Neuroscience Chapter: Somatic Nervous System

Somatic Nervous System (Part 2)

• The somatic nervous system is part of the peripheral nervous system (PNS)
• It's responsible for conscious perception of the environment and voluntary responses
• Sensory information (input) travels through afferent nerves to the central nervous system (CNS)
• Motor information (output) travels through efferent nerves from the CNS to skeletal muscles

Conscious Perception of the Environment

• Sensory input is collected through sensory neurons (afferent nerves)
• Sensory information is carried to the CNS (brain and spinal cord)

Voluntary Responses

• Motor output is transported through efferent nerves
• Efferent nerves carry motor information to skeletal muscles
• The CNS initiates a response based on environmental perception, causing voluntary movement

General Senses

• General senses are detected throughout the body
• These senses include touch, pressure, vibration, temperature, and pain perception
• They detect input from our visceral organs, such as hunger or thirst, balance etc

Special Senses

• Special senses involve specific organs dedicated to that sense - eye, inner ear, tongue, or nose
• Examples of special senses include:
  • Gustation (taste)
  • Olfaction (smell)
  • Audition (hearing)
  • Equilibrium (balance)
  • Vision (sight)

Vision (Sight)

• Visual perception involves the accessory structures of the eye, the anatomy of the eye, and photoreception
• Visual stimuli triggers change in membrane potentials in photoreceptors
• Light stimulates photoreceptors in the retina, triggering a response
• The human eye perceives visible light between 400 and 700 nanometers

Accessory Structures of the Eye

• Structures like eyelashes, eyebrows, eyelids protect the eye
• The conjunctiva is a mucus membrane providing lubrication
• Lacrimal glands produce tears with saline, mucus, antibodies, enzymes
• These tears drain into the lacrimal sac and nasal cavity through the nasolacrimal duct
• The orbit (surrounded by cranial bones) houses and supports the eye

Extraocular Muscles

• Originating from the orbit, extraocular muscles help maintain eye shape and move the eyes in response to stimuli

Disorders of the Extraocular Muscles

• Diplopia (double vision) arises from muscle weakness or neurological issues
• Strabismus (cross-eye) is caused by external eye muscle weakness

Anatomy of the Eye

• The eye is divided into anterior and posterior chambers
• The wall of the eye is composed of three layers:
  • Fibrous tunic (outer layer)
  • Vascular tunic (middle layer)
  • Neural tunic (inner layer)

The Fibrous Tunic

• Sclera, the white portion (5/6th of the ocular surface), is mostly not visible
• Cornea, clear portion covering the anterior tip, allows light into the eye

The Vascular Tunic

• Choroid provides blood supply to the eyeball
• Ciliary body, a muscular structure connected to the lens through suspensory ligaments, changes lens shape for focus
• Iris, the colored portion, controls pupil size (contracts or dilates)

The Neural Tunic

• Retina contains three layers of cells and synaptic layers
• Retina's photosensitive cells are stimulated by light, which affects the membrane potential
• Photoreceptors, rods and cones, convert light energy into neural signals

Photoreceptors

• Rods are sensitive to dim light and produce a black/white response
• Cones excel in bright light, enabling color vision

Organisation of the Retina

• Layers of the retina (ganglion cell, bipolar cell, and photoreceptor cells)
• Synaptic layers (inner plexiform and outer plexiform layers)
• Light stimuli travels through the retina's layers until photoreceptors are activated

Photoreception

• Rods contain rhodopsin, essential for dim-light vision
• Cones contain different opsins for distinct wavelengths, allowing us to see colors

Color Blindness

• Hereditary condition involving cone pigments deficiency, most common in males
• Common types include protanopia (red-blindness) and deuteranopia (green-blindness)

Color Blindness & Traffic Lights

• Horizontal red signals are displayed alongside other colors for clear differentiation by color-blind individuals

How does "Light" trigger a change in membrane potential?

• Opsins (transmembrane proteins) contain retinal
• Light alters retinal shape (photoisomerization), activating G-protein signaling
• The cascade of changes ultimately closes Na+ channels

Special Sensory Pathways to the CNS

• Includes auditory pathways, gustatory pathways, and optic pathways

Travelling from the Sensory Receptors to the CNS

• Sensory information travels back to the CNS along sensory nerves
• Sensory pathways connect receptor organs to the cerebral cortex, enabling perception
• Head and neck sensory information travels through cranial nerves
• Information from the rest of the body travels in through the spinal cord

Gustatory Pathway

• Sensory information travels along cranial nerves to the solitary nucleus in the medulla oblongata (3 steps)
• Information then projects to the thalamus
• Finally synapses in the gustatory cortex of the cerebral cortex

Auditory Pathway

• Sensory info travels along the auditory/vestibulocochlear nerve (VIII)
• These nerves synapse with neurons in the cochlear nuclei of the medulla oblongata
• Signals are relayed through the inferior colliculus and medial geniculate nucleus (in the thalamus)
• The auditory cortex processes auditory sensations

Optic Pathway

• Optic nerve pathways are quite complex and differ from other cranial nerve pathways.
• Processes from the left visual field project to the right side, vice versa
• These pathways ultimately synapse in the lateral geniculate nucleus of the thalamus
• Visual information continues to the visual cortex for further processing

Where is visual information delivered to?

• Visual information reaches the lateral geniculate nucleus, a crucial relay point within the thalamus (diencephalon)
• Suprachiastmatic nucleus primarily reacts to light presence/absence, relaying that info to other brain areas that establish our circadium rhythm

Topographic Mapping of the Retina to the Visual Cortex

• Left and right visual fields are sorted at the optic chiasm, mapped to their respective visual cortexes
• Superior and inferior visual fields are processed topographically in the corresponding cortex regions
• The image is inverted when it goes into the visual cortex

Defects in Image Formation

• Myopia (nearsightedness) shortens the eye
• Hyperopia (farsightedness) elongates the eye
• Astigmatism causes a distorted image due to the cornea or lens imperfection

Somatosensory Pathways to the CNS

• Somatosensory pathways include the dorsal column system and spinothalamic tract
• Somatosensory pathways send information from body parts to the CNS regarding touch, movement, pain, and temperature sensations

Cortical Responses

• Signals from sensory areas are processed through ascending pathways in the cerebral cortex
• Responses include voluntary skeletal muscle movement, with or without conscious control
• Sensory areas (occipital, temporal, and parietal lobes) receive and process sensory information
• Motor cortical areas (frontal lobe) initiate and control voluntary movement

Cortical Anatomy - Planning Voluntary Movement

• Prefrontal cortex plans and coordinates movements
• Rest of the frontal lobe coordinates movement production
• Posterior parietal cortex receives sensory information from multiple pathways and integrates this data to create a body/world representation
• This information is sent to the frontal lobe to execute movements

Descending Pathways

• The corticospinal tract is the major descending tract
• Important in controlling skeletal muscle movement