Kaap 309: Spinal Cord, PNS, and Synapses

Spinal Cord Anatomy

• Where it runs: From foramen magnum → L1/L2.

• Enlargements:

  Cervical: upper limbs.

  Lumbosacral: lower limbs.

• End structures:

  Conus medullaris: cone-shaped end.

  Filum terminale: string that anchors cord to coccyx.

  Cauda equina: bundle of nerves (horse’s tail).

Spinal Cord Anatomy (Internal Structure)

• Gray Matter (H-shaped in center):

  • Dorsal horn → sensory input

  • Ventral horn → motor output

  • Lateral horn → autonomic (only in thoracic & upper lumbar levels)

• White Matter (outer area):

  • Organized into funiculi (columns):

  • Dorsal funiculus

  • Lateral funiculus

  • Ventral funiculus

Spinal Cord Cross-Section

Spinal Nerve Roots

• Anterior (ventral) root → Motor (efferent) → to skeletal muscles

• Posterior (dorsal) root → Sensory (afferent) → from receptors

• Ventral root + Dorsal root = Spinal nerve (mixed motor + sensory)

Ventral = Motor → Muscles

Dorsal = Sensory → Detect

Functional Cross-Section

Spinal Nerve Rami

Dorsal root → sensory (afferent)

Ventral root → motor (efferent)

Dorsal root + Ventral root = Spinal nerve (mixed: sensory + motor)

After spinal nerve forms, it splits into:

• Dorsal rami → supply back muscles & skin of back

• Ventral rami → supply limbs & anterior/lateral trunk (form plexuses)

Naming Spinal Nerves

31 pairs total

• 8 Cervical (C1–C8)

• 12 Thoracic (T1–T12)

• 5 Lumbar (L1–L5)

• 5 Sacral (S1–S5)

• 1 Coccygeal (Co1)

“Breakfast at 8, Lunch at 12, Dinner at 5 + 5 + 1.”

Specific Spinal Nerves

• C6 → 6th cervical spinal nerve

• C7 → 7th cervical spinal nerve

• T1 → 1st thoracic spinal nerve

• T2 → 2nd thoracic spinal nerve

PNS Receptors (by Stimulus Type)

• Mechanoreceptors → respond to touch, pressure, vibration, stretch

• Thermoreceptors → respond to temperature

• Photoreceptors → respond to light (in the eye)

• Chemoreceptors → respond to chemicals (taste, smell, blood pH, O₂, CO₂)

• Nociceptors → respond to pain (damage, extreme heat/cold, chemicals)

“Many Tiny People Can Nap” = Mechanoreceptors, Thermoreceptors, Photoreceptors, Chemoreceptors, Nociceptors.

PNS Receptors (by Location)

• Exteroceptors → respond to stimuli outside the body

  • Example: skin (touch, pressure, pain, temperature), special senses (vision, hearing, smell)

• Interoceptors (visceroceptors) → respond to stimuli inside the body

  • Example: internal organs (stretch, chemical changes, temperature, pain)

• Proprioceptors → respond to body position/movement

  • Found in skeletal muscles, tendons, joints, ligaments, inner ear

PNS Receptors (by Structure)

• Nonencapsulated (free nerve endings):

  • Simple, unmyelinated endings

  • Found in epithelia & connective tissue

  • Detect pain, temperature, light touch, hair movement

• Encapsulated (nerve endings wrapped in connective tissue):

  • More complex, specialized.

Examples:

• Meissner’s corpuscles → light touch

• Pacinian corpuscles → deep pressure & vibration

• Muscle spindles, tendon organs, joint receptors → proprioception

PNS Receptors - Nonencapsulated

PNS Receptors – Encapsulated (I)

PNS Receptors – Encapsulated (II)

Brachial Plexus

• Function: Provides almost all nerves to the upper limb

• Spinal nerve contributions: (C4) C5–T1

• Location: Emerges between anterior & middle scalene muscles

Organization (Remember: “Randy Travis Drinks Cold Beer”):

• Roots (ventral rami C5–T1)

• Trunks (upper, middle, lower)

• Divisions (anterior & posterior)

• Cords (lateral, posterior, medial)

• Branches (terminal nerves: musculocutaneous, axillary, radial, median, ulnar)

