Biol 216 Notes: The Neuron, Topic 5

Topic 5 Learning Goals

  • Describe and differentiate the visible structure and anatomy of neurons, nerves, glia cells.

  • Identify different neuronal types and their function in the CNS and PNS.

  • Explain the electro-chemical basis of an action potential.

  • Understand the processes involved in neuronal signaling (reception, conduction, integration).

CNS and PNS Basics

  • Central Nervous System (CNS):

    • Includes the brain and spinal cord

    • Function involves interneurons

  • Peripheral Nervous System (PNS):

    • Comprised of afferent and efferent neurons

Afferent Neurons

  • Definition: Sensory neurons that pick up stimuli via sensory receptors.

  • Function:

    • Transmit sensory information to interneurons in the CNS.

Interneurons

  • Function:

    • Integrate information received from afferent neurons.

    • Formulate a response.

Efferent Neurons

  • Definition: Carry response signals to muscles and glands.

  • Motor Neurons: A type of efferent neuron specifically carrying signals to skeletal muscle.

Information Processing in the Nervous System

  • The process can be summarized in three steps:

    • Sense (detect stimuli)

    • Integrate (analyze information)

    • Act (execute a response)

Classes of Neurons

  1. Afferent

  2. Interneurons

  3. Efferent

Neuron Anatomy

Types of Neurons:

  • A. Motor Neuron

  • B. Neuron in the Cerebral Cortex and Hippocampus

  • C. Neuron in the Cerebellum

  • D. Neuron in the Retina

  • E. Afferent Neuron connecting skin and muscle

Afferent Neurons (PNS) Details

  • Afferent: Represents signals moving away from the stimulus.

  • Structure:

    • Generally has one axon with two branches:

    1. Peripheral Branch: Extends from the cell body to peripheral locations (skin, joints, muscles).

    2. Central Branch: Connects cell body with the spinal cord.

Receptors

  • Types of Sensory Receptors/Neuronal Endings:

    • Encapsulated and free.

    • Free Receptors: Represent types 1 and 2.

    • Encapsulated Receptors: Represent types 5.

  • Example:

    • Pacinian Corpuscle: Detects vibration and pressure; able to discern rough and smooth textures.

Neuron vs Nerve

  • Neuron: A single cell, often called a “nerve cell.”

  • Nerve: A cordlike structure containing many axons, providing a pathway for electrochemical nerve impulses.

  • Spinal Nerves: 31 pairs exist, located in the PNS.

  • CNS Equivalent Structure: Known as tracts.

Nerve Example: Radial Nerve

  • Function: Supplies the triceps brachii muscle and other posterior forearm muscles, providing a common pathway for transmitted impulses in the arm.

White/Gray Matter

  • White Matter: Composed of myelinated axons and glial cells.

  • Gray Matter: Contains neuronal cell bodies, includes both afferent and efferent neurons.

    • Dorsal Root: Afferent in CNS (brain, spinal cord).

    • Ventral Root: Efferent pathways.

Non-neuronal Cells of the Nervous System: Glial Cells

  • Definition: Non-neuronal cells providing nutrition and support to neurons.

  • Types of Glial Cells:

    • Ependymal Cells: Produce cerebrospinal fluid (CSF).

    • Microglia: Act as phagocytes, ingesting and breaking down pathogens and waste in CNS.

    • Astrocytes: Provide structural support and maintain ion concentrations surrounding neurons in CNS.

    • Satellite Cells: Perform a similar function as astrocytes in PNS.

    • Schwann Cells: Form the myelin sheath in PNS.

    • Oligodendrocytes: Form myelin sheath in CNS.

    • Myelin: High lipid content insulates electrical impulses along axons.

Signal Conduction

  • Presynaptic: Refers to the transmitting end of a synapse.

  • Postsynaptic: Refers to the receiving end of a synapse.

  • Node of Ranvier: Gaps in myelin that expose the axon membrane to extracellular fluid, helping increase the speed of signal conduction due to a high concentration of voltage-gated channels that propagate action potentials.

Axon Hillock

  • Function: Acts as a spike initiation zone for action potentials with a high concentration of voltage-activated sodium channels.

  • Signal Convergence: Signals from presynaptic terminals converge at the axon hillock; once they reach a threshold, an action potential is generated down the axon.

Synapse

  • The junction between the axon terminal of a neuron and the receiving cell (another neuron, muscle fiber, gland cell).

Types of Synapses:

  1. Axosomatic Synapse: Between presynaptic terminal and soma.

  2. Axoaxonic Synapse: Between the axons of presynaptic and postsynaptic cells.

Two Types of Synapse

  • Chemical Synapse:

    • Involves neurotransmitter release, traveling across a synaptic cleft (about 25 nm), to bind to the postsynaptic receptor and cause a response by generating new electrical impulses.

  • Electrical Synapse:

    • Found in tissues requiring synchronized electrical activity (e.g., heart); involves gap junctions allowing direct current flow between cells.

Quick Questions

  1. Which type of synapse enables faster conductance of an action potential?

    • Answer: Electrical synapse is faster than chemical synapse.

  2. Which allows better modulation?

    • Answer: Chemical synapse allows better modulation.

