Spinal Cord, Reflex arc and Spinal nerves

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.

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.

Spinal Cord Anatomy

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

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

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.

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

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

Pathways Between Brain and Spinal Cord

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

PNS: Spinal Nerves

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

PNS: Spinal Nerves Figure 7.25a

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

PNS: The Spinal Nerves Figure 7.25b

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

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

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

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

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