Anatomy and function of the spinal cord and nerves

1. Anatomy and function of the spinal cord and nerves Spinal cord Ch. 14, 17

2. Spinal cord is protected by vertebrae, meninges and CSF • Canal formed by foramina of vertebrae • Meninges • Dura mater • Arachnoid mater • Pia mater • CSF formed in brain • Found in subarachnoid space

3. Anatomy of the spinal cord and spinal nerves

4. 31 pairs of spinal nerves • 8 cervical • 12 thoracic • 5 lumbar • 5 sacral • 1 coccygeal • All are mixed nerves

5. Internal structure of spinal cord • Roots connect spinal nerves to spinal cord • Dorsal root- sensory neurons • Ganglion-cell bodies of sensory neurons • Ventral root- motor neurons • Horns- gray matter; integrates sensory and motor information • Anterior, posterior, lateral • White matter- myelinated; sensory and motor tracts • Columns- anterior, posterior, lateral

6. Regions of white and gray matter • Dorsal (posterior) horns- sensory neurons • Ventral (anterior) horns- motor neurons • Lateral horns- autonomic (motor) neurons • Columns- ascending (sensory) and descending (motor) tracts

7. Sensory and motor tracts • Named according to its position and pathway • Sensory impulses move up tracts and columns • Join the “pool” of sensory information • Voluntary motor output emanates from cerebral cortex (direct pathways) • Involuntary output originates in brain stem and hypothalamus (indirect pathways)

8. Spinal nerves • Comprise the peripheral nervous system (PNS) • Connect the CNS to muscles, sensory receptors and glands • 31 pairs of spinal nerves • Posterior and anterior roots (mixed nerves)

9. Distribution of spinal nerves • Branches (rami) • Posterior (muscles and surfaces; anterior) • Meningeal branch (CNS) • Rami communicantes- autonomic system • Plexuses- network of axons • Cervical, brachial, lumbar, sacral

10. “Map” of spinal nerves Motor fibers Sensory fibers

11. Major plexuses example: cervical plexus • Origin (C1- C5) • Superficial or deep • Superficial- skin; mostly sensory • Deep-muscle; largely motor • Severing spine above phrenic nerve causes respiratory arrest

12. More plexuses • Brachial • Shoulders, upper limbs • Axial, musculocutaneous, radial, medial, ulnar • Lumbar • Anterolateral abdominal wall, genitalia, part of lower limbs • Sacral and coccygeal • Buttocks, perineum, lower limbs • Damage can cause loss of sensation, palsy, loss of motor control • Intercostal nerves directly innervate muscles (no plexuses)

13. Dermatomes • Sensory neurons convey information form skin to CNS • Dermatomes- areas of skin that provide sensory input to a particular pair of nerves • Some overlap • Can be useful for diagnosis

14. Cervical plexus

15. Brachial plexus

16. Brachial plexus (C1-C8, T1)

17. Lumbar and sacral plexuses

18. Reflex arcs- fast involuntary responses to stimuli

19. Reflex arcs may be somatic or autonomic • Patellar reflex (knee-jerk) • Tap patellar ligament; quadriceps femoris contracts • Achilles reflex (ankle-jerk) • Tap calcaneal tendon; gastrocnemius and soleus muscles contract • Babinski sign- test for continuity of the corticospinal tract (goes from positive to negative with development) • Abdominal reflex • Can be useful for diagnosis of chronic disease or nerve damage

20. Types of reflexes

21. Somatic senses (pain, thermal, tactile, proprioreceptive) Somatic motor neurons- skeletal muscle Usually voluntary Motor units Acetylcholine Autonomic sensory (chemoreceptors, mechano- (stretch) receptors Motor neurons regulate visceral activities; can function independently of ANS Preganglionic and postganglionic neurons Acetylcholine or norepinephrine Autonomic vs. somatic nervous system

23. Organization of ANS

24. Different types of ganglia • Sympathetic • Sympathetic trunk • Innervate organs above the diaphragm • Superior, middle, inferior cervical ganglia • Prevertebral • Below the diaphragm • Celiac, superior and inferior mesenteric ganglia • Parasympathetic • Preganglionic axons are longer than in sympathetic ganglia • Preganglionic neurons are myelinated, postganglionic neurons are not

25. Review of spinal nerve distribution

26. Sympathetic (thoracolumbar) • Sympathetic chain ganglia, paired • Head and thoracic cavity • Collateral ganglia, unpaired • Abdominopelvic cavity • Suprarenal medullae, paired • hormonal

27. Functions of the sympathetic division • Sympathetic chain ganglia • Diversion of blood flow to skeletal muscles and brain • Sweating; pupil dilation • Collateral ganglia • Vasoconstriction; restrict urinary function • Suprarenal medullae • Release of epinephrine and norepinephrine

28. Parasympathetic (craniosacral) division of ANS

29. Parasympathetic system • Ganglia are located near, or in, target organs • Effects are directed more toward specific target organs

30. Autonomic plexuses along major arteries

31. Regulation by dual innervation • Systems intersect in plexuses • Cardiac • Pulmonary • Esophageal • Celiac • Inferior mesenteric • Hypogastric

32. Postganglionic neurons • Sympathetic system • One presynaptic neuron can diverge into many • Many organs can be affected at once (divergent) • May extend to adrenal medullae • Parasympathetic system • Many presynaptic neurons can converge on a single effector • Effect can be localized to a single effector

33. Action by neurotransmitters in ANS • Acetylcholine (cholinergic neurons) • All preganglionic • Sympathetic postganglionic innervation of sweat glands • All parasympathetic postganglionic neurons • Nicotinic, muscarinic receptors • Norepinephrine (adrenergic) • α and β receptors • Can be excitatory or inhibitory

34. Functions of ANS • “fight or flight” (sympathetic) • More widespread and longer lasting • Norepinephrine and epinephrine can act as hormones as well as neurotransmitters • “rest and digest” (parasympathetic) • Salivation, lacrimation, urination, digestion and defecation

35. Control of ANS

36. Control of autonomic functions • Reflexes • Blood pressure • Digestion • Defecation and urination • Control within brain • Brain stem (cardiovascular, swallowing , digestion) • Spinal cord (elimination) • Control and integration center is hypothalamus

37. Sympathetic vs parasympathetic effects • Parasympathetic tends to be organ-specific • More reflexes • Sympathetic tends to be more holistic • Divergence • Hormonal distribution