The Nervous System

The Two Divisions

The nervous system is the body's fast control system. It senses, decides and acts in milliseconds, in contrast to the endocrine system, which does similar work over minutes to days.

Central nervous system (CNS). The brain and the spinal cord. Everything that processes and decides.

Peripheral nervous system (PNS). Every nerve outside the brain and cord. Everything that carries information in and instructions out.

The peripheral system splits again.

• Somatic division. Voluntary. Carries sensation from the skin and moves skeletal muscle on command.
• Autonomic division. Involuntary. Runs the heart, vessels, airways, gut and glands without asking you.

The autonomic division splits once more, into sympathetic and parasympathetic, which is where most of the clinically useful material lives.

The Brain

Three major regions, each with its own job.

Cerebrum. The largest part, divided into left and right hemispheres. It handles conscious thought, voluntary movement, sensation, language and memory. Each hemisphere controls the opposite side of the body, which is why a stroke on the left produces weakness on the right.

Four lobes per hemisphere:

Cerebellum. Sitting at the back below the cerebrum. It coordinates movement, balance and fine motor control. Damage here does not paralyze, it makes movement clumsy and unsteady.

Brainstem. The stalk connecting brain to spinal cord, made of the midbrain, pons and medulla. The medulla oblongata is the one to know cold: it holds the respiratory center, the cardiac center and the vasomotor center. Breathing, heart rate and vessel tone are all controlled from that small area, which is why brainstem injury is so lethal.

The hypothalamus sits above the brainstem and regulates temperature, hunger, thirst and the endocrine system through its control of the pituitary. The thalamus below the cerebrum is the relay station routing sensory information to the right destination.

Protection and Blood Supply

The brain is wrapped in three membranes, the meninges. From outside in: dura mater, tough and fibrous; arachnoid mater, web like; pia mater, thin and clinging directly to the brain surface.

Cerebrospinal fluid circulates between the arachnoid and pia, cushioning the brain and cord and carrying nutrients.

Bleeding is named for which layer it lands between. An epidural bleed sits above the dura, is usually arterial and moves fast. A subdural bleed sits below the dura, is usually venous and can develop slowly, which is why it appears in older adults and in people on blood thinners after what seemed like a minor fall. A subarachnoid bleed sits in the cerebrospinal fluid space and classically presents as the sudden worst headache of a patient's life.

Blood arrives through two carotid arteries in front and two vertebral arteries behind, which join at the base of the brain in the circle of Willis. That ring means some regions have more than one source of supply.

The brain is roughly 2 percent of body weight and consumes about 20 percent of the body's oxygen. It stores essentially nothing, so it depends entirely on continuous flow of both oxygen and glucose. Any patient with an altered mental status gets a glucose check for exactly that reason.

Pressure Inside the Skull

The skull is a rigid box holding three things: brain tissue, blood and cerebrospinal fluid. The total volume cannot change. An increase in any one of them therefore has to come out of the other two.

Bleeding or swelling first displaces cerebrospinal fluid and some venous blood, and the patient may look reasonably well during that period. Once that compensation is used up, pressure rises quickly and the brain's own perfusion starts to fall.

Cushing's triad is the late sign of that failure:

• Rising blood pressure, with a widening pulse pressure, as the body raises systemic pressure to force blood into the skull
• Falling heart rate, a reflex response to that rising pressure
• Irregular breathing, as the brainstem centers lose control

Hypertension with bradycardia is the opposite of the fast and weak picture of blood loss, which is precisely what makes it recognizable. A head injured patient whose pressure is climbing while the pulse slows is not improving.

The Spinal Cord

The cord runs from the medulla down through the vertebral canal, ending around the first or second lumbar vertebra in an adult. Below that the canal holds a bundle of nerve roots rather than cord.

Thirty-one pairs of spinal nerves branch off: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal. Each pair serves a defined strip of skin, called a dermatome, and a defined group of muscles. Dermatomes are why a level of injury can be identified by where sensation stops.

Landmarks worth remembering: the nipple line is roughly T4, the umbilicus roughly T10.

The phrenic nerve supplying the diaphragm comes from C3, C4 and C5. An injury above that level takes away the ability to breathe. That is the origin of the line about C3, 4 and 5 keeping the diaphragm alive.

A reflex arc bypasses the brain entirely. Sensory information reaches the cord, connects there, and a motor response leaves immediately. Pulling your hand off a hot surface happens before you are aware of the heat.

The Autonomic Nervous System

This is the material that pays off most often, because nearly every early sign of shock is the autonomic response rather than the injury.

Sympathetic: fight or flight. Prepares the body for exertion or threat.

• Heart rate and contractility increase
• Blood vessels in the skin and gut constrict, shunting blood to muscle, heart and brain
• Bronchioles dilate to move more air
• Pupils dilate
• The liver releases glucose
• Sweat glands activate
• Digestion shuts down

Parasympathetic: rest and digest. Conserves and restores. Its main pathway is the vagus nerve.

• Heart rate slows
• Digestion and salivation increase
• Bronchioles constrict
• Pupils constrict

The two run continuously and in opposition, and the balance between them sets the resting state. Stimulating the vagus nerve, which straining or vomiting or suctioning too aggressively can do, slows the heart.

Receptors are how drugs act on this system. Alpha receptors constrict blood vessels. Beta 1 receptors are mostly in the heart and increase rate and contractility. Beta 2 receptors are mostly in the lungs and dilate the bronchioles. A medication described as a beta 2 agonist is therefore a bronchodilator, and the naming tells you the effect before you memorize the drug.

Cranial Nerves

Twelve pairs come directly off the brain rather than the cord. A handful come up constantly in assessment.

Cranial nerve III is the one behind a blown pupil. Rising pressure inside the skull presses on that nerve, and the pupil on the affected side dilates and stops reacting.

Assessing the Nervous System

Mental status is the most sensitive single indicator, because the brain is the first organ to complain when perfusion or oxygen falls. AVPU gives you a quick level: Alert, responds to Verbal, responds to Painful stimulus, Unresponsive. The Glasgow Coma Scale scores eye opening, verbal response and motor response, with a minimum of 3 and a maximum of 15.

Pupils should be equal, round and reactive. Unequal pupils in an unresponsive patient suggest pressure inside the skull. Pinpoint pupils suggest opioids or a brainstem problem. Widely dilated pupils suggest stimulants or severe low oxygen.

Motor and sensory function are checked in all four limbs. Ask the patient to move each one, and ask where they feel touch. A level at which sensation stops points to a level of cord injury.

Stroke screening uses facial droop, arm drift and speech. The F.A.S.T. approach adds time of onset, which is the piece that determines what the hospital can offer. The Cincinnati Prehospital Stroke Scale checks the same three physical findings. Establishing when the patient was last known well matters as much as the findings themselves.

A stroke can leave a patient fully alert or deeply unresponsive depending on which vessel and which region are involved. Level of consciousness tells you how much brain is affected, not whether it is a stroke.

What to Take Away

Central is brain and cord, peripheral is everything else, and the autonomic division runs the organs. The medulla controls breathing, heart rate and vessel tone. The skull cannot expand, so rising pressure eventually shows as increasing blood pressure with a slowing pulse and irregular breathing. C3, 4 and 5 supply the diaphragm. Sympathetic response explains the early findings in shock, and mental status is the earliest indicator you have.