Neuron Activation, Synapses, and Nervous System
Neuron Activation
A membrane also has channels through which ions pass. Na+ channels are closed, but upon appropriate stimulation, they open, and Na+ enters massively. The entry of Na+ changes the polarity of the membrane; the inside of the neuron becomes positive, and the outside becomes negative. This potential difference is called the nerve impulse, and the membrane is said to be depolarized. The opening of Na+ channels takes a short time, and when they close, K+ channels open, becoming positive again to repolarize the membrane. The sodium-potassium pump expels Na+ and brings in K+.
The Synapse
S. Ramon y Cajal found that the nervous system is a network of neurons separated by small spaces.
Nerve impulses are propagated between neurons via synapses. A synapse consists of three elements:
- Presynaptic: The part of the axon terminal through which the impulse arrives. It contains vesicles with chemicals called neurotransmitters, which can be excitatory or inhibitory, such as serotonin and acetylcholine, respectively.
- Synaptic Cleft: The space that separates the presynaptic and postsynaptic elements.
- Postsynaptic: The part of the neuron that has receptors.
Synapses are formed between:
- Axon – Dendrite / Axon – Cell Body
- Neurons – Muscle Fibers / Neurons – Glands
Synaptic Transmission
The arrival of a nerve impulse at the synaptic button causes the entry of calcium, which triggers the release of vesicles loaded with neurotransmitters. These neurotransmitters then diffuse to the receptors on the postsynaptic membrane.
Nervous Communication
Nervous communication requires the following elements:
- Receptors: Sensory structures capable of capturing environmental changes and generating impulses.
- Neurons: Neural structures that capture and interpret sensory messages and, depending on previous experience, prepare a response.
- Effectors: Motor structures (muscles) or secretory structures (glands) responsible for interpreting orders.
Types of Neurons
- Sensory: Allow the passage of messages generated in the sensory receptors to the nerve centers.
- Interneurons: Connect sensory and motor neurons.
- Motor: Carry nerve messages from the CNS to effector organs. Their axons are usually surrounded by Schwann cells, which form a myelin sheath. This sheath has discontinuities called nodes of Ranvier, resulting in saltatory conduction.
* Myelin is made of insulating lipids.
The Reflex Arc: The Shortest Path
Responses to a stimulus can be produced in the spinal cord and brain (rapid, involuntary, and reflex). The chain of neurons involved in this action is called the reflex arc, while the automatic nervous system responses to environmental changes are called reflex actions.
Complex Responses: Functions of the CNS
The CNS controls and coordinates all motor activity, including:
- Simple Movements: Involuntary and unconscious, involving the spinal cord.
- Complex Movements: Nerve impulses are directed to our muscles to produce a voluntary movement.
General Organization of the Brain and Spinal Cord
The brain and spinal cord are enclosed and protected by the skull and vertebrae, respectively. They are surrounded by the meninges, which, from outside to inside, are called the dura mater, arachnoid, and pia mater. Between the latter two is the cerebrospinal fluid (which acts as a shock absorber).
The brain has four cavities, the ventricles, connected to each other, and to the ependymal canal, which runs through the entire spinal cord.
Functions of the Spinal Cord
- It is the center of many reflex actions.
- It is the communication channel between the periphery and the brain. Sensory neurons enter the dorsal roots of the spinal nerves and synapse with neurons in the gray matter before exiting through the ventral nerve roots.
The transmission of impulses from the spinal cord to the brain is carried out through myelinated axons. Most ascending pathways cross to the opposite side before reaching their destination. Sensations coming from the right side end up on the left side of the CNS and vice versa. Descending axons (from the brain) follow the same pattern.