CBNS 116 Lecture 6: Spinal Cord Anatomy & Physiology (PDF)

CBNS 116 Lecture 6 - The Spinal cord: Gross anatomy, histology, cross-sectional anatomy, Autonomic Nervous System (ANS), the principal pathways, spinal reflexes, and clinical considerations Lecture notes by Todd Fiacco ASSIGNME...

CBNS 116 Lecture 6 - The Spinal cord: Gross anatomy, histology, cross-sectional anatomy, Autonomic Nervous System (ANS), the principal pathways, spinal reflexes, and clinical considerations Lecture notes by Todd Fiacco ASSIGNMENT: Read Nolte Chapter 10: “Spinal cord”. FOR NEXT WEEK, after Exam 1: Read Nolte Chapter 11: “Organization of the brainstem”. TODAY'S LECTURE: The big picture: The spinal cord is a primary component of the CNS by which the brain receives somatosensory information from the periphery and is the conduit through which the brain directs voluntary movement. Therefore the spinal cord is a good place to begin our study of 4 important pathways, called the principal pathways, 3 sensory and one motor: The posterior column-medial lemniscus system for fine touch, vibration, and propriocepion; the spinothalamic tract (anterolateral pathway) for nociceptive (pain and temperature) information, the spinocerebellar tracts, the pathways by which the cerebellum receives proprioceptive information to direct unconscious control of body position; and the corticospinal tract, the pathway from cerebral cortex to direct voluntary movement. The spinal cord is also the site of a number of important reflexes, the unconscious process by which the body reacts to incoming somatosensory stimuli for the overall well being and survival of the organism. In a similar fashion, the spinal cord plays a major role in the Autonomic (think “automatic”) Nervous System (ANS) which can be thought of as reflexive control of the body to maintain homeostasis. I. Gross anatomy and histology of the spinal cord [slides 2 - 6]. A. The spinal cord is a continuous longitudinally oriented structure with a cervical enlargement and a lumbar enlargement [slide 2]. These enlargements are due to the greater number of neurons (and their fibers) required for the processing of sensory and motor information associated with the limbs. Like the brain, the spinal cord possesses several important named sulci (though no gyri) that run the entire length of the cord. The most prominent of these is the deep anterior median fissure along the anterior (ventral) surface of the cord (can be seen in slide 2); its posterior (dorsal) counterpart the © Todd Fiacco 2020. This content is protected and may not be shared, uploaded or distributed.  posterior median sulcus; the posterolateral sulcus lateral to the posterior median sulcus, and the poorly defined anterolateral sulcus. The anterior median fissure and posterior median sulcus divide the spinal cord nearly in half, except for a small band of fibers called the anterior white commissure anterior to the central canal of the cord. Thus any fibers crossing the midline within the spinal cord do so within this narrow band. The posterolateral sulcus is where the dorsal rootlets emerge, while the anterolateral sulcus is where the ventral rootlets emerge. The dorsal root and the ventral root come together to form a spinal nerve. B. The spinal cord, like the brain, is covered by meninges (dura, arachnoid, and pia). These meninges continue along the length of each spinal (peripheral) nerve. Each spinal nerve contains all of the sensory and motor fibers of the joined ventral and dorsal roots of each segment. The meninges surrounding the peripheral nerves obtain special names, the epineurium (outer dural sheath of the nerve containing bundles of loose collagen), the perineurium (arachnoid layer surrounding nerve fascicles, which bundle together to form the nerve), and endoneurium, the innermost pial-like layer that surrounds each nerve fiber (‘nerve fiber’ is synonymous with ‘axon’). Thus, a peripheral nerve is a collection of many thousands of individual axons. Many of these are the heavily myelinated fibers of the primary sensory afferents on their way to the dorsal root ganglion (DRG) [slide 2, 3]. C. The spinal cord is divided into 31 functional segments [slide 6]. Thus, the spinal cord is not segmented like a centipede; rather each segment is defined by the 2 spinal nerves (one on each side) that emerge from it. The segments are: 1) 8 cervical 2) 12 thoracic 3) 5 lumbar 4) 5 sacral 5) 1 coccygeal The spinal cord ends caudally at the level of L1-L2 vertebra; thus the roots of segments L2 - coccygeal 1 continue caudally as a loose bundle of fibers called the cauda equina, or “horse’s tail”. Each spinal cord segment receives sensory information (via the dorsal © Todd Fiacco 2020. This content is protected and may not be shared, uploaded or distributed.  