BIOL 216- chapter 5- the neuron

Neurons are specialized cells that carry electrical signals.

Found in CNS and PNS

Functions:

• Receive, integrate, and transmit information within the nervous system

CNS- Central Nervous System

• Brain

• Spinal Cord

• Interneurons: process information and formula responses.

PNS- Peripheral Nervous System

• Composed of afferent (sensory) neurons and efferent (motor) neurons

Types of Neurons

• Afferent Neurons/ sensory neurons: 

  • Function: transmit sensory information from sensory receptors to the CNS

  • Structure: one axon, with two branches– one to peripheral body, one to spinal cord

• Interneurons:

  • Function: integrate and process information received from sensory neurons and relay commands to motor neurons.

  • Found in CNS

• Efferent neurons: 

  • Function: carry signals from the CNS to muscles and glands to initiate responses. 

  • Motor neurons are a type of efferent neuron that stimulates skeletal muscles. 

Dorsal and ventral roots: 

• Dorsal root:

  • afferent neurons

  • Carrying information towards the central nervous system

• Ventral root:

  • Efferent neurons

  • Motor nerve root

  • Carrying information away from the CNS

Information processing in the Nervous System: 

• Sense: Sensory input from receptors

• Integrate: Process information in the CNS

• Act: Efferent neurons initiate a response

Anatomy of a Neuron

• Cell body/Soma: Contains the nucleus and metabolic machinery.

• Dendrites: Branch-like extensions that receive signals from other neurons.

• Axon: A long, slender projection that transmits electrical impulses to other neurons or target tissues.

• Axon Hillock: Area where action potentials are initiated; high concentration of voltage-gated sodium channels.

Types of Neurons by Location: 

• Motor Neuron: Found in the spinal cord, transmits signals to muscles.

• Cerebral Cortex and Hippocampus Neurons: Involved in higher cognitive functions and memory.

• Cerebellum Neurons: Involved in motor coordination.

• Retina Neurons: Involved in vision processing.

• Sensory Neurons: Carry information from skin and muscles to the CNS.

Neurons vs. Nerves: 

• Neurons: single cell capable of transmitting electrical impulses

• Nerve: a bundle axons transmitting impulses to and from the CNS

  • Only in PNS

  • In CNS- called tracts

  • Radial nerve: supplies the triceps brachii muscle of the arm and all 12 muscles in the posterior osteofacial compartment of the forearm, as well as the associated joints and overlying skin. Common pathway for transmitted impulses in the arm.

• White matter: myelinated axons, glial cells 

• Grey matter: neuronal cell body

Glial cells- non-neuronal cells: 

• Functions: support and protect neurons

  • Ependymal Cells: Produce cerebrospinal fluid (CSF).

  • Microglia: Phagocytic cells that clean up waste and pathogens in the CNS.

  • Astrocytes: Provide structural support in the CNS, help maintain ion balance.

  • Satellite Cells: Similar to astrocytes but found in the PNS.

  • Schwann Cells: Form myelin sheath in the PNS.

  • Oligodendrocytes: Form myelin sheath in the CNS.

• Myelin sheath: 

  • Function: insulates axons, speeding up signal conduction

  • Composed of high lipid content

• Node of Ranvier: gaps in myelin where action potentials are accelerated 

Signal conduction

• Axon Hillock: 

  • Concentration of voltage-gated sodium channels

  • Main site for the initiate of APs

• Synapse

  • Junction between two neurons or between a neuron and a target cell

  • Types of synapses:

Electrical synapse

• Gap junctions: allow direct flow of current between cells

• Fast conduction- cardiac muscle and smooth muscle

• Enables synchronized activity

Chemical synapse

• Neurotransmitter is released from the presynaptic neuron, crosses the synaptic cleft and binds to receptors on the postsynaptic cell, generating a response

• Slower, but modulated signal

Resting Membrane Potential:

Neurons at rest have a membrane potential of about -70 mV.

Caused by the unequal distribution of ions across the cell membrane, with more K+ inside and Na+ outside the cell.

Na+/K+ Pump: Pumps 3 Na+ ions out and 2 K+ ions into the cell to maintain the resting potential.

Factors Affecting Membrane Potential:

• Electrochemical Gradient: The combination of the concentration gradient and electrical gradient that drives ion movement across the membrane.

