Nervous System: Structure, Function, and Sensory Mechanisms
The Nervous System: An Overview
The nervous system collects information about our body’s internal and external environment, processes that information, and responds to it. The peripheral nervous system consists of nerves and supporting cells. It gathers the information and sends it to the central nervous system. The central nervous system consists of the brain and spinal cord. It processes the information and forms a response.
Types of Neurons
- Sensory neurons: receive information from sense organs.
- Motor neurons: send commands from the brain and spinal cord to muscles and glands.
- Interneurons: process information from sensory neurons to send commands to other interneurons or motor neurons.
Neuron Structure
- Cell body: contains the nucleus of the neuron and the cytoplasm.
- Dendrites: receive impulses from other neurons and carry them to the cell body.
- Axon: carries impulses away from the cell body.
- Myelin sheath: surrounds the axon and contains gaps called nodes.
- Synapse: the space where a neuron transfers an impulse to another.
Nerve Impulse Transmission
A nerve impulse begins when a neuron is stimulated by another neuron or by the environment.
- Resting potential: the outside has more sodium ions, and the inside has more potassium ions, creating a voltage difference.
- Moving impulse: a reversal of charge occurs during action potential, from more negative to more positive. The normal charge of the axon is restored as the impulse passes.
- Threshold: impulses from dendrites reach the cell body and rest in the axon hillock, where the plasma membrane generates impulses.
Active transport: Na+/K+ pumps require ATP.
The synapse is where vesicles containing neurotransmitters release them into the synaptic cleft, transmitting information.
Brain Structure and Function
The cerebrum controls sensing, consciousness, thinking, voluntary activities, imagination, intelligence, learning, and judgment. The cerebellum controls balance and coordination.
The brain stem consists of the midbrain, pons, and medulla oblongata. It controls automatic functions of the body.
Brain Lobes and Functions
- Cerebrum:
- Temporal lobe: processes sound and balance.
- Parietal lobe: handles association, taste, temperature, and touch.
- Occipital lobe: processes vision.
- Frontal lobe: manages memory, impulse control, emotions, planning, and organization.
- Cerebellum: controls coordination and balance.
- Brain stem: regulates respiration, heartbeat, and blood pressure.
The frontal lobe is fully developed around age 18.
Peripheral Nervous System
The peripheral nervous system consists of nerves and associated cells that are not part of the brain or spinal cord. It connects us with the outside world. It is divided into the somatic and autonomic nervous systems.
Somatic Nervous System
Regulates body activities under conscious control. It includes:
- Voluntary control: impulses from the brain travel through the spinal cord to muscles, resulting in action.
- Reflexes: rapid responses that pass through the reflex arc, resulting in instant action.
Autonomic Nervous System
Regulates involuntary activities like respiration and heartbeat.
Sensory Systems
Five senses: touch, smell, taste, sight, and hearing.
Touch, Temperature, and Pain
Different sensory receptors in the body respond to touch, temperature, and pain. These receptors are found in the skin and other areas.
Smell and Taste
The sensations of smell and taste result from impulses sent to the brain by chemoreceptors. These chemoreceptors are found in taste buds, which help us perceive the taste of food.
Hearing and Balance
Mechanoreceptors in the ear transmit impulses to the brain, which are translated into sound and information about balance. The cochlea is a key part of the ear for sound and balance.
Vision
Vision occurs when photoreceptors in the eyes transmit impulses to the brain, which are translated into an image.
Structure of the Eye
- Cornea: where light enters the eye.
- Retina: focuses light and contains cones and rods that distinguish colors and black and white.
- Iris: the colored part of the eye.
- Lens: helps adjust the eye’s focus.
- Pupil: regulates the amount of light entering the eye.