Neurons notes

Chapter 10 Lecture Outline

10.1 General Characteristics of the Nervous System

  • Overview of Nervous System Function:

    • Master regulatory system.

    • Sends and receives information.

    • Functions include:

    • Sensory Input: Detects changes in the environment.

    • Integration and Processing: Involves decision-making based on sensory input.

    • Motor Output: Stimulates muscles and glands to respond accordingly.

    • Maintains homeostasis.

    • Acts as a center for thought, learning, and memory.

  • Main Cell Types of Nervous System:

    • Neurons (nerve cells):

    • Responds quickly to changes/stimuli.

    • Conducts electrical impulses via neurotransmitters.

    • Neuroglia:

    • Functions include protecting, supporting, insulating, and nourishing neurons.

    • Do not conduct electrical impulses like neurons.

10.2 Overview of Nervous System Structure

  • Central Nervous System (CNS):

    • Comprises the brain and spinal cord.

  • Peripheral Nervous System (PNS):

    • Connects CNS to other body parts.

    • Consists of cranial and spinal nerves.

    • Two subdivisions:

    • Afferent (Sensory): Carries signals towards the CNS.

    • Efferent (Motor): Carries signals away from the CNS.

Sensory (Afferent) and Motor (Efferent) Divisions

  • Sensory Division:

    • Sensory receptors perform sensory functions by detecting changes.

    • Receptors convert information into impulses for transmission to CNS.

  • Motor Division:

    • Transmits impulses from CNS to effectors (muscles or glands).

    • Two subdivisions:

    • Somatic: Transmits voluntary commands to skeletal muscles.

    • Autonomic: Transmits involuntary commands to viscera.

Clinical Application 10.1: Migraine

  • Affects 12% of the US population.

  • Signs:

    • Pounding headache, nausea, aura (shimmering images in visual field), light or sound sensitivity.

  • Environmental Triggers:

    • Bright light, certain foods, lack of sleep, stress, high altitude, stormy weather, excess caffeine, or alcohol.

    • Hormonal triggers in women; migraines often occur just before menstruation.

  • Duration: Lasts from 4 to 72 hours.

  • Mechanism of pain:

    • A period of excitation followed by unresponsiveness in specific neurons stimulates the production of pain sensations in areas at the base of the brain.

  • Treatment Approaches:

    • Identify and avoid triggers to reduce frequency of attacks.

Clinical Application 10.2: Treatment Options for Migraines

  • Drugs: Triptans can halt migraine attacks; some constrict blood vessels risking adverse effects.

  • Transcranial magnetic stimulation: Effective for both acute and chronic migraines.

  • Chronic migraine treatments include:

    • Injections of botulinum toxin (Botox).

    • Drugs binding to neurotransmitters causing dilation and inflammation of blood vessels.

    • "Repurposed" drugs such as certain antidepressants, anticonvulsants, and antihypertensives to decrease migraine frequency.

10.2 Nervous Tissue Cells: Neurons and Neuroglia

  • Neurons:

    • Vary in size and shape, differing in length, number, and size of axons and dendrites.

    • Common structural features include:

    • Cell Body (soma or perikaryon): Contains nucleus, cytoplasm, organelles, neurofilaments, and chromatophilic substances (Nissl bodies).

    • Dendrites: Branched structures serving as receptive surfaces; typically numerous per neuron.

    • Axon: Transmits impulses and releases neurotransmitters to other neurons or effectors; generally each neuron has one axon.

Axons

  • Structural features of axons include:

    • Axon Hillock: Cone-shaped area from which the axon arises.

    • Collaterals: Branches extending from the axon.

    • Axon Terminals: Specialized endings of axon extensions.

    • Synaptic Knob: Rounded terminal end of a synaptic terminal.

Schwann Cells

  • Type of neuroglia in the PNS that wraps around some axons in multiple layers.

  • Myelin: A mix of fats and proteins filling layers made by Schwann cell membranes.

  • Myelin Sheath: A wrapped coating around some axons composed of Schwann cell membranes and myelin; serves as electrical insulator.

  • Nodes of Ranvier: Gaps in the myelin sheath between Schwann cells.

Myelinated vs. Unmyelinated Axons

  • Myelinated Axons:

    • Coated by myelin sheath produced by Schwann cells in PNS and oligodendrocytes in CNS.

    • Comprise white matter in CNS and increase conduction speed for electrical impulses.

  • Unmyelinated Axons:

    • Encased by Schwann cell cytoplasm with no myelin coating.

    • Comprise gray matter in CNS.

