1. Spinal Cord, Reflex arc and Spinal nerves

2. Spinal Cord
The spinal cord suspended in the vertebral canal surrounded by the meninges and cerebrospinal fluid.
Extends from the foramen magnum of the skull to the first or second lumbar vertebra
31 pairs of spinal nerves arise from the spinal cord
Cauda equina is a collection of spinal nerves at the inferior end
It is approximately 45 cm long in an adult and is about the thickness of the little finger.

3. When a specimen of cerebrospinal fluid is required it is taken from a point below the end of the cord, i.e. below the level of the 2nd lumbar vertebra. This procedure is called lumbar puncture.
Some activities of the spinal cord are independent of the brain, i.e. spinal reflexes.

4. Spinal Cord Anatomy

6. Gray matter
The arrangement of grey matter in the spinal cord resembles the shape of the letter H
Internal gray matter is mostly cell bodies
Dorsal (posterior) horns
Anterior (ventral) horns
Gray matter surrounds the central canal
Central canal is filled with cerebrospinal fluid

7. The cell bodies may be:
• sensory cells, which receive impulses from the periphery of the body
• lower motor neurones, which transmit impulses to the skeletal muscles
• connector neurones, linking sensory and motor neurones, at the same or different levels, which form spinal reflex arcs

9. Sensory nerve tracts (afferent or ascending) in the spinal cord
There are two main sources of sensation transmitted to the brain via the spinal cord.
The skin. Sensory receptors (nerve endings) in the skin, called cutaneous receptors, are stimulated by pain, heat, cold and touch, including pressure.
The tendons, muscles and joints. Sensory receptors are nerve endings in these structures, called proprioceptors, and they are stimulated by stretch. Together with impulses from the eyes and the ears they are associated with the maintenance of balance and posture and with perception of the position of the body in space.

10. Motor nerve tracts (efferent or descending)in the spinal cord
Neurones which transmit nerve impulses away from the brain are motor (efferent or descending) neurones. Motor neurone stimulation results in:
• contraction of skeletal (striated, voluntary) muscle
• contraction of smooth (involuntary) muscle, cardiac muscle and the secretion by glands controlled by nerves of the autonomic nervous system

11. Meninges cover the spinal cord
Spinal nerves leave at the level of each vertebrae
Dorsal root
Associated with the dorsal root ganglia—collections of cell bodies outside the central nervous system
Ventral root
Contains axons

12. Pathways Between Brain and Spinal Cord

13. The Reflex Arc
Reflex—rapid, predictable, and involuntary response to a stimulus
Occurs over pathways called reflex arcs
Reflex arc—direct route from a sensory neuron, to an interneuron, to an effector

14. The Reflex Arc Figure 7.11a Stimulus at distal end of neuron Skin
Spinal cord
(in cross section)
Interneuron
Receptor
Effector
Sensory neuron
Motor neuron
Integration center
(a)
Figure 7.11a

15. The Reflex Arc Figure 7.11a, step 1 Skin
Stimulus at distal end of neuron
Skin
Receptor
(a)
Figure 7.11a, step 1

16. The Reflex Arc Figure 7.11a, step 2 Skin
Stimulus at distal end of neuron
Skin
Receptor
Sensory neuron
(a)
Figure 7.11a, step 2

17. The Reflex Arc Figure 7.11a, step 3 Skin Spinal cord
Stimulus at distal end of neuron
Skin
Spinal cord
(in cross section)
Interneuron
Receptor
Sensory neuron
Integration center
(a)
Figure 7.11a, step 3

18. The Reflex Arc Figure 7.11a, step 4 Skin Spinal cord
Stimulus at distal end of neuron
Skin
Spinal cord
(in cross section)
Interneuron
Receptor
Sensory neuron
Motor neuron
Integration center
(a)
Figure 7.11a, step 4

19. The Reflex Arc Figure 7.11a, step 5 Stimulus at distal end of neuron
Skin
Spinal cord
(in cross section)
Interneuron
Receptor
Effector
Sensory neuron
Motor neuron
Integration center
(a)
Figure 7.11a, step 5

20. Simple Reflex Arc Figure 7.11b–c
Sensory receptors (stretch receptors in the quadriceps muscle)
Sensory (afferent) neuron
Sensory receptors (pain receptors in the skin)
Spinal cord
Sensory (afferent) neuron
Synapse in ventral horn gray matter
Inter- neuron
Motor (efferent) neuron
Motor (efferent) neuron
Effector (quadriceps muscle of thigh)
Effector (biceps brachii muscle)
(b)
(c)
Figure 7.11b–c

