Peripheral Nervous System (PNS)

• The Peripheral Nervous System (PNS) facilitates communication between the CNS and the rest of the body

Sensory Division

• The sensory division is also know as the afferent division
• It conducts impulses from receptors to the CNS
• It informs the CNS of the state of the body's interior and exterior
• Sensory nerve fibers can be somatic, originating from skin, skeletal muscles, or joints
• Sensory nerve fibers can be visceral, originating from organs within the body cavity

Motor Division

• This is also known as the efferent division
• Motor division conducts impulses from the CNS to effectors, such as muscles and glands
• It is comprised of motor nerve fibers

Somatic Nervous System

• Somatic sensory neurons convey information to the CNS from sensory receptors in the skin, skeletal muscles, joints, and special senses
• Somatic motor neurons are voluntary
• They conduct impulses from the CNS to skeletal muscles

Autonomic Nervous System

• Autonomic, or visceral, sensory neurons convey information to the CNS from autonomic sensory receptors
• These autonomic sensory receptors are located primarily in the visceral organs (smooth muscle organs in the thorax, abdomen, and pelvis)
• The motor neurons of the autonomic nervous system are generally involuntary
• They conduct impulses from the CNS to smooth muscle, cardiac muscle, and glands

Sensory Pathway

• Types of neurons depend on the number of processes
• The reason there are many neurons between the sensory input and the cortex is because of synapses
• Synapses allow for modulation of the signal
• First-order neurons are located in the dorsal root ganglion
• Second-order neurons are found in the gray matter of the spinal cord
• Third-order neurons project to the thalamus
• This information then reaches the cortex
• Every place in the cortex represents a place in the body

Somatic Motor Pathways

• Upper motor neurons originate in the primary motor cortex or somatic motor nuclei of the brain stem
• In the somatic nervous system, an upper motor neuron in the CNS controls a lower-motor neuron in the brain stem or spinal cord
• Lower motor neurons extend from the spinal cord to the skeletal muscle
• The axon of the lower-motor neuron has direct control over skeletal muscle fibers
• Stimulation of the lower-motor neuron always has an excitatory effect on the skeletal muscle fibers

Autonomic Neural Pathways

• Visceral motor nuclei are located in the hypothalamus
• Preganglionic neurons are found within the gray matter of the spinal cord
• Autonomic nuclei are located in the brain stem
• In the autonomic nervous system (ANS), the axon of a preganglionic neuron in the CNS controls ganglionic neurons in the periphery
• Stimulation of the ganglionic neurons may lead to excitation or inhibition of the visceral effector innervated
• The axon of the first preganglionic neuron leaves the CNS to synapse with the second ganglionic neuron
• The axon of the second postganglionic neuron extends to the organ it serves
• Autonomic ganglia have synapses and cell bodies *motor ganglia
• Sensory ganglia have only cell bodies

Spinal Cord External Anatomy

• The spinal cord runs through the vertebral canal
• The spinal cord extends from the foramen magnum to the second lumbar vertebra
• The adult spinal cord ranges from 42 to 45 cm in length
• The conus medullaris is the tapered inferior end (conical structure) of the spinal cord and ends between L1 and L2
• The cauda equina is the origin of spinal nerves extending inferiorly from the conus medullaris and is called Cauda equina, a horse tail appearance
• The spinal cord is not uniform in diameter, instead, it has cervical and lumbar enlargements
• The spinal cord gives rise to 31 pairs of spinal nerves
• Spinal nerves are all mixed nerves
• Regions of the spinal cord include:
  • Cervical (8)
  • Thoracic (12)
  • Lumbar (5)
  • Sacral (5)
  • Coccygeal (1)
• Cervical enlargement supplies the upper limbs, it starts at C5-T1; it is the example innervation in the upper limbs - brachial plexus
• Lumbar enlargement supplies the lower limbs
• There are 31 spinal cord segments, which makes sense since Spinal nerves = vertebra

Spinal Nerve Info

• The area where the spinal nerve leaves is called the spinal segment
• The spinal nerves are called by a letter and a number, depending on their position
• C5 spinal nerve = cranial nerve 5, same as the segments
• If the spinal cord ends at L1 and L2, then the S5 segment is not necessarily corresponded to the position of the vertebra
• Instead, the S5 segment is found in the L4 vertebra

Meninges

• The meninges surround the CNS, but the CNS itself does not have connective tissue
• The CNS surroundings have connective tissue
• The three layers are Dura mater, arachnoid mater, pia mater

Dura Mater

• The dura mater the outermost layer and is continuous with the epineurium of the spinal nerves
• It is made of dense irregular connective tissue
• It extends from the level of the foramen magnum to S2
• The vertebra not the segment
• The closed caudal end is anchored to the coccyx by the filum terminale externum

Arachnoid Mater

• The arachnoid mater is a Thin web arrangement of delicate collagen and some elastic fibers
• It adheres to the inner surface of the dura mater

Pia Mater

• The pia mater is bound to the surface
• It is a thin, transparent connective tissue layer that adheres to the surface of the spinal cord and brain
• Forms the filum terminale and anchors the spinal cord to coccyx
• Pia mater also forms the denticulate ligaments that attach the spinal cord to the arachnoid mater and inner surface of the dura mater
• Pia mater is the innermost layer, very close to the dura mater, then gets attached to the spinal cord

Spaces Around Spinal Cord

• Epidural: space between the dura mater and the wall of the vertebral canal which is filled with fat
• Anesthetics injected here for epidural anesthesia - to block pain in Labor
• Subdural space: serous fluid which is not a true space - potential space
• Subarachnoid: between the pia and arachnoid - true space and important which is filled with CSF for physiological effect, cushioning effect *increases stability
• The best place to take the Lumbar Puncture Sample is L3+L4
• However if blood was found- bad sign there must have been bleeding or if bacteria was found its a sign of infection