• K+ Leak Channels: Allow K+ to move freely across the membrane, contributing to the negative charge inside the cell.

Equilibrium Potential: The membrane potential at which there is no net flow of an ion.

• Example: For K+, the equilibrium potential (EK) is -90 mV.

Goldman Equation: Predicts the membrane potential when the membrane is permeable to multiple ions, factoring in both the concentration gradient and ion permeability.

Resting Membrane Potential:

• Not exactly at the equilibrium potential for any single ion, as multiple ion channels contribute to the resting potential.

• A small influx of Na+ keeps the membrane potential slightly less negative than the equilibrium for K+.

A resting neuron

• Ion distribution: in a resting neuron, there is a unique distribution of ions inside and outside the cell membrane

• Ion channels:

  • Voltage-gated Na+ and K+ channels are closed during rest.

  • K+ leak channels allow the flow of K+ ions across the membrane.

  • Na+/K+ ATPase actively pumps Na+ out and K+ in to maintain resting potential (~-70 mV).

Types of Ion channels

• Ungated (leak) channels

• Voltage-gated channels (found in axon membranes)

• Ligand-gated channels (found at synapses)

• Mechanically gated channels (found in sensory receptors)

• Resting Neuron: primary ion channels open during rest are ungated channels, with more K+ leak channels than Na+

Ungated channels:

• Leak channels: allow ions to move according to their electrochemical gradients

• K+ leak channels: more open than Na+ channels

• Ion movement: Even with small differences in ion size, ions typically don't pass through the "wrong" channel, though exceptions exist.

Voltage gated Ion channel

• Integral membrane proteins that open or close in response to changes in membrane voltage

• Key aspects of voltage-gated channels:

  • Ion conductance

  • Pore gating

  • Regulation

• Voltage gates Na+ channels:  Open in response to repolarization and contribute to the rapid influx of Na+ during action potential generation

• Voltage-Gated K+ Channels: Have different types, some inactivate quickly (A-type currents) while others inactivate slowly or not at all. These channels contribute to repolarization.

• Single-Channel Current: The rate of ionic flow is influenced by the maximum channel conductance and the electrochemical driving force for the ion.

Action Potentials

• A sudden and brief change in the membrane potential that allows a neuron to conduct an electrical impulse.

  • A stimulus causes the flow of positive charges into the neuron.

  • The membrane potential becomes less negative (depolarized).

  • Depolarization continues until the threshold is reached (typically 10-20 mV more positive than resting potential).

  • A rapid influx of Na+ causes a sudden increase in membrane potential (firing).

  • Membrane potential falls below resting potential (hyperpolarization).

  • Membrane potential returns to resting potential.

• All-or-Nothing: Once the threshold is reached, depolarization occurs, and the action potential will fire regardless of the stimulus strength.

• Key features of action potential

  • All or nothing response

  • Maintain consistent size

  • Propagation: action potentials move along the axon without changing amplitude

• Propagation: 

  • Mechanism: Action potentials are generated locally and cause neighboring segments of the axon to fire, propagating the signal.

  • Unmyelinated Axon: Ions flow between firing and non-firing segments, triggering further action potentials in the adjacent regions.

Refractory period: 

• Absolute refractory period: a time when the neuron cannot generate a second action potential, no matter the stimulus

  • Occurs when voltage gates Na+ channels are open/ inactivated- they cant reopen until the membrane repolarize

• Relative refractory period: a time when a stronger-than-usual stimulus is needed to generate an action potential

  • Some Na+ channels have returned to the resting state, some K+ channels remain open

  • Neuron is more negative than usual, stronger stimulus to reach threshold 

  • A stimulus can generate an action potential, but it must be stronger than usual due to hyperpolarization and inactivation of Na+ channels

Conduction velocity: 

• Depends on: 

  • Diameter of axon

  • Myelination

• A-alpha fibers: large diameter: carry proprioception information

• A-beta fibers: transmit touch information

• A-delta fibers: transmit pain and temperature

• C-fibers: unmyelination, carry pain, temperature, and itch signals 

• Faster conduction velocity = larger axon diameter

  • Less internal resistance

  • Faster signal transmission

• Myelination increases conduction speed

  • Insulation

  • Signals jump between nodes of ranvier

• Cable theory: Describes the electrical properties of neurons, including resistance and capacitance.