root) from an area of skin called a dermatome (literally “skin section”) [slide 7]. Dermatomes are most evident at thoracic levels and can be useful clinically to determine if there is damage to a particular spinal nerve. II. Cross-sectional anatomy of the spinal cord. The organization of gray and white matter of the spinal cord is best appreciated in cross section [slides 8 - 11]. Recall that in the spinal cord, the white matter is on the outside and the gray matter is on the inside. The spinal cord sections, like those of the brain and brainstem, are stained for myelin, therefore white matter is dark, while gray matter is light. A. The white matter can be divided into three anatomical tracts (per side) called funiculi (Latin for “string”) [slide 8]: the posterior funiculus, the lateral funiculus, and the anterior funiculus. Realize that the lateral funiculus is large and therefore has posterior and anterior zones; the anterior funiculus is small and located medially (runs adjacent to the anterior median fissure). The large, myelinated sensory fibers carrying touch and proprioceptive information enter the cord in the lateral posterior funiculus (also called the posterior column) and then travel rostrally within the posterior column on their way the brain (with relays in the medulla and thalamus). The lateral funiculus carries spinothalamic pain and temperature fibers in its anterior compartment (the anterolateral pathway). The lateral funiculus also carries some spinocerebellar fibers, and in its posterior compartment, the bulk of corticospinal tract fibers on their way to synapse onto the lower motor neurons of the anterior horn. The anterior funiculus contains a small percentage of corticospinal tract fibers. B. The gray matter can be divided into a posterior horn and an anterior horn, with intermediate gray situated between the two. The small posterior horn contains cells receiving pain and temperature information located in a pale staining region called the substantia gelatinosa. The lightly myelinated nociceptive fibers on their way to substantia gelatinosa enter just superficial to it in a tract called Lissauer’s tract. The tract looks like part of the gray matter because the fibers are so thinly myelinated, so don’t stain as darkly by the myelin stain. The large anterior horn contains the large lower motor neurons (the α MNs) that supply skeletal muscle (visible in slide 11, arrows in L5, and at higher magnification in slides 12 - 14), while the intermediate gray contains mainly interneurons as well as 2 named columns of cells that are restricted to certain spinal cord levels [slides 9, 10]: © Todd Fiacco 2020. This content is protected and may not be shared, uploaded or distributed.  1) Clarke’s nucleus (a.k.a. Clarke’s column) is a relay nucleus of the spinocerebellar pathway for the lower limbs located from about T1 to L2. Axons from neurons in Clarke’s nucleus form the posterior spinocerebellar tract (PSCT), one of three named spinocerebellar pathways. 2) The intermediolateral (IL) cell column of T1 to L3 contains preganglionic sympathetic neurons of the autonomic nervous system, discussed in more detail below. The IL cell column forms a lateral horn of the spinal cord gray matter at this level. (For labeling purposes, “lateral horn” can be used instead of IL cell column). III. The autonomic nervous system (ANS) [slides 17-18] monitors and controls visceral activity in order to optimize survival. The ANS is divided functionally into 2 main parts: sympathetic and parasympathetic. Sympathetic outflow is from the spinal cord; specifically segments T1 to L3 (where the intermediolateral cell column is located) and is therefore also called the thoracolumbar outflow. Parasympathetics arise from spinal segments S2 - S4 but also from the brainstem (CNs III, VII, IX, and X) and is therefore also called the craniosacral outflow. Autonomic motor output to visceral smooth muscle relays through ganglia; the first neuron in this 2-neuron chain is called preganglionic, while the second neuron in the chain is called postganglionic. All preganglionic ANS axons use acetylcholine as their neurotransmitter. Sympathetics and parasympathetics differ in three important ways: 1) their functions are largely antagonistic; 2) the locations of their ganglia are different; and 3) they release different neurotransmitters onto their target organs from postganglionic axons. Generally speaking, the parasympathetic system enhances energy storage, while the sympathetic system prepares the body for situations in which energy needs to be expended. Thus, parasympathetic activation reduces heart rate and blood pressure, increases salivation and peristalsis in the gut, constricts the pupils and contracts the bladder (stimulates urination) and generally supports “rest & digest” activity; sympathetic activation increases attention, increases heart rate and blood pressure, decreases peristalsis, and diverts blood from the gut to skeletal muscle as a reflexive response to potentially life- threatening situations (“fight or flight” activity; think of the scene at the end of the movie © Todd Fiacco 2020. This content is protected and may not be shared, uploaded or distributed.  “Silence of the Lambs” when agent Starling (played by Jodie Foster) is in complete darkness with Buffalo Bill the skinner stalking her). The enteric nervous system (a third component of the ANS) is the extensive collection of nerves and their fibers controlling peristalsis of the gut via innervation of the longitudinal and circular smooth muscle. Recall that smooth muscle is controlled involuntarily (whereas skeletal muscle is under voluntary control). A. Preganglionic parasympathetic output neurons of the spinal cord are located in the intermediate gray of segments S2 - S4 in an analogous location to neurons in the intermediolateral cell column of T1 - L3. Axons leave through the ventral root and travel to their target ganglia, which for parasympathetics, are located adjacent to their target organs [slides 17, 18]. Postganglionic parasympathetic axons release acetylcholine. B. Spinal cord preganglionic sympathetic neurons are located in the lateral horn of T1- L3 and their axons leave the SC in these segments in the ventral root. These heavily myelinated axons reach the nearby sympathetic chain ganglia of the thorax or the prevertebral ganglia (the sympathetic ganglia that innervate abdominal and pelvic viscera). Postganglionic sympathetics then travel to their target organs where they release norepinephrine [slide 18]. IV. Overview of principal pathways in the spinal cord [slides 19 - 22]. The white matter and gray matter structures of the spinal cord already introduced are the components of 3 important sensory pathways and 1 motor pathway. These pathways also have structures at brainstem, cerebellar, thalamic and cortical levels (we will revisit these pathways throughout the course), but it is useful to introduce them now since they begin (or end) at the level of the spinal cord. A. The posterior column - medial lemniscus system [slide 19] is the pathway by which general sensory (touch, pressure, vibration) and proprioceptive information reaches the brain (and therefore consciousness). This information enters the spinal cord via large myelinated 1a fibers from the DRG central processes. The fibers enter in the lateral portion of the posterior funiculus and branch, with collaterals participating in the spinal reflexes (discussed below), as well as ascending rostrally within the posterior column. © Todd Fiacco 2020. This content is protected and may not be shared, uploaded or distributed.  Caudal to T6 the posterior funiculus is called fasciculus gracilis, which contains fibers from the trunk and lower limb sensory receptors, while rostral to T6, afferents accumulate in a separate bundle lateral to fasciculus gracilis called fasciculus cuneatus which contains fibers from the upper limbs. The fasciculus gracilis and fasciculus cuneatus are separated by a thin sulcus and therefore can be distinguished from one another at cervical levels. The DRG sensory afferents ascending in the fasciculus gracilis and fasciculus cuneatus synapse in nucleus gracilis or nucleus cuneatus, respectively, in the caudal medulla. From there, ascending fibers cross the midline to form the medial lemniscus, a fiber bundle that ascends to synapse onto neurons located in the ventral posterolateral (VPL) nucleus of the thalamus. VPL thalamocortical efferents then proceed to primary somatosensory cortex of the postcentral gyrus. B. The spinothalamic tract (anterolateral system) [slide 20] conveys nociceptive (pain and temperature) information. The cell bodies of nociceptive neurons are also located in the DRG. The thinly myelinated nociceptive afferents entering the SC are located in Lissauer’s tract, which synapse onto second order neurons in the substantia gelatinosa. Projections from substantia gelatinosa cross the midline in the spinal cord near the level at which they enter, and proceed rostrally within the (antero)lateral funiculus to ultimately reach the VPL of the thalamus. Realize, then, that fibers of the spinothalamic tract cross to the contralateral side within the spinal cord, while those of the posterior column-medial lemniscus pathway cross in the caudal medulla. Pain produces important effects other than simple conscious awareness of a painful stimulus. Pain leads to a rapid increase in level of attention, emotional reactions (think of a child crying or an adult swearing when they hurt themselves), autonomic responses to increase energy use if needed for survival, and a greater likelihood that the painful episode and its cause will be remembered. Thus, within the anterolateral column are spinoreticular fibers (to the reticular formation of the brainstem to increase attention), and spinohypothalamic fibers (to the hypothalamus for emotional and memory aspects of pain). There are also collaterals that reach the periaqueductal gray (gray matter surrounding the cerebral aqueduct) that play a role in pain suppression (discussed in more detail in a subsequent lecture). © Todd Fiacco 2020. This content is protected and may not be shared, uploaded or distributed.  C. The spinocerebellar tracts [slide 21] carry proprioceptive (muscle position) information to the cerebellum to control postural reflexes and motor coordination. There are three named tracts: 1) The anterior spinocerebellar tract (ASCT) ascends in the anterior part of the lateral funiculus and reaches the cerebellum via the superior cerebellar peduncle (SCP). This tract is very small and not very important. 2) The posterior spinocerebellar tract (PSCT) sends proprioceptive information from the lower limbs to the cerebellum. Axons from DRG neurons ascend in fasciculus gracilis to reach Clarke’s nucleus (T1 to L2), the axons of which form the PSCT (travel in the posterior part of the lateral funiculus). Realize then that the names “anterior” and “posterior” for the spinocerebellar tracts refer to their locations within the lateral funiculus, since both travel within the lateral funiculus. PSCT fibers enter the cerebellum in a bundle called the inferior cerebellar peduncle (ICP) located in the brainstem. 