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jjay / Jeremy's Nervous System

May 27, 2025 00:14
anatomy
nervous system
neuroscience
physiology
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Jeremy's Nervous System

A concise guide to the human nervous system, covering its structure, function, and key components like the auditory, visual, and motor systems.

Nervous System Overview

Divisions of the Nervous System

Central Nervous System (CNS)

Brain and spinal cord; responsible for processing and coordinating information.

Peripheral Nervous System (PNS)

Nerves outside the CNS; connects the CNS to the limbs and organs, serving as a communication relay.

Autonomic Nervous System (ANS)

Regulates involuntary functions (heart rate, digestion, etc.). Divided into sympathetic and parasympathetic branches.

Sympathetic Nervous System

‘Fight or flight’ response; prepares the body for action.

Parasympathetic Nervous System

‘Rest and digest’; conserves energy and promotes relaxation.

Cell Communication

Indirect

Gap Junctions

Transient linkup

Indirect

paracrine secretion

neurotransmitter secretion

Endocrine vs nervous signaling

Endocrine act through hormones controlling process that rely on duration rather than speed

Nervous acts through electrical signals for rapid responses

Sympathetic vs Parasympathetic

Sympathetic releases norepinephrine to Adrenergic receptors

Parasympathetic releases ACh to cholinergic receptors

The Auditory System

Components of the Ear

External Ear

Pinna, external auditory meatus, tympanic membrane

Middle Ear

Tympanic membrane, ossicles, oval window

Inner Ear

oval window, cochlea, vestibular apparatus, round window

Components of the Ear

Simplified Auditory Pathway

  1. Sound waves
  2. Vibration of Tympanic membrane
  3. Vibration of Middle Ear bones
  4. Vibration of Oval Window
  5. Fluid movement within the Cochlea
  6. Vibration of Basilar Membrane
  7. Bending of Hair cells
  8. Grade receptor potential
  9. Action Potentials generated in Auditory Nerve
  10. Propagation to Auditory Complex

Key Concepts

Auditory Receptors

Hair cells located in the cochlea. These hairs are bent when the basilar membrane is deflected which opens mechanically gated channels, leading to ion movements resulting in a receptor potential

Pitch

depends on the region of the basilar membrane that vibrates. Think “where”

Loudness

depends on the amplitude of vibration of the basilar membrane. Think “How Much”

The Visual System

Components of the Eye

Cornea

Transparent outer layer that refracts light.

Lens

Focuses light onto the retina.

Retina

Layer of photoreceptor cells (rods and cones) that transduce light into electrical signals.

Fovea

Central focal point in the retina, responsible for sharp central vision.

Optic Disk

The ‘blind spot’ in the retina where the optic nerve exits; lacks photoreceptors.

Optic Nerve

Transmits electrical signals from the retina to the brain.

Components of the Eye

Visual Pathway

  1. Light enters the eye and is refracted by the cornea and lens.
  2. Photoreceptors (rods and cones) in the retina transduce light into electrical signals.
  3. Signals are processed by retinal neurons and transmitted to ganglion cells.
  4. Optic nerve carries signals to the optic chiasm, where some fibers cross over.
  5. Optic tracts carry signals to the thalamus (lateral geniculate nucleus), then to the visual cortex in the occipital lobe.

Key Concepts

Rods

Photoreceptors sensitive to dim light
do not distinguish between different wavelengths of light
serve low acuity and peripheral field vision

Cones

Photoreceptors sensitive to bright light
distinguish between 3 different wavelengths of light
serve high acuity central field vision

Fovea

Central region of the retina with a high concentration of cones; responsible for high-acuity vision.

The Motor System

Components of the Motor System

Cerebral Cortex

Planning, initiating, and executing voluntary movements; primary motor cortex, premotor cortex, supplementary motor area.

Basal Ganglia

Involved in motor control, motor learning, executive functions and behaviors, and emotions.

Cerebellum

Coordinates movement and maintains balance.

Brainstem

Relays motor signals from the cerebral cortex and cerebellum to the spinal cord; controls basic motor functions (e.g., posture).

Motor Pathways

  1. action potential in motor neuron is propagated
  2. voltage-gated Ca2+ channels open and Ca2+ enters terminal button
  3. Ca2+ triggers ACh release
  4. ACh diffuses and binds to specific receptor channels on motor end plates
  5. binding cause opening of cation channels causing large movement of Na+ into muscle
  6. end plate potential is the result; local current flow occurs between depolarized end plate and adjacent membrane
  7. local current flow opens VG Na+ channels
  8. Na+ enters and reduces potential to threshold
  9. ACh is destroyed by acetylcholinesterase

Key Concepts

Motor Neuron

Nerve cell that directly innervates and controls muscle fibers
bring about movement

axons of motor neurons originate in the CNS and end on skeletal muscle
motor-neuron axon terminals release acetylcholine to stimulate muscle contraction

Neuromuscular Junction

Chemically links neurons and skeletal muscle fibers

each muscle cell has only 1

Muscle fiber: single, long, and cylindrical muscle cell

Terminal Button: enlarged knoblike structure at the end of axon terminal branches

Spinal Cord

each segment contains motor neurons that project to same side skeletal muscles

each segment also contains motor neurons that project to skeletal muscles via ventral roots

Neuromuscular Signaling Disruptions

Curare

poison that binds strongly to nicotinic ACh receptors

ACh therefore can not bind to receptors thus resulting in no muscle contractions

Myasthenia

autoimmune disease where antibodies attack nicotinic ACh receptors

causes muscle weakness commonly around facial expression muscles

can be treated with ACh-esterase inhibitors which reduce rate of ACh degradation

Tetanus

wound becomes contaminated with Clostridium Tetani

toxins block release of inhibitory neurotransmitter

causes stiffness and rigidity

Botulism

ingesting spores from Clostridium botulinum bacteria

inhibits release of excitatory neurotransmitter

causes muscle weakness