  • The length constant (λ) indicates how far electrical signals travel down an axon before decaying. Larger λ means faster signal propagation.

Neurotransmission:

• Chemical Synapses: Neurons communicate via neurotransmitters that bind to receptors on the postsynaptic neuron. This can lead to either excitatory or inhibitory responses.

  • Direct Neurotransmission: Neurotransmitters bind directly to ion channels, leading to rapid responses.

  • Indirect Neurotransmission: Neurotransmitters activate G-protein coupled receptors, which indirectly affect ion channels, leading to slower, longer-lasting effects.

Types of neurotransmitters:

• Excitatory:

  • Acetylcholine: Involved in muscle contraction and brain functions like learning and memory.

  • Glutamate: Major excitatory neurotransmitter involved in learning and memory.

  • Norepinephrine/Epinephrine: Involved in stress responses, attention, and focus.

  • Dopamine: Involved in movement, reward, and motivation.

• Inhibitory:

  • GABA: Major inhibitory neurotransmitter that opens Cl- channels to inhibit action potential.

  • Glycine: Also inhibits neurotransmission, increasing Cl- influx.

• Peptides:

  • Endorphins: Reduce pain perception and produce feelings of pleasure.

  • Substance P: Involved in the sensation of pain.

Graded potentials:  

• Changes in membrane potential that do not reach threshold for an action potential

  • EPSP- move membrane potential closer to threshold 

  • IPSP- move membrane potential further from threshold 

• Summation:

  • Temporal summation: graded potential can sum up over time, multiple EPSPs from one neuron

  • Spatial summation: graded potentials sum up across different synapses, EPSPs from multiple neurons

• The combination of EPSP and IPSP determine whether an AP is generated

Removal of neurotransmitters: 

• After neurotransmitters are released into the synaptic cleft, they are either broken down (e.g., acetylcholine is broken down by acetylcholinesterase) or taken back up (reuptake) by the presynaptic neuron to stop the signal.

Evolution of nervous system: 

• Invertebrates have simpler nervous system- fewer neurons and less complex networks

• Cephalization: concentration of sensory organs, and nervous tissue in the head, leading to more sophisticated nervous systems

• Nerve nets, Bilateral symmetry – better coordination and movement

Vertebrate nervous system:

• Neural tube formation: leads to development of CNS, with neural crest cells differentiating into various cell types

• Gene expression: ensures proper cell differentiation

Brain functions: 

• Receiving information

• Integrating information

• Storing information

• Sending out information

Key structures: 

• Blood brain barrier: protects the brain by restricting access to large molecules and microscopic objects, allowing essential small molecules like oxygen and glucose to pass.

• Meninges: connective tissue layers that cover the brain and spinal cord, providing structural support and protection. 

• CSF: circulates through the brain and spinal cord, providing nutrients and cushioning the brain.

• Ventricular system: cavities filled with CSF that protect the brain from injury.

  • 2 lateral ventricles

  • Third ventricle

  • Fourth ventricle

Brain anatomy: 

• Forebrain: 

  • Forms cerebrum- has left and right hemispheres

  • Cerebral hemispheres: 

    • Left hemisphere: focus on details, spoken and written language, abstract reasoning, math

      • Brocas and wernickes 

    • Right hemisphere: focus onboard background, relative position of objects, intuitive thinking, conceptualization, music, art

    • Lateralization: difference in function between the left and right hemisphere 

• Cerebral cortex: 

  • Outermost thin later of gray matter covering a core of white matter

    • Grey matter: neuron cell bodies and dendrites

    • White matter: axons

  • Convoluted to increase surface area

  • Regulates cognitive functions, such as thinking, learning, speaking, remembering, and making decisions

  • Has areas that: 

    • Primary somatosensory area: Receive and integrate sensory information 

    • Primary motor area: are involved in the planning, control, and execution of voluntary movements 

    • Association areas: integrate sensory information, formulate responses, relay responses to motor area

      • Brocas, wernickes

• Cerebrum

  • Frontal lobe

    • Executive function

  • Parietal lobe

    • Behind frontal lobe

    • Deals with perception and integration of stimuli from the senses

  • Occipital lobe

    • Back of brain

    • Concerned with vision

  • Temporal lobe

    • Long the side of the brain under the frontal and parietal lobes

    • Deals with senses of smell, sound, and the formation and storage of memories

• Cerebellum

  • Coordinates and refines body movements by information integration and comparison

