Overview of the Nervous System and Action Potentials

Chapter 11 Quick Overview

Overview of the Nervous System

  • Nervous System

    • Controls perception and experience of the world

    • Directs voluntary movement

    • Seat of consciousness, personality, learning, and memory

    • Regulates many aspects of homeostasis in conjunction with the endocrine system, including:

    • Respiratory rate

    • Blood pressure

    • Body temperature

    • Sleep/wake cycle

    • Blood pH

  • Source: Pearson Education, Inc. (2016)

Functional Divisions of the Nervous System

  • Central Nervous System (CNS)

    • Composed of brain and spinal cord

    • Integrates information

  • Peripheral Nervous System (PNS)

    • Composed of cranial and spinal nerves linking CNS to the rest of the body

    • Performs motor and sensory functions

    • Divided into:

    • Sensory (afferent) division: Carries sensory information to the CNS.

      • Somatic sensory division: Carries general sensory stimuli from muscles, bones, joints, and skin, as well as special sensory stimuli.

      • Visceral sensory division: Carries stimuli from internal organs.

    • Motor (efferent) division: Carries stimuli from the CNS to peripheral effectors.

      • Somatic motor division: Carries stimuli to skeletal muscles.

      • Autonomic nervous system (ANS): Carries stimuli to smooth muscle, cardiac muscle, and glands with involuntary control.

Neurons

  • Components of Neurons

    • Dendrites: Receive signals from other neurons

    • Cell body (soma): Contains nucleus and organelles

    • Axon hillock: Region where action potentials are initiated

    • Axon: Transmits impulses away from the cell body to other neurons or muscles

    • Axoplasm: Cytoplasm of the axon

    • Myelin sheath: Insulating layer around some axons to improve signal transmission speed

    • Axon terminals: End of axon, where neurotransmitters are released

    • Nissl bodies: Rough endoplasmic reticulum in neurons; involved in protein synthesis

    • Neurofibrils: Intermediate filaments providing structural support

Neuron Organization in the CNS and PNS

  • CNS:

    • Nuclei: Clusters of neuron cell bodies

    • Tracts: Bundles of axons

  • PNS:

    • Ganglia: Clusters of neuron cell bodies

    • Nerves: Bundles of axons

Neuroglia (Glial Cells)

  • Definition: Smaller supportive cells surrounding neurons

  • Functions:

    • Anchoring neurons and blood vessels

    • Monitoring composition of extracellular fluid

    • Speeding up rate of nerve impulse transmission

    • Circulating cerebrospinal fluid surrounding the brain and spinal cord

    • Capable of mitosis (unlike neurons)

Types of Neuroglia

  1. Astrocytes

    • Anchor neurons and blood vessels

    • Regulate extracellular environment

    • Facilitate formation of the blood-brain barrier

    • Repair damaged tissue

  2. Oligodendrocytes

    • Myelinate certain axons in the CNS

  3. Microglial Cells

    • Act as phagocytes, cleaning up debris in the CNS

  4. Ependymal Cells

    • Line cavities of the CNS

    • Cilia help to circulate cerebrospinal fluid

    • Some secrete cerebrospinal fluid

  5. Schwann Cells

    • Myelinate certain axons in the PNS

  6. Satellite Cells

    • Surround and support neuron cell bodies in ganglia

Gliomas and Astrocytomas

  • Primary brain tumors originating in the brain, most commonly gliomas.

  • Caused by an abnormally high rate of division of glial cells.

  • Predisposing conditions include:

    • Exposure to ionizing radiation

    • Certain diseases

  • Most affected cell type: Astrocytes; the resulting tumor is called Astrocytoma.

  • Severity varies from mild (good prognosis) to highly aggressive (poor prognosis).

  • Treatment options vary and generally involve:

    • Surgical removal of the tumor

    • Chemotherapy

    • Radiation therapy

Principles of Electrophysiology

  • Changes in Resting Membrane Potential

    • Resting membrane potential is generated due to the unequal distribution of ions and their differential ability to cross the plasma membrane.

    • Changes in ion permeability (via gated channels) alter the membrane potential.

Changes in Resting Membrane Potential

  1. Depolarization

    • Sodium channels open, allowing positively charged sodium ions (Na+) to flow into the cell; membrane potential becomes more positive.

  2. Repolarization

    • Potassium channels open; K+ flows out, returning the membrane potential toward resting, making it more negative.

  3. Hyperpolarization

    • Membrane potential becomes more negative than normal resting potential due to loss of K+ or gain of negatively charged ions (e.g., Cl-).

