The human body performs a vast number of functions simultaneously, and these functions must be coordinated for the organism to respond appropriately to changes in its internal and external environment. Coordination is the integration of the activities of the different organs to maintain homeostasis. In humans and other multicellular animals, this coordination is achieved by two major systems: the nervous system and the endocrine system. The nervous system provides rapid, point-to-point, short-lived signals, while the endocrine system provides slower, longer-lasting chemical signals through hormones.
The neural system is composed of highly specialised cells called neurons, which generate and transmit electrochemical impulses. The human nervous system is divided into the central nervous system (the brain and spinal cord) and the peripheral nervous system. This chapter describes the structure of the neuron, the mechanism of nerve impulse conduction, the synapse, the structure and functions of the brain and spinal cord, the reflex arc, and the sensory organs, including the eye and the ear.
The neuron is the structural and functional unit of the nervous system. A typical neuron consists of: - The cell body (cyton or soma): Contains the nucleus and other cytoplasmic organelles such as Nissl granules (clumps of rough ER). - Dendrites: Short, branched processes that receive information from other neurons or receptors and transmit it towards the cell body. - The axon: A long, single process that carries the nerve impulse away from the cell body. The axon may be myelinated (covered by a myelin sheath formed by Schwann cells) or non-myelinated. The gaps between the myelin segments are called nodes of Ranvier. The axon ends in a group of branches called axon terminals, which form synapses with other neurons or effector organs.
The nerve impulse is an electrochemical change that travels along the neuron. At rest, the membrane of the neuron is polarised: the inside of the axon is negatively charged compared to the outside. This resting membrane potential is about -70 millivolts, maintained by the unequal distribution of ions, especially sodium and potassium, with the sodium-potassium pump actively pumping sodium out and potassium in. The resting membrane is more permeable to potassium than to sodium.
When a stimulus is applied, the membrane becomes more permeable to sodium ions, which rush into the cell, depolarising the membrane. This generates an action potential, a brief reversal of the membrane potential, in which the inside becomes positive (about +40 mV). The action potential then propagates along the axon. In a myelinated axon, the impulse jumps from one node of Ranvier to the next, a process called saltatory conduction, which is faster and more energy-efficient than continuous conduction in non-myelinated fibres.
After the impulse has passed, the membrane is repolarised: sodium channels close, potassium ions leave the cell, and the sodium-potassium pump restores the resting potential. There is a brief period, the refractory period, during which the neuron cannot be stimulated again.
The synapse is the junction between the axon terminal of one neuron and the dendrite or cell body of the next neuron (or an effector). The neuron that transmits the impulse is the presynaptic neuron, and the receiving one is the postsynaptic neuron. The gap between them is the synaptic cleft.
When the action potential reaches the axon terminal, it causes the synaptic vesicles to release a neurotransmitter (such as acetylcholine) into the synaptic cleft. The neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic membrane. This binding opens ion channels, generating a new action potential in the postsynaptic neuron (or causing contraction in a muscle). After transmission, the neurotransmitter is destroyed by enzymes (e.g., acetylcholinesterase destroys acetylcholine) so that the synapse can be ready for the next impulse.
The central nervous system (CNS) consists of the brain and the spinal cord, which are protected by the bony skull and the vertebral column respectively. They are also surrounded by three membranes called the meninges (dura mater, arachnoid mater and pia mater), and by the cerebrospinal fluid (CSF), which cushions and nourishes the nervous tissue.
The brain is the central organ of the nervous system, divided into the forebrain, midbrain and hindbrain. - Forebrain: Consists of the cerebrum (the largest part), the thalamus and the hypothalamus. The cerebrum is divided into two cerebral hemispheres, which are connected by the corpus callosum. The outer layer, the cerebral cortex, is made of grey matter (neuron cell bodies) folded into convolutions, and it is responsible for conscious thought, memory, intelligence, voluntary movements and sensory perception. The inner part is white matter. The hypothalamus controls the internal environment, body temperature, hunger and thirst, and regulates the pituitary gland. The thalamus is a relay station for sensory impulses. - Midbrain: Contains the corpora quadrigemina (four optic lobes) and connects the forebrain to the hindbrain. - Hindbrain: Consists of the cerebellum, the pons and the medulla oblongata. The cerebellum coordinates voluntary movements and maintains body balance and posture. The pons connects the different parts of the brain and regulates respiration. The medulla oblongata contains the centres that control respiration, heart rate, blood pressure and other involuntary activities. The medulla passes into the spinal cord at the foramen magnum.