  • Receives sensory information from: 

    • Receptors in muscles and joints

    • Balance receptors in the inner eat

    • Touch, vision and hearing receptors

  • Information about body position, the directions of movement of limbs or trunk

  • Compares sensory input with signals from the cerebrum that control voluntary body movements

• Brain stem

  • Structures: 

    • Medulla

    • Pons

    • Midbrain

      • Smallest region of the brain

      • Acts as relay station for auditory and visual information

      • Controls eye movement

      • Ventral tegmental area- VTA

        • Dopamine and serotonin producing neurons

        • Involved in pleasure pathway/reward circuit

      • Substantia nigra

        • Control of body movement

        • Contains dopamine-producing neurons

        • Degeneration of neurons in the substantia nigra is associated with Parkinson's disease

  • connect forebrain with spinal cord

  • Functions: 

    • Heart and respiration rate

    • Blood pressure

    • Blood vessel dilation

    • Digestive system reflexes- vomiting 

  • Reticular formation: 

    • Network of neurons in the brain stem that connect the thalamus to the spinal cord

    • Integrate incoming sensory information

    • Filters incoming information 

    • Ascending reticular formation

      • Sends stimulatory signals to the thalamus to activate the cerebral cortex

      • Produces different levels of alertness or consciousness

      • Footers incoming stimuli to discriminate irrelevant background stimuli

      • abnormalities- comatose

    • Descending reticular formation

      • Receives information from the hypothalamus

      • Connects with interneurons of the spinal cord that control skeletal muscle contractions

• Thalamus

  • Structure between the cerebral cortex and midbrain

  • Function: relaying signals from the special sense and motor signals to the cerebral cortex

  • Regulates consciousness, sleep and alertness

• Hypothalamus

  • Below thalamus, above brainstem

  • Synthesizes and secrets neurohormones

  • Links nervous and endocrine systems via pituitary gland

  • Controls body temperature, hunger, thirst, fatigue, circadian cycles

  • Trigger swearing, shivering

  • Monitors the osmotic balance of the blood 

• Basal nuclei/ basal ganglia

  • Group of nuclei of varies origin in the brains of vertebrates that act as a cohesive functional unit

  • Contains substantia nigra

  • Surrounds thalamus

  • Involved with voluntary movement

  • Damage causes Parkinson’s disease

• Limbic system

  • Called emotional brain

  • Parts of thalamus, hypothalamus, basal nuclei

  • Amygdala- emotion, fear

  • Hippocampus- memory

  • Olfactory bulbs- smell 

  • Hippocampus

    • Part of the limbic system

    • Consolidation of information from short to long term memory and spatial navigation

    • Alzheimer’s- hippocampus is the first to suffer damage

      • Memory loss

      • Disorientation

The reward pathway

• VTA secretes dopamine

• Nucleus accumbens contains dopamine sensitive cells

• Causes feelings of pleasure

• Amygdala and hippocampus play roles in memory, and deciding is an experience is desirable

• Prefrontal cortex coordinates all the information and determines behavior of individual

• Pathway:

  • Triggering stimuli

    • Natural rewards activate the pathways

    • sensory inputs are processed in the brain

  • Dopamine release

    • VTA releases dopamine

    • Dopamine travels to the nucleus accumbens and prefrontal cortex

  • Reinforcement of behaviour

    • Nucleus accumbens processes the reward signal- pleasurable feeling

    • Reinforces behaviors that lead to reward- increasing likelihood to repeat

  • Cognitive and emotional integration

    • Prefrontal cortex assesses the value of the reward and its implications for future behavior 

    • The amygdala and hippocampus help attach emotional significance and memory to the reward experience. 

