Peripheral Nervous System: Sensory & Motor Divisions

• The peripheral nervous system (PNS) facilitates communication between the central nervous system (CNS) and the rest of the body
• The PNS divides into sensory and motor divisions

Sensory Division

• The Afferent division conducts impulses from receptors to the CNS
• The Afferent division informs the CNS of the body's internal and external states
• Sensory nerve fibers are either somatic (from skin, skeletal muscles, or joints) or visceral (from organs within the body cavity)

Motor Division

• The Efferent division conducts impulses from the CNS to effectors like muscles or glands
• The Efferent division contains motor nerve fibers

Somatic Nervous System

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

Autonomic Nervous System

• Autonomic (visceral) sensory neurons send information to the CNS from autonomic sensory receptors
• These autonomic sensory receptors are mainly in visceral organs like smooth muscle organs in the thorax, abdomen, and pelvis
• Autonomic motor neurons are generally involuntary
• They conduct impulses from the CNS to smooth muscle, cardiac muscle, and glands

Sensory Somatic Neurons

• Neuron types are dependent on the process numbers
• There are three orders of sensory somatic neurons to reach the cortex
• First-order neurons are found in the dorsal root ganglion
• The cortex represents every place in the body

Motor Somatic Neurons

• The motor direction is the opposite of the sensory direction
• The neuron starts at the cortex, with its cell body being found there, and travels down the spinal cord
• A neuron extends from the spinal cord to the skeletal muscle
• An upper motor neuron in the CNS controls a lower-motor neuron in the brain stem or spinal cord within the somatic nervous system (SNS)
• The lower-motor neuron's axon directly controls skeletal muscle fibers, stimulating an excitatory effect

Autonomic Neurons

• In the autonomic system, the cortex is not used, the higher center, hypothalamus is used
• The process starts in the gray matter of the spinal cord and doesn't directly go to the neuron
• The process goes to the ganglia
• A synapse occurs at the ganglion
• Post-ganglionic neurons then happen
• The autonomic nervous system (ANS): the axon of a preganglionic neuron in the CNS controls ganglionic neurons in the periphery
• The ganglionic neurons, when stimulated, may cause excitation or inhibition of the visceral effector being supplied

Neuron Connections

• Axons of 1st (preganglionic) neurons leave the CNS and synapse with 2nd (ganglionic) neurons
• Axons of 2nd (postganglionic) neurons extend to the organ they serve

Sensory Ganglion Synapses

• Ganglion cells in dorsal root ganglia do not receive synapses

Spinal Cord External Anatomy

• Runs through the vertebral canal

• Extends from the foramen magnum to the second lumbar vertebra

• Regions of the spine:

• Cervical (8)

• Thoracic (12)

• Lumbar (5)

• Sacral (5)

• Coccygeal (1)

• Gives rise to 31 pairs of spinal nerves

• All spinal nerves are mixed nerves

• Spinal cord is not a uniform diameter

• Cervical enlargement supplies upper limbs

• Lumbar enlargement supplies lower limbs

• The length of the adult spinal cord ranges 41 to 45 cm

• Conus medullaris is a tapered, inferior end (conical structure), which ends between L1 and L2

• Cauda equina refers to the origin of spinal nerves extending inferiorly from the conus medullaris

Spine Regions

• The spinal cord extends from the foramen magnum down through the vertebral canal until it reaches the level of L1 and L2 vertebrae
• There is no spinal cord below the area of L1+L2, just lower spinal nerves
• A horse tail appearance called Cauda equina occurs as thinner spinal nerves are present
• The spinal cord is not uniform in diameter
• There is cervical and lumbar enlargement, for example, innervation of the upper limbs starts at C5 - T1

Spinal Segments

• There are 31 segments, which relates to there being 31 spinal nerves
• Spinal segment is the area where spinal nerves leave
• Spinal nerves assigned a letter and a number correlating to their position on the spine C5 = spinal nerve & cranial nerve 5
• Cord ends at L1 and L2, so S5 segment is not correlated to the position of the vertebra
• S5 segment inside L1 vertebra

Meninges

• The CNS does not have connective tissue, but the surroundings do
• The meninges make three layers of connective tissue membranes

Dura Mater

• The outermost layer, continuous with epineurium of the spinal nerves
• It has a dense irregular connective tissue
• Dura mater extends from the level of the foramen magnum to S2
• The closed caudal end is anchored to the coccyx by the filum terminale externum

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

• Bound tightly to the spinal cord's surface
• Provides a thin, transparent connective tissue layer that adheres to the surface of the spinal cord and brain
• Forms the filum terminale, which anchors the spinal cord to the coccyx
• Pia mater forms the denticulate ligaments that attach the spinal cord to the arachnoid mater and inner surface of the dura mater

CNS Layers

• Outermost is Dura mater, layer near the bone
• Arachnoid mater is a layer close to the dura mater
• Innermost layer Pia mater, is attached to the spinal cord
• Foramen magnum goes to the S2 level
• The end connected by a fold that extends to the coccyx is filum terminale externum
• Externum layer outside the outer membrane
• Layers also cover the brain

Spaces in the CNS

• Epidural space between the dura mater and the vertebral canal wall, filled with fat and location of epidural anesthetic injection
• Subdural is a serous, potential space
• Subarachnoid space is a true space and important space between pia and arachnoid layers, filled with cerebrospinal fluid (CSF) has a physiological effect
• Lumbar puncture performed in this region at either L3 or L4 level