Chap 15

Sensory Pathways

- Deliver somatic and visceral sensory information to their final destinations inside the CNS using: - Nerves, nuclei and tracts

Proprioceptors

- Muscle Spindles Monitor skeletal muscle length - Golgi Tendon Organs Located at the junction between skeletal muscle and its tendon - Receptors in Joint Capsules Free nerve endings detect pressure, tension, movement at the joint

Sensory Receptors

- Specialized cells that monitor specific conditions in the body or external environment - When stimulated, a receptor passes information to the CNS in the form of action potentials along the axon of a sensory neuron

Levels of Processing and Motor Control

All sensory and motor pathways involve a series of synapses, one after the other General pattern Spinal and cranial reflexes provide rapid, involuntary, preprogrammed responses that preserve homeostasis over short term Cranial and spinal reflexes Control the most basic motor activities

The Somatic Nervous System (SNS)

Also called the somatic motor system Controls contractions of skeletal muscles Always involve at least two motor neurons Upper motor neuron Lower motor neuron

The Autonomic Nervous System (ANS)

Also called the visceral motor system Controls visceral effectors, such as smooth muscle, cardiac muscle, and glands (Ch. 16)

Tactile Receptors Types:vi. Ruffini corpuscles

Also sensitive to pressure and distortion of the skin Located in the reticular dermis Tonic receptors that show little if any adaptation

Tonic receptors

Are always active Show little peripheral adaptation Are slow-adapting receptors Remind you of an injury long after the initial damage has occurred

Fine touch and pressure receptors:

Are extremely sensitive and provide detailed information about a source of stimulation

Thermoreceptors

Are free nerve endings located in the dermis, skeletal muscles, liver and hypothalamus Temperature sensations conducted along the same pathways that carry pain sensations Sent to: The reticular formation The thalamus The primary sensory cortex (to a lesser extent)

Nociceptors (Pain Receptors)

Are free nerve endings with large receptive fields and common in In the superficial portions of the skin In joint capsules Within the periostea of bones Around the walls of blood vessels May be sensitive to temperature extremes, mechanical damage, and dissolved chemicals

Phasic receptors

Are normally inactive Become active for a short time whenever a change occurs Provide information about the intensity and rate of change of a stimulus Are fast-adapting receptors

Receptive field

Area is monitored by a single receptor cell The larger the receptive field, the more difficult it is to localize a stimulus

Second-Order Neuron

Axon of the sensory neuron synapses on an interneuron in the CNS May be located in the spinal cord or brain stem -Cross to the opposite side of the spinal cord before ascending Ascend within spinothalamic tracts The anterior tracts carry crude touch and pressure sensations The lateral tracts carry pain and temperature sensations

Anterior spinocerebellar tracts

Axons cross over to opposite side of spinal cord * Sensations reach the cerebellar cortex via superior cerebellar peduncle

Posterior spinocerebellar tracts

Axons do not cross over to the opposite side Axons reach cerebellar cortex via inferior cerebellar peduncle of that side

Reflexes

Can complement or increase the complexity of voluntary movements

Higher centers in the brain

Can suppress or facilitate reflex responses

Myelinated Type A fibers

Carry sensations of fast pain, or prickling pain, such as injection or deep cut Sensations relayed to the primary sensory cortex and receive conscious attention

Type C fibers

Carry sensations of slow pain, or burning and aching pain Cause a generalized activation of the reticular formation and thalamus You become aware of the pain but only have a general idea of the area affected

Somatic Sensory Pathways

Carry sensory information from the skin and musculature of the body wall, head, neck, and limbs

Upper Motor Neuron

Cell body lies in a CNS processing center Synapses on the lower motor neuron Innervates a single motor unit in a skeletal muscle Activity in upper motor neuron may facilitate or inhibit lower motor neuron

Lower Motor Neuron

Cell body lies in a nucleus of the brain stem or spinal cord Triggers a contraction in innervated muscle Only axon of lower motor neuron extends outside CNS Destruction of or damage to lower motor neuron eliminates voluntary and reflex control over innervated motor unit

