Nervous System 2 - Spinal Cord + Reflexes

Gross Anatomy of the Spinal Cord

  • extends from the brainstem, shorter than vertebral column, protected within vertebral column, gives rise to paired spinal nerves

  • Cauda Equina - consists of spinal nerve roots extending below the end of the cord

  • Spinal Nerves - 31 pairs, cervial, thoracic, lumbar, sacral, coccygeal

  • Vertebral Canal - spinal cord lives inside

  • Intervertebral Foramina - nerve roots exit through here


Spinal Meninges

  1. Dura Mater - outermost layer, tough fibrous sheath, provides protection

  2. Arachnoid Mater - middle membrane, lies deep in dura mater

  3. Pia Mater - innermost membrane, directly attached to the spinal cord, supports blood vessels


Spaces Around the Spinal Cord

  1. Epidural Space - between vertebrae and dura mater, contains adipose tissue + blood vessels + loose connective tissue - epidural administered into lumbar/sacral regions

  2. Subarachnoid Space - between arachnoid mater and pia mater, contains cerebrospinal fluid

CSF - shock absorption, nutrient and gas diffusion, protection of the CNS


Grey Matter of the Spinal Cord

  • contains neuron cell bodies and is organised into horns

  1. Dorsal (Posterior) Horn - contains interneurons, receives sensory information from the body

  2. Ventral (Anterior) Horn - contains interneurons, receives sensory information

  3. Lateral Horn - present in T1-L1 and S2-S4 spinal cord segments, contains autonomic motor neuron cell bodies


White Matter of the Spinal Cord

  • contains tracts (bundles of axons within the CNS)

  1. Ascending Tracts - carry sensory information toward the brain

  2. Descending Tracts - carry motor commands from the brain to the spinal cord


Spinal Roots

  • neurons enter and leave the spinal cord through roots

  1. Dorsal Root - contains sensory (afferent) axons only, carries information

  2. Dorsal Root Ganglion - contains sensory neuron cell bodies

  3. Ventral Root - contains motor (efferent) axons only, carries information away from the CNS


Spinal Nerves - form when one dorsal and one ventral root fuse together

  • 1-2 cm long, mixed nerves, contain sensory and motor fibres, 31 pairs

Peripheral Nerves

  1. Spinal nerves branch into rami

  2. Rami may join nerve plexus

  3. Peripheral nerves emerge from plexus


Reflexes

  • rapid, automatic, predictable responses to stimuli

  • processed by spinal cord or brain (spinal or cranial reflexes)

  1. Monosynaptic Reflex - single synapse between sensory and motor neuron, very fast response, no interneuron involved

  2. Polysynaptic Reflex - one or more interneurons, more complex, slower, long-lasting response


Reflex Arc - neural pathway responsible for a reflex; components: receptor, sensory neuron, integration centre, motor neuron, effector (muscle or gland)


Muscle Spindle

  • sensory receptors that detect changes in muscle length

  • proprioceptors, monitor muscle position + movement

  • intrafusal muscle fibres, wrapped by sensory nerve endings, surrounded by normal skeletal muscle (extrafusal fibres)

Mechanism

  1. muscle spindle stretches

  2. sensory neuron firing increases

  3. alpha motor neuron activity increases

  4. extrafusal muscle contracts


Stretch Reflex

  • maintains posture and muscle tone by detecting muscle stretch

  • monosynaptic, very rapid, causes contraction of the stretched muscle

Pathway

  1. muscle stretches

  2. muscle spindle activated

  3. sensory neuron sends APs to spinal cord

  4. sensory neuron synapses directly with alpha motor neuron

  5. alpha motor neuron stimulates the muscle

  6. muscle contracts


Withdraw Reflex

  • protects from harmful stimuli

  • polysynaptic, uses interneurons, produces rapid withdrawal of limb

Pathway

  1. pain receptor stimulated

  2. sensory neuron enters spinal cord

  3. excitatory interneuron stimulates flexor muscle

  4. inhibitory interneuron inhibits extensor muscle

  5. limb withdraws from stimulus


Cross Extensor Reflex

  • maintains balance during withdraw flex

  • occurs at same time as withdraw reflex

  • injured limb flexes and withdraws, opposite limb extends and supports


Reciprocal Inhibition - simultaneous contraction of one muscle and relaxation of its antagonist

Contralateral vs. Ipsilateral

Contralateral - response occurs on opposite side of stimulus (cross extensor)

Ipsilateral - response occurs on same side as stimulus (withdraw or stretch)