Chapter 5 Anatomy PDF

# Introduction The nervous system is highly complex, providing control and coordination among all body systems. Many disease processes and injuries affect the nervous system, with a wide range of clinical presentations based on the specific portion and location of the nervous system they affect. T...

# Introduction The nervous system is highly complex, providing control and coordination among all body systems. Many disease processes and injuries affect the nervous system, with a wide range of clinical presentations based on the specific portion and location of the nervous system they affect. The nervous system is divided anatomically into the central nervous system (CNS), the peripheral nervous system (PNS), and the autonomic nervous system (ANS). ## CNS - The CNS includes the brain and spinal cord. ## PNS - The PNS includes the motor and sensory peripheral nerves located outside the spinal cord. ## ANS - The ANS has two subdivisions: the sympathetic and the parasympathetic nervous systems. ### Diagram of the Nervous System | Autonomic Nervous System (ANS) | Peripheral Nervous System (PNS) | Central Nervous System (CNS) | |---|---|---| | Sympathetic NS | Cranial nerves | Brain | | Parasympathetic NS | Brachial plexus | Spinal cord | | | Spinal nerves | | | Lumbosacral plexus | ## Nervous Tissue - Neurons, nerve cells, are the fundamental unit of nervous tissue. - The different types of neurons are described by their function: - **Afferent (sensory) neurons** - **Efferent (motor) neurons** - **Interneurons (connecting neurons)** - **Upper motor neuron** cell bodies are located in the cerebral cortex, brainstem, and cerebellum. - **Lower motor neuron** cell bodies are located in the anterior horn of the spinal cord. - **Interneurons** function to transmit or integrate signals from one or more neurons, relaying impulses to other neurons or interneurons. - A neuron consists of three parts: a **cell body**, an **axon**, and **dendrites**. - **Cell body** (containing the nucleus and plasma) - **Axon** (conducts electrical impulses away from the cell body) - **Dendrites** (conduct impulses to the cell body from other structures) - **Synapse** is the junction between an axon, dendrite, interneuron, or other structures. - **Electrical impulses** that control and coordinate functions of the body are generated within a cell body and travel along an axon. - Impulses also arise in **sensory receptors**, each activated in response to specific sensory stimuli. - When activated, impulses are conducted along a dendrite to a sensory cell body. - At the synapse, a chemical termed a **neurotransmitter** is released. - When sufficient neurotransmitter reaches the other side of a synapse, an electrical impulse is generated in the structure receiving the neurotransmitter. - **Myelin** is a white, fatty substance covering axons and dendrites, speeding conduction of impulses. - **Nodes of Ranvier** are the interruptions in myelin. ## Central Nervous System Protection - The CNS has three levels of protection: - **Bony** - **Membranous** - **Fluid** - The **skull** surrounds the brain, and the **vertebrae** surround the spinal cord. - **Vertebral canal** is the canal collectively formed by the vertebral foramen of each vertebra. - **Meninges** are the three layers of membrane surrounding the brain and spinal cord: - **Dura mater** (Latin: "hard mother") is the thick, fibrous, tough outer layer of the meninges. - **Arachnoid** (Greek: "spider") is the middle, thinner, fragile layer, forming a spider-like web connecting the outer and inner layers. - **Pia mater** (Latin: "tender mother") is the innermost layer, contains blood vessels of the brain and spinal cord, and adheres closely to the cerebral cortex and spinal cord. - **Cerebrospinal fluid (CSF)** is the third level of protection. ## The Brain - The brain is the main part of the CNS. - Information received in the brain is analyzed, and acted on both consciously and subconsciously. - The brain is divided into two hemispheres, each containing the cortex, ventricles, and connecting fibers between the two hemispheres. - The **cerebrum** is the largest portion of the brain. - The cerebrum provides for the highest cognitive functions: - Motor control - Speech - Personality - Learning - The cerebrum is composed of right and left cerebral hemispheres. - The cerebral hemispheres are connected by the **corpus callosum**. - **Gyri** (singular: gyrus) and **sulci** (singular: sulcus) form the convoluted cortex of the cerebral hemispheres. - Each hemisphere is divided into four lobes: - **Frontal lobe** (anterior portion) - **Occipital lobe** (posterior portion) - **Parietal lobe** (located between the frontal and occipital lobes) - **Temporal lobe** (located under the frontal and parietal lobes just above the ear) - **Thalamus** is deep within the cerebral hemispheres, beneath the cortex. - The thalamus is the location for the perception of pain and is a relay station for body sensations. - **Hypothalamus** is near the thalamus. - The hypothalamus regulates