Synapses are the sites where one neuron contacts another neuron.
1. Composition
A synapse consists of three parts: presynaptic component (the spherical body at the end of the axon terminal), synaptic cleft (the gap between the end of one neuron and the beginning of another neuron), and postsynaptic component (a certain part of the dendritic terminal or soma).
2. Classification
(1) Excitatory synapse: When the presynaptic neuron is excited, synaptic vesicles release excitatory neurotransmitters, causing the postsynaptic neuron to become excited.
(2) Inhibitory synapse: When the presynaptic neuron is excited, synaptic vesicles release inhibitory neurotransmitters, causing inhibitory effects in the postsynaptic neuron.
3. Principle
Neurotransmitters are chemical substances that transmit nerve impulses between synapses. When a nerve impulse reaches the axon terminal, the stored neurotransmitters in the synaptic vesicles of the presynaptic component are released. They act on the postsynaptic component through the synaptic cleft, changing the permeability of the postsynaptic component's membrane and causing changes in the postsynaptic neuron's membrane potential, thus achieving the transmission of nerve impulses. The transmission of nerve impulses between synapses is also known as chemical transmission.
Peripheral Nervous System and Central Nervous System
1. Peripheral Nervous System
The peripheral nervous system is the neural structure that connects sensory input and motor output. It connects sensory organs, motor organs, visceral organs, and the central nervous system.
(1) Somatic Nervous System: It is composed of nerves that are connected to skeletal muscles and sensory receptors. It is mainly responsible for transmitting information between the central nervous system and sensory organs and skeletal muscles. It includes 12 pairs of cranial nerves and 31 pairs of spinal nerves.
(2) Autonomic Nervous System: It is mainly responsible for transmitting information between visceral organs and glands. It is divided into the sympathetic nervous system and the parasympathetic nervous system, which function in an antagonistic manner.
① Sympathetic Nervous System: It can increase the body's arousal level and prepare it for emergencies.
② Parasympathetic Nervous System: It can help the body relax and restore or maintain a state of calmness.
The somatic nervous system and the autonomic nervous system work together to coordinate the internal state and external behavior of the body. The main difference between the two is that the activity of the somatic nervous system is voluntary, while the activity of the autonomic nervous system is involuntary.
2. Central Nervous System★★
The central nervous system is the central part of neural activity, including the spinal cord and the brain.
(1) Spinal Cord
① Structure: Gray matter (cell bodies, in the middle) and white matter (axons, on the outside).
② Functions:
A. The spinal cord is the bridge between the brain and the peripheral nerves.
B. The spinal cord is a pathway for nerve signals and can analyze simple stimuli, produce responsive reactions, and perform simple reflex activities such as knee-jerk reflex and elbow reflex.
(2) Brain
① Brainstem
A. Medulla Oblongata: The part connected to the spinal cord. It contains the respiratory center and the cardiac center, also known as the vital center.
B. Pons: Located above the medulla oblongata, it is an ascending and descending pathway that connects the cerebellum and the cerebrum. It acts as a bridge and is a necessary passage for the transmission of information between the central nervous system and the peripheral nerves. It regulates and controls sleep.
C. Midbrain: Located above the pons, it is a pathway for nerves. It contains reflex centers for vision and hearing and controls the pupil, eyeball, and other functions.
D. Reticular Formation: A network-like structure distributed in the brainstem, it is a neural structure that controls arousal and consciousness. It is divided into the ascending system and the descending system. The ascending reticular formation, also known as the ascending activating system, controls the arousal and consciousness of the body and maintains the excitability of the cerebral cortex, which is closely related to maintaining attention. The descending reticular formation, also known as the descending activating system, has two effects on muscle tension: facilitation and inhibition, which can strengthen or weaken muscle activity.
② Diencephalon
A. Thalamus: It is a relay station for sensory nerves. All incoming information, except for olfaction, passes through the thalamus to the cerebral cortex. The thalamus is part of the reticular formation and also plays an important role in controlling sleep and wakefulness.
B. Hypothalamus: It is the main subcortical center that regulates the autonomic nervous system. It is of great significance in controlling the endocrine system, maintaining metabolism, regulating body temperature, and is closely related to physiological motivations such as hunger, thirst, and sex. The hypothalamus is also closely related to emotions. Stimulation of certain parts of the hypothalamus with weak electric currents can produce pleasure, while stimulation of adjacent areas can produce pain or unpleasant emotions.
③ Cerebellum: Located at the back of the brainstem, it is divided into two hemispheres. The gray matter on the surface of the cerebellum is called the cerebellar cortex, and the white matter inside is called the medulla. The cerebellum plays a role in maintaining body balance and mediating muscle tension.
④ Cerebrum: The interior of the cerebrum is white matter, and the exterior is gray matter. The cerebral cortex is the gray matter of the cerebrum and is the place where nerve cells are concentrated, making it the highest part of the nervous system. The cerebral cortex has grooves and fissures, and the raised areas between the grooves are called gyri. The lateral surface of the cerebral cortex is divided into frontal lobe, parietal lobe, occipital lobe, and temporal lobe.
⑤ Limbic System: Located deep inside the brain, it includes the cingulate gyrus, hippocampus, hippocampal fissure, nearby cerebral cortex, as well as the thalamus, subthalamus, and other structures. It is a unified functional system. The limbic system is associated with instinctive activities in animals and is closely related to memory and emotions. Organisms below mammals on the evolutionary ladder do not have a limbic system.