Spinal Cord, Spinal Nerves, and Somatic Reflexes

1. Spinal Cord, Spinal Nerves, and Somatic Reflexes

2. Functions of the Spinal Cord Conduction – sends information up and down the cord Locomotion Reflexes – involuntary responses to stimuli

3. Anatomy of the Spinal Cord Occupies C1 through L1 – starting at the foramen magnum Cauda equina Filum terminale Divided into 4 regions Cervical Thoracic Lumbar Sacral Cauda equina - Occupies the vertebral canal from L2 to S5 – bundle of nerve roots Nerves named for the levels of the vertebral column through which the spinal nerves emerge Cervical and lumbar are widest regions – for the emergence of nerves that control the limbsCauda equina - Occupies the vertebral canal from L2 to S5 – bundle of nerve roots Nerves named for the levels of the vertebral column through which the spinal nerves emerge Cervical and lumbar are widest regions – for the emergence of nerves that control the limbs

4. Meninges Enclose the spinal cord Three layers Dura mater – superficial Arachnoid mater Pia mater – deep Space between Epidural – between dura mater and vertebral bone Subarachnoid – between arachnoid and pia mater Dura mater – tough collagenous membrane Epidural space between – has blood vessels, adipose tissue, and connective tissue – anesthetics are introduced to this space to block pain signals – an epidural Arachnoid mater – loose mesh of collagenous and elastic fibers Subarachnoid – filled with CSF Pia mater – delicate, translucent membraneDura mater – tough collagenous membrane Epidural space between – has blood vessels, adipose tissue, and connective tissue – anesthetics are introduced to this space to block pain signals – an epidural Arachnoid mater – loose mesh of collagenous and elastic fibers Subarachnoid – filled with CSF Pia mater – delicate, translucent membrane

5. Nervous tissue Gray matter Contains somas, dendrites, and synapses Little myelin Site of information processing White matter Contains nerve fibers (axons) Abundance of myelin Carry signals from one part of the CNS to another Information processing in CNSInformation processing in CNS

6. Gray Matter Looks like a butterfly 2 dorsal horns 2 ventral horns Central canal Spinal nerve branches into a dorsal root and ventral root Dorsal root Ventral root Dorsal horns are posterior Ventral horns are anterior DORSAL root – carries sensory nerve fibers VENTRAL horns – contain large somas of the somatic morotr neurons – axons exit by way of the ventral root of the spinal nerve and lead to the skeletal muscleDorsal horns are posterior Ventral horns are anterior DORSAL root – carries sensory nerve fibers VENTRAL horns – contain large somas of the somatic morotr neurons – axons exit by way of the ventral root of the spinal nerve and lead to the skeletal muscle

8. White Matter Surrounds the gray matter Bundles of axons that provide avenues of communication up and down the spinal cord Bundles are arranged in three pairs of columns Dorsal Lateral Ventral Each column has subdivisions called tracts

9. Spinal Tracts Ascending – Travel across three neurons from their origin in the receptors to their destination in the sensory areas of the First-order neuron – detects stimuli and sends it to the spinal cord or brainstem Second-order – continues to the thalamus at the upper end of the brainstem Third-order – carries the signal to the sensory region of the cerebral cortex carry sensory information up the cord Braincarry sensory information up the cord Brain

10. Spinal Tracts Descending – Two neurons Upper motor neuron – begins with a soma in the cerebral cortex or brainstem Lower motor neuron – where the axon terminates from the above mentioned soma and travels to the muscle or target organ DESCENDING – carry motor impulses down the brainstem and spinal cord DESCENDING – carry motor impulses down the brainstem and spinal cord

11. Spinal Nerves 31 pairs 8 cervical 12 thoracic 5 lumbar 1 coccygeal Spinal nerves are mixed nerves – carries signals both ways Afferent signals approach the cord by the dorsal root and enter the dorsal horn of the gray matter Efferent signals begin at the somas in the ventral horn and leave the spinal cord by the ventral root Spinal nerves – emerge through intervertebral foraminaSpinal nerves – emerge through intervertebral foramina

13. Plexuses – weblike nerve bundle Cervical plexus Brachial plexus Lumbar plexus Sacral plexus Coccygeal plexus Dermatome – an area of skin innervated by a nerve Can assess spinal nerve damage

14. Reflexes What is a reflex? Four properties of a reflex Stimulation – response to sensory input Quick Involuntary Sterotyped Somatic versus Visceral reflexes Involuntary contraction of a muscle Unlearned skeletal muscle reflex that are mediated by the brainstem and spinal cord Quick, involuntary reaction of a gland or muscle to stimulation Involuntary – occur without intent, often without our awareness, and are difficult to suppress – can try to avoid stimulus Stereotyped – occur in essentially the same way every time – response is predictableQuick, involuntary reaction of a gland or muscle to stimulation Involuntary – occur without intent, often without our awareness, and are difficult to suppress – can try to avoid stimulus Stereotyped – occur in essentially the same way every time – response is predictable

15. Reflex Arc

16. Proprioceptors Somatic reflexes are initiated by proprioceptors Muscle spindles – proprioceptors in skeletal muscles that respond to stretching of the muscle Stretch reflex - Reciprocal inhibition – a reflex phenomenon that prevents muscles from working against each other by inhibiting antagonists Proprioceptors – monitor the position and movements of body parts Stretch reflex – tendency of a muscle to contract when it is stretched – jEXAMPLE – tendon reflex = patellar tendon reflex – tapping the patellar ligament suddenly stretches the quads – maintain equilibrium and posture EXAMPLE – if head starts to tip forward – posterior neck muscles stretch and stimulate their muscle spindles – afferent signals to the cerebellum by way of the brainstem….cerebellum integrates this information and relays it to the cerebral cortex, and the cortex sends signals back to the muscles = reaction = muscle contract and raise your head RECIPRICAL INHIBITION – with the knee jerk the quads would not produce much joint movement if its antagonist (the hamstrings) contracted at the same time. Reciprocal inhibition prevents that from happening hence, the hamstrings remain relaxed and allow the quads to extend the kneeProprioceptors – monitor the position and movements of body parts Stretch reflex – tendency of a muscle to contract when it is stretched – jEXAMPLE – tendon reflex = patellar tendon reflex – tapping the patellar ligament suddenly stretches the quads – maintain equilibrium and posture EXAMPLE – if head starts to tip forward – posterior neck muscles stretch and stimulate their muscle spindles – afferent signals to the cerebellum by way of the brainstem….cerebellum integrates this information and relays it to the cerebral cortex, and the cortex sends signals back to the muscles = reaction = muscle contract and raise your head RECIPRICAL INHIBITION – with the knee jerk the quads would not produce much joint movement if its antagonist (the hamstrings) contracted at the same time. Reciprocal inhibition prevents that from happening hence, the hamstrings remain relaxed and allow the quads to extend the knee

17. Proprioceptors Flexor reflex – withdrawal of a limb from an injurious stimulus Crossed extensor reflex – contraction of extensor muscles in the limb opposite from the one that is flexed Golgi tendon reflex – response to excessive tension on a tendon; inhibits muscle so it does not contract stronger Flexor reflex– pulling back from a hot stove Crossed extensor reflex – standing on one leg GTR – helps to prevent injury – but sometimes muscle contracts so quickly that reflex is not quick enoughFlexor reflex– pulling back from a hot stove Crossed extensor reflex – standing on one leg GTR – helps to prevent injury – but sometimes muscle contracts so quickly that reflex is not quick enough