Basics of the Nervous System

Question 1

How is the nervous system divided anatomically?

Answer 1

Central nervous system: brain and spinal cord

Peripheral nervous system: 12 pairs of cranial nerves from the brain and 31 pairs of spinal nerves from the spinal cord.

Question 2

How is the nervous system divided functionally?

Answer 2

Somatic nervous system: controls voluntary activities
Motor - control of skeletal muscles
Sensations - which reach consciousness

Autonomic nervous system: control involuntary activities
Motor - to glands, blood vessels, heart etc
Sensory - BP, CO2 in blood, fullness of duodenum

Further divided…
B1 Sympathetic - prepares body for fight/flight
B2 Parasympathetic - rest and digest

Question 3

What parts make up the brain?

Answer 3

Cerebral hemispheres
Brainstem - midbrain, pons and medulla
Cerebellum

Question 4

What are the characteristics of the cerebral hemispheres?

Answer 4

Superficially they are divided into frontal, parietal, temporal and occipital lobes.
Deep to these lobes are structures like the basal ganglia, thalamus and internal capsule.
Surface are made up of gyri (folds) and sulci (valleys).
Underneath the parietal bone, lies the central sulcus. In front of the central sulcus is the motor cortex and behind is the sensory gyrus. They correspond to control the opposite sides of the body.

Question 5

What is white and grey matter?

Answer 5

Grey matter is distinguished from white matter in that it contains numerous cell bodies and relatively few myelinated axons, while white matter contains relatively few cell bodies and is composed chiefly of long-range myelinated axon tracts. The colour difference arises mainly from the whiteness of myelin.
Grey matter does processing.
White matter does transmitting information.

Question 6

What does the brainstem do?

Answer 6

Mainly concerned with transmitting signals from the brain to the body (motor) and from the body to the brain (sensory).
Predominantly white matter.

Question 7

What does the cerebellum do?

Answer 7

Specialised area with no conscious input. It uses sensory information from the body to refine motor instruction which the brain sends to the muscles.

Question 8

What is the function of the meningeal layers?

Answer 8

To protect the brain
To provide a framework for blood supply
To enclose the fluid-filled subarachnoid space

Question 9

What are the different meningeal layers?

Answer 9

Dura - inner surface of the skull. Made up of two layers . Periosteal layer sticks to inside of skull, meningeal layer sticks to periosteal for the most part. When they separate it creates a venous sinus.
Arachnoid - layer deep to the dura
Pia mater - adherent to the surface of the brain. Very delicate and give spiny appearance.

Question 10

What are the potential spaces between the meningeal layers?

Answer 10

EXTRADURAL/EPIDURAL - outside the dura, only exists in pathology.
SUBDURAL - between dura and the arachnoid, only exists in pathology.
SUBARACHNOID - between arachnoid and pia mater, filled with CSF

Question 11

What is the function of the cerebrospinal fluid?

Answer 11

Clear fluid which provides nutrients to the brain and balances extracellular fluid levels.
Protects the brain by providing a cushion in trauma and prevents nervous and vascular structures being compressed against the skull.
Produced by the choroid plexus which is found within the ventricles of the brain.

Question 12

What are the brain’s ventricles?

Answer 12

Lateral ventricles lie within the cerebral hemispheres.
Third ventricle lies in the midbrain.
Fourth ventricle lies behind the pons and medulla and in front of the cerebellum.

Question 13

How does CSF move through the ventricles?

Answer 13

Lateral ventricles - 3rd ventricle - cerebral aquaduct - 4th ventricle - subarachnoid space - absorbed into the venous sinuses and the venous system.

Question 14

How is the brain supplied with blood?

Label a blank circle of Willis.

Answer 14

Two internal carotid arteries and two vertebral arteries.
These form a ring of blood vessels called the Circle of Willis, which contains branches which supply different parts of the cerebral hemispheres.
Anterior cerebral artery supplies frontal lobes. Middle cerebral artery supplies most of the motor and sensory complex, a blood clot here will cause the brain to infarct. Posterior cerebral artery supplies some of temporal and occipital lobes.

