Peripheral Nervous System - Lecture Slides PDF

Peripheral Nervous System  Learning Outcomes : After reading this Lecture , you should be able to ; 1. List the divisions of the peripheral nervous system, and describe the characteristics of each. 2. List the major ascending tracts. 3. State a function for...

Peripheral Nervous System  Learning Outcomes : After reading this Lecture , you should be able to ; 1. List the divisions of the peripheral nervous system, and describe the characteristics of each. 2. List the major ascending tracts. 3. State a function for each of the major ascending tracts. 4. Distinguish between upper and lower somatic motor neurons. 5. Distinguish between direct and indirect tracts. 6.Contrast the structure of the autonomic nervous system and the somatic motor system. 7. Name the two divisions of the autonomic nervous system, and describe the differences between them. 8. Compare and contrast the general functions of the parasympathetic and sympathetic nervous systems.  Somatic Sensory Pathways Transmit information from somatic sensory receptors to the primary somatosensory area in the cerebral cortex and to the cerebellum. The pathways to cerebral cortex consist of thousands of sets of three neurons: First-order Second-order Third-order neurons neurons neurons I. First-order neurons conduct impulses from the somatic receptors into brain stem or spinal cord. a. From face, mouth, teeth, and eyes → somatic sensory impulses propagate along cranial nerves into the brain stem. b. From the neck, trunk, limbs, and posterior aspect of the head → somatic sensory impulses propagate along spinal nerves into the spinal cord. II. Second-order neurons conduct impulses from the brain stem and spinal cord to the thalamus. Axons of second-order neurons decussate (cross over to the opposite side) in the brain stem or spinal cord before ascending to the ventral posterior nucleus of the thalamus. all somatic sensory information from one side of the body reaches thalamus on opposite side. III. Third-order neurons conduct impulses from the thalamus to primary somatosensory area of cortex on the same side. Somatic sensory impulses ascend to the cerebral cortex via three general pathways: 1. Posterior column–medial lemniscus pathway 2. Anterolateral (spinothalamic) pathway 3. Spinocerebellar tracts that relay somatic sensory impulses to the cerebellum.  Dorsal columns-medial lemniscal Pathway 1) Axons of second-order neurons cross to opposite side of medulla oblongata. 2) Large myelinated nerve fibers (CV = 30-110 m/sec). 3) More accurate intensity gradation (100 times). 4) Transmit the following sensory modalities :  Fine touch sensation.  Vibratory sensations.  Position sensation.  Pressure sensations  Clinical Consedration: Damage to this pathway can result in sensory ataxia (loss of coordinated movement) and impaired tactile discrimination. Conditions such as tabes dorsalis (seen in neurosyphilis) and multiple sclerosis can affect the dorsal columns, leading to significant sensory deficits. In clinical practice, tests like the Romberg test assess the integrity of proprioception by evaluating a patient’s ability to maintain balance with their eyes closed.  Anterolateral (spinothalamic ) pathway 1. Axons of second-order neurons cross to the opposite side of spinal cord. 2. Smaller myelinated nerve fibers (CV= 8-40m/sec). 3. Less accurate intensity gradation (10-20) 4. Transmit the following sensory modalities : Pain sensation. Crude touch and pressure sensations. Temperature sensation. Tickle and itch sensation Sexual sensations  Somatic sensory pathways to the cerebellum Carry proprioceptive impulses to the cerebellum from the spinal cord. Sensory impulses are critical for posture, balance, and coordination of skilled movements.  Student Activity Desribe the following Figures  Somatic motor pathways Neural circuits in brain & spinal cord coordinate all voluntary and involuntary movements. All excitatory and inhibitory signals converge on the motor neurons that extend out of brain stem and spinal cord to innervate body skeletal muscles. These neurons, also known as lower motor neurons (LMNs) have their cell bodies in brain stem and spinal cord. A. From brain stem: axons of LMNs extend through cranial nerves to innervate skeletal muscles of the face and head. B. From the spinal cord: axons of LMNs extend through spinal nerves to innervate skeletal muscles of the limbs and trunk. LMNs provide output from CNS to skeletal muscle fibers.  