The spinal cord is a cylindrical, elongated structure that extends from the medulla oblongata down the vertebral column. It is covered by the meninges and surrounded by the cerebrospinal fluid. The spinal cord has grey matter in the centre (H-shaped) and white matter outside. It conducts impulses to and from the brain and also controls reflex actions.
The peripheral nervous system (PNS) consists of the nerves that arise from the brain (cranial nerves) and the spinal cord (spinal nerves). The human PNS has 12 pairs of cranial nerves and 31 pairs of spinal nerves. The PNS is divided into the somatic neural system (transmits impulses from the CNS to the skeletal muscles for voluntary action, and from receptors to the CNS) and the autonomic neural system (transmits impulses to the involuntary organs and smooth muscles, heart and glands).
The autonomic nervous system is further divided into: - Sympathetic nervous system: Prepares the body for action (fight or flight response), increasing heart rate, dilating the pupils and bronchi. - Parasympathetic nervous system: Promotes rest and digest activities, decreasing heart rate and constricting the pupils.
A reflex action is a spontaneous, involuntary, rapid response to a stimulus. The reflex arc is the pathway of the reflex action, which involves: 1. A receptor that detects the stimulus. 2. A sensory (afferent) neuron that carries the impulse to the spinal cord. 3. An interneuron in the spinal cord (in some reflexes). 4. A motor (efferent) neuron that carries the impulse from the spinal cord to the effector. 5. An effector (muscle or gland) that responds.
An example is the withdrawal of the hand when it touches a hot object: the stimulus is detected by the receptors in the skin, the impulse travels to the spinal cord, and the motor response is generated immediately, even before the brain is aware of it. This reflex occurs in the spinal cord and is called a spinal reflex.
The eye is the organ of vision. The eyeball is enclosed in a bony socket and is composed of three layers: - The outer sclera (tough, white, protective) and the transparent cornea (front part through which light enters). - The middle choroid (rich in blood vessels and pigment) and the ciliary body, from which the lens is suspended by the suspensory ligaments. The iris, the coloured part, regulates the amount of light entering through the pupil. - The inner retina, which contains the photoreceptor cells, the rods (for vision in dim light, with the pigment rhodopsin) and cones (for colour vision and bright light, with the pigment iodopsin).
The lens forms an inverted image of the object on the retina, and the photoreceptors convert light into nerve impulses, which are transmitted by the optic nerve to the brain. The fovea is the region of the retina with the highest concentration of cones, responsible for sharp, detailed vision. The blind spot is the region where the optic nerve leaves the eye and has no photoreceptors.
The ear is the organ of hearing and balance. It is divided into three parts: - The outer ear: Consists of the pinna (auricle) and the external auditory canal. - The middle ear: Contains the three ear ossicles, the malleus, incus and stapes, which transmit and amplify the vibrations of the eardrum (tympanic membrane). The eustachian tube connects the middle ear to the pharynx, equalising the pressure. - The inner ear: Contains the cochlea (the organ of hearing) and the vestibular apparatus (the organ of balance, with the semicircular canals, utricle and saccule). The cochlea is filled with fluid and contains the organ of Corti, which has hair cells that convert sound vibrations into nerve impulses, transmitted by the auditory nerve to the brain.
| Region | Parts | Function |
|---|---|---|
| Forebrain | Cerebrum, thalamus, hypothalamus | Thought, memory, sensory relay, homeostasis |
| Midbrain | Corpora quadrigemina | Visual and auditory reflexes |
| Hindbrain | Cerebellum, pons, medulla | Balance, respiration, heart rate |
| Part | Function |
|---|---|
| Dendrites | Receive impulses |
| Cell body | Contains nucleus, Nissl granules |
| Axon | Carries impulse away |
| Axon terminals | Release neurotransmitters |
| Nodes of Ranvier | Saltatory conduction |
| Component | Role |
|---|---|
| Receptor | Detects stimulus |
| Sensory neuron | Carries impulse to CNS |
| Interneuron | Relay in spinal cord |
| Motor neuron | Carries impulse to effector |
| Effector | Muscle or gland response |
Neural control and coordination give the human body the ability to respond rapidly and precisely to the environment. The neuron, with its specialised structure and the electrochemical propagation of impulses, forms the basis of all nervous communication. The action potential, generated by ion movements across the membrane, and the synaptic transmission of neurotransmitters enable signals to travel from sensory receptors through the central nervous system to effectors. The brain, with its highly developed forebrain and the coordinating hindbrain, governs everything from conscious thought to the automatic control of respiration and heart rate. Reflex actions protect the body from harm, while the eye and ear translate light and sound into nerve impulses. Together with the endocrine system, the nervous system maintains the homeostasis that is essential for survival, setting the stage for the study of chemical coordination in the final chapter.