Question 15

What are all the different cranial nerves?

Answer 15

I - Olfactory: smell (sensory)
II - Optic: sight (sensory)
III - Oculomotor: movement of eye (motor), autonomic to pupil and lens
IV - Trochlear: movement of eye (motor)
V - Trigeminal: face, nose and oral cavity (sensory), chewing muscles (motor)
VI - Abducens: lateral eye movement (motor)
VII - Facial: facial expressions (motor), taste (sensory), autonomic to salivary glands
VIII - Vestibulocochlear: balance and hearing (sensory)
IX - Glossopharyngeal: tongue and pharynx (sensory), autonomic to salivary glands
X - Vagus: autonomic motor and sensory to thoracic and abdominal organs
XI - Accessory: head, shoulders, pharynx, larynx and soft palate (motor)
XII - Hypoglossal: tongue (motor)

Question 16

What are the clinical examinations of the cranial nerves?

Answer 16

I (Olfactory) - testing each nostril to smell different odours
II (Optic) - visual acuity test, pseudo-isochromatic plates, test visual fields using confrontation test or a perimeter, red pin test for central field, ophthalmoscopy to visual optic disc.
III (Oculomotor), IV (Trochlear), VI (Abducens) - follow your finger making an H-shape
V (Trigeminal) - test sensory at ophthalmic, maxillary and mandibular, clench their teeth for temporal and masseter muscles
VII (Facial) - Eyes tightly closed, raise your eyebrows, then smile and whistle. Taste sensation for anterior 2/3 of tongue
VIII (Vestibulocochlear) - whisper in each ear and ask to repeat. Rinne and Weber tests.
IX (Glossopharyngeal), X (Vagus) - tickle back of pharynx and note if reflex occurs, test gag reflex
XI (Accessory) - shrug their shoulders against resistance, turn head on each side against resistance
XII (Hypoglossal) - put tongue out as far as possible

Question 17

What makes up a spinal nerve?

Answer 17

Two rows of fine nerve fibres.
- Dorsal root fibres, enter the spinal cord (sensory)
- Ventral root fibres, leave the spinal cord (motor)
Fibres from a single spinal segment join to form a dorsal root and a ventral root which pass through the inter-vertebral foramina between vertebrae and form a spinal nerve.

Question 18

How are the spinal segments found in the vertebrae?

Answer 18

Area of spinal cord where spinal nerve emerges is defined as a spinal cord segment. The cord has 31 pairs of spinal nerves and hence 31 segments.
8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.
First 7 spinal nerves pass through foramina above the corresponding cervical vertebrae.
8th and the rest pass below the corresponding vertebrae.

Question 19

What are the boundaries of the inter-vertebral foramen?

Answer 19

Anteriorly inter-vertebral foramen is bound by the bodies of adjoining vertebrae and their intervertebral discs.
Posteriorly it is bound by synovial joints between superior and inferior articular processes.
Superiorly and inferiorly it is bound by the pedicles of the vertebra.

Question 20

What is slipped disc?

Answer 20

Herniation of the intervertebral disc can narrow the intervertebral foramen and cause compression of the nerve root. This causes pain in the associated dermatome supplied by that nerve root.
Commonest is Lumbar 5 which is part of the sciatic nerve, known as sciatica.

Question 21

What is a dermatome?

Answer 21

An area of skin which receives a sensory nerve supply from a single root of the spinal cord (although nerve fibres may travel through different peripheral nerves and branches may combine with other branches from different spinal roots).

Sensory abnormalities in the dermatomes means we can work out which spinal nerve is diseased.
Common dermatomes:
Clavicle: C4
Nipples: T5
Umbilicus: T10
Inguinal region: T12
Middle finger: C7
Patella: L3
Big toe: L5
Fifth toe: S1
Perianal region: S5

Question 22

What are myotomes?

