1. Neurophysiological Basis of Movement World II: Connections

2. The spinal cord has a laminar structure. It “flows” along the body, preserving the general picture of its cross-sections (upper figure). At each level, the gray matter forms a characteristic butterfly picture consisting of 10 Rexed’s laminae (lower figure). I II III IV V VI VII VIII IX X Front Back Lecture 7: Excitation and Inhibition Within the Spinal Cord

3. These geographical terms will be helpful in future descriptions. The figure of a person is drawn in a sagittal plane. Geographical Terms Dorsal Ventral Lateral Medial Ventral Rostral Distal Proximal Dorsal Caudal

4. Each vertebra has a body and a spinous process. Peripheral information gets into the spinal cord through the dorsal roots, while efferent signals are sent from the spinal cord through the ventral roots. Spinal Roots Vertebra Spinous process Dorsal root Spinal cord Ventral root

5. Starting from the skull, vertebrae are numbered from C 1 to L 5 and then end with the sacrum. Spinal segments are numbered from C 1 to S 5, but this classification does not correspond exactly to the vertebrae classification. The spinal cord ends at the L 1 vertebra; lower, the roots of the lower segments form the cauda equina. Vertebrae and Segments Spinal cordVertebrae Ventral roots C1C1 C7C7 T1T1 T 12 L1L1 L5L5 Sacrum Cauda equina Dorsal roots C1C1 C8C8 T1T1 T 12 L1L1 L5L5 S1S1 S5S5

6. A synapse consists of a presynaptic membrane, a synaptic cleft, and a postsynaptic membrane. Inhibition means a decrease in the efficacy of the synapse and may occur as a result of events on the presynaptic or the postsynaptic membrane. Excitatory and Inhibitory Synapses Presynaptic membrane Postsynaptic membrane Synaptic cleft Synaptic vesicles

7. EPSP and IPSP An excitatory synapse leads to a depolarization of the postsynaptic membrane (i.e., bringing it closer to the threshold), whereas an inhibitory synapse leads to a hyperpolarization of the postsynaptic membrane. Excitatory synapseInhibitory synapse V Time EPSP V Time IPSP Threshold

8. Axons of  -motoneurons branch very close to the cell body and make excitatory synapses on Renshaw cells. These cells, in turn, make inhibitory synapses on  - motoneurons of the same pool as well as on  -motoneurons, sending their axons to spindles in the same muscle. Recurrent Inhibition  -MN  Renshaw cell  -MN  Muscle

9. Ia interneurons receive excitatory inputs from Ia afferents and make inhibitory synapses on motoneurons innervating the antagonist muscle. Ia INs are inhibited by Renshaw cells and also receive descending inputs. Reciprocal Inhibition  -MN Renshaw cell  -MN Antagonist muscle Ia IN Ia afferent T-shaped axon Ganglion Antagonist pool Agonist pool Agonist muscle

10. Feedback loops involving Ia interneurons and Renshaw cells. A plus sign means that an increase in the input leads to an increase in the output. A minus sign means that an increase in the input leads to a decrease in the output. Schemes of Recurrent and Reciprocal Loops _ + Agonist force Antagonist length Ia afferents Ia INs  -MNs + + + + Renshaw cell  -MNs + _ Ia INs  -MNs Antagonist force + _ Agonist force

11. Recurrent inhibition:  Alpha-motoneurons of a pool fire.  They send axon branches to small inhibitory interneurons (Renshaw cells) in the ventral horns of the spinal cord.  Renshaw cells inhibit all motoneurons of the same pool. Reciprocal inhibition:  Small interneurons (Ia interneurons) are activated by primary spindle afferent fingers (Ia afferents).  The Ia interneurons inhibit motoneurons of the antagonist muscle. Recurrent and Reciprocal Inhibition

12. A postsynaptic inhibitory synapse hyperpolarizes the postsynaptic membrane and decreases its responsiveness to excitatory synapses (shown by open circles). Postsynaptic Inhibition Inhibitory synapse

13. Presynaptic inhibition acts selectively on certain synapses. It involves an excitatory synapse acting on the presynaptic membrane, inducing its steady subthreshold depolarization, thus decreasing the amount of mediator released in response to a single presynaptic action potential. Presynaptic Inhibition Presynaptic membrane Postsynaptic membrane Synaptic cleft Presynaptic inhibition

14. What Makes Presynaptic Inhibition Possible? A: A relatively small change in the peak-to-peak amplitude of the presynaptic action potential (  A) leads to a major change in the amount of released neurotransmitter (  NT). B: A small, steady depolarization leads to a drop in the peak-to-peak AP amplitude and a decrease in the synapse efficacy. A AP Neurotransmitter AA  NT A Time Voltage V EQ Threshold A AP B

15.  -MN Afferent fiber Muscle Presynaptic inhibition In this figure, an excitatory synapse is acting at a presynaptic afferent (sensory) fiber close to its synapse at the target  -motoneuron. An Example of Presynaptic Inhibition

16. An illustration of weak effects (solid line), moderate effects (dashed line), and strong effects (dotted line) of persistent inward currents (PIC). Voltage Current −70 PIC The Role of Persistent Inward Currents (PICs)