BIOL2301 Lecture 19, Fall 2024 (PDF)

Spinal nerve organization Motor axons originate from the spinal nerve Ventral rootlets contain the motor axons exiting from the spinal cord Ventral root forms...

Spinal nerve organization Motor axons originate from the spinal nerve Ventral rootlets contain the motor axons exiting from the spinal cord Ventral root forms from the ventral rootlets Cell bodies are found in the ventral and lateral horns of the spinal cord Sensory axons enter the spinal cord via the dorsal aspect Dorsal rootlets arise from the dorsal root Dorsal root contain the short central axons of sensory neurons Dorsal root ganglion houses the cell bodies of the sensory neurons Rami organization Spinal nerves form from the convergence of the dorsal and ventral roots Exits the vertebral canal through the intervertebral foramen Rami are distal to the spinal nerve and are formed by branches of the spinal nerves Dorsal ramus Innervate the deep muscles of the back & the skin of the back Ventral ramus Splits into multiple branches Innervate the ventral and lateral portions of the trunk, upper & lower limbs Form nerve plexuses Rami communicans Associated with autonomic nervous system Extends between the spinal nerve and the sympathetic ganglions Topographical regionalization Spinal nerves are organized topographically based on what they innervate Upper limbs enter and exit the inferior cervical region at the cervical enlargement Lower limbs enter and exit the inferior thoracic and superior lumbar region at the lumbar enlargement Dermatome and myotome Dermatome Area of skin supplied with sensory information by a pair of spinal nerves Myotome Muscles that a particular spinal nerve innervates Motor activity Nerve plexus Arise from the interweaving of the anterior rami of the spinal nerves on each side of the body Acts to combine several nerves that innervate the same part of the body into one larger nerve. Allow the axons of the spinal nerves to travel through various pathways (branches) to reach their destination Damage to a single spinal nerve does not result in complete loss in innervation to/from a particular region A nerve that arises from a plexus usually has axons from more than one spinal nerve and from more than one spinal segment Form the “named” nerves of the body Four main plexuses Cervical Plexus Intercostal Nerves Thoracic nerves do not form plexuses Exception: a portion of T1 forms part of the brachial plexus Anterior rami of the thoracic nerves travel between adjacent ribs thus termed intercostal nerves All innervate intercostal muscles T2 Sensory information from skin of axilla and medial surface of the arm T3 to T6 Sensory information from the anterior and lateral chest wall T7 to T12 Innervates abdominal muscles and overlying skin Cervicobrachial plexus anatomy Cervical Plexus Located deep at back of neck Formed primarily by anterior rami of C1 – C4 Nerves innervate superficial neck structures Muscles attached to the hyoid bone The surface of the neck, Lower portions of the head Cervical Plexus Cervical Plexus Cutaneous branches Occipital nerve C2-C3 Occipital scalp Auricular nerve C2-C3 Skin around the auricle and external auditory meatus Cervical nerve C2-C3 Skin of anterior neck Supraclavicular nerve C3-C4 Skin of Shoulder region Motor branches Ansa cervicalis C1-C3 Muscles of the throat Phrenic nerve C4 (some C3 & C5) Innervates the diaphragm to regulate breathing Cervical Plexus Brachial Plexus Located superior to the clavicle More complex than the cervical plexus 5 levels of organization Rami Continuation of the anterior rami of spinal Divisions nerves C5 – T1 Trunks Unification of the rami Superior: Formed by C5 & C6 Middle: Formed by C7 Inferior: Formed by C8 & T1 Divisions Fibers of the trunks diverge to travel to anterior or posterior portions of the upper arm Two divisions called anterior and posterior Found deep to the clavicle Brachial Plexus 5 levels of organization (cont.) Cords Convergence of anterior and posterior divisions Named for their position relative to the axillary artery Posterior Formed by the posterior divisions of all 3 trunks Contains portions of C5 – T1 Medial Formed by the anterior division of the inferior trunk Contains portions of C8 – T1 Lateral Formed by the anterior division of superior and middle trunks Contains portions of C5 – C7 Terminal Branches arise from the cords Five major branches Contains all the nerves of the upper limb Five major terminal branches of the brachial plexus Axillary nerve Arises from posterior cord Innervates deltoid Receives sensory from superolateral portion of arm Musculocutaneous nerve Arises from lateral cord Innervates anterior arm muscles Receives sensory from lateral forearm Radial nerve Arises from posterior cord Innervates posterior arm and forearm muscles Receives sensory from posterior arm and forearm and lateral 3 digits Median nerve Arises from medial and lateral cords Innervates most of anterior forearm lateral hand Receives sensory from lateral palmar and dorsal tips of lateral 3 fingers Ulnar nerve Arises from medial cord Innervates anteromedial forearm and intrinsic hand muscles Receives sensory from medial 1½ digits (anterior and posteriorly) Five major terminal branches of the brachial plexus Axillary nerve Arises from posterior cord Innervates deltoid Receives sensory from superolateral portion of arm Musculocutaneous nerve Arises from lateral cord Innervates anterior arm muscles Receives sensory from lateral forearm Radial nerve Arises from posterior cord Innervates posterior arm and forearm muscles Receives sensory from posterior arm and forearm and lateral 3 digits Median nerve Arises from medial and lateral cords Innervates most of anterior forearm lateral hand Receives sensory from lateral palmar and dorsal tips of lateral 3 fingers Ulnar nerve Arises from medial cord Innervates anteromedial forearm and intrinsic hand muscles Receives sensory from medial 1½ digits (anterior and posteriorly) Brachial Plexus Lumbosacral plexus anatomy Lumbar Plexus Located in the anterior pelvis Formed by the anterior rami