Neurophysiology

Major Divisions of the Nervous System

  1. Central Nervous System (CNS)

    • Composed of the brain and spinal cord.

    • Functions as the integrative and control center.

  2. Peripheral Nervous System (PNS)

    • Includes cranial and spinal nerves.

    • Serves as communication lines between the CNS and the rest of the body.

Divisions of the PNS

  • Sensory (Afferent) Division

    • Comprises somatic and visceral sensory nerve fibers.

    • Conducts impulses from receptors to the CNS.

  • Motor (Efferent) Division

    • Comprises motor nerve fibers.

    • Conducts impulses from the CNS to effectors (muscles and glands).

Autonomic Nervous System (ANS)

  • Sympathetic Division

    • Mobilizes body systems during activity ("fight or flight").

  • Parasympathetic Division

    • Conserves energy and promotes housekeeping functions during rest.

Types of Cells in the Nervous System

1. Neurons

  • Major functional units of the nervous system.

  • Electrically excitable and specialized in information processing.

  • Do not divide once matured; injuries can lead to permanent changes.

2. Neuroglial Cells (Glia)

  • Support cells in the nervous system involved in the nutrition and maintenance of nerve cells.

  • Major types include astrocytes, oligodendrocytes, and Schwann cells.

Neuron Classifications

Functional Classifications

  1. Sensory (Afferent) Neurons

    • Convey sensory information from receptors to CNS.

  2. Interneurons (Association Neurons)

    • Connect motor and sensory neurons within the CNS.

  3. Motor (Efferent) Neurons

    • Transmit information from the CNS to effectors.

    • Includes somatic (voluntary) and autonomic (involuntary).

Structural Classifications

  1. Unipolar (Pseudounipolar) Neurons

    • Single axonal process that branches into peripheral and central processes.

    • Found in sensory ganglia and cranial nerves.

  2. Bipolar Neurons

    • Have two processes (axon and dendrite).

    • Located in retina, inner ear, and olfactory epithelium.

  3. Multipolar Neurons

    • Possess one axon and multiple dendrites.

    • Most common neuron type, found throughout the body.

Neuron Anatomy

Components and Functions

  1. Dendrites

    • Receive information from presynaptic neurons.

  2. Cell Body (Soma)

    • Contains organelles: nucleus, ribosomes, rER, Golgi apparatus, and mitochondria.

  3. Axon

    • Conducts electrical signals away from the cell body.

  4. Presynaptic Terminals

    • Release neurotransmitters to send signals to neighboring neurons.

Myelin and its Functions

  • Myelin Sheath

    • Formed by Schwann cells in PNS and oligodendrocytes in CNS.

    • Acts as electrical insulation to speed up signal conduction.

    • Internodal segments of myelinated axons interspersed with nodes of Ranvier, supporting saltatory conduction.

Membrane Potential and Action Potentials

  1. Resting Membrane Potential (RMP)

    • The electrical potential difference across the plasma membrane when a cell is not excited; typically around -70mV (neuron).

    • Maintained by Na+/K+ pump and permeability differences across the membrane.

    • Ion concentrations: higher K+ inside and higher Na+ outside the cell.

  2. Action Potential

    • Triggered when the membrane potential reaches a threshold.

    • Involves sequential opening and closing of voltage-gated Na+ and K+ channels.

    • Propagates down the axon triggering neurotransmitter release at the synapse.

Neurotransmitter Release at Synapses

  • Action potentials trigger Ca2+ influx at axon terminals, leading to neurotransmitter release via exocytosis.

  • Major classes include:

    • Amino Acids: Glutamate, GABA

    • Amines: Acetylcholine, serotonin

    • Catecholamines: Dopamine, norepinephrine, epinephrine

    • Peptides: Endorphins, endogenous opioids.

Summary of Action Potential Stages

  1. Depolarization: Na+ channels open, Na+ flows in, causing less negative potential.

  2. Repolarization: Na+ channels close; K+ channels open to restore negative potential.

  3. Hyperpolarization: K+ continues to leave, making the inside more negative temporarily before returning to resting state.

  4. Propagation: Action potentials propagate down the axon via passive charge spread at myelinated segments.

Additional Information

  • Calcium Ions (Ca2+): Involved in synapse signaling; essential for neurotransmitter release.

  • Neurotransmitter Effects: Depends on type and receptor activated, influencing either excitatory (EPSP) or inhibitory (IPSP) potentials.

  • Conduction Velocity: Faster in myelinated fibers and larger diameters.

Contact Information

  • Happy Studying!

  • Clara Camargo, DVM

  • Email: ccamargo@rossvet.edu.kn

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