Upper motor neurons Upper motor neurons are those in the cortex and brainstem that activate the lower motor neurons. LMN receive input from upper motor neurons (UMNs). In which UMNs synapse directly & indirectly with LMNs. 1.From cerebral cortex: essential for performance of voluntary movements of body. 2.From motor centers of brain stem: regulate muscle tone, control postural muscles, and help maintain balance and orientation of head and body. Both the basal nuclei and cerebellum exert influence on UMNs.  LLMs versus UMNs lesion Damage or disease of LMNs: flaccid paralysis of muscles on the same side of the body. No voluntary nor reflex action of the innervated muscle fibers + lost or ↓ muscle tone. Injury or disease of UMNs (cerebral cortex): spastic paralysis of muscles on the opposite side of the body + ↑ tone, exaggerated reflexes + pathological reflexes, i.e., Babinski sign positive.  somatic motor pathways: Axons of UMNs extend from the brain to LMNs via two types of somatic motor pathways: i. Direct motor pathways: The corticospinal tracts are considered direct because they extend directly from upper motor neurons in the cerebral cortex to lower motor neurons in the spinal cord ii. Indirect motor pathways: Although they originate in the brainstem, these tracts are indirectly controlled by the cerebral cortex, basal nuclei, and cerebellum. These tracts are called indirect because no direct connection exists between the cortical and spinal neurons. Both pathways control generation of nerve impulses in LMNs (neurons that stimulate contraction of skeletal muscles).  Direct motor pathways Through which nerve impulses for voluntary movements propagate from cerebral cortex to LMNs. Also known as (pyramidal pathways) consist of axons that descend from pyramidal cells. Pyramidal cells are UMNs with pyramid-shaped cell bodies located in cerebral cortex. They consist of : 1. corticospinal pathways 2. corticobulbar pathway  90% of axons of UMNs decussate to the opposite side in MO and then descend into spinal cord.  10% remain on same side and decussate at spinal cord levels (synapse with a LMN). Thus: right cerebral cortex controls most of muscles on the left side of the body, and the left cerebral cortex controls most of muscles on the right side of the body.  Corticospinal Tracts Anterior Corticospinal Lateral Corticospinal Tract Tract  Anterior corticospinal tract  Formed by 10% of axons on same side and decussate at spinal cord levels.  At each spinal cord level, these axons decussate via the anterior white commissure.  They synapse with LMNs in the anterior gray horn and exit cord in anterior roots of spinal nerves.  They terminate in skeletal muscles that control movements of the trunk and proximal parts of the limbs (axial and girdle muscle) → concerned with postural control.  Lateral corticospinal tract  Formed by 90% of axons decussate to the opposite side in MO and then descend into spinal cord.  Its axons synapse with LMNs in anterior gray horn of spinal cord.  Axons exit the cord in anterior roots of spinal nerves and terminate in skeletal muscles (control movements of the distal parts of the limbs). Distal muscles are responsible for precise, and highly skilled movements of hands and feet. Example: movements needed to button a shirt or play the piano.  Damage of corticospinal pathway Motor weakness as hemiplegia (paralysis) or hemiparesis (weakness). Positive Babinski sign (extension of big toe and fanning of others in response to firmly stroking the sole of foot). Damage to the corticospinal fibers above to the pyramidal decussation → contralateral motor deficits. Lesions below to the decussation (in spinal cord) → ipsilateral motor deficits.  Indirect motor pathways Extrapyramidal (indirect) pathways are important in the control of: Upright posture Balance Walking. These tracts are: 1.Rubrospinal tract 2.Tectospinal tract 3.Vestibulospinal tract 4.Lateral reticulospinal tract 5.Medial reticulospinal tracts  The rubrospinal tract :Concerned with fine control of voluntary movements of distal muscles of upper limbs and facilitates flexion in upper extremities.  The tectospinal tract : mediate reflex postural movement of head, eyes, and trunk in response to visual or auditory stimuli.  The Vestibulospinal tract :Control excitatory signals to different antigravity muscles → maintain posture and balance in response to head movements.  Reticulospinal Tract : Maintaining posture Regulating muscle tone in response to ongoing body movements.  Amyotrophic lateral sclerosis Progressive degenerative disease attacks: 1.Motor areas of cerebral cortex 2.Axons of UMNs of corticospinal and rubrospinal tracts 3.LMN cell bodies. Causes progressive muscle weakness and atrophy. Often begins in sections of spinal cord that serve hands and arms but rapidly spreads to involve whole body and face, without affecting intellect or sensations. Death typically occurs in 2 to 5 years.  