there is no driving force for an ion to move across the cell membrane.

Action Potential Graph

The action potential graph shows the changes in membrane potential over time.

• Depolarization: The rapid increase in membrane potential due to the influx of sodium ions.

• Repolarization: The return of the membrane potential to its resting state.

• Hyperpolarization: The period of time during which the membrane potential is more negative than the resting potential.

Key Points

• Action potentials are rapid changes in membrane potential caused by the movement of ions across the cell membrane.

• The refractory period is the time during which the neuron is unable to fire another action potential.

• The equilibrium potential is the point at which there is no driving force for an ion to move across the cell membrane.

• The action potential graph shows the changes in membrane potential over time.## Refractory Periods and Electrical Synapses 🌐

Refractory Periods

The refractory period is the time during which a neuron is unable to fire again after an action potential. There are two types of refractory periods:

• Absolute Refractory Period: During this period, the neuron is completely unable to fire again, regardless of the strength of the stimulus.

• Relative Refractory Period: During this period, the neuron can fire again, but only if the stimulus is strong enough.

Electrical Synapses 🔌

An electrical synapse is a type of synapse where the neurons are physically connected through gap junctions. This allows for the direct transfer of ions between the neurons.

Characteristics of Electrical Synapses

Gap Junctions 🔗

Gap junctions are specialized channels that connect the cytoplasm of two adjacent cells, allowing for the direct transfer of ions and small molecules.

Importance of Gap Junctions in Cardiac Muscle Cells

Gap junctions play a crucial role in cardiac muscle cells, allowing for the coordinated contraction of the heart muscle.

• Coordinated Contraction: Gap junctions allow for the simultaneous contraction of cardiac muscle cells, ensuring a coordinated and efficient pumping of the heart.

• Action Potential Wave: Gap junctions can also allow for the propagation of action potentials in opposite directions, which can lead to arrhythmias if not regulated properly.

Chemical Synapses 🧬

A chemical synapse is a type of synapse where the neurons are not physically connected, but instead communicate through the release of neurotransmitters.

Structure of a Chemical Synapse

• Presynaptic Neuron: The neuron that releases the neurotransmitter

• Postsynaptic Dendrite: The dendrite that receives the neurotransmitter

• Synaptic Cleft: The space between the presynaptic neuron and the postsynaptic dendrite

Neurotransmitters 🧬

Neurotransmitters are chemical messengers that are released by the presynaptic neuron and bind to receptors on the postsynaptic dendrite.

• Examples of Neurotransmitters: Serotonin, dopamine, epinephrine, norepinephrine, GABA, glutamate

• Receptors: Proteins on the postsynaptic dendrite that bind to the neurotransmitter

Release of Neurotransmitters 🌈

The release of neurotransmitters is a complex process that involves the following steps:

• Vesicle Formation: The presynaptic neuron forms vesicles that contain the neurotransmitter

• Vesicle Release: The vesicles are released into the synaptic cleft

• Binding to Receptors: The neurotransmitter binds to receptors on the postsynaptic dendrite

Regulation of Neurotransmitter Release 🔒

The release of neurotransmitters is regulated by a variety of mechanisms, including:

• Calcium Channels: Calcium ions play a crucial role in the release of neurotransmitters

• Synapsin: A protein that helps to anchor and protect the neurotransmitter vesicles

• Cytoskeleton Filaments: Filaments that help to regulate the movement of the neurotransmitter vesicles## 🌐 Voltage-Gated Calcium Channels

Voltage-gated calcium channels are a type of ion channel that allows calcium ions to flow into the cell. These channels are typically found at the end of the axon, near the synaptic cleft.

🌈 Ligand-Gated Ion Channels

Ligand-gated ion channels are a type of ion channel that opens in response to the binding of a specific molecule, such as a neurotransmitter. These channels can be found on the postsynaptic neuron and are responsible for transmitting signals from one neuron to another.

📝 How Ligand-Gated Ion Channels Work

Here's a step-by-step explanation of how ligand-gated ion channels work:

• A neurotransmitter, such as serotonin, is released from the presynaptic neuron and binds to a receptor on the postsynaptic neuron.

• The binding of the neurotransmitter causes a conformational change in the receptor, which opens the ligand-gated ion channel.

• The opening of the ion channel allows ions to flow into or out of the cell, depending on the type of ion channel.

• The flow of ions can either excite or inhibit the postsynaptic neuron, depending on the type of ion and the direction of flow.

📊 Types of Ligand-Gated Ion Channels

Here are some examples of different types of ligand-gated ion channels:

🤝 Regulation of Synaptic Transmission

Synaptic transmission is regulated by the type of neurotransmitter released, the type of receptor on the postsynaptic neuron, and the type of ion channel opened. This allows for a high degree of specificity and control over the transmission of signals between neurons.

