The Nervous and Musculoskeletal Systems
Unit 1: Anatomy and Physiology. Lesson 4 of 60 in CBET Academy.
Nerves and muscles are electrical, which is why current shocks and stimulates, why ESUs run at high frequency, and why EEG and EMG exist.
The nervous system
- The central nervous system is the brain and spinal cord. The cerebrum handles thought, sensation, and voluntary movement; the cerebellum coordinates balance and fine movement; the brainstem runs breathing, heart rate, and consciousness.
- The peripheral nervous system carries signals to and from the body.
- The autonomic system works without conscious control. The sympathetic branch (fight or flight) raises heart rate and blood pressure; the parasympathetic branch (rest and digest) lowers them. This is why anxiety or pain shows up on the monitor.
Why electricity matters
A neuron sends a signal as an action potential: a brief electrical pulse traveling along the fiber. Muscle fibers contract in response to the same kind of pulse. External current at low frequencies mimics these pulses, which is why a 60 Hz shock causes muscle contraction, the let-go problem, and fibrillation. Electrosurgical units operate above about 100 kHz, faster than nerves can respond, so hundreds of milliamps pass through tissue without stimulation.
Measuring the nervous system
- EEG records the brain's electrical activity from scalp electrodes placed by the 10-20 system. Rhythms are grouped by frequency: delta below 4 Hz (deep sleep), theta 4 to 8, alpha 8 to 13 (relaxed, eyes closed), beta above 13 (alert).
- EMG records muscle electrical activity; nerve conduction studies time a stimulus traveling along a nerve.
- Evoked potentials measure the brain's response to a sound, flash, or stimulus.
The musculoskeletal system
About 206 bones give structure, protect organs, and make blood cells in the marrow. Joints connect bones; ligaments join bone to bone, tendons join muscle to bone. Three kinds of muscle: skeletal (voluntary, attached to bones), smooth (involuntary, in vessels and gut), and cardiac (the heart, involuntary but electrically driven).
Devices that touch these systems
Physical therapy stimulators (TENS for pain, muscle stimulators), traction, continuous passive motion machines, and nerve stimulators used to test paralysis during anesthesia. Every one delivers current and must be checked for output and safety.
When it goes wrong
Stroke is loss of blood flow to part of the brain. Seizures are abnormal electrical storms seen on EEG. Spinal cord injury blocks signals below the level of injury. Fractures and arthritis affect bones and joints.
The Nervous and Musculoskeletal Systems, explained slowly
The body's wiring, and why it responds to yours
Every nerve signal and every muscle contraction is electrical. That is the reason a 60 Hz shock makes muscles clamp, the reason an ECG can be recorded from the skin, and the reason electrosurgery is possible at all. Learn the biology and the electrical safety rules stop being arbitrary.
The parts
CENTRAL NERVOUS SYSTEM
Brain
cerebrum thinking, sensing, voluntary movement, speech
cerebellum balance, coordination
brainstem breathing, heart rate, blood pressure, consciousness (damage here is fatal)
Spinal cord the trunk line; injury blocks everything below the level
PERIPHERAL NERVOUS SYSTEM
sensory nerves in, from skin and organs
motor nerves out, to muscles
AUTONOMIC
sympathetic fight or flight: heart rate up, BP up, pupils wide, gut slows
parasympathetic rest and digest: heart rate down, gut active (the vagus nerve)The autonomic system is why a frightened or hurting patient's heart rate climbs on the monitor, and why pressing on the carotid or vagal stimulation slows it. Anesthesia and many drugs work through these branches.
How a nerve fires
A resting neuron holds a small voltage across its membrane, about minus 70 millivolts. A stimulus opens channels, the voltage flips positive for a millisecond (the action potential), and that pulse travels down the fiber to the next cell or to a muscle. Muscle fibers do the same thing and then contract.
Now the safety connection. An external current at 50 or 60 Hz changes direction slowly enough that nerve and muscle membranes respond to each cycle as if it were a stimulus. That is why:
- a few milliamps tingles (sensory nerves firing),
- 10 to 20 mA causes let-go failure (motor nerves fire, muscles clamp),
- 100 mA or so fibrillates the heart (cardiac cells fire chaotically).
Above roughly 100 kHz, membranes cannot follow the reversals, so current passes through tissue producing heat but no stimulation. Electrosurgical units run at 300 kHz to 3 MHz for exactly that reason: hundreds of milliamps cut and coagulate without contracting a single muscle.
Recording the nervous system
EEG scalp electrodes, 10-20 placement system brain rhythms in microvolts
delta < 4 Hz deep sleep | theta 4-8 | alpha 8-13 relaxed, eyes closed | beta > 13 alert
EMG needle or surface electrodes muscle electrical activity; neuromuscular disease
Nerve conduction stimulate, time the response slowed conduction = nerve damage
Evoked potentials flash, click, or shock, then EEG the brain's response; used during spine surgeryEEG signals are tens of microvolts, ten to a hundred times smaller than ECG, so EEG amplifiers need even more gain and even better noise rejection.
The frame and the motors
The skeleton's roughly 206 bones give shape, protect organs, store minerals, and make blood cells in the marrow. Joints connect bones; ligaments tie bone to bone; tendons tie muscle to bone. Three muscle types:
skeletal voluntary, attached to bone moves the body; what EMG usually records smooth involuntary, in vessels and gut controls blood pressure and digestion cardiac involuntary but electrically timed the heart; the only muscle that fibrillates
Devices that touch these systems
Physical therapy stimulators deliver controlled current: TENS for pain, neuromuscular stimulators to contract muscle. Traction and continuous passive motion machines move joints. Nerve stimulators in the OR check how deeply a paralytic drug is working. Each delivers current on purpose, so its output, timer, and leakage are checked at PM.
How the exam asks it
- "Which branch of the autonomic system increases heart rate?" Sympathetic.
- "Why does ESU current not cause muscle contraction?" Its frequency is too high for nerves to respond.
- "Which EEG frequency band is associated with a relaxed, awake state?" Alpha, 8 to 13 Hz.
- "Which part of the brain controls breathing?" The brainstem.
What to memorize
- Cerebrum thinks, cerebellum coordinates, brainstem keeps you alive. Sympathetic speeds, parasympathetic slows.
- Low frequency stimulates; ESU frequency does not.
- EEG delta, theta, alpha, beta from slow to fast. Skeletal, smooth, cardiac muscle.