Brain Storm During Intraoperative Monitoring of spinal cord by neurophysiologist doctor of patient underwent to spinal cord correction surgery ,the neurosurgean injured Rt Lateral corticospinal tract fibers at level of L3 ; Where the most probably motor deficits occur in Rt or Lt lower limb ?  Autonomic Nervous System (ANS) The ANS “self-regulating” consists of motor neurons that regulates the activity of: 1) Cardiac Muscle (Heart) 2) Smooth Muscle ( In Hollow Organs) a. Blood Vessels b. Digestive System c. Bronchioles d. Sphincters 3) Glands a. Adrenal b. Digestive glands  Autonomic Nervous System (ANS) Anatomical and physiological differences within the ANS are the basis for its further subdivision into: a) Sympathetic NS b) Parasympathetic NS c) Enteric NS  Autonomic Fibers Two types: Preganglionic “mostly myelinated” nerve fibers that passes between the CNS and the ganglia. Postganglionic “unmyelinated” nerve fibers that emerge from the ganglia and form terminal network in the target organ.  Vertebral Autonomic Ganglia They are of sympathetic type They are situated on the two sides of the vertebral column and they are known as sympathetic trunk. They have short preganglionic and long postganglionic nerve fibers They have divided into many branches so their action is diffused.  Terminal Autonomic Ganglia They are of parasympathetic type They are located near, on, or in the effector organ They have long preganglionic and short or no postganglionic nerve fibers. They have localized action.  Autonomic Higher Centers 1. Brainstem Medulla Oblongata (vasomotor centers, respiratory centers, swallowing centers, coughing centers, and vomiting centers). Pons (pneumotaxic centers). Midbrain (mictuirtion centers). 2. Hypothalamus Temperature regulation centers Thirst centers Satiety (food intake) centers. 3. Limbic System (anger, rage, emotion, and sexual behavior) 4. Cerebral Cortex can elicit any type of autonomic response.  Outflow of Autonomic Fibers Autonomic nerve fibers leave the CNS at different levels: i. Cranial Outflow (parasympathetic) ii. Thoraco-lumber Outflow (Sympathetic) iii. Sacral Outflow (parasympathetic). Thoracolumber Outflow Cranial Outflow Sacral Outflow  Autonomic Innervation 1) Dual Innervation (sympathetic and parasympathetic innervation). It could be: i. Antagonistic (mainly) ii. Complementary or cooperative (few cases) 2) Single Innervation iii. Only sympathetic. iv. Only parasympathetic  Autonomic Vs Somatic NS 1. Effectors Somatic innervates the skeletal muscles Autonomic innervates the glands, smooth and cardiac M. 2. Control Somatic concerned with the voluntary function Autonomic concerned with involuntary function 3. Number of Nerve Fibers Somatic has a single heavily myelinated neuron between the CNS and the effector organ Autonomic has two-neuron chain connected by synapse between the CNS and the effector organ. 4. Function Somatic NS is an operating system (damage lead to loss of function) Autonomic NS is regulatory system (damage lead to change in the function) AUTONOMIC VS SOMATIC NS Sympathetic Division Flight & Fight Reaction  Flight & Fight Reaction Involves E activities – exercise, excitement, emergency, and embarrassment This is called Sympathetic Stress Response The effects will be: a)  heart rate, COP, blood pressure and tachycardia b)  respiratory rate c)  blood glucose level d)  glycolysis in the muscles. e.  blood flow to active muscles. f.  cellular metabolic rate g.  muscle strength h.  mental activity i. Erection of the hairs j. Tremor of the fingers k. Exophthalmos, dilated pupil & lid retraction l. Skin pallor (vasoconstriction) due to pooling of blood to the internal organs. Parasympathetic Division ( Rest & Digest )  Rest & Digest Concerned with keeping body energy use low Involves the D activities – digestion, defecation, and diuresis Its activity is illustrated in a person who relaxes after a meal a) Low BP, heart rate, and respiratory rates b) High gastrointestinal tract activity c) Warm skin and constricted pupils  SUMMARY  The peripheral nervous system (PNS) consists of all the nervous tissue outside the CNS.  The Sensory division of the PNS transmits action potentials to the CNS  The Motor division carries action potentials away from the CNS.  The motor division is divided into somatic and autonomic systems. The somatic motor system innervates skeletal muscle and is mostly under voluntary control.  The autonomic nervous system innervates cardiac muscle, smooth muscle, and glands and is mostly under involuntary control.  The autonomic nervous system is divided into sympathetic and parasympathetic divisions. Thank you