The Spinocerebellar Pathway

Cerebellum receives proprioceptive information about position of skeletal muscles, tendons & joints

Visceral Sensory Pathways

Collected by interoceptors monitoring visceral tissues and organs, primarily within the thoracic and abdominopelvic cavities These interoceptors are not as numerous as in somatic tissues

Perception

Conscious awareness of a sensation

Conscious and Subconscious Motor Commands

Control skeletal muscles by traveling over three integrated motor pathways Corticospinal pathway Medial pathway Lateral pathway

Somatic Motor Portion of the Efferent Division

Controls peripheral effectors

Corticospinal Tracts cont'

Corticospinal Tracts Anterior: Control of more central axial and girdle muscles Lateral: controls fine movement of ipsilateral limbs - Lateral corticospinal tract: upper motor neuron: primary motor cortex, destination: anterior grey horn, crossover in medulla - Anterior corticospinal tract: Upper motor neuron: primary motor cortex, destination: anterior grey horn, crossover in level of lower motor neuron

Receptor specificity

Each receptor has a characteristic sensitivity

Tactile Receptors Types: Tactile discs (Merkel discs)

Fine touch and pressure receptors Extremely sensitive to tonic receptors Have very small receptive fields

Tactile Receptors Types: Tactile corpuscles (Meissner's corpuscles)

For fine touch, pressure, low-rate vibration Fairly large structures and adapt to stimulation within 1 second after contact Most abundant in the eyelids, lips, fingertips, nipples, & external genitalia

Third-Order Neuron

If the sensation is to reach our awareness, the second-order neuron synapses on a third-order neuron in the thalamus - Synapse in ventral nucleus group of the thalamus After the sensations have been sorted and processed, they are relayed to primary sensory cortex

Neurons of the primary motor cortex

Innervate motor neurons in the brain and spinal cord responsible for stimulating skeletal muscles

Visceral Sensory Pathways 1

Interoceptors include: Nociceptors Thermoreceptors Tactile receptors Baroreceptors Chemoreceptors Cranial Nerves V, VII, IX, and X Carry visceral sensory information from mouth, palate, pharynx, larynx, trachea, esophagus, and associated vessels and glands

Solitary nucleus

Large nucleus in the medulla oblongata Major processing and sorting center for visceral sensory information Extensive connections with the various cardiovascular and respiratory centers, reticular formation

Reticular formation

Loosely organized network of neurons that extends throughout brain stem Axons of upper motor neurons in reticular formation descend into reticulospinal tracts without crossing to opposite side - Upper motor neuron (reticular formation in brain stem), lower motor neuron (ant grey horn), no crossover

A. Tactile receptors

Mechanoreceptors

Baroreceptors

Monitor change in pressure Consist of free nerve endings that branch within elastic tissues In wall of distensible organ (such as a blood vessel) Respond immediately, but adapt rapidly

Tactile Receptors Types: Root hair plexus nerve endings

Monitor distortions and movements across the body surface wherever hairs are located Adapt rapidly, so are best at detecting initial contact and subsequent movements

The Cerebellum

Monitors: Proprioceptive (position) sensations Visual information from the eyes Vestibular (balance) sensations from inner ear as movements are under way

Primary motor cortex

Most complex and variable motor activities are directed by primary motor cortex of cerebral hemispheres

Sensory Information

Most somatic sensory information Is relayed to the thalamus for processing A small fraction of the arriving information Is projected to the cerebral cortex and reaches our awareness

Somatic Nervous System (SNS)

Motor neurons and pathways that control skeletal muscles

Efferent Division of the Nervous System

Nuclei Motor tracts Motor neurons

Special Senses

Olfaction (smell) Vision (sight) Gustation (taste) Equilibrium (balance) Hearing

Integrative centers in the brain

Perform more elaborate processing As we move from medulla oblongata to cerebral cortex, motor patterns become increasingly complex and variable

The Medial Pathway

Primarily concerned with control of muscle tone and gross movements of neck, trunk, and proximal limb muscles Upper motor neurons of medial pathway are located in: Vestibular nuclei Superior and inferior colliculi Reticular formation