📝 Key Points to Remember

• Neurons can only make one type of neurotransmitter, but they can respond to many different types of neurotransmitters.

• Dopamine is a type of neurotransmitter that is involved in reward pathways and motor control.

• Parkinson's disease is caused by the loss of dopaminergic neurons in the basal ganglia.

📊 Neurotransmitter and Ion Channel Table

Types of Neurotransmitters

• A neuron can respond to many types of neurotransmitters, but it can only produce one type.

• Neurotransmitters can be excitatory, inhibitory, or have other effects on the postsynaptic cell.

Neurotransmitter Recycling and Breakdown

• Neurotransmitters can be recycled or broken down in the synapse.

• Medicines can be used to change the amount of time a neurotransmitter spends in the synapse, adjusting the response.

Selective Serotonin Reuptake Inhibitors (SSRIs)

• SSRIs are a type of medication that prevents serotonin from being taken back up into the presynaptic neuron.

• This allows serotonin to act longer in the synapse, increasing its effect.

Monoamine Oxidase Inhibitors (MAOIs)

• MAOIs are a type of medication that blocks the enzyme that breaks down serotonin in the synapse.

• This allows serotonin to act longer in the synapse, increasing its effect.

📈 Postsynaptic Response

Dependence on Receptors

• A single neurotransmitter can have different effects on different cells, depending on the receptors present.

• For example, serotonin can bind to receptors that open sodium channels (excitatory) or calcium channels (inhibitory).

Threshold for Action Potential

• It takes more than one vesicle of a neurotransmitter to have a significant effect on a postsynaptic cell.

• This is to prevent accidental release of a single vesicle from triggering a big change in the postsynaptic cell.

🤝 Synaptic Neurotransmitter Disorders

📊 Excitatory and Inhibitory Postsynaptic Potentials (EPSPs and IPSPs)

• EPSPs and IPSPs are changes in the membrane potential of a postsynaptic cell in response to a single vesicle of a neurotransmitter.

• EPSPs are excitatory, causing the membrane potential to become more positive.

• IPSPs are inhibitory, causing the membrane potential to become more negative.

Summation of EPSPs and IPSPs

• Multiple EPSPs or IPSPs can summate to reach the threshold for an action potential.

• This allows for regulation of the postsynaptic cell's response to different stimuli.

📈 Action Potential and Calcium

• Calcium is released at the chemical synapse by calcium-gated ion channels to release neurotransmitters.

• This is a special case, as calcium is not part of the standard action potential.

🤝 Summation and Regulation

• Summation of EPSPs and IPSPs allows for regulation of the postsynaptic cell's response to different stimuli.

• This is an important mechanism for controlling the activity of neurons in the nervous system.## Spatial and Temporal Summation 📈

Spatial summation refers to the process by which multiple excitatory postsynaptic potentials (EPSPs) from different neurons are added together to generate a stronger signal. This can occur when multiple neurons are stimulated simultaneously, resulting in a stronger EPSP that can reach the threshold potential and trigger an action potential.

On the other hand, temporal summation occurs when a single neuron fires repeatedly, generating a series of EPSPs that can summate to reach the threshold potential. This can occur when a neuron is stimulated repeatedly, resulting in a stronger EPSP that can trigger an action potential.

Example: Heat on a Stove 🔥

Imagine putting your hand on a hot stove. The heat receptors in your skin send signals to your brain, which interprets the sensation as pain. The pain signal is transmitted through the nervous system, triggering a response to move your hand away from the stove. In this example, the heat receptors are like the excitatory neurons that fire repeatedly, generating a strong EPSP that reaches the threshold potential and triggers an action potential.

Functions of the Nervous System 🧠

The nervous system has three main functions:

• Sensory Input: receiving information from the environment through sensory receptors

• Integration: processing and interpreting the information in the central nervous system

• Motor Output: sending commands to the body to respond to the information

Types of Neurons 📚

Definitions 📝

Reflexes 🔄

Reflexes are rapid integrations that avoid potential injury. They involve a rapid response to a stimulus, often without conscious thought. Reflexes can be thought of as a "shortcut" through the nervous system, allowing for a quick response to a stimulus without the need for processing in the brain.

A reflex is a rapid integration to avoid potential injury. It is a complex process that involves the coordination of multiple neurons and muscles.

