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I bet that most people know how fast jetplanes go (500-600 mph), how fast their cars travel on highways (50-70 mph), and even how fast their internet connections transmits information (1-5 Mbits/sec) but very few people have any clue how fast and how much information their brains and spinal cords transmit. So, before you look at the answer below, vote in the poll above and then look below to see if you are correct.
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The fastest large myelinated axons of the spinal cord transmits action potentials (these are the signals that axons transmit) at the rate of 100-110 meters per second (m/sec). Since there are 3600 seconds in an hour, a conduction velocity of 100 m/sec means that the axons are transmitting at 360,000 meters per hour or 360 kilometers per hour (kph), which comes to about 216 miles per hour (mph).
The diameter of axons and myelination affect the speed of transmission. However, smaller myelinated axons that transmit at speeds of 50 m/sec or less. Likewise, there are many unmyelinated axons that transmit at 1 m/sec. Pain information tends to be be conducted by smaller and unmyelinated axons whereas proprioceptive information (touch, muscle position and length, joint angles) are carried by large myelinated axons. Therefore, the range of conduction velocities range from 1-110 m/sec or about 2-236 mph.
In order for calculate how fast sensory information to get from the tip of your toe all the way to your brain, let us assume that the distance is 2 meters. At about 100 meters/sec, the earliest information should arrive in about 0.02 seconds or 20 milliseconds (msec). However, the information carried by unmyelinated axons may take as long as 2 seconds. These calculations assume that there is no synaptic connection between the incoming signals and the brain.
Synapses are connections between neurons. Once an action potential arrives at the end of an axon contacting another neuron, neurotransmitter are released fairly quickly, within a millisecond (msec). However, it may take 5-10 msec for the neurotransmitters to cause the membrane potential change needed to activate an action potential potential in the receiving neuron.
Somatosensory evoked potentials are electrical signals recorded in the brain in response to stimulation of a peripheral nerve. When one stimulates the peripheral nerve in the feet, for example, the posterior tibialis nerve, it usually takes about 35 msec for the signal to reach the brain. This is because there is at least one synapse between the peripheral nerve and the brain.
One would expect children to have shorter latencies or the time between stimulus and arrival of the signal, because they are shorter and have less distance for the signal to transmit. However, it is interesting that young children under age 8 actually have much longer latencies of 50-60 msec for somatosensory evoked potentials, because the spinal tracts may not be fully myelinated.
Finally, how much information can the central nervous system transmit? This is not a trivial question. Action potentials are binary signals in the sense that they are all-or-none. A single action potential can activate the system but it does not carry all that information. Intensity is encoded in the frequency of bursts of action potential that code. Normally, action potentials themselves are at least 1-2 msec in duration. Therefore, the highest frequency bursts are typically no faster than 500 pulses per second or 500 Hertz (Hz) or 0.5 kilobits/second.
Computer generated signals are also binary and coded in bits. Eight bits is a byte and can encode as many as 256 characters. At 500 Hertz, the central nervous system can transmit at best about 60 bytes per second (e.g. 500 bits/second divided by 8 bits). By comparison, your computer communicates at rates of megabits/second or even gigabits/second. Computers can usually transfer data at rates of 1-10 Mbytes/sec or . Even through a telephone modem, computers achieve rates as high as 60 kilobytes per second.
So, a single wire connection can transmit at rates that are easily 1,000-100,000 more information per second than a single axon can. However, the central nervous system makes up for both the slow rate and slow bandwidth of information transmission by having millions of axons. It is estimated that the human spinal cord has about 20 million axons. Thus, the human spinal cord can transmit gibabits/second. At 0.5 kilobits/second, the maximum information transfer rate will be 10 gigabits per second or close to 1 gigabyte/second.
The injured spinal cord, however, conducts information much more slowly and less volume of information. In general, trauma to the spinal cord selectively damages larger myelinated axons. Thus, the average transmission speed may be as low as 10 m/sec. The number of surviving axons may be 10% of normal. Sensations may take a second or more to get to the brain and the volume of information will be 10 times less.