Clinical Application 10.2: Multiple Sclerosis

  • Condition characterized by destruction of myelin sheaths in CNS via immune response.

  • Myelin attacked by the individual’s own antibodies leading to scar formation (scleroses) and impeding neuronal impulse conduction.

  • Resulting symptoms include muscle atrophy due to lack of input from motor neurons, fatigue, mood variations, blurred vision, and weakness/numbness in limbs.

  • Treatments may incorporate drugs that suppress immune activity.

Classification of Neurons

  • By Structure:

    • Multipolar Neurons:

    • Many processes extend from the cell body (many dendrites, 1 axon).

    • Makes up 99% of neurons, mainly in CNS and some autonomic NS.

    • Bipolar Neurons:

    • Two processes extend from cell body (1 dendrite, 1 axon).

    • Uncommon; located in eyes, ears, and nose.

    • Unipolar (Pseudounipolar) Neurons:

    • One process extends from cell body, with two branches acting as one axon (peripheral and central processes).

    • Mainly found in ganglia of PNS.

By Function:

  • Sensory (Afferent) Neurons:

    • Carry impulses from periphery to CNS.

    • Contain sensory receptors to detect changes at distal ends; most are unipolar, some bipolar.

  • Interneurons (Association or Internuncial Neurons):

    • Link neurons within CNS; relay information between areas.

    • Typically multipolar; some cell bodies cluster into nuclei within CNS.

  • Motor (Efferent) Neurons:

    • Carry impulses from CNS to effectors (muscles or glands).

    • Primarily multipolar.

    • In somatic NS, control voluntary skeletal muscle.

    • In autonomic NS, control involuntary smooth and cardiac muscle, as well as glands.

Classification of Neuroglia

  • General Functions of Neuroglia:

    • Provide structural support for neurons.

    • In embryos, help guide neurons into position while stimulating specialization.

    • Produce growth factors to nourish neurons and remove excess ions and neurotransmitters.

    • Aid in synapse formation.

Neuroglia of the CNS:

  • Astrocytes:

    • Connect neurons to blood vessels, facilitating nutrient and growth factor exchange.

    • Form scar tissue, help in the metabolism of some substances, and regulate ion concentrations (e.g., part of the blood-brain barrier).

  • Oligodendrocytes:

    • Myelinate CNS axons and provide structural support.

  • Microglia:

    • Phagocytic cells providing structural support.

  • Ependyma or Ependymal Cells:

    • Line central canal of spinal cord and ventricles of brain; cover choroid plexuses.

    • Help regulate cerebrospinal fluid composition and are ciliated cuboidal or columnar cells.

Neuroglia of the PNS:

  • Schwann Cells:

    • Produce myelin sheath on some peripheral axons; speed up nerve impulse transmission.

  • Satellite Cells:

    • Support clusters of neuron cell bodies (ganglia) and maintain ionic balances.

Types and Functions of Neuroglia (Table 10.2):

  • CNS:

    • Astrocytes: Star-shaped, structurally supportive, involved in nutrient transport and formation of scar tissue.

    • Oligodendrocytes: Myelinate axons in CNS, structural support.

    • Microglia: Immune protection through phagocytosis.

    • Ependyma: Form porous layers for substance diffusion between interstitial fluid and cerebrospinal fluid.

  • PNS:

    • Schwann Cells: Myelinate peripheral neurons, support neuronal regeneration.

    • Satellite Cells: Support ganglia, manage excess ions and neurotransmitters.

Neuronal Regeneration

  • Mature Neurons:

    • Do not divide; if cell body is injured, the neuron typically dies.

  • Neuron Regeneration in PNS:

    • Peripheral axon injury may allow regeneration if Schwann cells remain; damaged axons generate a guiding sheath.

  • Neuron Regeneration in CNS:

    • Regeneration is unlikely as CNS axons lack neurilemma and oligodendrocytes do not proliferate after injury.

10.3 Cell Membrane Potential

  • A cell's membrane is typically electrically charged (polarized).

  • Polarity: Refers to charge difference.

  • Resting Potential: Negatively charged interior compared to the exterior, about −70 mV.

    • Results from an unequal ion distribution: K+ ions are more concentrated inside, while Na+ ions exist in higher concentrations outside.

    • Negativity within the cell is due to large numbers of negatively charged impermeant proteins and phosphate/sulfate ions.

  • Importance of Membrane Potential:

    • Essential for impulse conduction in neurons and muscle fibers, allowing for rapid internal charge changes and communication between cells.

  • Resting Membrane Potential Characteristics:

    • Potassium (K+) is more permeable than Sodium (Na+).