3) The cuneocerebellar tract (CCT) is the upper limb version of the PSCT. Proprioceptive DRG afferents travel rostrally within fasciculus cuneatus to reach the lateral cuneate nucleus, which lies just lateral to the nucleus cuneatus in the caudal medulla (nucleus cuneatus contains second order neurons for fine touch). From the lateral cuneate nucleus, the ascending fibers form the CCT and reach the cerebellum, again via the ICP. D. Finally, the corticospinal tract [slide 22] is the motor pathway by which motor cortex controls voluntary movement. From the precentral gyrus of the frontal lobe (primary motor cortex), upper motor neurons descend through the internal capsule and form the large cerebral peduncles of the midbrain. These axons continue caudally through the pons, then form the pyramids in the medulla. At the brainstem-SC junction, the corticospinal (pyramidal) tract axons cross the midline to the contralateral side at the pyramidal decussation. From there they descend in the lateral funiculus (posterior part) to reach the lower motor neurons of the anterior horn. V. Overview of spinal reflexes: The stretch reflex and the flexor reflex. © Todd Fiacco 2020. This content is protected and may not be shared, uploaded or distributed.  A. The stretch (myotatic) reflex [slide 23] involves only 2 neurons: a 1a primary sensory proprioceptive afferent from a muscle spindle, stimulated when the muscle is stretched, and a lower motor neuron that innervates the corresponding muscle group causing it to contract. The patellar tendon (knee-jerk) reflex that you get with your physical is a stretch reflex. The stretch reflex is thought to be important for the constant, automatic corrections we perform during movements and postures. During the stretch reflex, contraction of the synergist muscle group is accompanied by inhibition of the antagonist muscle group. This facilitates the performance of the reflex. Thus, in the case of the knee-jerk reflex, contraction of the quadriceps (synergist) group is accompanied by inhibition of the hamstrings (antagonist) group. This is called reciprocal inhibition [slide 24]. B. The flexor, or withdrawal reflex [slide 25] involves retraction of an entire limb in response to a noxious (painful) stimulus. The most commonly used examples of the flexor reflex are retraction of the hand from the hot stove or pulling the foot away from the tack. Because the flexor reflex involves an entire limb, its pathway must spread over several spinal segments to include the motor neurons innervating the required muscles of that limb. This is accomplished by: 1) collaterals of primary afferent fibers extending one or more segments in the rostral or caudal directions; and 2) primary afferent collaterals activating interneurons which have processes extending over several segments. The flexor reflex includes crossed effects, whereby extensor muscles (the quadriceps) of the opposite leg are stimulated [slide 26] so that the body’s weight can continue to be supported during the withdrawal of the opposite leg from the painful stimulus. VI. Clinical considerations of spinal cord damage - very predictable with good understanding of the underlying anatomy. A. Damage to the posterior column-medial lemniscus system initially results in complete loss of fine touch, pressure, and proprioceptive information. Over a period of weeks, most general tactile function is recovered, meaning that there must be fibers carrying touch information that reach the cortex by other routes besides the posterior columns. However, proprioception remains affected long-term, leading to ataxia (discoordination of movement), and subjects never recover complex tactile discrimination tasks such as stereognosis (recognizing an object by touch alone). © Todd Fiacco 2020. This content is protected and may not be shared, uploaded or distributed.  B. Lower motor neuron (LMN) damage vs. upper motor neuron (UMN) damage. 1) Damage to lower motor neurons (neurons of the anterior horn) innervating a muscle results in flaccid paralysis (muscle is weak, limp and uncontracted) and eventually, atrophy of the muscle on the ipsilateral side of the body. 2) Damage to upper motor neurons whose axons comprise the corticospinal tract results in spastic paralysis, characterized by increased resting muscle tension, muscle weakness, and hyperactive reflexes. C. Brown-Sequard syndrome [slide 27] occurs following complete transection of one half of the spinal cord. It is rare but very instructive, having the following symptoms: 1) An initial period of spinal shock characterized by flaccid paralysis of musculature and areflexia (absence of reflexes) (Symptoms resemble LMN damage). 2) After recovery from spinal shock, there is long-term spastic paralysis of muscles below the level of the lesion ipsilaterally (due to UMN damage of corticospinal tract). 3) Autonomic spinal shock. Cervical and upper thoracic transection results in hypotension (low blood pressure) and bradycardia (slowed heart rate) due to loss of inputs from the hypothalamus, the autonomic control center in the brain. 4) Loss of fine discriminatory touch and proprioception ipsilaterally due to transection of the posterior columns. 5) Loss of pain and temperature sensation contralateral to the hemisection due to interruption of the spinothalamic tract. Why is the loss of sensation contralateral? © Todd Fiacco 2020. 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