The Lateral Pathway

Primarily concerned with control of muscle tone and more precise movements of distal parts of limbs Axons of upper motor neurons in red nuclei cross to opposite side of brain and descend into spinal cord in rubrospinal tracts - upper motor neuron (midbrain), lower motor neuron (ant grey horn), crossover in midbrain

The Basal Nuclei

Provide background patterns of movement involved in voluntary motor activities Some axons extend to the premotor cortex, the motor association area that directs activities of the primary motor cortex Alters the pattern of instructions carried by the corticospinal tracts Other axons alter the excitatory or inhibitory output of the reticulospinal tracts

1. Corticobulbar Tracts

Provide conscious control over skeletal muscles that move the eye, jaw, face, and some muscles of neck and pharynx - Upper motor neuron: motor cortex, destination: cranial nerve nuclei of brain stem (crosses in brainstem)

The Corticospinal Pathway (Sometimes called the pyramidal system)

Provides voluntary control over skeletal muscles - System begins at pyramidal cells of primary motor cortex - Axons of these upper motor neurons descend into brain stem and spinal cord to synapse on lower motor neurons that control skeletal muscles Contains three pairs of descending tracts 1. Corticobulbar tracts 2. Lateral corticospinal tracts 3. Anterior corticospinal tracts

Vestibular nuclei

Receive information over the vestibulocochlear nerve (VIII) from receptors in inner ear that monitor position and movement of the head Primary goal is to maintain posture and balance Descending fibers of spinal cord constitute vestibulospinal tracts - Located at the border of medulla and pons (upper motor neuron), anterior grey horn (lower motor neuron), no crossover

Afferent Division of the Nervous System

Receptors Sensory neurons Sensory pathways

Adaptation

Reduction in sensitivity of a constant stimulus The nervous system quickly adapts to stimuli that are painless and constant

Chemoreceptors

Respond only to water-soluble and lipid-soluble substances dissolved in surrounding fluid Receptors that monitor pH, carbon dioxide, and oxygen levels in arterial blood are located in: Carotid bodies Near the origin of the internal carotid arteries on each side of the neck Aortic bodies Between the major branches of the aortic arch

The Basal Nuclei and Cerebellum

Responsible for coordination and feedback control over muscle contractions Whether contractions are consciously or subconsciously directed

Tactile Receptors Types: Lamellated (Pacinian) corpuscles

Sensitive to deep pressure Fast-adapting receptors Most sensitive to pulsing or high-frequency vibrating stimuli

Mechanoreceptors

Sensitive to stimuli that distort their plasma membranes Mechanically gated ion channels gates open or close due to stretching, compression, or twisting Three types of mechanoreceptors: Tactile receptors Baroreceptors Proprioceptors

Tactile Receptors Types: Free nerve endings

Sensitive to touch and pressure Situated between epidermal cells They are tonic receptors with small receptive fields

First-Order Neuron

Sensory neuron delivers sensations to the CNS Cell body of a first-order general sensory neuron is located in dorsal root ganglion or cranial nerve ganglion - Axons of first-order sensory neurons enter spinal cord and synapse on second-order neurons within posterior gray horns

The Medial and Lateral Pathways

Several centers in cerebrum, diencephalon, and brain stem may issue somatic motor commands as result of processing performed at subconscious level

The Medial Pathway

Superior and inferior colliculi Are located in the roof of the mesencephalon, or the tectum Colliculi receive visual (superior) and auditory (inferior) sensations Axons of upper motor neurons in colliculi descend in tectospinal tracts These axons cross to opposite side, before descending to synapse on lower motor neurons in brain stem or spinal cord -Upper motor neuron (tectum), lower motor neuron (ant grey horn), crossover at midbrain

General Senses

Temperature Pain Touch Pressure Vibration Proprioception

Sensation

The arriving information from these senses

Somatic Motor Commands

Travel from motor centers in the brain along somatic motor pathways of: Motor nuclei, tracts, nerves

Interoceptors

monitor visceral organs and functions

Exteroceptors

provide information about the external environment

Crude touch and pressure receptors

provide poor localization and give little information about the stimulus

Proprioceptors

report the positions of skeletal muscles and joints