The Peripheral Nervous System and Reflex Activity: Part A 13 The Peripheral Nervous System and Reflex Activity: Part A

Peripheral Nervous System (PNS) All neural structures OUTSIDE the brain and those starting in and ending outside of the spinal cord Sensory nerves contain Neurons receiving info in senses, skin, organs, muscles Motor nerves contain neurons going to muscles and glands

Structure of a nerve Axon Myelin sheath Endoneurium Perineurium Epineurium Fascicle Blood vessels (b) Figure 13.3b

How is a nerve different than a neuron? A nerve is a group of axons Most nerves have both Sensory and Motor neurons Cranial nerves carry information to and from the head and shoulders and brain Spinal nerves carry info to and from the body and the spinal cord.

Sensory “Receptors” Are just modified dendrites of sensory neurons that respond to things other than just chemical neurotransmitters They respond to changes in their environment (stimuli) rather than NT.

Perception: Modified dendrites respond to many things Mechanoreceptors—respond to touch, pressure, vibration, stretch, and itch Thermoreceptors—sensitive to changes in temperature Photoreceptors—respond to light energy (e.g., retina) Chemoreceptors—respond to chemicals (e.g., smell, taste, changes in blood chemistry) Nociceptors—sensitive to pain-causing stimuli (e.g. extreme heat or cold, excessive pressure, inflammatory chemicals)

Receptors can be classified by Location 1. Exteroceptors -Respond to stimuli arising outside the body on skin and in eyes/ears/mouth/nose 2. Interoceptors (visceroceptors) -Respond to stimuli from organs and blood vessels (low pressure=empty, high pressure= full) 3. Proprioceptors -Respond to stretch in skeletal muscles, tendons, joints -they Inform the brain of where your arms and legs are. -How do you know where your hand is with your eyes closed? Proprioceptors!

Receptors can be classified by how they are made 1. Unencapsulated dendrites -free dendrites -temperature, pain, chemicals 2. Encapsulated dendrites -have specialized coverings - to distinguish deep touch, light touch

Table 13.1

How do we perceive the environment? Levels of neural integration in sensory systems: Receptor level—the sensory receptors (dendrites or special receptors) (1st order) Circuit level—ascending pathways through spinal cord/brainstem (2nd order) Perceptual level—neuronal circuits in the cerebral cortex (3rd order)

Perceptual level (processing in cortical sensory centers) 3 Perceptual level (processing in cortical sensory centers) Motor cortex Somatosensory cortex Thalamus Reticular formation Cerebellum Pons Medulla 2 Circuit level (processing in ascending pathways) Spinal cord Free nerve endings (pain, cold, warmth) Muscle spindle 1 Receptor level (sensory reception and transmission to CNS) Joint kinesthetic receptor Figure 13.2

Adaptation of Sensory Receptors Have you ever gotten “used to” an itchy sweater, tight necktie or bra? Adaptation is a change in sensitivity in the presence of a constant stimulus Some receptors become less responsive over time and may turn off.

Adaptation of Sensory Receptors Phasic receptors signal adapt quickly only signal the beginning or end of a stimulus receptors for pressure, touch, and smell Tonic receptors adapt slowly or not at all Nociceptors (you want to be aware of pain) Proprioceptors (you want to know where your limbs are)

How does the brain know what stimuli are coming in? Different senses GO to different places in the brain Eye nerves go to back of brain (vision) Touch nerves go to side of brain (feeling) So different parts of the brain expect different information The more intense the stimulus the more action potentials are received The bigger the area it is received from

Processing a the perceptual level When information gets to the brain it needs to decode it: By the frequency of APs sent by one or more neurons (how loud you shout!) By the number of neurons sending information from a narrow or broad area of the body (how many people are shouting)

Perceptual level (processing in cortical sensory centers) 3 Perceptual level (processing in cortical sensory centers) Motor cortex Somatosensory cortex Thalamus Reticular formation Cerebellum Pons Medulla 2 Circuit level (processing in ascending pathways) Spinal cord Free nerve endings (pain, cold, warmth) Muscle spindle 1 Receptor level (sensory reception and transmission to CNS) Joint kinesthetic receptor Figure 13.2

Pain is an important sense Warns of actual or impending tissue damage Stimuli include extreme pressure and temperature, and chemicals Glutamate and substance P are NT that relay pain info Some pain impulses are blocked by inhibitory NT called endorphins

Visceral Pain Organs have pain, but there are very few sensory receptors in organs. Stimulation of visceral organ receptors is: Felt as vague aching, gnawing, burning Activated by tissue stretching, ischemia, chemicals, muscle spasms © 2013 Pearson Education, Inc.

Referred Pain Referred pain Pain from one body region perceived from different region Since visceral and somatic pain fibers travel in same nerves the brain assumes stimulus from commonly felt areas, like skeletal muscles or skin Ex., left arm pain (commonly felt in life) is perceived during heart even though the pain is actually from the heart (hardly ever felt in life!).

© 2013 Pearson Education, Inc. Figure 13.3 Map of referred pain. Lungs and diaphragm Heart Gallbladder Liver Appendix Stomach Pancreas Small intestine Ovaries Colon Kidneys Urinary bladder Ureters © 2013 Pearson Education, Inc.

© 2013 Pearson Education, Inc. Regeneration of a nerve axon in a peripheral nerve. (1 of 4) Endoneurium Schwann cells The axon becomes fragmented at the injury site. 1 Droplets of myelin Fragmented axon Site of nerve damage © 2013 Pearson Education, Inc.

© 2013 Pearson Education, Inc. Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. (2 of 4) Macrophages clean out the dead axon distal to the injury. 2 Schwann cell Macrophage © 2013 Pearson Education, Inc.

© 2013 Pearson Education, Inc. Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. (3 of 4) 3 Aligning Schwann cells form regeneration tube Axon sprouts, or filaments, grow through a regeneration tube formed by Schwann cells. Fine axon sprouts or filaments © 2013 Pearson Education, Inc.

© 2013 Pearson Education, Inc. Figure 13.5 Regeneration of a nerve fiber in a peripheral nerve. (4 of 4) Schwann cell New myelin sheath forming The axon regenerates and a new myelin sheath forms. 4 Single enlarging axon filament © 2013 Pearson Education, Inc.

Regeneration of damaged axons:summary If a cell body of a damaged nerve cell is still intact, peripheral axon might regenerate The axon fragments (Wallerian degeneration) Macrophages clean dead axon The Schwann cells are still in place and form a “regeneration tube” or tunnel. Axon filaments grow through regeneration tube Axon regenerates; new myelin sheath form © 2013 Pearson Education, Inc.

Regeneration of CNS neurons Most CNS neurons never regenerate if damaged CNS oligodendrocytes bear growth-inhibiting proteins that prevent CNS fiber regeneration Astrocytes at injury site form scar tissue containing chondroitin sulfate that blocks axonal regrowth Treatment Neutralizing growth inhibitors, blocking receptors for inhibitory proteins, destroying chondroitin sulfate promising © 2013 Pearson Education, Inc.