behavior, hormonal regulation, and contributes to coordination of movement. - **Brainstem** is located below the cerebrum. - Most cranial nerves arise from nuclei in the brainstem. - All fiber tracts from within the brain and spinal cord pass through the brainstem. - The brainstem is divided into three parts, from superior to inferior: - **Midbrain** - **Pons** (Latin: "bridge") - **Medulla oblongata** (Latin: "long" innermost") - The **midbrain** controls visual reflexes. - The **pons** connects various nuclei to the cerebellum. - The **medulla oblongata** regulates ventilation and respiration, blood pressure, and heart rate. - The **foramen magnum** is where the medulla oblongata becomes the spinal cord. - **Cerebellum** is located behind the pons and medulla, covered superiorly by the posterior portion of the cerebrum. - The cerebellum controls muscle tone, coordination, and posture. ## The Spinal Cord - The spinal cord transmits information between the brain and the rest of the body. - The spinal cord is a continuation of the medulla and is located within the vertebral canal. - In an adult, the **conus medullaris** (cone-shaped end) is at approximately the level of the second lumbar vertebra. - Below the second lumbar vertebra, spinal nerve roots extend from the spinal cord to their respective intervertebral foramina. - The collection of spinal nerve roots below the second lumbar vertebra is termed the **cauda equina** (Latin: "horse's tail"). - The **filum terminale** (Latin: "thread" "end") is a thread-like non-neural filament that extends from the conus medullaris and attaches to the coccyx. ## Spinal Nerves - Anterior and posterior roots join, forming spinal nerves. - Each spinal nerve contains both sensory and motor nerve fibers. - There are 31 pairs of spinal nerves: - 8 cervical - 12 thoracic - 5 lumbar - 5 sacral - 1 coccygeal - Cervical nerves (C1-C7) exit the vertebral column above the corresponding vertebra. - For example, the C3 nerve exits above the C3 vertebra. - Because there is one more cervical nerve than cervical vertebrae, the C8 spinal nerve exits below the C7 vertebra and above the T1 vertebra. - Upon exiting the intervertebral foramen, spinal nerves branch, forming a **dorsal (posterior) ramus** and a **ventral (anterior) ramus**. - The dorsal rami innervate muscles and skin of the posterior trunk. - Except in the thoracic region, anterior rami form plexuses (singular: plexus), ending as peripheral nerves. - **Cervical plexus** (C1-C4) innervates the neck. - **Brachial plexus** (C5-T1) innervates the neck and upper extremity. - **Lumbosacral plexus** (LI-S3) innervates the lower extremity. - **Thoracic nerves** maintain their segmental relationships and do not join with other nerves to form a plexus. - The posterior rami innervate the muscles of the back and the overlying skin. - The anterior rami become **intercostal nerves**, which innervate the muscles and skin of the anterior and lateral trunk. ## Dermatomes and Myotomes - **Dermatomes** are areas of skin supplied with sensory fibers of specific spinal cord levels or specific peripheral nerves. - Dermatomes are described by either the specific spinal cord level involved in the innervation or the peripheral nerve involved. - Adjacent dermatomes may have overlapping innervations, and specific dermatome boundaries vary slightly among individuals. - Complete anesthesia of an area will not occur unless two or more spinal nerves or a spinal cord level are not intact. - When an injury involves only one spinal nerve, sensation will be decreased or altered but not lost completely. - Injury to a peripheral nerve results in loss or change of its function distal to the site of injury. - Methodical testing of sensation and comparison of results to dermatome charts are used to determine the location of the injury causing the sensory loss: spinal cord level or peripheral nerve. - **Myotomes** are all of the muscles that receive motor innervation from a specific spinal cord level. - Loss of full motor function of a muscle will not occur unless all spinal levels from which the muscle receives motor innervation are involved. - Injury to a peripheral nerve may result in weakness or paralysis, depending on the site of injury and involvement of specific spinal cord levels. ## Plexus Formation - **Plexus** (Latin: "braid") is a network of nerves that branches and combine to form peripheral nerves. - Branches from anterior rami of spinal nerves combine either with fibers from other branches or directly to other anterior rami. - This branching and recombining create peripheral nerves consisting of a mix of fibers from multiple spinal nerves (spinal cord levels). - The result of the formation of peripheral nerves in a plexus is that motor and sensory innervation to a region of the body arises from more than one spinal cord level. - An advantage of plexus formation of peripheral nerves is that not all innervation of specific muscles or sensory regions is lost in the event of trauma or disease. - **Cervical plexus** is formed by the anterior rami of the first