Chapter 22 Signal Transduction Mechanisms

Chapter 22: Signal Transduction Mechanisms: Electrical and Synaptic Signaling in Neurons

1. Functions of the Nervous System

  • Sensory Input:

    • The nervous system collects information from the environment via receptors (e.g., eyes, ears, skin).

  • Integration:

    • The brain and spinal cord process and interpret sensory input, deciding if an action should be taken.

  • Motor Output:

    • The nervous system sends signals to muscles or glands in response to decisions made about the sensory input.

2. Organization of the Nervous System

A. Central Nervous System (CNS)

  • Composed of the brain and spinal cord.

    • Receives sensory information and coordinates appropriate responses.

B. Peripheral Nervous System (PNS)

  • Comprises all neural tissue outside the CNS, connecting it to organs, muscles, and glands.

    • Divided into:

    • Somatic Nervous System: Voluntary control.

    • Autonomic Nervous System: Involuntary control.

C. Signal Transmission

  • Afferent Signals: Carry information from sensory receptors to the CNS.

  • Efferent Signals: Exit the CNS and travel to the PNS to elicit a response.

D. Neurons Structure

  • Cell Body (Soma):

    • The control center that processes signals; contains the nucleus.

  • Dendrites:

    • Receive signals and send them towards the soma.

  • Axon:

    • Conducts electrical impulses away from the soma to muscles or other neurons.

  • Axon Terminals:

    • Contain synaptic vesicles filled with neurotransmitters for transmission to the next cell (neuron, muscle, gland).

  • Myelin Sheath:

    • A protective fatty layer around axons that helps speed up signal transmission and protects the axon.

  • Nodes of Ranvier:

    • Small gaps between myelin sheaths along the axon that facilitate saltatory conduction by allowing action potentials to jump from node to node.

    • Packed with voltage-gated sodium (Na⁺) and potassium (K⁺) channels, which recharge the action potential.

  • Neurons that are unmyelinated (e.g., pain and temperature sensory neurons) receive slow signals because fast conduction is not always necessary.

E. Neuroglia (Glial Cells)

  • Supporting cells in the nervous system that outnumber neurons 10:1 and do not conduct impulses. Functions include:

    • Providing structure and support.

    • Supplying nutrients to neurons.

Types of Neuroglia in CNS:

  • Astrocytes:

    • Most abundant glial cells.

    • Anchor neurons to blood vessels.

    • Regulate chemical balance.

    • Assist in forming the blood-brain barrier.

  • Oligodendrocytes:

    • Produce myelin sheath in CNS.

  • Microglia:

    • Specialized immune cells that act as phagocytes responding to injury and inflammation.

  • Ependymal Cells:

    • Line the ventricles of the CNS.

    • Produce and circulate cerebrospinal fluid (CSF).

    • Create a barrier between CSF and nervous tissue.

F. Schwann Cells (PNS)

  • Form myelin sheaths around peripheral axons.

  • Aid in axon regeneration following injury.

G. Satellite Cells (PNS)

  • Surround neuron cell bodies in ganglia.

  • Regulate microenvironment (nutrient and ion exchange) and provide structural support.

3. Resting Membrane Potential

  • Defined as the electrical charge difference across the plasma membrane of a resting neuron, typically around -70 mV.

  • Essential for generating action potentials (nerve impulses).

  • Maintained by:

    • Na⁺/K⁺ ATPase Pump:

    • Pumps 3 Na⁺ ions out and 2 K⁺ ions into the cell, contributing to the negative resting potential.

    • Selective permeability of the membrane: More Na⁺ outside the cell and more K⁺ inside, but the membrane is more permeable to K⁺, allowing its leakage and creating a net negative charge.

4. Depolarization

  • Defined as becoming less negative (closer to zero).

  • The initial step in action potential generation; triggered by the opening of voltage-gated Na⁺ channels.

  • Process:

    1. A stimulus arrives (e.g., binding of neurotransmitter).

    2. Na⁺ rushes into the cell (down its electrochemical gradient).

    3. Membrane potential shifts from approximately -70mV toward +30mV.

5. Action Potential

  • A rapid, temporary change in a cell's membrane potential that propagates along the axon once it reaches a critical threshold.

  • Threshold Potential:

    • Critical voltage (around -55 mV) needed to trigger an action potential. If reached, Na⁺ channels further open leading to a full spike in action potential.

6. Refractory Periods

  • Following an action potential, there are two types of refractory periods:

    • Absolute Refractory Period:

    • No new action potential is possible (no matter how strong the stimulus) as Na⁺ channels are open or inactive, ensuring each action potential is separate.