The Reflex Arc

The reflex arc is the pathway that a reflex signal takes from the sensory neuron to the motor neuron. It consists of the following components:

• Sensory Neuron: a neuron that detects a stimulus and sends a signal to the spinal cord

• Interneuron: a neuron that integrates the signal from the sensory neuron and sends a signal to the motor neuron

• Motor Neuron: a neuron that receives the signal from the interneuron and stimulates a muscle to contract

The Knee Jerk Reflex

The knee jerk reflex is a common reflex that occurs when the tendon below the kneecap is tapped. This causes the quadriceps muscle to contract and the hamstring muscle to relax, resulting in a sudden extension of the knee.

Medical Significance of Reflexes

Reflexes can be used to diagnose problems with the nervous system. For example:

• If a reflex is absent or diminished, it may indicate a problem with the peripheral nervous system

• If a reflex is exaggerated or prolonged, it may indicate a problem with the central nervous system

Brain Anatomy 🧠

The brain is a complex organ that consists of several distinct regions, each with its own unique functions.

Brain Regions

The Cerebellum

The cerebellum is a region of the brain that is responsible for coordinating movement. It is involved in complex motions such as walking, balance, and fine motor skills.

The Limbic System

The limbic system is a region of the brain that is involved in emotion and memory. It is responsible for processing sensory information and storing memories.

The Diencephalon 🤔

The diencephalon is a region of the brain that is involved in sensory processing and motor control. It consists of several distinct structures, including the thalamus.

The Thalamus

The thalamus is a structure in the diencephalon that is responsible for relaying sensory information to the cortex.

The diencephalon is a division of the brain that includes the thalamus, epithalamus, and hypothalamus.

Thalamus

The thalamus is a sensory relay station that receives sensory inputs from most senses except smell. It then sends these signals to the appropriate part of the brain for processing.

Epithalamus

The epithalamus is located above the thalamus and contains the pineal gland, which helps regulate melatonin levels and sleep-wake cycles.

Hypothalamus

The hypothalamus is located below the thalamus and is responsible for maintaining body homeostasis. It regulates the "4 F's":

• Feeding

• Fleeing

• Fighting

• Fornication

Pituitary Gland

The pituitary gland is connected to the hypothalamus and releases hormones that regulate various body functions.

📦 White Matter vs Gray Matter

White Matter

White matter is found in the brain, spinal cord, and peripheral nervous system. It is called a:

• Tract in the brain

• Tract or column in the spinal cord

• Nerve in the peripheral nervous system

Gray Matter

Gray matter is found in the brain, spinal cord, and peripheral nervous system. It is called a:

• Nucleus in the deep brain

• Cortex on the brain surface

• Horn in the spinal cord

• Ganglion in the peripheral nervous system

🧠 Brain Lobes

The telencephalon is the highest functioning part of the brain and is divided into four lobes:

The frontal lobe is responsible for problem solving, which involves coordinating sensory input and motor responses to achieve a goal.## 🧠 The Central Nervous System (CNS) 🧠

The CNS consists of the brain and spinal cord. It is responsible for processing and integrating information from the body.

Lobes of the Cerebrum

The cerebrum is divided into four lobes: Frontal, Parietal, Temporal, and Occipital. Each lobe has distinct functions:

• Frontal Lobe: responsible for motor control, decision-making, and problem-solving

• Parietal Lobe: involved in sensory processing, spatial awareness, and attention

• Temporal Lobe: plays a key role in auditory processing, memory, and language

• Occipital Lobe: primarily responsible for visual processing

Functions of the CNS

The CNS is responsible for:

• Controlling voluntary movements

• Processing and integrating sensory information

• Regulating emotions and behavior

• Maintaining homeostasis

Key Structures of the CNS

White and Gray Matter

• White Matter: composed of myelinated nerve fibers, responsible for transmitting signals

• Gray Matter: composed of non-myelinated nerve fibers, responsible for processing and integrating information

The Peripheral Nervous System (PNS)

The PNS consists of nerves that connect the CNS to the rest of the body. It is divided into two main categories:

• Somatic Nervous System: responsible for voluntary movements and sensory input from skeletal muscles

• Autonomic Nervous System: regulates involuntary functions, such as heart rate, blood pressure, and digestion

Autonomic Nervous System

The autonomic nervous system is further divided into two branches:

• Sympathetic Nervous System: responsible for "fight or flight" responses, such as increased heart rate and blood pressure

• Parasympathetic Nervous System: promotes relaxation and restoration, such as decreased heart rate and blood pressure

Neurotransmitters

Comparison of Somatic and Autonomic Nervous Systems

The autonomic nervous system (ANS) is a branch of the nervous system that controls involuntary actions of the body. It has two main branches: the sympathetic nervous system and the parasympathetic nervous system.

Receptors and Neurotransmitters

The ANS uses two main types of receptors: muscarinic and adrenergic. These receptors respond to the neurotransmitters acetylcholine (ACh) and norepinephrine (NE).