Terminal Branches of the Brachial Plexus

• Musculocutaneous nerve → anterior arm (flexors, skin of lateral forearm)

• Axillary nerve → deltoid & teres minor, skin of shoulder

• Radial nerve → posterior arm & forearm (extensors, back of hand)

• Median nerve → anterior forearm (flexors), most hand lateral palm & fingers

• Ulnar nerve → forearm & most hand medial side (funny bone nerve)

Lumbar Plexus

• Function: Provides nerves to the lower abdomen, anterior & medial thigh

• Spinal nerve contributions: L1–L4

• Major Terminal Branches:

  • Femoral nerve → anterior thigh muscles, skin of anterior thigh & medial leg

  • Obturator nerve → medial thigh muscles, skin of medial thigh

Lumbar Plexus

Sacral Plexus

• Function: Provides nerves to the pelvis, posterior thigh, and most of the leg & foot

• Spinal nerve contributions: L4–S5

• Location: Emerges through the pelvis inferior to the piriformis muscle

• Major terminal branch: Sciatic nerve (splits into tibial & common fibular nerves)

Sacral Plexus

Sacral Plexus – Sciatic Nerve Branches

• Sciatic nerve → largest nerve in the body, posterior thigh & leg

  • Tibial nerve → posterior leg & plantar foot

  • Common fibular (peroneal) nerve → lateral & anterior leg

    • Superficial fibular nerve → lateral leg muscles, dorsum of foot

    • Deep fibular nerve → anterior leg muscles, skin between first two toes

Synapses

• Definition: Junction that transmits information from one neuron to another

1. Location (where the synapse occurs):

• Axosomatic → axon → cell body

• Axodendritic → axon → dendrite

• Axoaxonal → axon → axon

2. Type (how the signal is transmitted):

• Electrical → direct current flow through gap junctions

• Chemical → via neurotransmitters across a synaptic cleft

Key Terms:

• Presynaptic neuron → sends the signal

• Postsynaptic neuron → receives the signal

Chemical Synapses

• Definition: Use neurotransmitters to transmit signals between neurons

Components:

1. Axon terminal (presynaptic) → releases neurotransmitters

2. Receptor region (postsynaptic) → usually on dendrite or cell body

3. Synaptic cleft → small gap between presynaptic and postsynaptic neurons

• Synaptic delay: Time it takes for signal to cross the synapse (0.3–5.0 ms) → rate-limiting step in transmission

Chemical Synapse Transmission

1. Action potential arrives at the axon terminal.

2. Voltage-gated Ca²⁺ channels open, allowing Ca²⁺ to enter the terminal.

3. Ca²⁺ triggers synaptic vesicles to release neurotransmitters via exocytosis into the synaptic cleft.

   • More frequent impulses → more vesicles released.

4. Neurotransmitters diffuse across the synaptic cleft and bind to postsynaptic receptors.

5. Ion channels open on the postsynaptic neuron → graded potentials form → neuron may be excited or inhibited depending on the neurotransmitter.

6. Neurotransmitter effects end via:

   • Reuptake by cells (e.g., astrocytes)

   • Enzymatic degradation

   • Diffusion away from the synapse

Specific chemical synapses

Excitatory adrenergic synapse (Norepinephrine, NE)

1. Resting state: NE receptor is bound to a G protein.

2. NE binds to the receptor → G protein dissociates.

3. G protein activates adenylate cyclase, which converts ATP → cAMP.

4. cAMP effects in the postsynaptic cell:

   • Opens ligand-gated ion channels → depolarizes cell

   • Activates cytoplasmic enzymes → metabolic changes

   • Induces gene transcription → produces new enzymes → metabolic effects

Excitatory adrenergic synapse

Excitatory Postsynaptic Potential (EPSP)

• Definition: Neurotransmitter binding depolarizes the postsynaptic membrane

• Key point: Produces EPSPs, not action potentials

• Goal: Bring the axon hillock closer to threshold so an AP can be triggered

Inhibitory Postsynaptic Potential (IPSP)

• Definition: Neurotransmitter binding hyperpolarizes the postsynaptic membrane

• Key point: Produces IPSPs, not action potentials

• Goal: Make the membrane more negative, moving it away from threshold