Submitted by: Isidoro Ayson, Jr.

Sec C1 Grp. 2

INTRODUCTION

Fear as defined in the dictionary as the anxious anticipation of danger, pain, lost or death. We’ve all experienced fear, maybe, a hundred times is an understatement. We’ve experienced our heart pounding against the walls of our chest, sweating and our hair raising because of fear. This literature will discuss in particular the sympathetic nervous system which is primarily responsible for the physical manifestations in a frightened individual. The psychological aspects of fear will not be discussed but rather the biochemistry behind.

REVIEW OF THE SYMPATHETICS NRVOUS SYSTEM

Sympathetic nervous system or "Alarm" or "Stress" response is a major subdivision of the autonomic system (the other subdivision is the parasympathetic system). Its fibers originate in the spinal cord between the segments T-1 and L-2. Sympathetic fibers are mutisynaptic. It has a short preganglionic neuron in which its cell bodies lies in the intermediolateral horn of the spinal cord and its fibers passes through an anterior root of the cord into the corresponding spinal nerve. As the spinal nerve leaves the spinal column the preganglionic fibers of the sympathetics separates from the spinal nerve and pass through the white ramus into the ganglia of the sympathetic chain. The fibers will synapse to postganglionic neurons, to other sympathetic ganglia, or terminate in one of the prevertebral ganglia. The postganglionic fibers of the sympathetic system origenates either from the sympathetic chain ganglia or from one of the prevertebral ganglia and terminate to various organs.

The preganglionic fibers secrete a neurotransmitter, acetylcholine to the nicotinic receptors of the proceeding postganglionic fiber. The postganglionic fibers on the other end secrete two different nerotransmitters. Catecholamines which is secreted by postganglionic fibers to alpha and beta receptors of effector cells and acethylcholine which is secreted to muscarinic receptors of effector organs. [Guyton]

Biosynthesis of Sympathtic Neurotransmitters

Acetylcholine is synthesized by the condensation of choline and acetyl CoA. This reaction is catalyzed the enzyme choline acetyltransferase which is present in the cytosol of the cell. The choline substrate is derived mainly from the diet and some come from reabsorption from synaptic junction. Acetyl CoA comes from the decarboxylation of pyruvate by the pyruvate dehydrogenase complex in the mitochondria. It exits the mitochondria in the form of citrate and then reconverted back to Acetyl CoA in the cytosol by the enzyme ATP-citrate lyase.

Catecholamines are derivatives of 3,4-dihydroxyphenethylamine (dopamine). Among the catecholamine neurotransmitters in the sympathetics are norepinephrine and epinephrine. The precursor for the synthesis of catecholamines is tyrosine. Tyrosin is first converted to L-dopa by the enzyme tyrosine hydroxylase and then dopa decarboxylase converts L-dopa to dopamine. Dopamine is converted to norepinephrine by dopamine hydroxylase and norepinephrine maybe converted to epineohrine by transmethylase.

These neurotransmitters are stored in vesicle near the synaptic knob of the nerve fiber. Newly synthesized neurotransmitters are carried by proteins into the vesicles by exchanging it for protons. The protons are pumped into the vesicles by a vacuolar ATPase.

Release of Neurotransmitters

The depolarization of the cell membrane cannot cause synaptic vesicles to fuse with the plasma membrane and release neurotransmitters. These action potentials must be transduced into a chemical signal, - a localized rise in Ca²⁺ concentration, in order to trigger vesicle fusion. Action potentials open voltage-gated Ca²⁺ channels which allows influx of Ca ions in to the cytosol near the synaptic knob. When the concentration of Ca ions is sufficient, some Ca²⁺ binds to proteins that connects the synaptic vesicles to the plasma membrane and releases the neurotransmitter it contains. The extra Ca ions in the cytosol is pumped out by Ca²⁺ ATPasese lowering the concentration Ca ions in the cytosol and prepare for another action potential.

Nicotinic Receptors in Postganglionic Fibers

Nicotinic receptors are ligand-gated channels. These channels open, when acetylcholine binds to it and allow Na ions to enter the cell and generate action potentials.

How is the Sympathetic Nervous System Triggered?

It is said that the center (the brain) triggers the release of these neurotransmitters. If the individual suddenly felt afraid, this stimulates the brain and the reflex action of the body is to release these neurotransmitters and so altering the normal state of the individual’s body.

MARKED EFFECTS OF FEAR IN THE HUMAN BODY

Increased Heart Rate and Blood Pressure

Sympathetic stimulation of the heart will increase the rate of contraction of cardiac muscle cells. The increase in heart rate is facilitated by catecholamines binding to beta 1 receptor of the SA node allowing ions to enter the cell decreasing the negativity of the cell resulting to increase excitability. Catecholamines also bind to beta 1 receptors of the ventricles of the heart increasing the force of contraction. The same neurotransmitter binds to alpha1 and alpha2 receptors of the smooth muscles of the arteries constriction the vessel and increasing the resistance of flow. The overall effect to the blood circulation is increase in blood pressure.

Sweating and Goose flesh

Sweating on the event of fear is brought about catecholamines binding to alpha1 receptors of the sweat glands. The effect of which is increase localized secretion of the sweat glands. The same complex, catecholamines binding to alpha1 receptors pilomotor muscles of the skin will initiate contraction of the smooth muscles resulting to what is commonly known as the goose flesh.

SUMMARY

In summary, the part of the body primary responsible for the manifestation of fear is the sympathetic nervous system. There are many reactions occurring in the body of a frightened individual. This literature discussed only the physical signs that manifest (increase blood pressure and heart rate, sweating and goose flesh) in a frightened individual.