four cervical spinal nerves (C1-C4). - The cervical plexus innervates the neck. - **Brachial plexus** is formed by the anterior rami of spinal nerves C5-T1. - The brachial plexus innervates the neck and upper extremity. - The brachial plexus is organized from central to peripheral into roots, trunks, divisions, cords, and peripheral nerves. - The five spinal nerve roots of the brachial plexus consist of the anterior rami of spinal nerves C5-T1. - The five spinal nerve roots combine to form three trunks. - **Superior trunk** from C5 and C6 - **Middle trunk** from C7 - **Inferior trunk** from C8 and T1 - Each of the three trunks divides, forming anterior and posterior divisions, resulting in six divisions. - The six divisions combine, forming the three cords: - **Lateral cord** - **Posterior cord** - **Medial cord** - The three cords divide into the five major peripheral (terminal) nerves of the brachial plexus: - **Axillary nerve** (a branch of the posterior cord) - **Musculocutaneous nerve** (a branch of the lateral cord) - **Radial nerve** (a continuation of the posterior cord) - **Median nerve** (a combination of the lateral and medial cords) - **Ulnar nerve** (a continuation of the medial cord) - **Lumbosacral plexus** has eight roots, and each root branches into upper and lower rami. - The divisions combine to form the peripheral nerves innervating the lower extremity. - The lumbosacral plexus may also be divided into two plexuses: - **Lumbar plexus** (L1-L4) - **Sacral plexus** (L5-S3) - The lumbar plexus provides motor and sensory innervation of the thigh. - The sacral plexus provides motor and sensory innervation of the leg and foot. - Spinal nerves S3-S5 innervate the perineum and pelvic floor. - The lumbosacral plexus spinal nerves do not branch and combine as much as the spinal nerves of the brachial plexus. - The seven peripheral nerves of the lumbosacral plexus are: - **Femoral nerve** (formed from posterior divisions of L2-L4) - **Obturator nerve** (formed from anterior divisions of L2-L4) - **Superior gluteal nerve** (formed from posterior divisions of L4-S1) - **Inferior gluteal nerve** (formed from posterior divisions of L5-S2) - **Sciatic nerve** (formed from the division of L4-S3 and separates into the tibial and common fibular nerves) - **Tibial nerve** (formed from anterior divisions of L4-S3) - **Common fibular (peroneal) nerve** (formed from posterior divisions of L4-S2) ## Stretch Reflex - A stretch reflex is a reflex arc of sensory input resulting in motor output. - Many tissues, including muscles and blood vessels, have sensory receptors that respond to stretch or tension imposed on the tissue. - In muscle, these sensory receptors are termed **muscle spindles**. ## The Autonomic Nervous System (ANS) - The ANS regulates the function of visceral structures such as the digestive, reproductive, cardiac, and pulmonary systems. - When spinal nerves branch into anterior and posterior rami, just outside the intervertebral foramen, additional rami are formed, ultimately becoming the ANS. - The ANS has two components: - **Sympathetic nervous system** - **Parasympathetic nervous system** - The **sympathetic nervous system** prepares the body's response to stress ("fight or flight") by regulating such functions as blood pressure, increasing heart rate, and bronchodilation. - The **parasympathetic nervous system** controls smooth muscle contraction, constriction of the bronchioles, and slowing of heart rate. - The sympathetic and parasympathetic nervous systems operate as a check-and-balance for each other. ## Common Pathologies - **Spina bifida** is a congenital defect in which posterior segments of some of the vertebrae fail to close during embryonic development. - **Cerebral palsy** is a term used to describe a group of non-progressive upper motor neuron disorders of the brain that result from abnormal development or physical damage occurring in utero, during birth, or soon after birth. - **Spinal cord injury (SCI)** can be caused by disease (cancer), blood clots, or trauma. - The impairments associated with SCI depend on the spinal level at which the injury occurs and the extent of tissue damage. - Impairments include both sensory and motor loss at and below the level of the spinal cord lesion. - Assessment of sensory and motor function is used to determine spinal cord level of injury, establish goals for rehabilitation, and predict functional outcomes. - SCI above spinal level L2 may affect spinal cord function at lower levels. - **Tetraplegia (quadriplegia)** is an SCI occurring at T1 or above, affecting function of all four extremities. - **Paraplegia** is an SCI occurring at T2 or below, affecting function of the trunk and lower extremities. - **Incomplete SCI** is when only part of the spinal cord is damaged. - Some, but not all, sensory and motor nerve impulses can still pass through the site of injury. - This will result in partial paralysis or sensory loss. - **Amyotrophic lateral sclerosis (ALS)**, also know as Lou Gehrig's disease, is a progressive degenerative motor disease