    • Na+/K+ Pump maintains balance by pumping 3 Na+ out and 2 K+ into the cell, using ATP energy.

Action Potential

  • Involves a sequence of electrical events in an excitable cell leading to a rapid change in membrane potential.

    • Transition from negative resting potential to positive action potential, followed by return to resting potential.

Illustration of Ion Channels in Cell Membrane (Figure 10.11)

  • Action Potential Mechanics:

    • Neurons can change their resting potential due to stimuli, leading to local potential changes which may add up to trigger an action potential if threshold is reached (−55 mV).

    • Depolarization: Na+ enters cell when chemically gated channels open, making the interior less negative.

    • Repolarization: After reaching +30 mV, K+ channels open, allowing potassium ions to exit and return to resting potential.

    • Hyperpolarization: May overshoot below −70 mV briefly before returning.

10.4 The Synapse

  • Communication Between Neurons at Synapse:

    • Site for impulse transmission where presynaptic neuron sends impulse to postsynaptic neuron.

    • The synaptic cleft separates the two neurons.

    • Process of synaptic transmission is one-way, utilizing neurotransmitters for signal transfer.

Synaptic Transmission Process

  • Traverse across synaptic cleft reacting with receptors on postsynaptic neuron.

  • Neurotransmitter effects may vary:

    • Some neurotransmitters increase ion channel permeability (e.g., Na+), making action potentials more likely (excitatory), while others can have inhibitory outcomes by moving membranes further from threshold.

Synaptic Potentials: EPSP and IPSP

  • EPSP (Excitatory Postsynaptic Potential):

    • Membrane changes where neurotransmitters open Na+ channels, leading to depolarization of the postsynaptic neuron, increasing the chance for action potential generation.

  • IPSP (Inhibitory Postsynaptic Potential):

    • Membrane changes where neurotransmitters open K+ or Cl− channels causing hyperpolarization, decreasing likelihood of action potential generation.

  • EPSPs and IPSPs can sum for net excitatory or inhibitory effects, usually occurring at trigger zone.

10.5 Neurotransmitters

  • There are at least 100 identified neurotransmitters.

  • Acetylcholine: Stimulates skeletal muscle contraction, influencing CNS and peripheral systems.

  • Types of Neurotransmitters:

    • Monoamines, Amino Acids, Peptides.

    • Synthesized in the rough ER or cytoplasm.

    • Released through exocytosis when impulses reach synaptic knobs.

Neurotransmitter Functions (Table 10.4):

  • Acetylcholine:

    • Controls skeletal muscle actions, stimulates contraction, can have excitatory or inhibitory effects based on the synapse.

  • Biogenic Amines (Examples):

    • Norepinephrine: Mood regulation; low levels linked to depression.

    • Dopamine: Associated with feelings of pleasure; deficiency in certain areas linked to Parkinson's disease.

    • Serotonin: Primarily inhibitory, affects sleep; blocked by LSD but enhanced by SSRIs.

Disorders Related to Neurotransmitter Imbalance (Table 10.5):

  • Clinical Depression: Lack of norepinephrine and serotonin leads to debilitating sadness.

  • Epilepsy: Excess of GABA or norepinephrine and dopamine can result in seizures.

  • Parkinson Disease: Related to deficient dopamine causing motor symptoms.

Altering Neurotransmitter Levels (Table 10.6):

  • Various drugs influence neurotransmitter actions:

    • Tryptophan: Increases serotonin synthesis, potentially leading to sleepiness.

    • Cocaine: Blocks dopamine reuptake leading to euphoria.

10.6 Impulse Processing

  • The nervous system organizes neurons and axons in the CNS, reflecting a strategic system for processing nerve impulses.

Neuronal Pools

  • Closure Rings of Interneurons: Groups of interneurons forming synaptic connections in the CNS.

  • Facilitation through increased neurotransmitter release in response to repeated activity, promoting action potentials.

Convergence and Divergence

  • Convergence: Multiple neurons provide signals to one neuron, allowing for integration of diverse sensory inputs.

  • Divergence: One neuron influences multiple neurons, amplifying signals to affect numerous targets, such as in muscle activation.

Clinical Application 10.5: Drug Addiction Impacts on Synaptic Transmission

  • Drugs may modify neurotransmitter activities, either enhancing or inhibiting synaptic transmission.

    • Antagonists block receptors, while agonists activate them, often resulting in altered behavior and responses to stimulation.

    • Over time, addiction leads to reduced receptor sensitivity, requiring increased dosage to obtain effects, notably with stimulants like amphetamines and nicotine.