IPHY 3450 Muscle and Coordinated Movement
Coordinated movement is produced by neural output to skeletal muscle. The organization of the nervous system provides a simple look at how input and output are arranged to produce this activity.
The functional unit of the nervous system is called the Reflex arc Neuronal circuit that connects sensory input to motor output.
There are three types of reflex arc
(see fig 8-2, pg 278) The sequence of events from environmental cue to effector response.1)Single-cell connection from receptor cell to effector cell.
2)Monosynaptic reflex arc from receptor cell to motor neuron to effector cell.
3)Polysynaptic reflex arc from receptor cell to afferent [sensory neuron] to interneuron to efferent motor neuron to effector cell.
The single-cell connection has the advantage of speed while the polysynaptic connection provides for integration of information, necessary for coordinated responses.
Recall that animals are segmented (see fig 8-7, pg 285). Each segment is contains clusters of reflex units. Communication between
segments provides coordination.
In polysynaptic reflexes (see fig 8-8, pg 286) sensory input is via sensory neurons (myelinated) that enter the spinal cord via the dorsal root. These inputs synapse with interneurons in the gray matter of the spinal cord. The interneurons are the seat of integration in the nervous system. This integration is achieved by excitatory and inhibitory synaptic connections between interneurons and either adjacent interneurons or motor neurons (myelinated) that exit the spinal cord via the ventral root.
Typically the ratio of interneurons to sensory and motor neurons is about 10 : 1.
Example of a polysynaptic reflex (see handout *note - Fig 8-8(b) on pg 286 shows the general organization of the cross-extensor reflex and Fig 11-2(a) on pg 428 shows the relationship between excitatory/inhibitory synaptic connections in antagonistic muscle groups)
A polysynaptic reflex (the cross-extensor reflex) an example of the integration of information. Identify each of the following on the handout sheet.
1)Sensory input ex., pain
2)Afferent synapse with integration neuron
3)Integration neuron activates flexor neuron outflow, inhibits extensor neuron outflow
4)Integration neuron activates cross-extensor neuron
5)cross-extensor neuron activates contralateral excitatory/inhibitory neurons
6)contralateral integration neuron activates extensor neuron outflow, inhibits flexor neuron outflow
.
**Can you envision how walking can be generated from this example of a simple reflex?
Sodium Channelopathies: With single amino acid substitutions in the protein that makes up membrane sodium channels the nature of the channel can be dramatically altered. Two examples will be discussed.
Example #1: Hyperkalemic periodic paralysis this pathology is found in quarter horses. Animals with this trait have voltage-gated sodium channels that become irritable when exposed to elevated potassium. Recall that KCl (potassium chloride) is plant salt. Some plants such as alfalfa may be very rich in KCl, enough so to trigger paralysis in quarter horses.
Example #2: Paramyotonia congenita this pathology is found in humans. Individuals with this trait have voltage-gated sodium channels that become irritable when exposed to cold. When this happens muscles of exposed surfaces will contract.