Chapter 13 PNS

PNS Cranial nerves Spinal nerves

Sensory Receptors Specialized cells that monitor specific conditions in the body or external environment Activation of sensory receptors results in depolarizations that trigger impulses to the CNS Sensation: activation of sensory receptor cells

Sensation vs. Perception Activity in sensory cells Perception: Conscious awareness of a sensation (in cortex)

Signaling Stimulation of a receptor produces action potentials along the axon of a sensory neuron Action potentials are all the same so: The frequency and pattern of action potentials contains information about the strength, duration, and variation of the stimulus

Senses General senses: temperature Special senses Olfaction (smell) pain touch pressure vibration proprioception Special senses Olfaction (smell) Vision (sight) Gustation (taste) Equilibrium (balance) Hearing

General receptor types Exteroceptors Provide information about the external environment Proprioceptors Report the positions of skeletal muscles and joints Interoceptors Monitor visceral organs and functions

Modality Your perception of the nature of a stimulus (its modality) depends on the path it takes inside the CNS, especially, where in the brain the information ends up.

Adaptation of Sensory Receptors Adaptation occurs when sensory receptors are subjected to an unchanging stimulus Receptor membranes become less responsive Receptor potentials decline in frequency or stop Receptors responding to pressure, touch, and smell adapt quickly Pain receptors and proprioceptors do not exhibit adaptation

Spinal Nerves Figure 13–6

Spinal Nerves 31 pairs one per segment on each side of the spine dorsal and ventral roots join to form a spinal nerve Carry both afferent (sensory) and efferent (motor) fibers = mixed nerves

Spinal Nerve Organization Every spinal nerve is surrounded by 3 connective tissue layers that support structures and contain blood vessels (just like muscles) Epineurium: outer layer dense network of collagen fibers Perineurium: middle layer divides nerve into fascicles (axon bundles) Endoneurium: inner layer surrounds individual axons

Peripheral Distribution of Spinal Nerves start where dorsal and ventral roots unite (just lateral to the vertebral column), then branch and form pathways to destination

Spinal Nerves: Rami The short spinal nerves branch into three or four mixed, distal rami Small dorsal ramus Larger ventral ramus Rami communicantes at the base of the ventral rami in the thoracic region

Nerve Plexuses All ventral rami except T2-T12 form interlacing nerve networks called plexuses Plexuses are found in the cervical, brachial, lumbar, and sacral regions Each resulting branch of a plexus contains fibers from several spinal nerves Each muscle receives a nerve supply from more than one spinal nerve Damage to one spinal segment (gray matter) cannot completely paralyze a muscle

Peripheral Distribution of Spinal Nerves Motor fibers Figure 13–7a

Motor fibers: First Branch From the spinal nerve, the first branch (blue): carries visceral motor fibers to sympathetic ganglion of autonomic nervous system (More about this later)

Communicating Rami Also called Rami Communicantes, means “communicating branches” made up of gray ramus and white ramus together

Communicating Rami White Ramus: Gray Ramus Preganglionic branch Myelinated axons (hence: white) Going “to” the sympathetic ganglion Gray Ramus Unmyelinated nerves (so: gray) Return “from” sympathetic ganglion Rejoin spinal nerve, go to target organ

Dorsal and Ventral Rami Both are somatic and visceral outflow to the body Dorsal ramus: contains somatic and visceral motor fibers that innervate the back Ventral ramus: larger branch that innervates ventrolateral structures and limbs

Peripheral Distribution of Spinal Nerves Sensory fibers Figure 13–7b

Sensory Nerves Dorsal, ventral, and white rami (but not gray) also carry sensory information in addition to motor efferent outflow.

Dermatomes Bilateral region of skin Each is monitored by specific pair of spinal nerves Figure 13–8

Peripheral Neuropathy Regional loss of sensory or motor function Due to trauma, compression, or disease

Reflexes

Reflexes Rapid, automatic responses to specific stimuli coordinated within the spinal cord (or brain stem) Occurs via interconnected sensory, motor, and interneurons Can be a movement, like a knee jerk, or visceral, like pupil dilation or swallowing

Functional Organization of Neurons in the NS Sensory neurons: about 10 million that deliver information to CNS Motor neurons: about 1/2 million that deliver commands to peripheral effectors Interneurons: about 20 billion that interpret, plan, and coordinate signals in and out = information processors

The Reflex Arc The wiring of a single reflex Begins at sensory receptor Ends at peripheral effector (muscle, gland, etc) Generally opposes original stimulus (negative feedback)

5 Steps in a Neural Reflex Step 1: Arrival of stimulus, activation of receptor physical or chemical changes Step 2: Activation of sensory neuron graded depolarization Step 3: Information processing by postsyn. cell triggered by neurotransmitters Step 4: Activation of motor neuron action potential Step 5: Response of peripheral effector

5 Steps in a Neural Reflex Figure 13–14

Classification of Reflexes There are several ways to classify reflexes but most common is by complexity of the neural circuit: monosynaptic vs polysynaptic

Monosynaptic Reflexes Have the least delay between sensory input and motor output: e.g., stretch reflex (such as patellar reflex) Completed in 20–40 msec No interneurons involved

Monosynaptic Reflex A stretch reflex Figure 13–15

Muscle Spindles The receptors in stretch reflexes Bundles of small, specialized muscle fibers Sense passive stretching in a muscle

Polysynaptic Reflexes More complicated than monosynaptic reflexes Interneurons involved that control more than 1 muscle group Produce either EPSPs or IPSPs Examples: the withdrawal reflexes

Withdrawal Reflexes Move body part away from stimulus (pain or pressure): flexor reflex: pulls hand away from hot stove crossed extensor reflex Strength and extent of response depends on intensity and location of stimulus

A Flexor Reflex Figure 13–17

Key = Reciprocal Inhibition For flexor reflex to work: the stretch reflex of the antagonistic (extensor) muscles must be inhibited reciprocal inhibition by interneurons in spinal cord causes antagonistic extensors to be inhibited

Reflex Arcs Crossed extensor reflexes: involves a contralateral reflex arc occurs on side of body opposite from the stimulus

Crossed Extensor Reflexes Occur simultaneously and coordinated with flexor reflex Example: flexor reflex causes leg to pull up: crossed extensor reflex straightens other leg to receive body weight

The Crossed Extensor Reflex Figure 13–18

Integration and Control of Spinal Reflexes Though reflex behaviors are automatic, processing centers in brain can facilitate or inhibit reflex motor patterns based in spinal cord

Reinforcement of Spinal Reflexes Higher centers can reinforce spinal reflexes: by stimulating excitatory neurons in brain stem or spinal cord creating EPSPs at reflex motor neurons facilitating postsynaptic neurons

Inhibition of Spinal Reflexes Higher centers can inhibit spinal reflexes: by stimulating inhibitory neurons creating IPSPs at reflex motor neurons suppressing postsynaptic neurons

Voluntary Movements and Reflex Motor Patterns Higher centers of brain incorporate lower, reflexive motor patterns Automatic reflexes: can be activated by brain as needed use few nerve impulses to control complex motor functions e.g. walking, running, jumping