Chapter 15 Sensory Pathways and the Somatic Nervous System

Sensory Receptors

Specialized cells (receptor cells or neurons) unique to each sense organ that respond to a particular form of sensory stimulation and transmit it to sensory (afferent) division and the brain.

Mechanoreceptors

Sensory receptors responsible for sensing distortion (respond to touch, pressure, vibration, and stretch) in body tissues.

Thermoreceptors

respond to changes in temperature

Photoreceptors

rod and cone cells in the retina; respond to light

Chemoreceptors

chemical sensors in the brain and blood vessels that identify changing levels of chemicals (e.g. oxygen, carbon dioxide, pH)

Nociceptors

sensory receptors that enable the perception of pain in response to potentially harmful stimuli

Exteroceptors

Respond to stimuli arising outside the body
Receptors in the skin for touch, pressure, pain, and temperature
Most special sense organs

Interoceptors

monitor visceral organs and functions

Proprioceptors

monitor the position and movement of skeletal muscles, tendons, and joints

Nonencapsulated (free) nerve endings

Abundant in epithelia and connective tissues
Respond mostly to temp, pain, or light touch

Encapsulated nerve endings

dendrites enclosed in connective tissue capsule for pressure, vibration, and some touch sensations

Sensation

the process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment

Perception

The act of becoming aware through the senses

Somatosensory system

sensory network that monitors the surface of the body and its movements

Receptor level

processing at this level involves a stimulus that must excite a receptor in order for sensation to occur

Generator potential

a local change in the resting potential of a receptor cell in response to stimuli, which may initiate an action potential

Receptor potential

An initial response of a receptor cell to a stimulus, consisting of a change in voltage across the receptor membrane proportional to the stimulus strength.

Peripheral Adaptation

when sensory receptors decrease their level of activity even when the stimuli continues

Central Adaptation

-sensory neurons are still active
-CNS neurons reduce activity, cause reduced perception

Tonic receptors

a receptor in which the frequency of action potentials declines slowly or not at all as stimulation is maintained

Phasic receptors

Receptors that respond intensely to the onset of stimulation but quickly stop responding.

Motor endings

axon terminals of motor neurons
innervate effectors (muscle fibers and glands)

Innervation

Supply or distribution of nerves to a specific body part

Neuromuscular junction

point of contact between a motor neuron and a skeletal muscle cell

End plate potential

the depolarizations of skeletal muscle fibers caused by neurotransmitters binding to the postsynaptic membrane in the neuromuscular junction

Varicosities

abnormally enlarged, twisted veins

Motor control

the ability to regulate or direct the mechanisms essential to movement

Segmental level

consists of reflexes and spinal cord circuits that control automatic movements

Central pattern generators

neural mechanisms in the spinal cord that generate rhythmic patterns of motor output

Brain stem

midbrain, pons, medulla oblongata

Cerebellum

the "little brain" at the rear of the brainstem; functions include processing sensory input and coordinating movement output and balance

Basal nuclei

Controls muscle activity and posture; largely inhibits unintentional movement when at rest