lecture 23, the peripheral nervous system

cranial nerves

• located in the peripheral nervous system

• there are 12 pairs of cranial nerves

  • 10/12 are associated with the brain stem: III-XII

  • 3/12 carry only sensory impulses

    • I: olfaction (smell)

    • II: optic (vision)

    • VIII: vestibulocochlear

  • 9/12 are mixed nerves → carry both sensory and motor information

    • the cell bodies of the motor neurons in mixed nerves are located in nuclei of the brain stem

    • the cell bodies of the sensory neurons in mixed nerves are located in ganglia outside of the central nervous system

spinal nerves

• there are 31 pairs of spinal nerves

  • all are mixed nerves containing both a sensory neuron and a motor neuron

• there are

  • 8 cervical (C1-C8)

  • 12 thoracic (T1-T12)

  • 5 lumbar (L1-L5)

  • 5 sacral (S1-S5)

  • 1 coccygeal (C0)

all spinal nerves have two points of attachment

1. the dorsal root

   • receives sensory information (carries in from the back)

   • the cell bodies are located in the dorsal root ganglion (has to go up → synapse)

2. the ventral route

   • carries motor output

   • the cell bodies are located in the ventral or the lateral horn

spinal nerves are located at the joint of the dorsal and the ventral root

• the spinal nerves immediately branch into

  • the dorsal ramus → innervates the skin and the muscles of the back

  • the ventral ramus → innervates the thoracic nerves T1-T12 and the plexuses

  • the rami communicants: forms a component of the autonomic nervous system

• all but the first spinal nerve leave through the intervertebral foramina of the vertebrae

  • branching very useful to have one wire do many things

the spinal plexuses

• are formed from the ventral rami of the spinal nerves (except T2-T12)

  • a plexus is a nerve network

  • there are four plexuses

    1. the cervical plexus: C1-C4 → the phrenic nerve → the diaphragm (if damage can’t breath on your own)

    2. the branchial plexus: C5-T1 → the axillary, the radius, the ulna, and the median nerves

    3. the lumbar plexus: L1-L5 → the femoral nerve

    4. the sacral plexus: L4-S4 _. the sciatic nerve → a combination of the tibial and the fibular nerves

       • terminal

       • sciatic nerve → pinched nerve

  • (high to low of how bad it would be to damage)

wrappings of nervous tissue

• The entire nerve is wrapped in epineurium connective tissue

• Each nerve fascicle is wrapped in perineurium

• Each axon and corresponding myelin is wrapped in endoneurium

receptors

• Afferent pathways move sensory information to the CNS

  • Receptors will detect a change in the environment

  • Stimulus → receptor → CNS

  • Receptors can be classified according to:

    1. Location

    2. Type of stimulus

    3. Structure

receptor: location

• a. Exteroceptors: detect stimuli in the external environment or very close to the body’s surface

  • Located in the skin (in dermis)

  • Example: pain, touch, etc.

• b. Interoceptors: detect stimuli in the internal environment

  • Proprioceptors located in the joints and the muscles

  • Example: blood pressure

receptor: type of stimulus

• Mechanoreceptors: detect pressure, touch

• Thermoreceptors: detect heat and cold

• Chemoreceptors: detect chemicals (have in olfactory mucosa)

• Photoreceptors: detect light

receptors: structure

• a. Free nerve endings

  • Dendrites of sensory neurons

  • Example: pain, itchiness

  • blunted and structured

• b. Encapsulated nerve endings:

  • The terminal dendrites of these nerves are enclosed in connective tissue

  • Example: Meissner’s corpuscle for touch

sensory pathways

• ascending or sensory pathways

  • general senses contain 3 different neurons from the receptor to the cortex of the brain

  1. 1st order neuron

  2. 2nd order neuron

  3. 3rd order neuron

sensory neuron: 1st order neuron

• Located in a spinal nerve

• Part of the peripheral nervous system

• Unipolar neurons containing receptors (sensory)

• Cell body is located in ganglia outside of the central nervous system (outside)

• Synapse onto 2nd order neurons in the dorsal horn of the spinal cord or the brain stem

  • neuronal synapse

• turning the signal to an electrical signal

sensory neuron: 2nd order neuron

• Cell body is located in the dorsal horn of the spinal cord or the medulla

• Multipolar interneurons that carry impulses to the thalamus

• Located in tracts

• Decussates in the spinal cord or the medulla

• post-office

• motor pathway are always multipolar

• in come in via brain

sensory neuron: 3rd order neuron

• Cell body is located in the thalamus

• Multipolar interneurons that carry impulses to the sensory cortex of the CNS

• Located in tracts → because in CNS

• interpret specifically

ascending tracts located in spinal cord

1. non-specific ascending pathway

   • when you are aware of a sensation but are unable to detect its origin

   • ex. pain

   • ex. spinothalamic tracts of the spinothalamic pathway

     • decussates in the spinal cord (crosses over)

     • picks up pain, temperature and sends the information to the thalamus

2. specific ascending pathways

   • sensations that you are accurately able to detect the origin of

   • ex. touch

   • ex. dorsal columns in the dorsal column pathway

     • decussates in the medulla

the spinocerebellar pathways

• Consists of ascending tracts from the spinal cord to the cerebellum (compare plan to what is happening)

• The receptor is located on the 1st order neuron

  • Proprioceptors that detect changes in balance and body position (present in joints)

• The second order neuron goes to the cerebellum (normally goes in thalamus)

• This pathway does not have a 3rd order neuron

• You do not have any conscious perception of the activities in this pathway

motor pathways

• send output away from the CNS

  • Efferent pathways (exiting)

  • CNS → effector cells, muscles, etc.

  • All efferent neurons are multipolar

  • Consists of the:

  1. Somatic nervous system (within your control)

  2. Autonomic nervous system (involuntary)

the somatic nervous system

• Effector cells are all skeletal muscles

• The somatic motor pathways consist of two neurons

i. Upper motor neurons

ii. Lower motor neurons

upper motor neurons

• Cell bodies of these neurons are located in the cerebral cortex and the basal nuclei

• Multi-polar interneurons

• The descending tract of the pathway

  • a. Corticospinal (pyramidal tracts) tracts:

    • Cell bodies are located in the cortex

    • 85% of these tracts decussate in the medulla (because they start high)

  • b. Indirect tracts:

    • Cell bodies are located in the brainstem

• Upper motor neurons synapse onto lower motor neurons

  • decussate in spinal cord

lower motor neurons

• Located in the peripheral nervous system

• The cell bodies of these neurons are located in the ventral horn of the spinal cord

• These neurons transmit information from the spinal nerves to the effectors (skeletal muscles)

the autonomic nervous system

• consists of two divisions:

1. The sympathetic nervous system (SNS) (fight or flight)

2. The parasympathetic nervous system (PSNS) (rest and digest)

• the effector cells are cardiac muscle, smooth muscle, and glands

  • can’t control

  • “send message to bottle and squeeze”

• consists of

  • a. preganglionic neurons

    • myelinated (insulated)

    • cell body is located in the brain stem or the spinal cord

    • also pre-synaptic

  • b. postganglionic neurons

    • unmyelinated

    • cell body is located in the ganglia

    • also post-synaptic

the autonomic nervous system chart