Learning Outcomes
- Understand the basic structure and function of the neural system.
- Explore the human neural system’s major components and divisions.
- Study the structure and function of a neuron and mechanisms of nerve impulse transmission.
- Comprehend reflex actions and how the central neural system coordinates these responses.
- Learn about sensory reception and processing, including the role of the eye and ear in perceiving stimuli.
The human body maintains homeostasis through the coordination of various organs and organ systems. This coordination ensures that different organs interact and complement each other in a synchronized manner. For instance, during physical exercise, increased energy demand leads to higher oxygen needs, which in turn results in an increase in respiration rate, heart rate, and blood flow. Once physical activity ceases, these processes gradually return to their normal state. The neural system and endocrine system work together to integrate and coordinate all activities, ensuring the organs function in harmony. The neural system provides a rapid and organized response network through point-to-point connections, while the endocrine system controls chemical integration via hormones.
The neural system across the animal kingdom varies in complexity. In lower organisms like Hydra, the neural system is a simple network of neurons. Insects exhibit more organization with a brain, ganglia, and other neural structures. Vertebrates possess a highly developed neural system with more complex functions.
The human neural system is divided into two major parts:
The PNS is further classified into two divisions:
A neuron is the structural and functional unit of the neural system. Neurons consist of three primary components:
Axons are either myelinated or unmyelinated. Myelinated fibers are surrounded by Schwann cells, which form a myelin sheath. The gaps between myelin sheaths are called nodes of Ranvier. Unmyelinated fibers are enclosed by a Schwann cell but lack a myelin sheath. Myelinated fibers are present in spinal and cranial nerves, while unmyelinated fibers are common in the autonomic and somatic neural systems.
Important Note:
Myelination speeds up the transmission of nerve impulses, while unmyelinated fibers transmit impulses more slowly.
Neurons are excitable cells due to their polarized membrane state. The resting membrane has more K+ inside the axoplasm and more Na+ outside. This creates an ionic gradient, which is maintained by the sodium-potassium pump. When a stimulus is applied, the membrane becomes permeable to Na+, allowing an influx of sodium ions and reversing the polarity at that site. This is called depolarization, and the resulting potential difference is termed the action potential or nerve impulse. The action potential travels along the axon through the following mechanism:
After depolarization, K+ diffuses out of the membrane to restore the resting potential.
Synapses are junctions where nerve impulses transfer between neurons. There are two types of synapses:
At a chemical synapse, neurotransmitters are released into the synaptic cleft, bind to specific receptors on the post-synaptic membrane, and trigger the opening of ion channels. This may generate either an excitatory or inhibitory potential in the post-synaptic neuron.
The CNS controls all voluntary and involuntary actions, integrating complex sensory, motor, and cognitive functions.
The forebrain comprises the cerebrum, thalamus, and hypothalamus. The cerebrum is divided into left and right hemispheres connected by the corpus callosum. The outer layer, the cerebral cortex, contains motor, sensory, and association areas. Below the cerebrum is the thalamus, a coordination center for sensory and motor signals. The hypothalamus below the thalamus controls body temperature, hunger, thirst, and hormonal secretions.
Important Note:
The limbic system, including the amygdala and hippocampus, alongside the hypothalamus, regulates emotions, motivation, and sexual behavior.
The midbrain lies between the forebrain and hindbrain, consisting of corpora quadrigemina, which processes visual and auditory inputs.
The hindbrain includes the pons, cerebellum, and medulla oblongata. The pons connects various brain regions, the cerebellum helps maintain balance, and the medulla regulates respiration, heart rate, and gastric secretions.
A reflex action is an automatic, involuntary response to a stimulus, typically involving the spinal cord and not requiring conscious thought. The reflex arc consists of an afferent neuron that carries the stimulus to the CNS and an efferent neuron that transmits the response to the target organ.
Sensory organs detect environmental changes and send signals to the CNS for processing. The CNS then interprets these signals and coordinates appropriate responses.
The human eye is a complex organ responsible for vision. It is a nearly spherical structure with three layers:
There are two types of photoreceptor cells:
Light entering the eye through the cornea and lens is focused on the retina, generating impulses in the rods and cones. The photopigments, composed of opsin and retinal, undergo structural changes, leading to the generation of action potentials. These impulses travel to the visual cortex of the brain, where the image is processed and recognized.
The ear functions in hearing and balance and is divided into three sections:
apes).
Sound waves collected by the pinna cause the tympanic membrane to vibrate. These vibrations pass through the ossicles to the oval window of the cochlea, where they generate waves in the inner ear fluids. The basilar membrane movements bend the hair cells, generating nerve impulses sent to the auditory cortex for sound interpretation.
| Aspect | Rods | Cones |
|---|---|---|
| Function | Twilight (scotopic) vision | Daylight (photopic) vision and color |
| Sensitivity | High sensitivity in dim light | Low sensitivity in bright light |
| Pigment | Rhodopsin (visual purple) | Three pigments (for red, green, and blue) |
The vestibular apparatus in the inner ear, consisting of semi-circular canals and the otolith organ, maintains balance by detecting changes in body position and motion.
MCQ
Which part of the ear is responsible for determining the pitch of a sound?
Answer: Cochlea.