Action Potentials

  • Definition: A uniform, rapid depolarization and repolarization of the membrane potential; generated only in trigger zones (axon hillock and initial segment of axon).

  • Phases of Neuronal Action Potential:

    1. Depolarization Phase:

    • Membrane potential rises toward zero and becomes temporarily positive.

    1. Repolarization Phase:

    • Membrane potential returns to a negative value.

    1. Hyperpolarization Phase:

    • Membrane potential temporarily becomes more negative than resting potential.

Steps of Action Potential Generation

  • Triggered by the depolarization of the axolemma to threshold (usually around -55 mV).

  • Once threshold reached:

    • Voltage-gated sodium ion channels open, Na+ flows into the axon causing depolarization.

    • Increased Na+ permeability due to positive feedback leads to rapid depolarization to about +30 mV.

  • Repolarization begins when sodium channels inactivate and potassium channels open, allowing K+ to exit the axon.

  • Hyperpolarization may occur before returning to resting potential.

Local Anesthetic Drugs

  • Local anesthetics (e.g., lidocaine) produce temporary numbness by blocking voltage-gated sodium channels in treated area.

  • This prevents depolarization, inhibiting action potentials that relay pain sensation to the CNS.

  • Nonselective effect may also lead to temporary weakness or paralysis.

Refractory Period

  • Definition: Time after an action potential during which a neuron cannot be stimulated to generate another action potential.

  • Phases:

    1. Absolute Refractory Period: No additional stimulus will produce an action potential.

    2. Relative Refractory Period: A stronger than normal stimulus can cause an action potential.

Action Potential Propagation

  • Mechanism: Action potentials propagate down the axon from the trigger zone to axon terminals.

  • Each action potential triggers another in the adjacent segment of the axon, making them self-propagating due to local depolarization, leading to depolarization of neighboring areas.

Clinical Considerations

Multiple Sclerosis

  • Definition: An autoimmune disorder where the immune system attacks myelin sheaths in the CNS, leading to progressive loss of myelin and conduction slowing.

  • Symptoms may include changes in sensation, cognitive alterations, and motor dysfunction (e.g., paralysis).

Overview of Neuronal Synapses

  • Synapse: The junction where a neuron communicates with another neuron or target cell.

  • Types of synapses:

    • Electrical Synapse: Cells are electrically coupled via gap junctions; transmission is bidirectional and instantaneous.

    • Chemical Synapse: The majority of synapses; utilize neurotransmitters to convey signals; characterized by:

    • Synaptic vesicles (containing neurotransmitters)

    • Synaptic cleft (space between pre and postsynaptic neurons)

    • Unidirectional transmission

    • Presence of synaptic delay.

Events at a Chemical Synapse

  1. An action potential triggers voltage-gated calcium channels to open in the presynaptic neuron.

  2. Ca2+ influx causes synaptic vesicles to release neurotransmitters into the synaptic cleft.

  3. Neurotransmitters bind to receptors on the postsynaptic neuron, opening ion channels and producing local potentials or action potentials as needed.

Major Neurotransmitters

  • Many neurotransmitters are classified based on structure and function. Characteristics include:

    • Synthesized in neurons and released at axon terminals

    • Binds to receptors, leading to excitatory postsynaptic potentials (EPSPs) or inhibitory postsynaptic potentials (IPSPs).

  • Major categories:

    1. Acetylcholine: Excitatory, found in CNS (brain and spinal cord) and PNS (neuromuscular junction).

    2. Biogenic Amines: Includes catecholamines (e.g., norepinephrine, dopamine) with excitatory effects.

    3. Amino Acids: Glutamate (excitatory), GABA (inhibitory), glycine (inhibitory).

    4. Neuropeptides: Include substance P (pain perception) and opioids (pain control).

Psychiatric Disorders and Treatments

  • Treatments often involve modifying synaptic transmission such as:

    • Schizophrenia: Management mainly involves blocking postsynaptic dopamine receptors.

    • Depressive Disorders: Treated with SSRIs that block the reuptake of serotonin.

    • Anxiety Disorders: Treated with antidepressants and GABA enhancers.

    • Bipolar Disorders: Often treated via blocking sodium channels to decrease action potential generation.

Epileptic Seizures

  • Definition: Recurrent episodes of abnormal electrical activity in the brain (seizures).

  • Caused by excessive excitation in a neural network, overcoming inhibitory controls, resulting in widespread excitation.

  • Symptoms range from mild disturbances to loss of consciousness.

  • Treatment focuses on preventing seizure activity and restoring normal inhibitory function.