Biceps Curls, Banjo Playing And Bladder Control …
…all rather obviously need a brain.
But that 3lb squidgy ball of nerve cells (neurons) that takes the credit for our ability to do, well, anything?
It’s only as thoughtful as the data it receives and only as useful as the instructions it sends out.
And that’s where the rest of our nervous system components come in – our nerves and spinal cord.
(Truthfully, they didn’t really need to have added the word “peripheral” here as nerves are always “peripheral” - head to the Geek notes to find out more.)
The data a brain needs to successfully move a weight, play a chord or start emptying a bladder starts its journey from perhaps a muscle, a region of skin, or an organ and travels first, along a nerve, to the spinal cord, from where it’s relayed upwards, for analysis by the brain.
However, this input data, or as scientists like to call it, sensory information, comes in two varieties.
There’s the information that we’re conscious of - is our bath warm enough, is that irresistible new shoe purchase truthfully too tight, is our partner’s snoring too loud to ignore?
But there’s an absolute ton of sensory information being sent via nerves and our spinal cord, that we’re completely ignorant of. Updates from our internal environment, like the amount of glucose and oxygen in our blood and the pressure in our blood vessels have to be frequently dispatched to our brain, if we’re to remain healthy and function optimally yet we’ll have no idea of these messages.
When conscious sensory information arrives in the brain, we might register a sensation, like a delightfully warm bath or a scratchy tag on our t-shirt and that’s the end of it.
When our brain receives that unconscious sensory information, such as our current blood pressure numbers, it compares these to a pre-programmed value hardwired into our brain’s internal database and if the numbers match, nothing needs to happen.
Sometimes though, our brain will need to change things up – increase our breathing rate if the oxygen report indicates insufficient blood oxygen or muscle action to cut out that pesky label. The brain’s commands travel down our spinal cord, ultimately arriving at the appropriate lung-inflating or scissor-operating muscles through the very same nerves that started the whole body-brain-body conversation.
For example, the infamous sciatic nerves that run down the back of each leg.
These nerves deliver reports into the spinal cord about everything from skin temperature, or the angle of the knee, to the calf pain from yesterday’s excessive work out. Any instructions that the brain generates after examination of this information, travel down the spinal cord and along that same sciatic nerve to all the bits and pieces making up our leg. Using these nerves, our brain can both order leg muscles to move us up and off the sofa, as well as message our blood vessel walls to squeeze closer together, pushing more oxygen-rich blood up to our brain.
(Had that slightly woozy feeling when standing up too quickly? That’s the nervous system being a little slow getting the message out to blood vessels to squeeze blood up to the brain. Our brain becomes slower at activating this blood vessel response as we age, contributing to our increased tendency to fall as we mature!)
The eagle-eyed might have noticed a parallel here – just as we have two kinds of sensory input, the sort we’re aware of (touch, pain, taste etc) and the sort we’re not (oxygen levels, blood pressure etc), our brain also generates two types of output or commands: voluntary and autonomic.
Whoa.
Let’s de-science that a little.
When we decide to pick up a pen, chew our food or play our musical instrument, our brain sends orders to muscles attached to our bones (skeletal muscles) through neurons making up our voluntary nervous system
On the other hand, using an output system called the autonomic nervous system, our brain takes care of regulating our organs and pipes (blood vessels, intestines, airways) without any conscious decision-making from us! We don’t need to choose to move food through our digestive system or increase our heart rate when exercising or narrow our pupils in bright light, because our brain quietly and without demanding acknowledgment, does that all for us.
For anyone wondering “how can a single nerve send all these different types of inputs and commands”?
Great question.
It can because every nerve is a bundle of thousands of neurons, although some send sensory information whilst others bring instructions, each neuron has just one, unchanging job.
(Here’s a cross-section of a nerve where each one of the tiny circles is the end of a neuron - I said there were a lot!)
A neuron that lives to relay temperature reports from the foot to the spinal cord, will never send any other type of information into the spinal cord, nor will it ever, ever, bring a command out from the spinal cord to the foot.
A neuron that gets its kicks from helping the brain boss around our biceps muscle?
That neuron will never go rogue and start sending information into the spinal cord about how hard our biceps are having to work to curl that weight.
There’s a different type of neuron to send that information.
Although our various neurons have their individual roles, what they do have in common is the method they use to transmit whatever their particular information is – a series of electrical pulses.
Creating these pulses is a super-complex process involving chemicals like dopamine, serotonin and melatonin (the one that helps get us off to sleep) and sodium (one half of salt), potassium (lots in bananas) and calcium (yum, Greek yoghurt).
(Our kidneys are vital for the body’s regulation of the levels of these chemicals, and so, failing kidneys can be disastrous for the nervous system.)
These electrical pulses move along and between the billions of neurons in our nervous system at an almost unbelievable rate, at times almost twice that of the ….
….. fastest formula 1 and Indy cars.
That information input and output speed is vital for our brain’s seamless adjustment of our body processes, that 99.9% of the time, we’re completely oblivious of. Different neurological conditions can affect different aspects of our nervous systems input and output pathways:
Motor neuron diseases for example, typically affect only the neurons controlling skeletal muscles – although “only” includes all the muscles of walking, talking, chewing and ultimately, breathing.
Actions including crying, sweating, even goosebumps that we don’t consciously choose to set in motion won’t be affected and neither will the delivery of sensory information – we’ll still feel the sting of an insect bite but be able to enjoy the chill of a snowflake or the smell of festive baking.
On the flip side, the dizziness, palpitations, and racing heart experienced by some long-COVID patients are thought to be the result of the virus singling out the brain’s autonomic output for its unwanted attention, whilst leaving the voluntary control of skeletal muscles intact.
Sever our spinal cord though and we’re disconnecting the body and brain. That loss of ability to transfer sensory input up to the brain and brain commands out to the body may well result in an inability to feel or move anything below the level of cord damage.
Understandably, I think the first question most of us would have for our health care professionals would be “will I walk again”?
Would we be thinking about the potential impact on functions controlled by our autonomic nervous system?
The functions we don’t normally consciously control like keeping an even body temperature, producing an erection, maintaining blood pressure, emptying our bladder and bowels?
Some or all of those are likely to be impacted too
As if we needed them, even more reasons to look after our nervous system!!
Stay curious,