PNS divisions

• Afferent neurons: transmit signals to CNS

• Efferent neurons: transmit signals from CNS

  • Somatic system

    • Voluntary

    • Conscious body movements

    • Motor neurons

    • Efferent signals from CNS to skeletal muscles 

  • Autonomic system

    • Refers to collections of motor neurons (ganglia)

    • In head, neck, thorax, abdomen, pelvis

    • Axonal connections of these neurons

    • Involuntary movements:

      • Controls visceral functions 

      • Heart rate

      • Digestion 

      • Respiration rate

    • Some actions work in tandem with the conscious mind

      • Sympathetic system

        • Utilized situations involving stress, strenuous physical activity, danger, excitement 

        • Fight or flight response

        • Increases force and rate of heartbeat

        • Increased blood pressure constricts blood vessels, dilates bronchioles

        • Suppresses digestion

      • Parasympathetic system

        • Housekeeping functions- like digestion

        • Utilized during quiet, low stress times

        • Rest and digest

        • Nerves of parasympathetic division are located around the sympathetic nerves

Vagus nerve

• Cranial nerve 10

• Contributes to innervation of the viscera

• Conveys sensory information about the state of the body’s organs of the CNS

• Responsible for: 

  • Heart rate

  • GI peristalsis

  • Sweating

  • Muscle movements in mouth, speech and keeping the larynx open for breathing 

Spinal cord

• Carries impulses from the brain to PNS

• Sensory info to brain

• Motor info to periphery

Sensory regions in the brain

• Primary somatosensory area

  • Located in the parietal lobes of each hemisphere

  • Integrates information regarding touch, pressure, temp, pain

  • Causes tingling in related body parts on the opposite side of the body- if stimulated

• Primary motor area

  • Located anterior to the primary somatosensory area

  • Stimulation of portions of the primary motor area causes movements of specific body parts on opposite sides of the body

• Homunculus

  • Representation of correlation between areas of the body from which sensory information projects to areas in the primary somatic sensory cortex

  • Size is related to the various regions correlated to the number of sensory receptors in the corresponding part of the body

• Association areas- integration

  • Areas surrounding the sensory and motor areas

  • Function: 

    • Integrating information from the sensory areas

    • Formulate responses

    • Transmit the response to the motor cortex

• Association areas- language

  • Broca’s 

    • Expressing language

    • Coordination of lips, tongue, jaw 

    • Initiates the complex series of movements necessary for speech

    • Damage- speak few words which are poorly pronounced- comprehend written and spoken words

    • Broca’s aphasia- hesitant and distorted speech

  • Wernicke’s

    • Understanding and formulating coherent speech

    • Coordinates input from auditory and visual areas

    • Damage- can speak but words make no sense

    • Wernicke’s aphasia- fluent language with made up or unnecessary words with little or no meaning to speech, difficulty understanding other’s speech with unawareness of mistakes

  • Arcuate fasciculus is believed to connect wernicke’s and broca’s area

  • How does speech work?

    • AP from eye reach the primary visual cortex- word is seen here

    • Word is recognized in visual association area

    • Word is understood in parts of Wernicke’s area

    • APs representing the word are conducted through association fibers that connect the wernicke’s to broca’s

    • Word is formulated in Broca's area

    • APs are conducted to the premotor area where movements are programmed

    • Movements are triggered in primary motor cortex

• Association areas- memory

  • Storage and retrieval of sensory or motor experiences

  • Short term memory: depends on transient changes in neurons, such as changes in membrane potential and reversible changes in ion transport

  • Long term memory: storage of memories for days and years

    • Permanent biochemical, molecular or structural changes that establish signal pathways that cannot be easily terminates

  • Long term potentiation: caused by a short burst of repetitive firing in the presynaptic neurons such that when there is single AP later, it will evoke a greatly enhanced response in the post synaptic cells

    • Effects last in relation to the number and intensity of repetitive firing

    • Occurs when a presynaptic cell fires at a time when the post synaptic membrane is strongly depolarized due to recent repetitive firing of the same presynaptic cell or other means

    • Late LTP:

      • Permanent alterations: 

        • Number and area of synaptic connections

        • Number and branching of dendrites

        • In gene transcription

        • Protein synthesis

      • Pathway

        • Repeated stimulation of presynaptic cell reaches a threshold such that dopamine, a modulatory neurotransmitter is released

        • Dopamine acts on GPCR that is coupled to adenyl cyclase

        • Increases level of cAMP

        • Activates protein kinase

        • Activates CREB which is a transcription factor

        • Turns on genes that make proteins involved in generating new synaptic connections

• Association areas- learning

  • Involves a change in the response to a stimulus based on information or experiences stored in memory

    • Store a memory

    • When a stimulus is encountered, scan your memories

    • Modify your response according, this means you learn

Consciousness

• EEG- recording of electrical activity along the scalp produced by the firing of neurons within the brain

• Sleep is semi conscious