    • Relative Refractory Period:

    • A new action potential can occur if the stimulus is stronger than usual, typically occurring during K⁺ efflux and hyperpolarization.

7. Ion Channels

A. Voltage-Gated Ion Channels

  • Open/close in response to changes in membrane potential.

  • Example:

    • Na⁺ channels involved in depolarization; K⁺ channels involved in repolarization.

B. Ligand-Gated Ion Channels

  • Open when a specific chemical (ligand) binds to them.

  • Example: Acetylcholine receptors at synapses which lead to muscle contraction.

8. Transmission of a Nerve Impulse

  • Sequence of events in nerve impulse transmission through a neuron:

    1. A stimulus (pressure, temperature, neurotransmitter, etc.) disturbs the neuron's membrane, opening Na⁺ channels at the site.

    2. Na⁺ flows inward; the membrane becomes less negative, moving towards 0 mV (depolarization). If sufficient, this depolarization spreads to the axon hillock.

    • Axon Hillock: Specialized part where the soma transitions to the axon; initiates action potential if the threshold is reached.

    1. Local depolarization triggers voltage-gated Na⁺ channels, causing more Na⁺ influx; the inside becomes positively charged, reaching +30 mV at the peak of action potential.

    2. “Repolarization”: Na⁺ channels inactivate, K⁺ channels open, K⁺ flows out, returning the membrane potential towards -70 mV.

    3. Refractory period follows, where Na⁺/K⁺ pumps restore original gradients, and the neuron recovers before being ready to fire again.

9. Types of Synapses

A. Electrical Synapses

  • Direct, physical connections between two neurons allowing ions to flow instantly via gap junctions.

B. Chemical Synapses

  • Junctions for communication using neurotransmitters across a synaptic cleft.

Comparison: Electrical vs. Chemical Synapses

  • Transmission Method:

    • Electrical: Direct ion flow via gap junctions.

    • Chemical: Neurotransmitters across synaptic cleft.

  • Speed:

    • Electrical: Very fast, almost instantaneous.

    • Chemical: Slower (milliseconds).

  • Directionality:

    • Electrical: Often bidirectional.

    • Chemical: Typically one-way.

  • Flexibility:

    • Electrical: Less flexible, mainly excitatory.

    • Chemical: Highly flexible (excitatory, inhibitory, modulatory).

  • Locations:

    • Electrical: Retina, neuron-neuron or glia-glia networks, interneurons.

    • Chemical: Dendrites, soma, axon terminals.

  • Distance Between Cells:

    • Electrical: Approximately 3.5 nm.

    • Chemical: Approximately 20–40 nm.

10. Neurotransmitters

  • Definition: Chemical messengers released from presynaptic neurons into the synaptic cleft that bind to receptors on the postsynaptic cell. Each neuron is wired to produce a specific type of neurotransmitter.

  • Examples:

    • GABA: Inhibitory neurotransmitter that prevents propagation of signals.

    • Acetylcholine: Involved in muscle contraction, learning, and memory.

    • Biogenic Amines: e.g., Dopamine, Serotonin, Histamine; regulate mood and more.

    • Amino Acids: Quick signaling messages such as excitatory (e.g., Glutamate) and inhibitory (e.g., GABA).

    • Neuropeptides: e.g., Endorphins; send slower messages related to stress control, mood, and pain regulation.

11. Summation of Signals

A. Temporal Summation

  • Occurs when multiple signals arrive at a neuron in rapid succession from the same presynaptic input; their effects add together, increasing the chance of reaching threshold. This allows weak signals to cause results over time (e.g., light touch on the skin).

B. Spatial Summation

  • Occurs when multiple presynaptic neurons release neurotransmitters simultaneously onto a single postsynaptic neuron, which adds these inputs across space.

12. Psychotropic Drugs and Neuro Diseases

A. Tranquilizers

  • Psychoactive drugs that act on the CNS to reduce anxiety, fear, and agitation. Types include:

    • Minor Tranquilizers: Used as anti-anxiety agents.

    • Major Tranquilizers: Block dopamine receptors for treatment of disorders such as bipolar disorder and schizophrenia.

B. Myasthenia Gravis (MG)

  • A rare, chronic autoimmune disease causing muscle weakness (especially in the eyes, face, throat, limbs) by blocking or destroying acetylcholine receptors.

C. Multiple Sclerosis (MS)

  • A chronic autoimmune disease where the immune system targets myelin, forming scar tissue that blocks nerve signals and damages nerve fibers over time, leading to permanent disability. Symptoms may include vision problems, muscle weakness, numbness, and balance issues.