Sympathetic and Parasympathetic Nervous Systems

The sympathetic nervous system is responsible for the "fight or flight" response, while the parasympathetic nervous system is responsible for the "rest and digest" response.

Parasympathetic Nervous System

• Decreases heart rate

• Decreases respiratory rate

• Decreases blood pressure

• Increases blood flow to digestive organs

• Uses acetylcholine (ACh) as its primary neurotransmitter

Sympathetic Nervous System

• Increases heart rate

• Increases respiratory rate

• Increases blood pressure

• Decreases digestion

• Uses norepinephrine (NE) as its primary neurotransmitter

Adrenal Medulla

The adrenal medulla is a part of the sympathetic nervous system that releases epinephrine (also known as adrenaline) into the bloodstream. Epinephrine has a longer-lasting effect than norepinephrine and is used to stimulate the body's "fight or flight" response.

Sensory Receptors 🎯

Sensory receptors are specialized nerve endings that respond to different types of stimuli.

Types of Sensory Receptors

• Mechanoreceptors: respond to physical shape changes, such as pressure and vibration

• Chemoreceptors: respond to chemicals, such as pH and oxygen levels

• Thermoreceptors: respond to temperature changes

• Nociceptors: respond to painful stimuli

• Photoreceptors: respond to light

Sensory Processing

• Absolute Threshold: the minimum amount of stimulus required to activate a receptor

• Difference Threshold: the amount of stimulus change required for us to notice a difference

• Just Noticeable Difference: a measure of the difference threshold

Bottom-Up and Top-Down Processing 🤔

There are two types of processing: bottom-up and top-down.

• Bottom-Up Processing: Sensory receptors register information, which is then sent to the brain for identification.

• Top-Down Processing: The brain applies prior knowledge and experience to interpret sensory information.

Example of Bottom-Up and Top-Down Processing 📸

Imagine seeing a picture of a man and a woman. A bottom-up approach would involve taking in individual details, such as the man's bent knee and the sparkly ring in the box. A top-down approach would involve recognizing the scene as a marriage proposal.

Eye Anatomy 👀

The eye has several important structures:

Important Words to Know 📚

The following words are important to know for the MCAT:

• Iris

• Lens

• Cornea

• Pupil

• Ciliary Muscle

• Fovea

• Retina

• Optic Disc

• Optic Nerve

Organization of the Retina 📊

The retina is organized in the following way:

• Photoreceptors (rod and cone cells) convert light into electrical signals

• Bipolar Cells transmit signals from photoreceptors to ganglion cells

• Ganglion Cells transmit signals to the optic nerve

• Optic Nerve carries visual information to the cerebral cortex for processing

Rod and Cone Cells 🌈

Rod and cone cells are two types of photoreceptors in the retina.

• Cone Cells: Respond to high light intensities and are responsible for color vision

• Rod Cells: Respond to low light intensities and are responsible for black and white vision

Bipolar cells are a type of neuron in the retina that play a crucial role in the visual pathway. They have different functions depending on the type of neurotransmitter they release.

• On Bipolar Cells: These cells are inhibited by the neurotransmitter in the dark and do not respond. However, when the neurotransmitter is off, they are no longer inhibited and turn on to send the signal.

• Off Bipolar Cells: These cells are triggered in the dark and are not active when it's light.

👂 The Structure of the Ear 👂

The ear is composed of three main parts: the outer ear, middle ear, and inner ear.

👂 How Hearing Works 🎧

Sound waves enter the ear through the auditory canal and hit the tympanic membrane, causing it to vibrate. These vibrations are transmitted through the middle ear bones to the oval window, which separates the middle ear from the inner ear.

• The cochlea is a spiral-shaped structure that contains the hearing receptors.

• The basilar membrane is a thin, flexible structure that runs along the length of the cochlea and helps determine the pitch of sound.

• Hair cells are mechanoreceptors that are embedded in the basilar membrane and are responsible for converting sound vibrations into electrical signals.

👂 Sound Interpretation 🎵

The body interprets different sounds based on the vibrations of the basilar membrane and the stimulation of the hair cells.

• Pitch: Different pitches move different places on the basilar membrane. Low-frequency sounds move the stiffer parts of the membrane, while high-frequency sounds move the floppier parts.

• Loudness: The louder the sound, the bigger the amplitude of the vibrations. This produces more frequent action potentials and determines the intensity of the sound.

👂 Equilibrium and Balance ⚖

The semicircular canals in the inner ear are responsible for maintaining equilibrium and balance. They are filled with a fluid called endolymph and are lined with hair cells that detect changes in movement and acceleration.