of spinal nerves L1 – L4 Overlaps with sacral plexus Less complex than brachial but has anterior and posterior divisions Contains nerves innervating the anterior aspect of lower limbs as well as the abdomen, pelvis and buttock Femoral nerve Main nerve of the posterior division Innervates muscles of the anterior thigh Allows flexion and extension of the knee and hip Receives sensory from the anterior and inferomedial thigh and medial aspect of the leg Obturator nerve Main nerve of the anterior division Innervates the medial thigh Receives sensory from the superomedial thigh Lumbar Plexus Located in the anterior pelvis Formed by the anterior rami of spinal nerves L1 – L4 Overlaps with sacral plexus Less complex than brachial but has anterior and posterior divisions Contains nerves innervating the anterior aspect of lower limbs as well as the abdomen, pelvis and buttock Femoral nerve Main nerve of the posterior division Innervates muscles of the anterior thigh Allows flexion and extension of the knee and hip Receives sensory from the anterior and inferomedial thigh and medial aspect of the leg Obturator nerve Main nerve of the anterior division Innervates the medial thigh Receives sensory from the superomedial thigh Sacral Plexus Located in the posterior pelvis (inferior to the lumbar pelvis) Overlaps with lumbar plexus Formed by the anterior rami of spinal nerves L4 – S4 Organized into anterior and posterior divisions Anterior division tend to innervate muscles that flex Posterior division tend to innervate muscles that extend Contains nerves innervating the posterior aspect of the lower limbs as well as the abdomen, pelvis and buttock Sacral Plexus Sciatic (aka ischiatic) nerve Longest and largest nerve in the body Formed from portions of both anterior and posterior divisions of the sacral plexus Has two divisions wrapped in a common sheath, tibial division and common fibular division that give rise to the tibial and common fibular nerve. Tibial Nerve Formed by anterior division of sciatic nerve Innervates posterior thigh and leg and plantar foot muscles Hip extension, thigh adduction, knee and foot flexion, and toe flexion Plantar nerves cause flexion and adduction of the toes Receives sensory input from the plantar region of the foot (via sural nerve) Common fibular nerve Formed by posterior division of sciatic nerve Innervates a knee muscle, anterior and lateral leg muscles Receives sensory input from the anteroinferior region of the foot (via its branches) and the dorsal interspace between the 1st and 2nd toe Lumbosacral plexus anatomy Reflexes Reflex: a rapid, pre-programmed, involuntary reaction of muscles or glands to a stimulus Stimulus: sensory input that initiates reflex Rapid response: requires few neurons involved Pre-programmed response: response is the same every time Involuntary response: Requires no conscious intent, can’t be suppressed Spinal cord is responsible for the integration of many basic reflexes Kinds of a reflex Basic: unlearned, built-in response Conditioned: acquired after practice & learning The reflex arc Basic functional unit of the nervous system Smallest, simplest circuit capable of Afferent pathway receiving a stimulus and producing a response Action potential never travels to the brain for processing Neural pathway involved in reflex activity Ipsilateral: Receptor and effector are Efferent pathway on the same side of the spinal cord Contralateral: receptor and effector are on opposite sides of the spinal cord Five parts Mono- v. Polysynaptic reflexes Monosynaptic Most simple reflex Have only one synapse in between receptor and effector Direct communication between sensory and motor neuron Cannot be modified by other input from the CNS Polysynaptic More complex Have more than one synapse between the receptor and effector Interneurons facilitate communication between the sensory and motor neuron May be affected by other input from the CNS Autonomic v. Somatic reflexes Autonomic Affects organs Somatic Affects muscles Stretch Reflex Monosynaptic Monitors and regulates muscle length Monitored by stretch receptors called muscle spindles Internal (2 items): Intrafusal muscle fibers Innervated by γ motor neurons Sensory neurons Wrapped around the intrafusal muscle fibers Surrounded by a CT capsule External: Extrafusal muscle fibers Innervated by α motor neurons Stretch Reflex Monosynaptic Monitors and regulates muscle length When Muscle spindle is stretched Sensory neurons detect the stretch of the intrafusal muscle fibers Signal sent to spinal cord (CNS) Synapse with α motor neurons Extrafusal fibers contract to resist stretch Indirectly involved in reciprocal inhibition, inhibition of the antagonistic muscle Requires an interneuron and is thus polysynaptic Golgi tendon reflex Polysynaptic Prevents excessive contraction in response to increased tension Sensory receptor is the Golgi tendon organ Sensory nerve endings found within a tendon or muscle-tendon junction Golgi tendon reflex Polysynaptic Prevents excessive contraction in response to increased tension When a muscle contracts, the tendon stretches Golgi tendon organ detects the stretch Sends the sensory information to the CNS Synapse with interneurons in spinal cord Interneurons inhibit alpha motor neurons to prevent muscle contraction Sensory neuron also stimulates the alpha motor neuron in the antagonistic muscle Called reciprocal activation More reflexes Withdrawal (flexor) reflex Polysynaptic Reflex that is initiated by a painful stimuli Causes a flexor muscle to contract (and the antagonist muscle to relax) thus causing the limb to be removed from the offending stimulus Crossed-extensor reflex Polysynaptic Occurs in conjunction with the withdrawal reflex When the withdrawal reflex is initiated in one limb, the opposing limb is stimulated to extend Sensory neurons branch to synapse with interneurons involved in the withdrawal activity and the crossed-extensor activity Interneuron involved in crossed-extensor activity crosses to the other side of the spinal cord to synapse with alpha neurons innervating the opposing limb