nervous system
- Peripherical Nervous System (PNS)
- ==Nerves== → Nervous tissue → Neurons
- Central Nervous System (CNS)
- Made up of relay neurons
- Brain
- Spinal Chord
Neurons
Sensory neurons: From sense organs to the Central Nervous System
- Its shape is long with a receptor on one end.
Relay neurons: Impulses can travel both ways.
- Its shape is small with lots of dendrites and a central nucleus.
- Connects the motor and sensory neurons.
- Only found in the CNS, and they are multipolar.
Motor neurons: From the central nervous system towards effectors (muscles and glands)
- Its shape is long, with dendrites from one end
Impulse (also called action potential)
Is a series of electrical pulses that travel really quickly carrying information towards and from the central nervous system.
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parts of the neurons
- Dendrites: Extensions that mainly receive and improve synaptic neuronal communication.
- Axon: Long fiber that allows a quicker carrying process from neuron to neuron
- It is mostly insulated due to a fatty sheath, but where it is not insulated, the impulses can jump from one “node” to another.
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Voluntary and involuntary actions
An involuntary action is any action that a body makes without thinking about it and making the conscious decision to do it.
Therefore, a reflex is an involuntary action. But what makes a reflex a reflex is that it always responds to stimuli, it is automatic.
Examples of reflexes:
- Blinking
- Flinch
reflex arc
Stimulus → Receptor → Sensory Neuron → Co-ordinator (CNS) // Relay neurons → Motor neuron → Efector → Response
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Synapse
A method of active transport to communicate neurons between themselves so that they can pass on impulses even though they always have a cleft between them.
- Presynaptic neuron: Has vesicles that are carrying the neurotransmitters, the vesicles are triggered by the electrical impulse.
- Synaptic cleft.
- Postsynaptic neuron: Has receptors with whom specific neurotransmitters bind.
- The neurotransmitters that do not bind, are uptaken again into the presynaptic neuron, or broken down by enzymes.
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Sense organs
A sense organ is a group of tissues with a specific task, that has a bunch of sensory neurons and receptors responding to a specific stimulus. They are mainly known for the five senses.
| Sense organ | Stimulus |
|---|---|
| mouth | chemicals (taste) |
| nose | chemicals (smell) |
| eye | light |
| ears | sound and balance |
| skin | touch, pain, temperature, and pressure. |
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The eye👁️🗨️
The pupil reflex
- Mechanism to protect the retina from damage in bright light and to allow vision with dim-light.
- Photoreceptors detect the change in the environment’s light conditions.
| Dim light | Bright light |
|---|---|
| Pupil dilates to enter as much light as possible. | Pupil constricts preventing too much light entering and damaging the retina. |
| Radial muscles contract | Radial muscles relaxed. |
| Circular muscles relax | Circular muscles contracted. |
Circular and Radial muscels; The iris (circular and radial muscles) and pupil; The cilliary muscles and suspensory ligaments have an ANTAGONISTIC RESPONSE.
Lens’ accommodation
- Its purpose is to focus according to the distance of the object willing to be seen.
- The Lens is elastic and has a changing shape, relying on how tight the suspensory ligaments are.
- The change in lens’ accommodation is mainly causes bythe relaxation or contraction of the ciliary muscles
| Near object | Distant object |
|---|---|
| Light needs to be more refracted. | Light is less refracted. |
| Ciliary muscles contracted | Ciliary muscles relaxed |
| Suspensory ligaments slacked | Suspensory ligaments contracted |
| Fatter lens | Thinneer lens |
| Structure | Function |
|---|---|
| Cornea (Scelera) | A transparent lens that refracts light to focus images. |
| Iris | Controls the amount of light entering the pupil |
| Lens | A transparent disc that changes its shape through accommodation with the help of the suspensory ligaments and ciliary muscles to regulate the light that enters the retina. |
| Retina | When light falls on it, rods and/or cones are stimulated and they send impulses. |
| Optic nerve | A nerve that carried impulses from the retina towards the brain, producing the images. |
| Pupil | A hole that allows light to enter the rest of the eye. |
- Why the eye cannot easily identify different colors in low levels of light?
- Because the coness (that enable color distinguishment) are not sufficiently stimulated due to low light levels. In contrast, most light in a dim environment is used to stimulate rodes so that the eye can see.
- Cells in the retina:
- Rods: Sensible to light changes.
- Rods are everywhere in the retina except for the blind spot
- Cones: Sensible to colors.
- Cones are mainly found in the Fovea