peripheral & central nervous system

what are the 2 anatomical subdivisions of the nervous system?

1. central nervous system (CNS)

2. peripheral nervous system (PNS)

what is the central nervous system made up of?

brain, spinal cord
*contained within the skull & vertebral column

how is the central nervous system connected to the body?

through the peripheral nervous system

what is the peripheral nervous system made up of?

nerves

how is the peripheral nervous system connected to the central nervous system?

through nerves
*outside of CNS

what are nerves?

bundles of hundreds/thousands of axons (nerve fibers) widely distributed throughout the body and internal organs

what are the 3 types of nerves?

1. cranial nerves

2. spinal nerves

3. autonomic nerves

what are ganglia?

collections of neurons outside the CNS

what are the 2 types of ganglia?

1. spinal ganglia

2. autonomic ganglia

what are spinal ganglia?

• sensory neurons associated with spinal nerves

• cell bodies located in the spinal ganglia

• part of the dorsal root of the spinal nerve (also called: dorsal root ganglia)

what are autonomic ganglion?

• associated with autonomic nerves

• two-neuron chain of command

  • cell body of second neuron located in the autonomic ganglion

what are the 2 functional subdivisions of the nervous system?

1. somatic (voluntary) nervous system (SNS)

2. autonomic (involuntary/visceral) nervous system (ANS)

what is the sympathetic nervous system made up of?

somatic motor, somatic sensory

what does the somatic motor in the sympathetic nervous system provide?

• provides motor information from the CNS → skeletal muscles

  • voluntary control of movement and posture

what does the somatic sensory in the sympathetic nervous system provide?

provides conscious sensation from skin, mucosal surfaces, and organs → CNS

what is the function of the autonomic nervous system?

• reflex control of smooth muscle in organs, cardiac muscle, and secretory glands

  • primarily in viscera

• sensory innervation of viscera

what are the components of the brain?

• cerebrum

• cerebellum

• brain stem

  • midbrain, pons, medulla oblongata

where does the spinal cord start and end?

• continuation of the brainstem at the foramen magnum

• extends to level of the disc between vertebrae L1 and L2

where is the spinal cord located?

upper 2/3 of the vertebral canal

what is a spinal segment?

length of the spinal cord to which one pair of spinal nerves is attached

how many pairs of cranial nerves are there?

12 pairs
(one on each side, total 24)

how many pairs of spinal nerves are there?

31 pairs
(62 total)

how many cervical spinal nerves are there?

8 pairs

how many thoracic spinal nerves are there?

12 pairs

how many lumbar spinal nerves are there?

5 pairs

how many pairs of sacral spinal nerves are there?

5 pairs

how many pairs of coccygeal spinal nerves are there?

1 pair

how do the spinal nerves exit the vertebral column?

through the intervertebral foramina
*the spinal nerves are vulnerable to injury by herniated intervertebral disc or pathological narrowing of the intervertebral foramen

what aspect does motor rootlets emerge along?

ventrolateral aspect of the entire spinal cord
(from anterior horn)

what aspect does the sensory rootlets emerge along?

dorsolateral aspect of the entire spinal cord
(from posterior horn)

what does the motor rootlets converge to form?

31 pairs of ventral roots

what does the sensory rootlets converge to form?

31 pairs of dorsal roots

what does the ventral roots innervate?

skeletal muscle in the neck, trunk, and limbs

what does the dorsal roots originate as?

• sensory endings in the skin

  • send information through the cell body located in the dorsal root ganglion → dorsal root → spinal cord

what is a spinal nerve?

• ventral root + dorsal root (mixed motor & sensory information)

• very short

• immediately divides into 2 primary rami

what are the 2 primary rami from a spinal nerve?

1. dorsal ramus

   1. ventral ramus

what does the dorsal ramus innervate?

erector spinae muscles of the back, skin lying over those muscles

what does the ventral rami innervate?

