bio 231 last lecture

Sensory Pathways and Reflexes

• The concept of labeled lines in sensory pathways- The direct route from a receptor to the cerebral cortex which processes perceptions by neuronal connections.

General Terminology

• Neuronal pools: a group of interconnected neurons that work together in pathways, though the term is not frequently used.

• Circuits: refer to the complexity of neuronal connections in reflex pathways:

  • Monosynaptic: A simple circuit with one synapse (e.g., knee-jerk reflex).

  • Polysynaptic: A complex circuit with more than one synapse (e.g., withdrawal reflex).

Types of Circuits

Presynaptic to Postsynaptic Interactions

• One-to-One Circuit:

  • One presynaptic cell connects to one postsynaptic cell, lacking convergence or divergence.

Diverging Circuits

• Characteristics:

  • A divergence circuit has more postsynaptic cells.

  • The signal spreads broadly from a single neuron to multiple target neurons.

• Example: Autonomic nervous system pathways, particularly the sympathetic division which has a greater degree of divergence.

Converging Circuits

• Characteristics:

  • More presynaptic neurons funneling to fewer postsynaptic cells.

• Example: Retinal rod cells synapsing on bipolar neurons for vision in low light; numerous rods connecting to a single neuron increases sensitivity but decreases resolution.

Reflexes

Overview

• Monosynaptic Reflexes:

  • Involve one synapse and respond to stretching (e.g., patellar reflex).

Polysynaptic Reflexes:

• More complex; involve interneurons.

• Involve withdrawal reflexes where sensory neurons trigger a response to ice burns or sharp objects.

Contraction and Relaxation of Muscles

• Muscle Spindles (intrafusal fibers) are activated by stretch and facilitate contraction through alpha motor neurons, which cause extrafusal fibers to contract.

• Reciprocal Inhibition: Mechanism to ensure antagonistic muscles (e.g., hamstrings) are inhibited while muscles (e.g., quadriceps) contract during reflex actions.

Sensory Tracts

Gracile and Cuneate Tracts

• Function: Carry sensory information (e.g., touch, proprioception) to the medulla.

Spinothalamic Tract

• Carries information regarding pain, temperature, and light touch to the thalamus before reaching the cerebral cortex.

Motor Tracts

Corticospinal Tract

• Connects from the brain (primary motor cortex) down to the spinal cord, responsible for voluntary motor control of skeletal muscles.

Reticulospinal Tracts

• Facilitate postural adjustments and modulate motor responses.

Vestibulospinal Tracts

• Help maintain balance by sending signals from the vestibular apparatus to extensor muscles in response to balance-related proprioceptive information.

Reflexes in Detail

Stretch Reflex

• Initiates in response to muscle stretch via muscle spindles, leading to contraction of the muscle via alpha motor neurons.

• Example: Patellar reflex, which is monosynaptic, involving direct connection between sensory and motor neurons.

Withdrawal Reflex

• Initiates a withdrawal response and involves polysynaptic pathways with interneurons; inhibits conflicting muscle responses.

Crossed Extension Reflex

• Pulls the body away from a painful stimulus while maintaining balance through contraction of opposing muscles on the opposite limb.

Golgi Tendon Reflex

• Monitors and protects tendons from damage by inhibiting muscle contraction if tension exceeds a threshold, utilizing Golgi tendon organs.

Additional Considerations

• Clinical Implications of Reflex Activity: Understanding reflex actions aids in diagnosing neurological conditions based on sensory and motor responses.

• Dermatome Maps: Areas of skin supplied by specific spinal nerves; useful in identifying nerve damage or compression.