Neurophysiology Introduction

Neurophysiology Study Notes

Nervous System Organization

• The nervous system is divided into:

  • Afferent Division: Responsible for sensory input; consists of sensory neurons that convey signals from sensory organs to the CNS.

  • Efferent Division: Responsible for motor output; consists of:

    • Somatic Nervous System (controls skeletal muscles)

    • Autonomic Nervous System (controls cardiac and smooth muscles, and glands)

      • Effectors for the somatic system: Skeletal muscles

      • Effectors for the autonomic system: Cardiac muscles, smooth muscles, and glands

Glial Cells

• Central Nervous System (CNS) Glial Cells:

  • Astrocytes: Support neurons, maintain blood-brain barrier.

  • Oligodendrocytes: Myelinate CNS axons.

  • Microglia: Immune defense in the CNS.

  • Ependymal Cells: Line ventricles and produce cerebrospinal fluid (CSF).

• Peripheral Nervous System (PNS) Glial Cells:

  • Schwann Cells: Myelinate PNS axons.

  • Satellite Cells: Support and protect neuron cell bodies in ganglia.

Oligodendrocytes vs. Schwann Cells

• Oligodendrocytes (CNS): Myelinate multiple axons.

• Schwann Cells (PNS): Myelinate a single axon segment.

Resting Membrane Potential (RMP)

• RMP of a typical neuron: Approximately -70 mV.

• Determined by ion concentrations across the membrane, primarily Na⁺ (sodium) and K⁺ (potassium).

• Channels Involved:

  • Leak Channels: Allow passive diffusion of K⁺ out and Na⁺ into the neuron,

  • Na⁺/K⁺ Pump: Actively transports K⁺ in and Na⁺ out.

Graded Potentials

• Definition: Changes in membrane potential that can lead to action potentials.

• Summation: When two or more graded potentials combine, it’s called temporal (timing) or spatial (location) summation.

• Subthreshold Stimulus: A stimulus that does not reach the threshold for an action potential.

• Threshold Stimulus: The minimal stimulus required to generate an action potential.

Action Potentials

• All or None: Action potentials occur fully or not at all, once the threshold is reached, typically -55 mV.

• Distance Dependence: Action potentials do not fade with distance while graded potentials do.

Action Potential Phases

• Phases:

  1. Depolarization: Na⁺ channels open, Na⁺ enters the neuron.

  2. Repolarization: K⁺ channels open, K⁺ exits the neuron.

  3. Hyperpolarization: Overshoot as K⁺ continues to leave.

• Ionic Movement: Na⁺ influx during depolarization and K⁺ efflux during repolarization.

Refractory Period

• Phases:

  • Absolute Refractory Period: No action potential can occur, Na⁺ channels are inactivated.

  • Relative Refractory Period: Action potential can occur with a stronger stimulus, K⁺ channels still open.

Propagation of Action Potentials

• Definition: The transmission of action potentials along the axon.

• Types:

  • Continuous Conduction: Occurs in unmyelinated axons.

  • Saltatory Conduction: Occurs in myelinated axons, faster due to nodal gaps.

  • Myelination increases conduction speed, with channels concentrated at the nodes of Ranvier.

Synapses

• Definition: Junctions where neurons communicate.

• Structures:

  • Presynaptic Neuron

  • Synaptic Cleft (the space between)

  • Postsynaptic Neuron

Chemical vs. Electrical Synapses

• Chemical Synapses: Use neurotransmitters; slower transmission.

• Electrical Synapses: Direct electrical signals; faster transmission.

Transmission at Chemical Synapses

• Story for Acetylcholine (ACh):

  1. Action potential reaches the axon terminal.

  2. Voltage-gated Ca²⁺ channels open; Ca²⁺ enters the terminal.

  3. ACh is released from synaptic vesicles into the synaptic cleft.

  4. ACh binds to nicotinic receptors on the postsynaptic membrane, leading to depolarization.

  5. If depolarization exceeds threshold, an action potential ensues in the postsynaptic neuron.

Graded Potentials

• Hyperpolarizing Graded Potential: Receives negative signals, causing an inhibitory effect; receptors involved could include GABA receptors.

• Depolarizing Graded Potential: Receives positive signals; involved receptors could include nicotinic ACh receptors.

Summation Types

• Spatial Summation: Multiple signals at different locations on the neuron.

• Temporal Summation: Multiple signals from the same location over time.

Integration of Concepts

• Complete Story:

  1. Action potential phases

  2. Propagation along myelinated axon

  3. Transmission of electrical signals at chemical synapse

  4. Production of graded potentials = summation of EPSP (excitatory) and IPSP (inhibitory) signals that affect the postsynaptic neuron's excitability.