• innervate muscles and skin of remaining anterior & lateral aspects of the neck/trunk

• form plexuses for the motor & sensory innervation of the limbs

  • cervical, brachial, lumbar, and sacral

*brachial plexus → bulk innervation for upper extremity
lumbar/sacral plexus → lower extremity, pelvis

what is the dorsal root connected to?

dorsal horn of spinal cord and corresponding spinal ganglion

what is the dorsal root composed of?

central processes of spinal ganglion cells

what does dorsal roots end as?

the peripheral processes end as sensory (afferent) nerves in the skin that respond to sensory stimuli

what is the flow of information for the dorsal root?

sensory information → dorsal horn of spinal cord → multiple levels of the CNS (includ. cerebral cortex for conscious perception of sensation)

where does the ventral root arise from?

ventral horn of the spinal cord

what does the ventral root join with to form the spinal nerve?

the dorsal root distal to the ganglion

what is the flow of information for ventral roots?

motor (efferent) fibers send action potentials that terminate in skeletal muscles of the limb, trunk, and neck that causes contractions

what are the 2 branches of the spinal nerves?

1. ventral ramus

2. dorsal ramus

what does the ventral ramus supply and form?

• supplies skin & muscles of the anterior and lateral neck & trunk

• forms plexuses that innervate the limb

what does the dorsal ramus supply?

supplies skin and muscles of the back

what are the connected sensory and motor neurons within the CNS for?

reflex pathways

what is an example of a reflex pathway?

patellar reflex

how does the patellar reflex work?

1. tap on the patellar ligament

2. stretch the quadriceps femoris muscle

3. sensory neurons in the femoral nerve detect the stretch → transmit information to the spinal cord

4. motor neurons in the femoral nerve cause contraction of the quadriceps femoris muscle (motor response)

what is a dermatome?

a given spinal cord segment and its corresponding spinal nerves that provide sensory innervation to a particular region of the skin

what is a dermatomal map?

describes location of each dermatome

what is a dermatomal map used for?

used during sensory testing to localize sensory loss to specific spinal cord segments

how can the segmental pattern of the spinal nerves innervation be observed in the lab?

note how the spinal nerves emerge from the spinal cord

what are the key dermatomes?

• thumb (C6)

• index, middle finger (C7)

• ring, pinky (C8)

• upper limb (C5-T2)

• nipple (T4)

• umbilicus (T10)

what are spinal meninges?

supportive layers of fibrous and elastic connective tissues that surround the spinal cord
(continuous with the cranial meninges that encircle the brain)

what are the 3 layers of the spinal meninges?

1. dura mater

2. arachnoid mater

3. pia mater

what is the dura mater?

• outermost covering of the spinal cord

• separated from bone by the epidural space

where does the dura mater extend to?

• extends to the S2 level of the vertebral column

  • ends as a blind sac

• thin tubular extension attaches to the coccyx

what is the arachnoid mater?

• inner layer to the dura mater

• separated from the dura mater by the potential subdural space

where does the arachnoid mater extend to?

• extends to the S2 level of the vertebral column

  • lines the dura

  • ends as blind sac

what is the pia mater?

• microscopic layer that closely invests the spinal cord

• separated from the arachnoid mater by the subarachnoid space

• contains blood vessels

where does the pia mater extend to?

• extends to L2 of the vertebral column

• from there, it continues as a thin extension (pia filum terminale) which perforates the arachnoid mater and dura mater at S2

  • continues within the coccygeal ligament

what are the 3 spaces in relation to the meninges?

1. epidural space

2. subdural space

3. subarachnoid space

what is the epidural space?

space between the bone and dura mater
(contains venous plexuses)

what is the subdural space?

potential space between the dura mater and arachnoid mater

what is the subarachnoid space?

• space between the arachnoid mater and pia mater

• contains CSF

why is the subdural space a “potential” space?

• the dura mater and arachnoid mater are normally adhered together

• an artificial space can be created if there is trauma

where does the spinal cord end?

at the level of the disc between L1 and L2 vertebrae

what is the conus medullaris?

tapering end of the spinal cord

what are the development stages of the spinal cord & vertebral column?

