chp 14 contd

Sensory Neurons

• Two types of sensory neurons:

  • Somatic Sensory Neurons: Responsible for sensation from the skin, muscles, and joints.

  • Visceral Sensory Neurons: Responsible for sensation from internal organs.

Cell Bodies Location

• Dorsal Root Ganglia: Houses cell bodies for both somatic and visceral sensory neurons.

• Pseudounipolar Neurons: This type of sensory neuron has a single axon that divides into two branches, one to the periphery and one to the spinal cord.

Sensory Signals Pathway

• Somatic Sensory Pathway:

  • Somatic sensory axons synapse in the posterior gray horn (somatic sensory nuclei).

  • Dorsal rami produce nerves for the back body wall; ventral rami produce nerves for the front body wall, arms, and legs.

Sensory Processing

• All sensory input eventually travels to the parietal lobe in the brain for processing and awareness.

Interneurons

• Interneurons: Located in the somatic sensory nuclei, they relay signals from sensory neurons to the brain.

• Interneurons are responsible for independent processing of sensory input before reaching the central nervous system (CNS).

Distinction Between CNS and PNS

• CNS: Central Nervous System, including the spinal cord and brain. It processes sensory input and coordinates motor output.

• PNS: Peripheral Nervous System, including nerve networks and ganglia outside the CNS, where sensory neurons cell bodies reside.

Motor Neurons

• Ventral Gray Horn: This is where the cell bodies of motor neurons reside.

• Motor neurons are multipolar and their axons exit through the ventral roots then travel via dorsal rami to the effectors.

Nerve Functions

• Dorsal Roots: Carry sensory information into the spinal cord.

• Ventral Roots: Carry motor information out to muscles and glands.

Reflex Arc Overview

• Reflex arcs bypass the brain to provide a quicker response to stimuli.

• Monosynaptic Reflex: One synapse (e.g., knee jerk reflex).

• Polysynaptic Reflex: More than one synapse (e.g., stepping on a tack).

Types of Reflexes

• Developmental:

  • Innate Reflexes: Reflexes that are present at birth (e.g., suckling).

  • Acquired Reflexes: Learned reflexes (e.g., riding a bicycle).

• Response Type:

  • Somatic Responses: Involve skeletal muscle. (Baby drinking milk from moms breast)

  • Visceral Responses: Involve smooth muscles or glands. ( milk being released from breast)

Complexity of Reflexes

• Monosynaptic vs. Polysynaptic:

  • Monosynaptic: Simple reflex involving one synapse (e.g., knee jerk).

  • Polysynaptic: More complex; may involve multiple synapses and interneurons.

Processing Sites

• Cranial Reflexes: Reflexes mediated by cranial nerves (e.g., blinking).

• Spinal Reflexes: Reflexes mediated by spinal nerves (e.g., withdrawal reflex).

Motor Output

• After sensory processing, if voluntary action is needed, the motor output is processed, and the corresponding region of the motor cortex sends signals back down through descending tracts to the relevant ventral rami.

Important Terminology

• Tracks (CNS): Bundles of axons in the CNS.

• Nerves (PNS): Bundles of axons in the PNS.

• Ganglia (PNS): Collections of neuron cell bodies outside the CNS.

• Nuclei (CNS): Collections of neuron cell bodies within the CNS.

Blood-Nerve Barrier

• Protects nerves from blood composition using the endoneurium and perineurium.

• Endoneural Fluid: Bathes axons, maintaining their environment.

• Endoneurial tissue creates the endoneural fluid from the matter that was allowed to enter from the diffusion barrier

• Perineurial tissue is the diffusion barrier” decides what enters

Plexuses

• Groups of nerves formed from multiple spinal sections that coordinate movements and sensory information across different body regions (e.g., cervical, brachial, lumbar plexuses).

Reflex Centers

• Reflexes occur in the spinal cord and brainstem with no involvement of the cerebrum; this allows for rapid reflex actions.