  3. Which enables synchronized electrical activity?

    • Answer: Electrical synapse allows for synchronized activity.

Membrane Potential

  • Definition: Arises from a difference in electrical charge across the membrane, with typical measurements around -70 to -80 mV.

  • Implication: Neurons maintain a resting membrane potential due to ion movement influenced by both concentration gradients and electric gradients.

Ion Movement and Membrane Potential

  • Driving Forces for Ion Movement:

    • Electrochemical Gradient: Includes concentration gradient and electrical gradient.

Membrane Potential Characteristics at Rest

  • Membrane potential at rest is typically -70 mV due to:

    1. Na+/K+ Pump: Pumps 3 Na+ out and 2 K+ in, creating higher Na+ outside and K+ inside.

    2. Selective Permeability: The membrane is more permeable to K+ than to Na+ at rest.

Distribution of Charged Particles

  • Na+/K+ Pump:

    • Creates uneven distribution of Na+ (higher outside) and K+ (higher inside).

  • Selectivity of Membrane:

    • Freely allows certain ions through specific channels allowing K+ to leave and Na+ to flow in (passively), leading to net charge differences across the membrane.

Role of K+ in Membrane Potential

  • When resting potential is 0, K+ will leak out down its concentration gradient, causing unbalanced negative charge inside. The resulting membrane potential builds an electrical field that counteracts further efflux of K+.

Voltage-Gated Ion Channels

  • Resting Neuron: Features closed voltage-gated Na+ and K+ channels but open K+ leak channels.

  • Diversity of Ion Channels: More than 140 voltage-gated ion channels involved, classified by structure and function.

Types of Channel:

  1. Voltage-Gated Channels: Situated in axon membranes; open and close in response to voltage changes.

  2. Ligand-Gated Channels: Primarily located at synapses; open in response to chemical messengers.

  3. Mechanically Gated Channels: Present in sensory receptors responding to mechanical stimuli (e.g., sound).

  4. Ungated Channels: Allow passive ion flow, most open at rest.

Action Potentials

  • Definition: Abrupt and transient change in membrane potential that occurs during neuron impulse conduction.

  • Six Steps of Action Potential:

    1. A stimulus causes positive charges to enter the neuron, leading to depolarization.

    2. Depolarization gradually reaches 'threshold' before a sudden increase in membrane potential occurs due to Na+ influx.

    3. Membrane potential falls back (repolarization), often going below resting potential (hyperpolarization).

    4. The membrane potential returns to resting potential after hyperpolarization.

    5. The process is all-or-nothing, dependent on reaching threshold.

Key Features in Neurons

  • All or None: Action potential occurs if threshold is reached.

  • Size Maintenance: The action potential maintains its size during propagation.

Action Potential Propagation

  • Action potentials are generated locally but can trigger others in neighboring segments.

  • Mechanism: Ion currents from one firing segment induce the next.

  • Refractory Period: The sensitivity of an area to further stimulation decreases after an action potential, divided into absolute and relative refractory periods:

    • Absolute Refractory Period: Complete insensitivity to further stimulation.

    • Relative Refractory Period: A larger-than-threshold stimulus can initiate another action potential.

Graded Potentials

  • Graded potentials occur when a sensory stimulus excites a sensory cell or when a neurotransmitter binds to a postsynaptic cell:

    • Can result in depolarization (EPSP) or hyperpolarization (IPSP).

    • The strength of graded potentials correlates with stimulus intensity.

    • Summation: Can occur temporal (from one neuron over time) or spatial (from multiple neurons at once).

  • Summary of Graded Potentials Characteristics:

    1. Response to stimuli alters ion permeability.

    2. Size corresponds to stimulus strength.

    3. Conducted decrementally across the membrane.

    4. A depolarizing graded potential can lead to action potentials.

Neurotransmitters

  • Types and Functions:

    • Acetylcholine: Important for muscle activation and cognitive functions (Alzheimer's affects these neurons).

    • GABA: Inhibitory neurotransmitter, opens Cl- channels.

    • Glutamate: Important for learning and memory, primarily excitatory.

    • Dopamine, Norepinephrine, Serotonin: Involved in mood, attention, reward pathways, etc.

Process of Neurotransmission

  • Steps:

    1. Action potential reaches the axon terminal, opening voltage-gated Ca2+ channels.

    2. This allows Ca2+ influx, promoting vesicle fusion and neurotransmitter release.

    3. Neurotransmittters bind to receptors on the postsynaptic neuron, leading to possible ion flow and an action potential.

Removal of Neurotransmitters from Synaptic Cleft

  • Mechanisms include enzymatic breakdown or reuptake into the presynaptic terminal. Examples include SSRIs and acetylcholinesterase inhibitors.

Stimulating and Inhibiting during Neurotransmission

  • Stimulation: Involves Na+ influx, depolarizing the membrane (EPSP).

  • Inhibition: Involves K+ exiting or Cl- entering, hyperpolarizing the membrane (IPSP).

Topic 5 Key Concepts and Take-home Messages

  • Central and Peripheral Nervous System components and their functions.

  • Neuron and glial cell characteristics.

  • Action potentials and their propagation mechanisms, including the role of ion channels.

  • Understanding neurotransmitter functions and neurotransmission mechanisms, including graded potential summation.