Answer 22

A group of muscles supplied by the same spinal root. Some nerves have mixed spinal root, the pattern of muscle supplied by a spinal root is different to the pattern of muscles supplied by a single peripheral nerve.
To assess motor function, patient is asked to perform movement of the relevant group of muscles against gravity and then against resistance.

Question 23

How do spinal nerves supply the thoracic and abdominal regions?

Answer 23

A single spinal nerve will pass from the spine to the front and supply a strip of skin with sensation (dermatome) and a strip of muscle underneath (myotome).

Question 24

How do spinal nerves supply the limbs? e.g. upper limb

Answer 24

Because of the complexity of the limbs the peripheral nerves need a mix of different spinal segments.
Mixing of spinal nerves is achieved in a plexus. There are cervical, brachial, lumbar and sacral plexuses supplying the upper and lower limbs.
The spinal nerves supplying the brachial plexus are from C5-C8 and T1. The peripheral nerves contain fibres from a number of different spinal segments, giving them a unique pattern of sensory and motor supply.

Question 25

What happens when there is damage to nerve fibres from C8 and/or T1?

Answer 25

Damage to T1: motor deficit to the small muscles of the hand and a sensory deficit in the medial aspect of the arm.
Damage to C8/T1: Klumpke’s palsy. Upper limb is pulled superiorly suddenly. Paralysis of the small muscles of the hand and sensory deficit in the medial aspect of the arm.
Damage to ulnar nerve: motor deficit resulting in a ‘claw’ hand and loss of sensation of the lateral one and half digits.

Question 26

What is the sympathetic nervous system?

Answer 26

Prepares the body for emergencies: fight, fright, flight, frolic.
Heart rate increases, peripheral vasoconstriction, blood pressure increases, respiratory rate increases, bowel is suppressed and sphincters close, hair stands on end, you sweat in preparation for vigorous exercise.
Emerge from the spinal cord in the thoracic and lumbar area (thoracolumbar outflow), terminating at L2-3.

Question 27

What is the parasympathetic nervous system?

Answer 27

Prepares the body for rest and sleep.
Heart and respiratory rate falls, bowel and glandular activity increases, sphincters relax, blood pressure falls, you feel sleepy, vessels in the erectile tissue dilate causing penile erection.
Emerge at the III, V, IX, X cranial nerves and S2-S4 spinal cord (craniosacral outflow).

Question 28

How does the sympathetic nervous system connected to the central nervous system?

Answer 28

• Preganglionic axons pass out of cell bodies found in the intermediolateral column.
• Move through anterior root to join the spinal nerve.
• Travels to paravertebral ganglion via the white ramus.
• At the paravertebral ganglion there are 4 possible outcomes.
  1) Synapse at the level it entered the paravertebral ganglion and postganglionic axon leaves the grey ramus via the posterior or anterior ramus to supply target organs.
  2) Move up and down the sympathetic trunk and synapse at another paravertebral ganglion.
  3) Synapse at the level it entered the paravertebral. Postganglionic axon leaves the paravertebral ganglia NOT via the grey ramus. A splanchnic nerve. In thorax.
  4) Preganglionic nerve does not synapse at the paravertebral ganglia. Instead it moves out of the ganglia NOT via the grey ramus (becomes a splanchnic) and synapses at a prevertebral ganglion. In abdomen and pelvis.

Question 29

Where is the sympathetic trunk located?

Answer 29

Runs along the side of the vertebral bodies in the chest and the abdomen with an extension into the neck.
Trunk distributes nerve fibres to different levels, and there are swellings where different nerve fibres occur.
Gives off multiple branches which supply the internal organs and all arteries, sweat glands and arrector pili muscles.

Question 30

How does the parasympathetic nervous system connected to the central nervous system?

Answer 30

Connected to the CNS through the 3rd, 7th, 9th and 10th cranial nerves and the 2nd, 3rd and 4th sacral nerves. The sacral nerves pass out of the spinal nerves to form the pelvic nerves which pass to the hind gut and the pelvic organs. Cranial nerves pass WITH these nerves to the organs that they innervate.
These preganglionic neurons synapse with a parasympathetic ganglion close to the organ of innervation.