Synaptic Transmission: Temporal and Spatial Summation

Temporal and Spatial Summation

of Excitation and Inhibition

A. Resting state: motor nerve cell shown with synaptic boutons of excitatory and inhibitory nerve fibers ending close to it

Axon

Inhibitory fibers

C. Temporal excitatory summation: a series of impulses in one excitatory fiber together produce a suprathreshold depolarization that triggers an action potential

Axon

Inhibitory fibers

B. Partial depolarization: impulse from one excitatory fiber has caused partial (below firing threshold) depolarization of motor neuron

Axon

Inhibitory fibers

D. Spatial excitatory summation: impulses in two excitatory fibers cause two synaptic depolarizations that together reach firing threshold triggering an action potential

Axon

Inhibitory fibers

E. Spatial excitatory summation with inhibition: impulses from two excitatory fibers reach motor neuron but impulses from inhibitory fiber prevent depolarization from reaching threshold

FIGURE 2.9 TEMPORAL AND SPATIAL SUMMATION •

Neurons receive multiple excitatory and inhibitory inputs. Temporal summation occurs when a series of subthreshold impulses in one excitatory fiber produces an action potential in the postsynaptic cell (panel C). Spatial summation occurs when subthreshold impulses from two or more different fibers trigger an action poten-

tial (panel D). Both temporal and spatial summation can be modulated by simultaneous inhibitory input (panel E). Inhibitory and excitatory neurons use a wide variety of neurotransmitters, some of which are summarized here.

Left lateral phantom view

Right lateral ventricle

Left interventricular foramen (Monro)

3rd ventricle

CHART 2.2 CSF COMPOSITION

Cerebral aqueduct (Sylvius)

4th ventricle

Left lateral aperture (foramen of Luschka)

Left lateral recess

Median aperture (foramen of Magendie)

Central canal of spinal cord

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CSF circulates through the four brain ventricles (two lateral ventricles and a third and fourth ventricle) and in the subarachnoid space surrounding the brain and spinal cord. The electrolyte composition of the CSF is regulated by the choroid plexus, which

secretes the CSF. Importantly, the CSF has a lower [HCO3 ] than plasma and therefore a lower pH. This allows small changes in blood PCO2 to cause changes in CSF pH, which in turn regulates the rate of respiration (see Chapter 5).

Choroid plexus of lateral ventricle (phantom)

Cistern of corpus callosum

Dura mater

Arachnoid

Interventricular foramen (Monro)

Choroid plexus of 3rd ventricle

Cerebral aqueduct (Sylvius)

Lateral aperture (foramen of Luschka)

Choroid plexus of 4th ventricle

Dura mater

Arachnoid

Subarachnoid space

Central canal of spinal cord

Superior sagittal sinus

Subarachnoid space

Arachnoid granulations

Median aperture (foramen of Magendie)

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FIGURE 2.11 CIRCULATION OF CEREBROSPINAL FLUID•

CSF circulates through the four brain ventricles (two lateral ventricles and a third and fourth ventricle) and in the subarachnoid space surrounding the brain and spinal cord. Most of the CSF is

reabsorbed into the venous system through the arachnoid granulations and through the walls of the capillaries of the central nervous system and pia mater.

Base of skull

C1 spinal nerve

C8 spinal nerve

T1 spinal nerve

1st rib

Intercostal nerves

T12 spinal nerve

12th rib

Conus medullaris

L1 spinal nerve

Cauda equina

S1 spinal nerve

Sacrum (cut away)

Termination of

dural sac

Coccygeal nerve

FIGURE 2.12 SPINAL CORD AND VENTRAL RAMI IN SITU•

C1 vertebra (atlas)

Cervical plexus

Brachial plexus

Spinal dura mater

Filaments of spinal nerve roots (T7 and T8)

L1 vertebra

Lumbar plexus

L5 vertebra

Sacral plexus

Sciatic nerve

Coccyx

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The spinal cord gives rise to 31 pairs of spinal nerves that distribute segmentally to the body. These nerves are organized into plexuses that distribute to the neck (cervical plexus), upper limb (brachial plexus), and pelvis and lower limb (lumbosacral plexus). Motor

fibers of these spinal nerves innervate skeletal muscle, and sensory fibers convey information back to the central nervous system from the skin, skeletal muscles, and joints.