The Mammalian Nervous System: Structure and Higher Functions

Nervous Systems: Cells and Functions Neurons are specialized cells of the nervous system that receive, encode, and transmit information. Neurons with their support cells (glial cells) make up nervous systems. Information is received by sensory cells and converted into electrical signals that are transmitted and processed by neurons. To cause behavioral or physiological responses, a nervous system communicates these signals to effectors, such as muscles and glands.

Nervous Systems: Cells and Functions The neuron’s plasma membrane generates electrical signals called nerve impulses (or action potentials) and conducts the signals from one location on a cell to the most distant reaches of that cell. Neurotransmission is the electrochemical process by which nerve cells communicate. Most neurons have four regions: a cell body, dendrites, an axon, and axon terminals.

Nervous Systems: Cells and Functions The cell body contains the nucleus and most of the cell’s organelles. Many projections sprout from the cell body; most of them are dendrites, which bring information from other neurons or sensory cells to the cell body.

Nervous Systems: Cells and Functions The axon usually carries information away from the cell body. Axons conduct information to target cells, which can be other neurons, muscle cells, or gland cells. At its end, the axon divides into many fine nerve endings. At the tip of each nerve ending is a swelling called the axon terminal. The axon terminal is positioned very close to the target cell. At the axon, terminal nerve impulses cause the release of neurotransmitters (chemical messengers) into the synapse.

Figure 44.2 Neurons (Part 1)

Nervous Systems: Cells and Functions The synapse is a specialized region in which nerve cells make contact with one another. It is here that information, in chemical form, is transferred between neurons.

Nervous Systems: Cells and Functions Variation between different types of neurons is considerable. Length of the axon differs in different cell types. Some axons can be very long. Gilal cells supply neurons with nutrients. Some consume foreign particles, and some maintain ionic balance around neurons.

Neurons with fewer dendrites process fewer imputs. Figure 44.2 Neurons (Part 2) Neurons with fewer dendrites process fewer imputs. Neurons with bushy dendrites collect information from many other cells.

Figure 44.2 Neurons (Part 3) Some neurons branch over a broad area. Some neurons provide local links to a small number of cells. Some communicate long distances via long axons.

Nervous Systems: Cells and Functions It is important to remember that nervous systems depend on neurons working together. The simplest neural network consists of three cells: a sensory neuron connected to a motor neuron connected to a muscle cell. Most neuronal networks are more complex. The human brain has an estimate 1011 neurons and 1014 synapses. The neurons and synapses in the human brain are divided into thousands of distinct but interacting networks that function in parallel.

Neurons: Generating and Conducting Nerve Impulses The difference in voltage across the plasma membrane of a neuron is called its membrane potential. In an unstimulated neuron, the voltage difference is called a resting potential. An action potential is the sudden and rapid reversal in voltage across a portion of the plasma membrane. For 1 to 2 milliseconds, the inside of the cell becomes more positive than the outside. Nerve impulses are action potentials that travel along axons.

Figure 44.13 Synaptic Transmission Begins with the Arrival of a Nerve Impulse

The Nervous System: Structure, Function, and Information Flow The brain and spinal cord together constitute the central nervous system (CNS). Information is transmitted from sensory cells to the CNS and from the CNS to effectors via neurons, which extend or reside outside of the brain and spinal cord. In the CNS, sensory information is assimilated, responses formulated, and motor responses sent out. In addition to controlling physiological activities, the brain encodes and stores information in the form of memory, and generates ideas.

The Nervous System: Structure, Function, and Information Flow Information is brought to and from the CNS by means of the enormous network of nerves that make up the peripheral nervous system. The peripheral nervous system reaches every tissue of the body; it connects to the CNS via spinal and cranial nerves. The PNS consists of pairs of nerve extensions from the brain, the spinal cord, and the autonomic nervous system. Some axons in a nerve may be carrying information from the CNS while other axons in the same nerve may be carrying information to the CNS.

Figure 44.1 Nervous Systems Vary in Size and Complexity (Part 2)

The Nervous System: Structure, Function, and Information Flow The nervous system is a complicated information processing system. The afferent portion of the peripheral nervous system carries information to the CNS. We are consciously aware of vision, hearing, pain, and limb position, which are types of information that move through afferent pathways. We are unaware of other afferent information such as blood pressure, deep body temperature, and blood oxygen supply.

The Nervous System: Structure, Function, and Information Flow The efferent portion of the peripheral nervous system carries information from the CNS to the muscles and glands of the body. Efferent pathways can be divided into a voluntary division (conscious movements) and an involuntary, or autonomic, division (physiological functions). In addition to receiving neural information, the CNS receives chemical information from hormones circulating in the blood or from neurohormones released by neurons into the extracellular fluids of the brain.

The Nervous System: Structure, Function, and Information Flow The Autonomic Nervous System is the portion of the peripheral nervous system that regulates the activity of the human body’s internal organs. It is in direct connection with the cardiac muscle, smooth muscles, and glands. It controls motor activity such as speeding up and slowing down heartbeat and dilating and constricting the pupils. It also works on the conscious level by relaying sensory information, such as the stuffed feeling after eating a big meal, to the CNS.

Figure 46.1 Organization of the Nervous System

The Nervous System: Structure, Function, and Information Flow Early in vertebrate development, a hollow tube of neural tissue forms. At the anterior end, the neural tube forms three swellings that become the basic divisions of the brain: the hindbrain, midbrain, and forebrain.

Figure 46.2 Development of the Human Nervous System (Part 1)

Figure 46.2 Development of the Human Nervous System (Part 3)

The Nervous System: Structure, Function, and Information Flow Different brain structures develop from the three embryonic regions. The medulla, pons, and cerebellum are derived from the hindbrain. The medulla and pons contain distinct groups of neurons that are involved in the control of physical functions such as breathing and circulation. All information traveling from the spinal cord must pass through the pons and medulla. The cerebellum refines motor commands to the joints and muscles.

The Nervous System: Structure, Function, and Information Flow The part of the brain from which the spinal cord extends is called the brain stem. The brain stem controls the functioning of vital body organs. It is the site of the reticular activating system, which regulates the brain’s level of awareness. The brain stem filters sensory information that stimulates the reticular activating system, which then stimulates activity and alterness in the brain.

The Nervous System: Structure, Function, and Information Flow The core of the forebrain and consists of the thalamus and the hypothalamus. Cerebrum consists of two cerebral hemispheres, left and right. In humans, the cerebrum is the largest part of the brain and plays major roles in sensory perception, learning, memory, and conscious behavior.

Functional Subsystems of the Nervous System The spinal cord conducts information between the brain and organs of the body, integrates information coming from the peripheral nervous system, and issues motor commands. In the nervous system, gray matter is tissue rich in neuronal cell bodies, and white matter contains axons. Spinal nerves leave the spinal cord at regular intervals; each one has two roots, one connecting it to the dorsal horn of the gray matter and the other connecting with the ventral horn.

Figure 46.3 The Spinal Cord Processes Information

Functional Subsystems of the Nervous System The cerebral cortex is the largest part of the central nervous system. It is divided into two hemispheres and four paired lobes, which are separated in terms of function but which share information. It is the highest level at which sensory information is perceived and identified and motor responses initiated. Higher functions such as thought, speech, and music appreciation take place here.

Figure 46.5 The Human Cerebrum (Part 1)

Functional Subsystems of the Nervous System Proprioception is the sense of position and movement of the body. Sense receptors located in muscles, tendons, and joints, relay information concerning the position of body parts, muscular coordination, and the degree of stretch in muscles and tendons.