Nervous System Chapter 4: Spinal Cord

Spinal Cord

• Long, slender cylinder of nerve tissue, extends from brain stem, approximately 45cm long and 1-1.5cm wide
• Vertebral canal: a space formed by the vertebral column, contains the spinal cord, and exits through a large hole in the base of the skull
• Descends through the vertebral canal, enclosed by the protective vertebral column

Spinal Nerves

• Emerge from the spinal cord through spaces between the bony, wing-like arches of adjacent vertebrae
• Named according to the region of the vertebral column from which they emerge:
  • Cervical nerves (neck): 8 pairs, C1-C8
  • Thoracic nerves (chest): 12 pairs, T1-T12
  • Lumbar nerves (abdominal): 5 pairs, L1-L5
  • Sacral nerves (pelvic): 5 pairs, S1-S5
  • Coccygeal nerves (tailbone): 1 pair

Differential Growth

• During development, the vertebral column grows 25cm longer than the spinal cord
• Spinal cord segments giving spinal nerves are not aligned with corresponding intervertebral spaces
• Most spinal nerves descend before emerging from the vertebral column at their corresponding space
• The spinal cord extends only to the L1-L2 vertebra level (waist level)

Spinal Cord Grey Matter

• Centrally located, functionally organized
• Forms an inner butterfly-shaped region surrounded by outer white matter
• Consists of neuronal cell bodies, their dendrites, and glial cells
• Central canal: lies in the center of the grey matter, filled with CSF
• Each half of the grey matter is divided into:
  • Dorsal (posterior) horn: contains cell bodies of interneurons, on which sensory neurons terminate

Spinal Nerves and Reflexes

• Spinal nerves are part of the peripheral nervous system, consisting of 31 pairs of nerves
• Each spinal nerve has dorsal and ventral roots at each level, joining to form a spinal nerve
• Emerges from the vertebral column, traverses between the spinal cord and a particular region of the body
• Enclosed by connective tissue, following the same pathway
• Does not contain complete nerve cells, only axonal portions of many neurons
• Progressively branches, forming a vast network of peripheral nerves supplying the tissues

Reflexes

• Reflex: any response that occurs automatically without conscious effort
• Spinal cord is responsible for integrating many innate reflexes
• Strategically located between the brain and afferent and efferent fibers of the PNS
• Enables the spinal cord to fulfill its two primary functions:
  • Serving as a link for transmission between the brain and the rest of the body
  • Integrating reflex activity between afferent input and efferent output without involving the brain

Reflex Arc

• The neural pathway involved in accomplishing reflex activity
• Includes five basic components:
  • Sensory receptor: responds to stimulus, detects changes in the environment
  • Afferent pathway: relies on the action potential produced by the receptor
  • Integrating center: processes all information, makes a decision about the appropriate response
  • Efferent pathway: transmits instructions from the integrating center to the effector organ
  • Effector organ: muscle or gland that carries out the desired response

Reflex Categories

• Depending on the CNS level at which the reflex is integrated:
  • Spinal reflexes: integrated by the spinal cord, example: withdrawal reflex
  • Cranial reflexes: subconsciously integrated by lower levels of the brain (brainstem or hypothalamus), example: pupillary constriction
• Depending on whether the reflex is inborn or learned:
  • Innate (simple/basic) reflexes: built-in, unlearned responses, integrated by the spinal cord and brain stem, examples: withdrawal reflex, micturition reflex, pupillary constriction
  • Conditioned (acquired) reflexes: result of learning, processed by higher brain levels, examples: increased salivation on smelling food
• Depending on which efferent division of the PNS and which effector organs are involved:
  • Somatic reflex: output transmitted by motor neurons to skeletal muscles, example: withdrawal reflex
  • Autonomic (visceral) reflex: output carried via ANS to smooth muscles, cardiac muscle, or glands, examples: micturition reflex, pupillary constriction reflex, salivary reflex, baroreceptor reflex

Reflex Examples

• Stretch reflex: involves afferent neurons originating at stretch-detecting receptors in skeletal muscles, terminating on efferent neurons supplying the same skeletal muscles, causing contraction to counteract the stretch
• Monosynaptic reflex: only one synapse in the reflex arc, between afferent and efferent neurons
• Withdrawal reflex: initiated to withdraw from a painful stimulus, involves receptors stimulated enough to reach threshold, action potential generated in afferent neurons, and reciprocal innervation of agonist and antagonist muscles