Synapse-Neurotransmitters

SYNAPSE-NEUROTRANSMITTERS

  • A transmitter cause excitation or inhibition is determined by
    • The transmitter released at the synaptic terminal
    • The nature of the receptor present in the post synaptic membrane
  •  A single neuron may either be excited or inhibited. When excited, a transmitter is released at the synaptic knob which acts on the post synaptic excitatory receptors of the knob (eg. Acetyl choline). When inhibited, a transmitter is released at certain other knobs which acts on the post-synaptic inhibitory receptors of the knob (eg. glycine). Similarly, in the CNS norepinephrine released in the synaptic knob when acted on excitatory post synaptic receptors of the knob causes excitation and when the receptors are inhibitory causes inhibition.
  • Usually, only a single type of transmitter substance is released at the nerve terminal by each neuron and the concept is known as “Dale's principle”. However, there are exceptions in which the same nerve terminal might secrete another transmitter which may be excitatory or inhibitory, and yet another that acts as a modulator.
  • For example, the excitatory transmitter might be glutamate which causes immediate excitation as it secretes one of the neuropeptides might also be secreted as a modulator and increases the number of glutamate receptors and this increases the sensitivity of the synapse for days or weeks.

The transmitter substances

  • More than 30 different biochemical substances have  been  identified. These substances are grouped as  follows :
    • Class 1: Acetylcholine 
    • Class 2: Several  amines
    • Class 3: Several  amino acids
    • Class 4: Neuroactive peptides.
  • The following are the important transmitters :
  • Acetylcholine : It is secreted by neurons of the following areas: 
  •  Parts of the brain such as
  • Preganglionic neurons of the autonomic nervous system.
  • Post ganglionic neurons of the parasympathetic nervous system
  • Some of the post ganglionic neurons of the sympathetic nervous system.
  • Glycine: Glycine mainly secreted at synapses in the spinal cord and probably always acts as an inhibitor transmitter.
  • Gamma-aminobutyric acid (GABA): In addition to its secretion in the neurons of the spinal cord, it is secreted in the neurons of the cerebellum, basal ganglia and many areas of the cortex. It always causes inhibition.
  • Glutamate: Glutamate is secreted by the presynaptic terminals in most of the sensory pathways and also in many areas of the cortex. It is an excitatory transmitter.
  • Substance P: It is released by pain fiber terminals present in the dorsal horn of the spinal cord. It is also found in the basal ganglia and hypothalamus. Generally it is a excitatory transmitter.
  • Enkephalins: Enkephalins are secreted by the nerve terminals, in the spinal cord, brain, stem, thalamus and hypothalamus. Act as excitatory transmitters to the system that inhibit the transmission of pain.
  • Serotonin : serotonin is secreted by the nuclei that present in the median rapne of the brain stem and the fibers of the nuclei project to many brain areas such as hypothalamus and dorsal horn of the spinal cord. Serotonin acts as an inhibitor of the pain pathways.
  • The esterase: Hydrolyses both, the noradrenaline and the adrenaline into the principal end product 3-Methoxy, 4-hydroxy mandalic acid which is secreted along the urine. The esterase, is found in the adrenergic nerve ending.
  • Post-ganglionic: The transferase is found predominantly in the adrenal medulla as the norepinephrine is the precursor of epinephrine in the medulla and the transferase-methylates the former to the later. On generalised sympathetic stimulation adrenal medulla releases 80% epinephrine and 20% norepinephrine into the circulation.
  • In the nerve endings the enzymatic inactivation of the norepinephrine accounts only 15% of the total transmitter released on stimulation. The rest 85% is reincorporated into the storage vesicles of the nerve endings. The receptors bind the transmitter and prevents it’s diffusion away from the nerve terminals, the later help the reincorporation of the transmitter.
  • Examples of neurotransmitters are
    • Dopamine
    • Norepinephrine

Dopamine

  •  It is secreted by the neurons of the substantia nigra and  the basal ganglia. The effect is usually inhibition.
  • There are about 3000 molecules of acetylcholine are present in each vesicle that store the transmitter. Enough number of such vesicles are present in the presynaptic terminal of a neuron to transmit few thousand impulses.
  • Acetylcholine is synthesized from acetyl-CoA and choline with the help of the enzyme choline acetyl transferase which is present in abundance in the cytoplasm of the cholenergic type of presynaptic terminal. The released acetylcholine present in the cleft is rapidly split again to acetate and choline by the enzyme cholinesterase which is present in the proteoglycon reticulum filling the cleft. The choline is transported back into the knob and is reutilized for resynthesis of new ach. molecules.

Norepinephrine

  • It is secreted by many neurons of the brain stem, hypothalamus and pons. The later sends nerve fibers to extensive areas of the brain and function in controlling the overall activity and mood of the mind. Mostly it causes excitation in the areas and however some inhibition too. Most of the post ganglionic nerve endings of the sympathetic system secrets norepinephrine and excite some organs and inhibits the others. The adrenaline acts on the other post ganglionic nerve endings.
  • Catechol-O-methyl transferase and catechole-O-methyl esterase are the two enzymes that metabolise the noradrenaline and adrenaline respectively, in the cord and also helps to control the mood and cause sleep.
  • Other substances such as peptides, other amino acids, histamine, prostaglandins, cyclic AMP and many others also act as neurotransmitters.
Last modified: Monday, 6 June 2011, 7:25 AM