Neurohumoral Transmission

Nervous System

• A network of nerve cells and fibers that transmits nerve impulses between parts of the body.

Classification of the nervous system

Motor: carries neuronal impulses away from the CNS to different parts.

Sensory: Carry impulses from sensory stimuli towards the CNS and Brain.

Introduction to Autonomic Nervous System Drugs

Neurohumoral transmission

• Neurohumoral transmission implies that nerves transmit their message across synapses and neuroeffector junctions by the release of a humoral (chemical) message.

Steps in Neurohumoral Transmission

1. Impulse conduction

• Resting transmembrane potential: -70 mV (inside) (high concentration of K⁺ ions).
• Electrical impulse causes an increase in Na⁺ conductance, so depolarization occurs.
• Now the voltage becomes +20 mV.
• K⁺ ions then move out in the direction of their concentration, and repolarization is achieved.
• The ionic distribution is normalized during the refractory period by activation of Na⁺ K⁺
• The action potential thus generated sets of local circuit currents, which activate ionic channels at the next excitable part of the membrane.

2. Impulse release

• Transmitted stored pre-junctional nerve ending with a synaptic cleft.
• Entry of Ca²⁺: all contents of the vesicle are extruded.

3. Transmitter action on post-junctional member

• Release transmitter combines with a specific receptor on the postsynaptic membrane and, depending on its nature induce an excitatory postsynaptic potential (EPSP) or inhibitory postsynaptic potential (IPSP)

EPSP

• Increase permeability of cations, so that Na⁺ and Ca²⁺ influx

IPSP

• Increase permeability of anions (Cl^– ions move inside)
• Increase in permeability to K⁺ ions, which move out carrying positive charges.

4. Postjunctional activity

• EPSP: propagated post-junctional AP results in nerve impulse secretions.
• IPSP: stabilize the post-junctional membrane. It resists depolarization stimuli.

5. Termination of transmitter action

• The transmitter is locally degraded or partly taken back into the pre-junctional neuron (e.g., Ach).
• Specific carrier proteins, e.g., NET (norepinephrine transporter), DAT (dopamine transporter), SERT (serotonin transporter), amino acid transporter, enkephalins, VIP (vasoactive intestinal peptides), and NPY (neuropeptide-y).

6. Co-transmission

• The co-transmitter is stored in the pre-junctional nerve terminal along with the primary but in a separate vesicle.
• Nerve impulse releases both transmissions concurrently.
• It may also act on pre-junctional receptors and modulate the release of the transmitters.