of both upper and lower motor neurons. - ALS results in increasing paralysis of all skeletal muscle, including the muscles of ventilation. - **Alzheimer disease** is one of a number of progressive brain disorders causing dementia and loss of intellectual function, personality changes, and language disorders. - These impairments eventually destroy a person’s ability to live independently. - Death often results from complications of immobility. - **Multiple sclerosis (MS)** is a progressive disorder characterized by destruction of the myelin sheath. - Tremors and lack of coordination are typical signs of MS. - **Myasthenia gravis** is a disease involving the neuromuscular junction, the site at which peripheral nerves synapse with muscles. - The resulting weakness and fatigue of skeletal muscles are reversible over time. - **Muscular dystrophy (MD)** is a hereditary and progressive disease of muscle tissue. - There are several forms of MD, all resulting in progressive loss of motor function. - **Duchenne muscular dystrophy** is characterized by weakness of proximal muscles, followed by progressive involvement of distal musculature. - Duchenne muscular dystrophy affects males, and onset is during childhood. - **Neuropathy** of a peripheral nerve may result in sensory and motor deficits at the level of, and distal to, the site of the injury. - Loss of sensory and motor function may be partial or complete, depending on the severity of the neuropathy. - **Bell palsy** is an injury to the facial nerve (cranial nerve VII). - The facial nerve innervates the facial muscles. - Bell palsy is usually temporary and typically affects only one side of the face. - **Scapular winging** is a sign of injury to the long thoracic nerve, producing weakness or paralysis of the serratus anterior muscle. - **Thoracic outlet syndrome** occurs when the nerves of the brachial plexus, the subclavian artery, or subclavian vein, or a combination of these structures, are compressed in the thoracic outlet, the space between the clavicle, first rib, and scalene muscles. - The signs and symptoms associated with thoracic outlet syndrome depend on the structures involved and severity of compression. - Thoracic outlet signs and symptoms may include pain, sensory and motor loss, and interference with blood flow. - **Burners, or stingers**, occur after overstretch or compression of the brachial plexus. - The injury may be the result of a blow to the head or shoulder, commonly associated with the activities of football, wrestling, and gymnastics. - Symptoms may include immediate burning pain, paresthesia radiating from the neck, numbness, and brief paralysis of the upper extremity. - These signs and symptoms usually resolve within minutes, although shoulder weakness and muscle tenderness of the neck may continue for a few days. - **Erb palsy** is a traction injury to the upper portions of the brachial plexus. - This brachial plexus birth injury occurs to infants during a difficult childbirth. - When this injury occurs, the affected arm has weakness or paralysis of muscles of the shoulder and elbow flexion. - A position of shoulder extension and medial rotation, elbow extension, forearm pronation, and wrist flexion results. - Thus the hand is positioned behind the body with the wrist flexed and the palm facing superiorly. - **Saturday night palsy** occurs when the radial nerve is compressed as it spirals around the mid-humerus. - **Sciatica** is the result of irritation of the sciatic nerve roots (L4-S5) or of the sciatic nerve itself. - Irritation of the sciatic nerve roots is often a result of compression by a herniated lumbar disc. - Signs and symptoms of sciatica include radiating pain along the path of the sciatic nerve and loss of sensory and motor function of the region of the lower extremity. - **Foot drop** is the result of injury to the common fibular (peroneal) nerve. - Compression of the common fibular nerve is caused by pressure on the nerve at the head of the fibula. - This may occur when a shoe is applied too tightly or without sufficient padding to protect the nerve. - **Neuroma** is a mass of neurovascular tissue. - A Morton’s neuroma is compression of a neuroma, most often between the third and fourth toes. - Symptoms include pain and paresthesia. - Compression may occur as a result of flattening of the metatarsal arch, transfer of weight to the forefoot when wearing high heels, or wearing shoes with a tight toe box. - **Degenerative Diseases** occur when the nervous system starts deteriorating. - **Amyotrophic lateral sclerosis (ALS)**, also known as Lou Gehrig's disease, is a progressive degenerative motor disease of both upper and lower motor neurons. - ALS results in increasing paralysis of all skeletal muscle, including the muscles of ventilation. - **Alzheimer disease** is one of a number of progressive brain disorders causing dementia and loss of intellectual function, personality changes, and language disorders. - These impairments eventually destroy a person's ability to live independently. - Death often results from