1. (fetal life) spinal cord and vertebral column are co-extensive

   1. spinal nerves arise opposite their corresponding intervertebral foramina

2. (during development) vertebral column grows more extensively than the spinal cord

   1. roots of the lower spinal nerves must travel downward within the vertebral canal to reach their appropriate intervertebral foramina [cauda equina]

what is the cauda equina?

descending roots of the lower spinal nerves

why does the arachnoid membrane and the subarachnoid space continue down to the S2 vertebrae?

nerve roots of the cauda equina are within the subarachnoid space
(bathed in CSF)

which space does a lumbar puncture occur?

• subarachnoid space

  • area below L2, above S2

what is a lumbar puncture ?

introduce a needle into the subarachnoid space and draw CSF

in a lumbar puncture, between which spinous processes is the needle introduced to?

1. L3-L4 spinous processes

2. L4-L5 spinous processes

why is a lumbar puncture done in the subarachnoid space?

no penetration of the spinal cord (terminates L1-L2 level)

what is used as a landmark for a lumbar puncture?

an imaginary line connecting the highest points on the iliac crests corresponding to L4 spine

what are the 3 clinical conditions of the spinal cord?

1. polio

2. herpes zoster

3. herniated disc

what is polio?

• viral infectious disease

• invades nervous system, causes paralysis

• affects mainly children under 3

• virus enters mouth → multiplies in the intestines → travels to the CNS → destroys anterior horn motor neurons

• result: paralysis, severe atrophy of the denervated muscles

• primarily in the lumbosacral part of spinal cord

  • paralyzes muscles of the lower limbs

• purely motor lesion

what is herpes zoster?

• shingles

  • same virus that causes chicken pox

• virus lays dormant in the dorsal root ganglia after chicken pox infection

• re-emerges in peripheral sensory nerves

• symptoms: shooting pains, tingling, loss of sensation

  • in dermatomes innervated by the affected dorsal roots

• purely sensory lesion

what is a herniated disc?

• bulging of the intervertebral disc posterolaterally

• compresses the spinal nerve in/near the intervertebral foramen

• results: pain/weakness in distribution of the affected spinal nerve

• mixed motor and sensory lesion

what clinical disease is purely a motor lesion?

polio

what clinical disease is purely a sensory lesion?

herpes zoster

what clinical disease is a mixed motor and sensory lesion?

herniated disc

is the autonomic nervous system voluntary or involuntary?

involuntary
(reflexes, unconscious)

what does the autonomic nervous system regulate?

• visceral function

  • innervates smooth muscle, cardiac muscle, and secretory glands

how is the autonomic nervous system organized?

• chain of two neurons

  • preganglionic, postganglionic

what is a preganglionic neuron?

• first neuron in a chain of two neurons (autonomic nerves)

  • located in the CNS

what is a postganglionic neuron?

• second neuron in a chain of two neurons (autonomic nerves)

• found in autonomic ganglia outside the CNS

what are the 2 subdivisions of the autonomic nervous system?

1. sympathetic nervous system (SNS)

2. parasympathetic nervous system (PNS)

how does the sympathetic nervous system and parasympathetic nervous system coordinate together?

• maintain a stable internal environment despite changes in external conditions

• constantly adjusts factors to keep them within a narrow, healthy range

  • ex. body temperature, blood pressure, etc.

what is the sympathetic nervous system also called?

thoracolumbar outflow

where are the cell bodies of preganglionic neurons of the sympathetic nervous system located?

• exclusively in the T1-L2 spinal cord segments

  • lateral horn of the gray matter

what is the sympathetic trunk?

• string of ganglia

• entered by preganglionic axons

• exited by postganglionic axons

what is the flow of information for axons of preganglionic neurons?

• axons leave spinal cord through the ventral roots

• enter the sympathetic trunk

• travel to appropriate level

• synapse with postganglionic neurons located with the sympathetic trunk ganglia

what is the flow of information for axons of postganglionic neurons?

• axons leave the sympathetic trunk

• join any of the 31 corresponding spinal nerve pairs

where does sympathetic innervation of smooth muscle and glands in the head come from?

postganglionic neurons in the superior cervical sympathetic ganglion
(axons follow internal and external carotid arteries to reach the head)

what is the parasympathetic nervous system also called?

craniosacral outflow