Spinal Cord

Describe the organization of the nervous system
Describe the relationship between the spinal cord and vertebral canal
Explain the function of the spinal cord
Describe the structure and function of the somatosensory pathways in the spinal cord
Contrast between upper and lower motor neurons
Contrast between flaccid and spastic paralysis
Describe the central motor program and elaborate on the conversion of environmental stimuli into motor activity
Describe the motor pathways in the spinal cord and resultant behaviours produced by damage to these pathways along their length

Function:
- Cylindrical cable of pathways to and from the brain and the rest of the body
- Extends from the base of the skull (medulla) to the L1-L2 level of the vertebral column
Features:
- Cervical enlargement (brachial plexus)
- Lumbar enlargement (lumbosacral plexus)
- Covered by meninges, similar to the brain
- Central canal is continuous with the ventricles of the brain

Organized into:
- Inner Grey Matter: Consists of:
- Posterior (Dorsal) Horn: Receives sensory information
- Anterior (Ventral) Horn: Sends motor information
- Outer White Matter: Contains ascending and descending nerve pathways
Dorsal and ventral roots lead to:
- Spinal Nerve: Formed by the joining of dorsal and ventral roots

Continuous flow of information between the brain, spinal cord, peripheral nerves, and organ systems
- Sensory Neurons: Deliver incoming information to processing centers in the CNS
- Motor Neurons: Control or adjust the activities of peripheral effectors (muscles)
- This flow of information continues around the clock

Involves:
- Ascending Pathways: Relay sensory information to brain centers
- Descending Pathways: Transmit motor commands from the brain to the periphery
Pathways are named according to their origin and destination:
- Example: Spinothalamic pathway: From spinal cord to thalamus
- Example: Corticospinal pathway: From cerebral cortex to spinal cord

Three Major Sensory Pathways:
1. Dorsal Column/Medial Lemniscal Pathway:
- Functions:
  - Joint (limb) position and motion sense (proprioception)
  - Light touch sensations (discriminative touch, stereognosis, graphesthesia)
1. Spinothalamic (Anterolateral) Pathway:
- Functions:
  - Pain, temperature, and deep touch (pressure)
1. Spinocerebellar Pathway:
- Function:
  - Unconscious proprioception

Conveys joint position and discriminative touch (stereognosis) from the trunk and extremities
Pathway details:
- 1st Order Neuron: From skin/joint receptor to medulla (same side) via fasciculus gracilis (lower limbs and trunk) & fasciculus cuneatus (upper limbs and trunk)
- 2nd Order Neuron: Crosses to opposite side at medulla and ascends as medial lemniscus to thalamus
- 3rd Order Neuron: From thalamus to primary sensory cortex (thalamocortical projections)

Carries unconscious proprioception to the cerebellum
Pathway details:
- 1st Order Neuron: From receptors in joint capsules, tendons, and muscle spindles to dorsal horn spinal grey
- 2nd Order Neuron: From dorsal horn to cerebellum via spinocerebellar tracts

CNS issues motor commands which are distributed to the body by the PNS
Somatic Motor Commands: Affect contraction of skeletal muscle
Involve at least 2 motor neurons:
- Upper Motor Neuron (UMN): Cell body in CNS processing centers
- Lower Motor Neuron (LMN): Cell body in the ventral horn of the spinal cord or motor cranial nerve nucleus in brainstem

UMN can modulate LMN activity through either:
- Excitatory Synapse: Increase activity
- Inhibitory Synapse: Decrease activity
Lesions impacting UMN results in spastic paralysis (rigidity due to loss of inhibition)
Lesions impacting LMN result in flaccid paralysis (no response due to total neuron destruction)

Corticospinal (Pyramidal) Pathway:
- Involves upper motor neurons extending from the cerebral cortex to the spinal cord
- Regulates distal musculature in limbs and trunk
Corticonuclear (Corticobulbar) Pathway:
- Involves upper motor neurons extending from the cerebral cortex to motor cranial nerve nuclei in brainstem
- Regulates muscles of head and neck

Extrapyramidal Pathways:
- Important subsidiary descending pathways for motor function
Basal Nuclei and Cerebellum Modulation:
- Ensure smooth, coordinated, purposeful movements through modulation of the corticospinal tract

Central Motor Program:
1. Identification and Localization of Targets in Space: Involves the posterior parietal cortex
2. Formulation of a Plan of Action: Involves the premotor cortex and supplementary motor cortex
3. Execution of the Movement: Involves primary motor cortex

Stimuli from Environment: Transmitted as sensory input
Cortex Communication: Information conveyed via association fibers
Motor Command Initiation: Central motor program fed into primary motor cortex
Muscle Contraction Execution: Leads to physical energy output via muscle contraction

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