complications of immobility. - **Demyelinating Diseases** occur when the myelin sheath is destroyed. - **Multiple sclerosis (MS)** is a progressive disorder characterized by destruction of the myelin sheath. - Tremors and lack of coordination are typical signs of MS. - **Disorders of Muscle and the Neuromuscular Junction** occur when the neuromuscular junction is impaired. - **Myasthenia gravis** is a disease involving the neuromuscular junction, the site at which peripheral nerves synapse with muscles. - The resulting weakness and fatigue of skeletal muscles are reversible over time. - **Muscular dystrophy (MD)** is a hereditary and progressive disease of muscle tissue. - There are several forms of MD, all resulting in progressive loss of motor function. - **Duchenne muscular dystrophy** is characterized by weakness of proximal muscles, followed by progressive involvement of distal musculature. - Duchenne muscular dystrophy affects males, and onset is during childhood. - **Neuropathy** of a peripheral nerve may result in sensory and motor deficits at the level of, and distal to, the site of the injury. - Loss of sensory and motor function may be partial or complete, depending on the severity of the neuropathy. - Neuropathies are identified by either cause (diabetic neuropathy) or anatomical location (carpal tunnel syndrome) and are often named for the person first describing the pathology (e.g., Bell palsy). - **Neuropathy of peripheral nerves** results in typical patterns of motor and sensory impairment. - Impairments resulting from peripheral neuropathies may resolve or persist, depending on the severity of injury to the nerve(s) involved. - **Bell palsy** is an injury to the facial nerve (cranial nerve VII). - The facial nerve innervates the facial muscles. - This condition is usually temporary and typically affects only one side of the face. - **Scapular winging** is a sign of injury to the long thoracic nerve, producing weakness or paralysis of the serratus anterior muscle. - The result is rising of the medial border of the scapula away from the rib cage. - **Thoracic outlet syndrome** is a group of disorders occurring when the nerves of the brachial plexus, the subclavian artery, or subclavian vein, or a combination of these structures, are compressed in the thoracic outlet, the space between the clavicle, first rib, and scalene muscles. - Signs and symptoms associated with thoracic outlet syndrome depend on the structures involved and severity of compression. - Thoracic outlet signs and symptoms may include pain, sensory and motor loss, and interference with blood flow. - **Burners, or stingers, ** occur after overstretch or compression of the brachial plexus. - The injury may be the result of a blow to the head or shoulder, commonly associated with the activities of football, wrestling, and gymnastics. - Signs and symptoms may include immediate burning pain, paresthesia radiating from the neck, numbness, and brief paralysis of the upper extremity. - These signs and symptoms usually resolve within minutes, although shoulder weakness and muscle tenderness of the neck may continue for a few days. - **Erb palsy** is a traction injury to the upper portions of the brachial plexus. - This brachial plexus birth injury occurs to infants during a difficult childbirth. - When this injury occurs, the affected arm has weakness or paralysis of muscles of the shoulder and elbow flexion. - A position of shoulder extension and medial rotation, elbow extension, forearm pronation, and wrist flexion results. - Thus the hand is positioned behind the body with the wrist flexed and the palm facing superiorly. - **Saturday night palsy** occurs when the radial nerve is compressed as it spirals around the mid-humerus. - The name derives from the mechanism of injury. This injury may occur when a person, perhaps intoxicated, falls asleep with their arm over the back of a chair or other hard surface, compressing the radial nerve. - Signs and symptoms include weakness or paralysis of wrist and finger extensor muscles. - **Sciatica** is the result of irritation of the sciatic nerve roots (L4-S5) or of the sciatic nerve itself. - Irritation of the sciatic nerve roots is often a result of compression by a herniated lumbar disc. - Signs and symptoms of sciatica include radiating pain along the path of the sciatic nerve and loss of sensory and motor function of the region of the lower extremity. - **Foot drop** is the result of injury to the common fibular (peroneal) nerve. - Compression of the common fibular nerve is caused by pressure on the nerve at the head of the fibula. - This may occur when a shoe is applied too tightly or without sufficient padding to protect the nerve. - **Neuroma** is a mass of neurovascular tissue. - A **Morton’s neuroma** is compression of a neuroma, most often between the third and fourth toes. - Symptoms include pain and paresthesia. - Compression may occur as a result of flattening of the metatarsal arch, transfer of weight to the forefoot when wearing high heels, or wearing shoes with a tight toe box.