Week 5 Bioscience Flashcards
The spinal cord
The longitudinal cord of nerve tissue enclosed in the spinal canal. It serves not only as a pathway for nerve impulses to and from the brain, but also as a center for operating and coordinating reflex actions independent of the brain.
Spinal Nerves
- Emerge from the vertebral column
- Branch to form the peripheral nerves that innervate all parts of the body (except the head)
- Are mixed nerves - contain the axons of sensory & motor (somatic & autonomic) neurons
dorsal roots
contains the axons of the sensory neurons
dorsal root ganglion
contains the cell bodies of sensory neurons
ventral roots
contain the axons of motor neurons (somatic & autonomic)
Cervical
8 cervical nerves
Thoracic
12 thoracic nerves
Lumbar
5 lumbar nerves
Sacral
5 sacral nerves
Coccyx
1 coccygeal nerve
Protection of the Spinal Cord
The spinal cord is protected from mechanical forces by:
- vertebral column
- cerebrospinal fluid - also nourishes the neural.
tissue & removes waste products
- spinal meninges
The Spinal Meninges
- Are continuous with the cranial meninges
- Extend beyond the spinal cord to the second sacral vertebra (S2)
- a lumbar puncture (spinal tap) involves the insertion of a needle into the subarachnoid space beyond L3
- this procedure:
-does not damage the spinal cord
-is used to withdraw CSF fluid for diagnostic testing (e.g. meningitis) or reducing intracranial pressure
-is used to administer medications
Internal Anatomy of the Spinal Cord
The neural tissue (neurons & neuroglia) of the spinal cord:
- is divided into a left and right side by two grooves
- anterior (ventral) median fissure
- posterior (dorsal) median sulcus
- is organised into areas of gray matter & white
matter
- identical arrangement of gray & white matter, and the structures within them, on each side of the spinal cord
Gray Matter
- Composed of neuron cell bodies (mainly), unmyelinated axons & neuroglia
- Varies in size and shape down the length of the spinal cord
- Encloses a central canal - filled with cerebrospinal fluid
Sensory Nuclei of the Posterior Gray Horns
Contain the cell bodies of spinal cord interneurons
Somatic Motor Nuclei of the Anterior Gray Horns
Contain the cell bodies of lower motor neurons
Autonomic Motor Nuclei of the Lateral Gray Horns
Contain the cell bodies of preganglionic neurons
Spinal Reflexes
Gray matter is the site of integration for spinal reflexes.
Reflex integration centres:
* receive & interpret incoming sensory input
* decide on & generate the motor output that causes the reflex response
These reflexes rapidly and involuntarily stimulate:
- skeletal muscle contractions = somatic reflex (e.g. patellar reflex)
- smooth muscle contractions of visceral organs (e.g. bladder) or gland secretions = autonomic reflex
White Matter ”conduction highway”
- Superficial to gray matter
- Composed of myelinated (mainly) & unmyelinated axons
- Anterior white commissure - site where axons cross from one side of the CNS to the other
- Subdivided into white columns
Spinal Cord Tracts of the White Matter
- Conduct sensory or motor information between the spinal cord & brain
- ascending tracts - conduct sensory input to the brain
- descending tracts - conduct motor output away from the brain
- Tract name often reveals the origin, destination & white matter location e.g. lateral spinothalamic tract
- origin = spinal cord
- destination = thalamus
- tract location = lateral white column
Fasciulus gracilis
Fasciulus cuneatus
- ascending spinal cord tracts
- posterior white columns
- fine
- vibration
- light pressure
- proprioception
Lateral spinothalamic
- ascending spinal cord tracts
- lateral white columns
- pain & temp
anterior spinocerebellar
posterior spinocerebellar
- ascending spinal cord tracts
- lateral white columns
- proprioception
Anterior spinothalamic
- ascending spinal cord tracts
- anterior white columns
- crude touch
- deep pressure
Lateral corticospinal tracts
- descending spinal cord tracts
- lateral white columns
somatic motor output that controls the skeletal muscles of limbs
anterior corticospinal tracts
- descending spinal cord tracts
- anterior white columns
somatic output that controls the skeletal muscles of the trunk
Spinal Cord Injuries
- Result in sensory and/or motor losses at or below the level of the injury
- damage to the posterior gray horns or ascending spinal cord tracts = loss of sensation
- damage to the anterior gray horns or corticospinal tracts = muscle weakness or paralysis
Neural Pathways
Spinal cord tracts belong to sensory pathways that connect receptors to the brain, and motor pathways that connect the brain to skeletal muscles.
These pathways:
- conduct sensory input from receptors to the brain
- conduct motor output from the brain to skeletal muscles
- consist of a chain of 2-3 neurons
- usually cross over (decussate) within the CNS
Somatosensory Pathways
- Conduct general sensory input from receptors to the brain
- Involve a relay of neurons called:
- first-order neuron
- second-order neuron - third-order neuron
Three main somatosensory pathways:
- Spinothalamic pathway
- Posterior column pathway (a.k.a. dorsal column-medial lemniscal pathway)
- Spinocerebellar pathway
Spinothalamic Pathway
Conducts sensations of crude touch, deep pressure, pain & temperature.
First-order neurons:
- conduct sensory input from receptors into a posterior gray horn
- synapse with second-order neurons within a sensory nucleus
Second-order neurons:
- conduct sensory input to the thalamus
- ascend spinal cord in a/an:
- anterior spinothalamic tract (crude touch & deep pressure)
- lateral spinothalamic tract (pain & temperature)
- synapse with third-order neurons in the thalamus
Third-order neurons:
- conduct sensory input to the primary somatosensory cortex (PSC)
Posterior Column Pathway
Conducts sensations of fine touch, light pressure, vibration & proprioception.
First-order neurons:
- conduct sensory input into a posterior gray horn and up to the medulla oblongata
- ascend spinal cord in a fasciculus gracilis or fasciculus cuneatus tract
- synapse with second-order neurons in the medulla oblongata
Second-order neurons:
- conduct sensory input to the thalamus
- synapse with third-order neurons in the thalamus
Third-order neurons:
- conduct the sensory input to the PSC
Spinocerebellar Pathway
Conducts sensations of proprioception.
First-order neurons:
- conduct sensory input from proprioceptors into a posterior gray horn
- synapse with second-order neurons within a sensory nucleus
Second-order neurons:
- conduct sensory input to the cerebellum
- ascend spinal cord in an anterior or posterior spinocerebellar tract
Somatic Motor Pathways
- Conduct somatic motor output from the primary motor cortex (PMC) to skeletal muscles
- Involve a relay of two neurons:
- upper motor neuron
- lower motor neuron
- Two main somatic motor pathways:
1. Lateral corticospinal pathway
2. Anterior corticospinal pathway
Lateral Corticospinal Pathway
Conducts the somatic motor output that controls the skeletal muscles of the limbs.
Upper motor neurons:
- conduct somatic motor output from the PMC to an anterior gray horn
- descend spinal cord in a lateral corticospinal tract
- synapse with lower motor neurons in a motor nucleus
- Lower motor neurons:
- conduct somatic motor output to skeletal muscles of the limbs
Anterior Corticospinal Pathway
Conducts the somatic motor output that controls the skeletal muscles of the trunk (axial skeleton).
Upper motor neurons:
- conduct somatic motor output from the PMC to an anterior gray horn
- descend the spinal cord in an anterior corticospinal tract
- synapse with lower motor neurons in a motor nucleus
- Lower motor neurons:
- conduct somatic motor output to skeletal muscles of the trunk
