Bio 2870 Lecture No. 11- Integration in CNS and PNS

**Excitatory and Inhibitory postsynaptic potentials

At the synapsis, a chemical neurotransmitter is released and its effect either depolarizes or hyperpolarizes the postsynaptic membrane, thereby exciting or inhibiting the post synaptic neuron.

When the depolarization of postsynaptic membrane occurs, the response is stimulatory, and the local depolarization is an excitatory postsynaptic potential (EPSP).

The neuron that release neurotransmitter causing EPSP is an excitatory neuron.

Recall that EPSP is the result of increasing permeability to Na+.

Glutamine in CNS and ACh in PNS are some of the examples.

Contrary to the above, when a neurotransmitter causes hyperpolarization of the postsynaptic membrane, the result will be inhibitory and an inhibitory postsynaptic potential (IPSP) is observed.

The neuron is an inhibitory neuron.

The IPSP is the result of an increased permeability to Cl- or K+.

In the spinal cord glycine bind to the postsynaptic membrane to increase permeability to Cl-, the ion will flow inward to the cell according to the concentration gradient. Thus, increases negative memrane potential.

In the cardiac muscles, acetylcholine binds its receptors causing G protein-mediated opening of K+ ion channels. Thus, K+ ions flow outward from the cells leading to hyperpolarization.

**Presynaptic inhibition and facilitation

Many of the synapses of the CNS are axoaxonic synapses.

In other words, the axon of one neuron synapses with presynaptic terminal of another.

The axoaxonic synapses can control the amounts of neurotransmitter release from the presynaptic membrane to the cleft.
Thus, it could become either presynaptic inhibitor or facilitator.

** Summation of postsynaptic local potentials

Action potential is all-or-none.

But, within CNS and in many PNS a single presynaptic action potential may not cause enough local depolarization of the postsynaptic membrane reach to the threshold.

Multiple of presynaptic actions must be summed to provide enough depolarization on the soma of the membrane to have an action potential at the axon hillock.

** Neurotransmitters    
     
Substance Location Effect
Acetylcholine CNS, N/M junction
many ANS		 E/I
Monoamines    
Norepinephrine CNS, some ANS E/I
Serotonin CNS I
Dopamine CNS, some ANS E
Histamine CNS I
Amino Acids
GABA CNS brain post syn I
Spin pre syn I
Glycine CNS local effect Spin post synI
Glutamate CNS E
Aspartate CNS E
Nitric oxide CNS, glands E
Neuro Peptides
Endorphins CNS, PNS I
Enkephalins CNS, PNS I
Substance P CNS, pain sensory n. E


PNS AND INTEGRATED NEURONAL FUNCTIONS


Another important function of the neuronal system is the communication between the CNS and PNS.

They create pathways, circuits.

A sensory receptor ascends sensory pathway (afferent division), through the medulla and the thalamus before reaching the cerebral cortex for final processing.

Then descends through motor pathway (efferent division) to effector tissue and organs.

We will first study the peripheral nervous system. then their integrated functions with CNS.

1. The peripheral nervous system

The peripheral nervous system includes all the neurons other than those of CNS, and inclusive of the cell bodies and the axons of sensory neurons.

a. The cranial nerves

The peripheral nerves may be connected to CNS through the spinal cord or directly.

The cranial nerves are those directly connect to the brain.

There are 12 pairs (Fig. 9-1) and identified with N and Roman numerals, i.e. N I - N XII.

b. The spinal nerves

A cross sectional Fig of the spinal cord here. (Fig. 8-15)

All together 31 pairs of spinal nerves grouped nerves are identified. According to the region of the spinal cord, they are broken down into more groups. (Fig. 9-2)

8 pairs of cervical nerves: up to C8
12 pairs of thoracic nerves: Up to T12
5 pairs of lumbar nerves: Up to L5
5 pairs of sacral nerves: Up to S5
A pair of coccygeal nerves: Co1

c. Nerve plexuses

Fused compound nerves

2. The CNS and PNS: Integrated functions


a. Simple reflexes

We have already studied a reflex arc.

There was only one synapsis between the sensory neuron and the motor neuron, thus a monosynaptic reflex.

An example is the stretch reflex, i.e. knee jerk.

A Fig of stretch receptor and reflex arc here.

b. Complex reflexes

Addition of an interneruon(s) to a monosynaptic reflex makes the reflex arc a complex reflex, or polysynaptic reflex.

We have already seen withdrawal reflexes, such as the flexor reflex, could be complex reflexes.

Study Fig. 9-3 and observe the sequence of events. Also note that the signal could ascend through the spinal cord.

Why not both the flexor muscle and extensor muscles are simultaneously stimulated? There is reciprocal inhibition.


c. Integration and control of spinal reflexes

Automatic reflexes may involve higher centers, thus making it possible for the effector or affector neurons to control the reflex arc.

Example: Babinski sign of fanning of the baby toes. After developing inhibitory synapses in adult, the funning changes to curling toes.

Used as test for any damage to the neuronal ckt.

3. Sensory and Motor pathways

A number of sensory and motor pathways have been discovered and some of their examples are shown in Table 9-3.

a. Sensory pathways

Pay spatial attention to the posterior column pathway, which is described in Fig. 9-4.

The sensors respond to: localized touch, pressure, vibration and position sensations

Through dorsal roots of the spinal nerves

Ascend through the column to the medulla oblongata and cross over.

The thalamus, where positions are sorted.

To the sensory cortex according to the positions in the body.

Relative sizes in the cortex responding to the specific regions of the body are in proportion not to the size of the region, but the number of sensors present in that region.

b. Motor pathways

Responding the stimulation by the sensory systems, primary motor cortex or the pyramidal cells, of the cerebral cortex start sending signals through the somatic nervous system (SNS) and the autonomic nervous system (ANS).

Note the pyramidal system Fig. 9-5 as an example of somatic nervous system

It is essentially in reverse to the sensory pathway.

Note there are cranial nerves coming out directly from the brain.


4. The autonomic nervous system

As shown in Fig. 9-6, the autonomic nervous system has autonomic ganglia after preganglionic neurons.

The post ganglionic fibers innervate cardiac muscles and smooth muscles.

There are two types of ANS; sympathetic division and parasympathetic division.

The sympathetic division: Preganglionic fibers from the T and L region of the spinal cord make ganglia. Fig. 9-7. They usually stimulates tissue metabolism, increases alertness, get ready for emergencies.

The parasympathetic division: They originate from the brain and the S region of the spinal cord. Conserve energies and sedentary. Also digestion.

The neurotransmitters in the ANS.

Preganglionic uses ACh

Post ganglionic parasympathetic fibers are also ACh. But, could be E or I depending on the target cells.


Most postganglionic sympathetic terminal adrenergic.

a. The relationships of the two

Study Table 9-4.


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