21. Simple Reflex Arc Figure 7.11b, step 1
Sensory receptors (stretch receptors in the quadriceps muscle)
Spinal cord
(b)
Figure 7.11b, step 1

22. Simple Reflex Arc Figure 7.11b, step 2
Sensory receptors (stretch receptors in the quadriceps muscle)
Sensory (afferent) neuron
Spinal cord
(b)
Figure 7.11b, step 2

23. Simple Reflex Arc Figure 7.11b, step 3
Sensory receptors (stretch receptors in the quadriceps muscle)
Sensory (afferent) neuron
Spinal cord
Synapse in ventral horn gray matter
(b)
Figure 7.11b, step 3

24. Simple Reflex Arc Figure 7.11b, step 4
Sensory receptors (stretch receptors in the quadriceps muscle)
Sensory (afferent) neuron
Spinal cord
Synapse in ventral horn gray matter
Motor (efferent) neuron
(b)
Figure 7.11b, step 4

25. Simple Reflex Arc Figure 7.11b, step 5
Sensory receptors (stretch receptors in the quadriceps muscle)
Sensory (afferent) neuron
Spinal cord
Synapse in ventral horn gray matter
Motor (efferent) neuron
Effector (quadriceps muscle of thigh)
(b)
Figure 7.11b, step 5

26. Simple Reflex Arc Figure 7.11c, step 1
Sensory receptors (pain receptors in the skin)
Spinal cord
(c)
Figure 7.11c, step 1

27. Simple Reflex Arc Figure 7.11c, step 2
Sensory receptors (pain receptors in the skin)
Spinal cord
Sensory (afferent) neuron
(c)
Figure 7.11c, step 2

28. Simple Reflex Arc Figure 7.11c, step 3
Sensory receptors (pain receptors in the skin)
Spinal cord
Sensory (afferent) neuron
Inter- neuron
(c)
Figure 7.11c, step 3

29. Simple Reflex Arc Figure 7.11c, step 4a
Sensory receptors (pain receptors in the skin)
Spinal cord
Sensory (afferent) neuron
Inter- neuron
Motor (efferent) neuron
(c)
Figure 7.11c, step 4a

30. Simple Reflex Arc Figure 7.11c, step 4b
Sensory receptors (pain receptors in the skin)
Spinal cord
Sensory (afferent) neuron
Inter- neuron
Motor (efferent) neuron
Effector (biceps brachii muscle)
(c)
Figure 7.11c, step 4b

31. Simple Reflex Arc Figure 7.11b–c
Sensory receptors (stretch receptors in the quadriceps muscle)
Sensory (afferent) neuron
Sensory receptors (pain receptors in the skin)
Spinal cord
Sensory (afferent) neuron
Synapse in ventral horn gray matter
Inter- neuron
Motor (efferent) neuron
Motor (efferent) neuron
Effector (quadriceps muscle of thigh)
Effector (biceps brachii muscle)
(b)
(c)
Figure 7.11b–c

32. Types of Reflexes and Regulation
Spinal reflex: When integration takes place in the spinal cord gray matter, the reflex is a. An example the familiar patellar reflex (knee jerk)
Cranial reflex: If integration occurs in the brain stem rather than the spinal cord, the reflex is an example is the tracking movements of your eyes as you read this sentence.

33. Types of Reflexes and Regulation
Autonomic reflexes
Smooth muscle regulation
Heart and blood pressure regulation
Regulation of glands
Digestive system regulation
Somatic reflexes, which involve contraction of skeletal muscles

34. Types of Reflexes and Regulation
Patellar, or knee-jerk, reflex is an example of a two-neuron reflex arc
Figure 7.11d

35. PNS: Spinal Nerves

36. PNS: Spinal Nerves
There is a pair of spinal nerves at the level of each vertebrae for a total of 31 pairs
Formed by the combination of the ventral and dorsal roots of the spinal cord
Named for the region from which they arise

37. PNS: Spinal Nerves
Figure 7.25a

38. PNS: Anatomy of Spinal Nerves
Spinal nerves divide soon after leaving the spinal cord
Dorsal rami—serve the skin and muscles of the posterior trunk
Ventral rami—form a complex of networks (plexus) for the anterior

39. PNS: The Spinal Nerves
Figure 7.25b

41. PNS: Spinal Nerve Plexuses
Table 7.2 (1 of 2)

42. PNS: Spinal Nerve Plexuses
Table 7.2 (2 of 2)

44. PNS: Nerves of the Upper and Lower Limbs
Figure 7.26a

45. PNS: Nerves of the Upper and Lower Limbs
Figure 7.26b

46. PNS: Nerves of the Upper and Lower Limbs
Figure 7.26c