In summary, I believe that the best answer to the above poll is that the spinal cord transmits information at 100 meters/second and can carry as much as 1 gigabyte/second. However, injured spinal cords conduct about 10 times slower and can transmit 10 times less information.
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The fastest large myelinated axons of the spinal cord transmits action potentials (these are the signals that axons transmit) at the rate of 100-110 meters per second (m/sec). Since there are 3600 seconds in an hour, a conduction velocity of 100 m/sec means that the axons are transmitting at 360,000 meters per hour or 360 kilometers per hour (kph), which comes to about 216 miles per hour (mph).
The diameter of axons and myelination affect the speed of transmission. However, smaller myelinated axons that transmit at speeds of 50 m/sec or less. Likewise, there are many unmyelinated axons that transmit at 1 m/sec. Pain information tends to be be conducted by smaller and unmyelinated axons whereas proprioceptive information (touch, muscle position and length, joint angles) are carried by large myelinated axons. Therefore, the range of conduction velocities range from 1-110 m/sec or about 2-236 mph.
In order for calculate how fast sensory information to get from the tip of your toe all the way to your brain, let us assume that the distance is 2 meters. At about 100 meters/sec, the earliest information should arrive in about 0.02 seconds or 20 milliseconds (msec). However, the information carried by unmyelinated axons may take as long as 2 seconds. These calculations assume that there is no synaptic connection between the incoming signals and the brain.
Synapses are connections between neurons. Once an action potential arrives at the end of an axon contacting another neuron, neurotransmitter are released fairly quickly, within a millisecond (msec). However, it may take 5-10 msec for the neurotransmitters to cause the membrane potential change needed to activate an action potential potential in the receiving neuron.
Somatosensory evoked potentials are electrical signals recorded in the brain in response to stimulation of a peripheral nerve. When one stimulates the peripheral nerve in the feet, for example, the posterior tibialis nerve, it usually takes about 35 msec for the signal to reach the brain. This is because there is at least one synapse between the peripheral nerve and the brain.
One would expect children to have shorter latencies or the time between stimulus and arrival of the signal, because they are shorter and have less distance for the signal to transmit. However, it is interesting that young children under age 8 actually have much longer latencies of 50-60 msec for somatosensory evoked potentials, because the spinal tracts may not be fully myelinated.
Finally, how much information can the central nervous system transmit? This is not a trivial question. Action potentials are binary signals in the sense that they are all-or-none. A single action potential can activate the system but it does not carry all that information. Intensity is encoded in the frequency of bursts of action potential that code. Normally, action potentials themselves are at least 1-2 msec in duration. Therefore, the highest frequency bursts are typically no faster than 500 pulses per second or 500 Hertz (Hz) or 0.5 kilobits/second.
Computer generated signals are also binary and coded in bits. Eight bits is a byte and can encode as many as 256 characters. At 500 Hertz, the central nervous system can transmit at best about 60 bytes per second (e.g. 500 bits/second divided by 8 bits). By comparison, your computer communicates at rates of megabits/second or even gigabits/second. Computers can usually transfer data at rates of 1-10 Mbytes/sec or . Even through a telephone modem, computers achieve rates as high as 60 kilobytes per second.
So, a single wire connection can transmit at rates that are easily 1,000-100,000 more information per second than a single axon can. However, the central nervous system makes up for both the slow rate and slow bandwidth of information transmission by having millions of axons. It is estimated that the human spinal cord has about 20 million axons. Thus, the human spinal cord can transmit gibabits/second. At 0.5 kilobits/second, the maximum information transfer rate will be 10 gigabits per second or close to 1 gigabyte/second.
The injured spinal cord, however, conducts information much more slowly and less volume of information. In general, trauma to the spinal cord selectively damages larger myelinated axons. Thus, the average transmission speed may be as low as 10 m/sec. The number of surviving axons may be 10% of normal. Sensations may take a second or more to get to the brain and the volume of information will be 10 times less.
In summary, I believe that the best answer to the above poll is that the spinal cord transmits information at 100 meters/second and can carry as much as 1 gigabyte/second. However, injured spinal cords conduct about 10 times slower and can transmit 10 times less information.
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